Some photos speak for themselves.Some gestures communicate morethan words ever could, like these
clasped hands, which seek comfort in theface of fear of the unknown. The picture wastaken Oct. 8, 2005, when aftershocks werestill being felt from the strongest earthquakeever to strike Kashmir, in northern India.Those clasped hands symbolize terror andpanic; they speak of fragility and
helplessness, of endurance in the face ofchaos. Unlike storms and volcanic eruptions,earthquakes are unpredictable, unleashedwithin seconds, and without warning. Theyspread destruction and death, forcingmillions to flee from their homes. The dayafter the catastrophe revealed a terrifyingscene: debris everywhere, a number ofpeople injured and dead, others wanderingdesperately, children crying, and over threemillion survivors seeking help after losingeverything. Throughout history Earth hasbeen shaken by earthquakes of greater orlesser violence. These earthquakes havecaused great harm. One of the most famousis the earthquake that rocked San Franciscoin 1906. Registering 8.3 on the Richter scale,the temblor left nearly three thousand deadand was felt as far away as Oregon to thenorth, and Los Angeles in southernCalifornia.
The purpose of this book is to help youbetter understand the causes offractures and the magnitude and
violence of the forces deep within the earth.The full-color, illustrated book you hold inyour hands contains shocking scenes ofcities convulsed by earthquakes andvolcanoes, natural phenomena that, in mereseconds, unleash rivers of fire, destroybuildings, highways and bridges, and gas andwater lines and leave entire cities withoutelectricity or phone service. If fires cannotbe put out quickly, the results are even moredevastating. Earthquakes near coastlandscan cause tsunamis, waves that spreadacross the ocean with the speed of anairplane. A tsunami that reaches a coast can
be more destructive than the earthquakeitself. On Dec. 26, 2004, the worldwitnessed one of the most impressivenatural disasters ever. An undersea quakewith a magnitude of 9 on the Richter scaleshook the eastern Indian Ocean, causingtsunamis that reached the coastal areas ofeight Asian nations, causing about 230,000deaths. The earthquake was the fifthstrongest since the invention of theseismograph. Satellite images show theregion before and after the catastrophe.
Throughout history, nearly all ancientpeoples and large societies havethought of volcanoes as dwelling
places of gods or other supernatural beingsto explain the mountains' fury. Hawaiianmythology, for instance, spoke of Pele, thegoddess of volcanoes, who threw out fire tocleanse the earth and fertilize the soil. Shewas believed to be a creative force.Nowadays, specialists try to find out when avolcano might start to erupt, because withinhours after an eruption begins, lava flowscan change a lush landscape into a barrenwilderness. Not only does hot lava destroyeverything in its path, but gas and ashexpelled in the explosion also replace oxygenin the air, poisoning people, animals, andplants. Amazingly, life reemerges once againfrom such scenes of destruction. After atime, lava and ash break down, making thesoil unusually fertile. For this reason manyfarmers and others continue to live nearthese “smoking mountains,” in spite of thelatent danger. Perhaps by living so close tothe danger zone, they have learned that noone can control the forces of nature, and theonly thing left to do is to simply live.
The Power of Nature
Kashmir, 2005Farmer Farid Hussain, 50, graspsthe hand of his wife, Akthar Fatma,after the earthquake that rockedthe Himalayas on the Indian subcontinent. Eighty thousand people were killed, and thousandsof families were left homeless.
OCEAN TRENCHES 16-17
WRINKLES IN THE EARTH 18-19
WHEN FAULTS RESOUND 22-23
In the volatile landscape ofVolcano National Park in Hawaii,the beginning and end of lifeseem to go hand in hand.Outpourings of lava often reach
the sea. When the molten rock entersthe water, the lava quickly cools andhardens into rock that becomes partof the coastline. By this process,volcanic islands grow constantly, and
nothing stays the same from onemoment to another. One day rivers oflava blaze down the volcano's slopes,and the next day there are new, silver-colored rocks. The ongoing
investigation of lava samples under themicroscope helps volcanologistsdiscover the rock's mineralcomposition and offers clues abouthow the volcano may behave.
SCORCHING FLOW 8-9
THE LONG HISTORY OF THE EARTH 10-11
STACKED LAYERS 12-13
THE JOURNEY OF THE PLATES 14-15
PAHOEHOE LAVAA type of Hawaiian lava that flows down the slopes of Mt. Kilauea to the sea.
VOLCANOES AND EARTHQUAKES 98 CONTINUOUS MOVEMENT
ost of the Earth's interior is in a liquid and incandescent state at extremely hightemperatures. This vast mass of molten rock contains dissolved crystals andwater vapor, among other gases, and it is known as magma. When part of the
magma rises toward the Earth's surface, mainly through volcanic activity, it is called lava.As soon as it reaches the surface of the Earth or the ocean floor, the lava starts to cooland solidify into different types of rock, according to its original chemical composition.This is the basic process that formed the surface of our planet, and it is the reason theEarth's surface is in constant flux. Scientists study lava to understand our planet better.
is the average temperatureof liquid lava.
1,800º F(1,000º C)
TYPES OF LAVABasaltic lava is found mainly in islands and in mid-ocean ridges; it is so fluid that it tends to spread asit flows. Andesitic lava forms layers that can be up to 130 feet (40 m) thick and that flow very slowly,whereas rhyolitic lava is so viscous that it forms solid fragments before reaching the surface.
Streams of FireLava is at the heart of every volcanic eruption. The characteristics of lava vary, depending onthe gases it contains and its chemical composition. Lava from an eruption is loaded with water
vapor and gases such as carbon dioxide, hydrogen, carbon monoxide, and sulfur dioxide. As thesegases are expelled, they burst into the atmosphere, where they create a turbulent cloud that sometimes discharges heavy rains. Fragments of lava expelled and scattered by the volcano are classified as bombs, cinders, and ash. Some large fragments fall back into the crater. The speed atwhich lava travels depends to a great extent on the steepness of the sides of the volcano. Some lavaflows can reach 90 miles (145 km) in length and attain speeds of up to 30 miles per hour (50 km/hr).
Rock CycleOnce it cools, lava forms igneous rock.This rock, subjected to weathering and
natural processes such as metamorphism and sedimentation, will form other types ofrocks that, when they sink back into theEarth's interior, again become molten rock.This process takes millions of years and isknown as the rock cycle.
Mineral CompositionLava contains a high level of silicates, light rocky mineralsthat make up 95 percent of the Earth's crust. The second
most abundant substance in lava is water vapor. Silicates determine lava's viscosity, that is, its capacity to flow. Variationsin viscosity have resulted in one of the most commonly used classification systems of lava: basaltic, andesitic, and rhyolitic, in order from least to greatest silicate content. Basaltic lavaforms long rivers, such as those that occur in typical Hawaiianvolcanic eruptions, whereas rhyolitic lava tends to erupt explosively because of its poor fluidity. Andesitic lava, namedafter the Andes mountains, where it is commonly found, is anintermediate type of lava of medium viscosity.
INTENSE HEATLava can reach temperaturesabove 2,200º F (1,200º C). Thehotter the lava, the more fluid it is.When lava is released in greatquantities, it forms rivers of fire.The lava's advance is slowed downas the lava cools and hardens.
SOLID LAVALava solidifies at temperatures below1,700º F (900º C). The most viscoustype of lava forms a rough landscape, littered with sharp rocks; more fluidlava, however, tends to form flatter andsmoother rocks.
LAVAThe state in which magmaflows to the Earth's outercrust, either reaching thesurface or getting trappedwithin the crust.
IGNEOUS ROCKRock formed when lavasolidifies. Basalt and granite are good examplesof igneous rocks.
SEDIMENTARY ROCKRock formed by eroded and compacted materials.
METAMORPHICROCKSTheir original structure is changedby heat and pressure.
VOLCANOES AND EARTHQUAKES 1110 CONTINUOUS MOVEMENT
4.5BILLION YEARS AGO
4BILLION YEARS AGO
The accumulation of matter into solidbodies, a process called accretion, ended,and the Earth stopped increasing in volume.
COOLINGThe first crust formed as itwas exposed to space and cooled. Earth's layers becamedifferentiated by their density.
WARMINGEarth warmed again, and theglaciers retreated, giving way tothe oceans, in which neworganisms would be born. Theozone layer began to form.
METEORITE COLLISIONMeteorite collisions, at a rate150 times as great as that oftoday, evaporated the primitiveocean and resulted in the rise ofall known forms of life.
The oldest rocksappeared.
CONTINENTSThe first continents, made of lightrocks, appeared. In Laurentia (nowNorth America) and in the Baltic,there are large rocky areas thatdate back to that time.
Hypothesis of a first,great glaciation.
1.8BILLION YEARS AGO
FOLDING IN THE TERTIARY PERIODThe folding began that would producethe highest mountains that we nowhave (the Alps, the Andes, and theHimalayas) and that continues togenerate earthquakes even today.
60MILLION YEARS AGO
The Long History of the EarthT
he nebular hypothesis developed by astronomers suggests that the Earth was formedin the same way and at the same time as the rest of the planets and the Sun. It allbegan with an immense cloud of helium and hydrogen and a small portion of heavier
materials 4.6 billion years ago. Earth emerged from one of these “small” revolving clouds,where the particles constantly collided with one another, producing very high temperatures.Later, a series of processes took place that gave the planet its present shape.
Earth was formed 4.6 billion years ago. In the beginning it was a body ofincandescent rock in the solar system. The first clear signs of life appeared in
the oceans 3.6 billion years ago, and since then life has expanded and diversified.The changes have been unceasing, and, according to experts, there will be many more changes in the future.
From Chaos to Today's Earth
When the first crust cooled, intense volcanicactivity freed gasesfrom the interior of theplanet, and those gasesformed the atmosphereand the oceans.
3.8BILLION YEARS AGO
2.3BILLION YEARS AGO
THE AGE OF THESUPER VOLCANOES
STABILIZATIONThe processes that formedthe atmosphere, the oceans,and protolife intensified. At the same time, the cruststabilized, and the first plates of Earth's crustappeared. Because of theirweight, they sank intoEarth's mantle, making wayfor new plates, a processthat continues today.
FRAGMENTATIONThe great landmass formed that would later fragment to provide the origin of thecontinents we have today. The oceans reached their greatest rate of expansion.
540MILLION YEARS AGO
PALEOZOIC ERA 2.2BILLION YEARS AGO
Indications of komatite,a type of igneous
rock that no longerexists.
SUPERCONTINENTSRodinia, the first supercontinent, formed, but itcompletely disappeared about650 million years ago.
1.0BILLION YEARS AGO
VOLCANOES AND EARTHQUAKES 1312 CONTINUOUS MOVEMENT
Earth's crust is its solid outer layer, with a thickness of 3 to 9 miles (4 to 15 km) under the oceans and up
to 44 miles (70 km) under mountain ranges. Volcanoes on land and volcanic activity in the mid-ocean ridges generatenew rock, which becomes part of the crust. The rocks at the bottom of the crust tend to melt back into the rocky mantle.
The air and most of the weather events that affect our lives occur only in the lower layer of the Earth's atmosphere. This relatively thin layer, called
the troposphere, is up to 10 miles (16 km) thick at the equator but only 4 miles (7 km) thick at the poles. Each layer of the atmosphere has a distinct composition.
The Gaseous Envelope
Composed mainly of molten iron and nickelamong other metals attemperatures above8,500º F (4,700° C).
The inner core behaves as a solid because it is under enormous pressure.
Composition similar to that of the crust, but in a liquid state and under great pressure, between 1,830° and 8,130° F(1,000° and 4,500° C).
THE MID-OCEAN RIDGESThe ocean floor is regenerated with newbasaltic rock formed by magma that solidifiesin the rifts that run along mid-ocean ridges.
THE CONTINENTALSHELFIn the area wherethe oceanic crustcomes in contact witha continent, igneousrock is transformedinto metamorphic rockby heat and pressure.
KEY Sedimentary Rock
PLUTONSMasses of risingmagma trappedwithin the Earth'scrust. Their name isderived from Pluto,the Roman god ofthe underworld.
INTERNAL ROCKThe inside of amountain rangeconsists of igneousrock (mostly granite) and metamorphic rock.
GRANITIC BATHOLITHSPlutons can solidifyunderground asmasses of granite.
COASTAL ROCKLithified layers ofsediments, usuallyclay and pebbles,that come from theerosion of highmountains.
OCEANIC ISLANDSSome sedimentary rocks areadded to the predominantlyigneous rock composition.MOUNTAIN RANGES
Made up of the threetypes of rock in aboutequal parts.
Contains 75 percent of the gas and almostall of the water vaporin the atmosphere.
Every 110 feet (33 m) below the Earth's surface, the temperature increases by 1.8 degreesFahrenheit (1 degree Celsius). To reach the Earth's center—which, in spite of temperaturesabove 12,000° F (6,700° C), is assumed to be solid because of the enormous pressure
exerted on it—a person would have to burrow through four well-defined layers. The gases thatcover the Earth's surface are also divided into layers with different compositions. Forces act onthe Earth's crust from above and below to sculpt and permanently alter it.
LITHOSPHERE 93 miles(150 km)
(450 km)280 milesASTHENOSPHERE
Very dry; water vaporfreezes and falls out of this layer, which contains the ozone layer.
The temperature is -130º F (-90° C), butit increases graduallyabove this layer.
THE SOLID EXTERIORThe crust is made up ofigneous, sedimentary, andmetamorphic rock, ofvarious typical compositions,according to the terrain.
Includes the solidouter part of theupper mantle, as well as the crust.
Underneath is theasthenosphere,made up of partiallymolten rock.
Metamorphic RockIgneous Rock
6 miles Less than
31 milesLess than
62 milesLess than
Very low density. Below155 miles (250 km) it is made up mostly of nitrogen; above thatlevel it is mostly oxygen.
310 milesLess than
No fixed outer limit. Itcontains lighter gasessuch as hydrogen andhelium, mostly ionized.
310 milesGreater than
VOLCANOES AND EARTHQUAKES 1514 CONTINUOUS MOVEMENT
CONVECTION CURRENTSThe hottest molten rock rises; once it rises, it cools and sinks again. This process causes continuous currents in the mantle.
CONVERGENT BOUNDARYWhen two plates collide, one sinksbelow the other, forming a subductionzone. This causes folding in the crustand volcanic activity.
OUTWARD MOVEMENTThe action of the magma causes the tectonic plate to move toward a subduction zone at its far end.
WIDENINGAt divergent plate boundaries the magmarises, forming new oceanic crust. Foldingoccurs where plates converge.
DIVERGENT BOUNDARYWhen two plates separate, arift is formed between them.Magma exerts great pressure,and it renews the ocean flooras it solidifies. The AtlanticOcean was formed in this way.
The number of years it will take for the continents to drift together again.
2 inches(5 cm)Typical distance the platestravel in a year.
The Journey of the PlatesW
hen geophysicist Alfred Wegener suggested in 1910 thatthe continents were moving, the idea seemed fantastic.There was no way to explain the idea. Only a half-century
later, plate tectonic theory was able to offer an explanation of thephenomenon. Volcanic activity on the ocean floor, convectioncurrents, and the melting of rock in the mantle power the continental drift that is still molding the planet's surface today.
Convection currents of molten rock propel the crust. Rising magma
forms new sections of crust at divergentboundaries. At convergent boundaries, the crust melts into the mantle. Thus, the tectonic plates act like a conveyor belt on which the continents travel.
The Hidden Motor
The landmass today's continents come from wasa single block (Pangea) surrounded by the ocean.
...180 MILLION YEARS AGO The North American Plate has separated, as hasthe Antarctic Plate. The supercontinent Gondwana(South America and Africa) has started to divideand form the South Atlantic. India is separatingfrom Africa.
…100 MILLION YEARS AGOThe Atlantic Ocean has formed. India is headedtoward Asia, and when the two masses collide,the Himalayas will rise. Australia is separatingfrom Antarctica.
... 60 MILLION YEARS AGO MILLIONYEARSThe continents are near their current location. India
is beginning to collide with Asia. The Mediterranean is opening, and the folding is already taking place that will give rise to the highest mountain ranges of today.
250 MILLION YEARS AGO
The first ideas on continental drift proposed that the continents floated on the ocean.
That idea proved inaccurate. The seven tectonic plates contain portions of ocean beds and continents.They drift atop the molten mantle like sections of agiant shell. Depending on the direction in which they move, their boundaries can converge (whenthey tend to come together), diverge (when theytend to separate), or slide horizontally past eachother (along a transform fault).
Inside and Outside the RidgeThe abyssal (deep-ocean) plains of the Atlantic are theflattest surfaces on Earth; for thousands of miles, the
elevation varies by only about 10 feet (3 m). The plains aremade mostly of sediment. Variations in the ocean's depth aremainly the result of volcanic activity, not just within the mid-Atlantic Ridge but elsewhere as well.
A spongy layer of rock severaldozen miles wide rises above the
rift. As the layer fractures and movesaway from the fissure, it solidifies intoangled blocks that are parallel to thefissure and separated by dikes. Thus theocean widens as the ridge spreads. Themagma exists in a fluid form 2 miles(3.5 km) below the crest of the ridge.
Magnetic ReversalsThe Earth's magnetic field changes directionperiodically. The magnetic north pole changes
places with the magnetic south pole. Rock that solidifiedduring a period of magnetic polarity reversal wasmagnetized with a polarity opposite that of newly formingrocks. Rocks whose magnetism corresponds to the presentdirection of the Earth's magnetic field are said to havenormal polarity, whereas those with the opposite magneticpolarity are said to have reversed polarity.
The constant generation of newocean crust along rift zones
powers a seemingly endless process thatgenerates new lithosphere that is carriedfrom the crest of the ridges, as if on aconveyor belt. Because of this, scientistshave calculated that in about 250 millionyears, the continents will again join andform a new Pangea as they are pushed
by the continually expanding ocean floor.Ocean plates are in contact with landplates at the active boundaries ofsubduction zones or at passivecontinental boundaries (continentalshelves and slopes). Underseasubduction zones, called ocean trenches,also occur between oceanic plates: theseare the deepest places on the planet.
OCEAN MOUNTSIsolated volcanic cones. Some riseabove the ocean's surface tobecome islands, such as the Azores.
ATOLLSAlso called coral reefs, atolls areformations of coral depositedaround a volcanic cone in warmseas. They form ring-shaped islands.
29,035 feet (8,850 m)Highest point (Mount Everest)
2,900 feet (870 m)Average land elevation
7,900 feet (2,400 m)Earth's average elevation
12,240 feet (3,730 m) Average depth
Greatest depth (Mariana Trench)About 36,000 feet(11,000 m)
0 feet (0 m) Sea level
Dikes withinhost rock
HEIGHTS AND DEPTHSDeep-ocean basins cover 30 percent of the Earth'ssurface. The depth of theocean trenches is greater thanthe height of the greatestmountain ranges, as shown inthe graphic below at left.
Cracks in the Ocean Floor
16 CONTINUOUS MOVEMENT VOLCANOES AND EARTHQUAKES 17
The concept that the ocean floor is spreading was studiedfor many years: new crust constantly forms at thebottom of the ocean. The ocean floor has deep trenches,
plains, and mountain ranges. The mountain ranges are higherthan those found on the continents but with differentcharacteristics. The spines of these great mountain ranges,
The Crust Under the Oceans
called mid-ocean ridges, exhibit incredible volcanic activity inrift zones. The rift zones are fissures in relatively narrowregions of the crust, along which the crust splits and spreads.One hundred eighty million years ago, the paleocontinentGondwana broke apart, forming a rift from which the AtlanticOcean grew, and is still growing.
How the Mid-OceanRidge Was Formed
The Three Greatest Folding Events The Earth's geological history has included three major mountain-building processes, called “orogenies.” The mountains created during
the first two orogenies (the Caledonian and the Hercynian) are much lowertoday because they have undergone millions of years of erosion.
Formation of the HimalayasThe highest mountains on Earth were formed following the collision ofIndia and Eurasia. The Indian Plate is sliding horizontally underneath
the Asiatic Plate. A sedimentary block trapped between the plates is cuttingthe upper part of the Asiatic Plate into segments that are piling on top ofeach other. This folding process gave rise to the Himalayan range, whichincludes the highest mountain on the planet, Mount Everest (29,035 feet[8,850 m]). This deeply fractured section of the old plate is called anaccretion prism. At that time, the Asian landmass bent, and the plate doubledin thickness, forming the Tibetan plateau.
A portion of the crust subjected to a sustainedhorizontal tectonic force is met by resistance,and the rock layers become deformed.
The outer rock layers, which are often more rigid,fracture and form a fault. If one rock boundaryslips underneath another, a thrust fault is formed.
1 2 3
430 Million YearsCALEDONIAN OROGENYFormed the Caledonian range.Remnants can be seen in Scotland, the
Scandinavian Peninsula, and Canada(which all collided at that time).
HERCYNIAN OROGENYTook place between the late Devonic andthe early Permian periods. It was moreimportant than the Caledonian Orogeny. Itshaped central and western Europe and
produced large veins of iron ore and coal.This orogeny gave rise to the UralMountains, the Appalachian range in NorthAmerica, part of the Andes, and Tasmania.
300 Million Years
MATERIALS Mostly granite, slate,amphibolite, gneiss, quartzite, and schist.
MATERIALSHigh proportions ofsediment in Nepal,batholiths in the AsiaticPlate, and intrusions of newgranite: iron, tin, and tungsten.
Distortions of the CrustThe crust is composed of layers of solid rock. Tectonicforces, resulting from the differences in speed and direction
between plates, make these layers stretch elastically, flow, orbreak. Mountains are formed in processes requiring millions of
years. Then external forces, such as erosion from wind,ice, and water, come into play. If slippage releases rockfrom the pressure that is deforming it elastically, the rocktends to return to its former state and can cause earthquakes.
VOLCANOES AND EARTHQUAKES 1918 CONTINUOUS MOVEMENT
MATERIALS Mudstone, slate, andsandstone, in lithified layers.
60 MILLION YEARS AGOThe Tethys Sea gives way as the platesapproach. Layers of sediment begin to rise.
40 MILLION YEARS AGOAs the two plates approach each other,a subduction zone begins to form.
20 MILLION YEARS AGOThe Tibetan plateau is pushed up bypressure from settling layers of sediment.
THE HIMALAYAS TODAYThe movement of the plates continues to fold thecrust, and the land of Nepal is slowly disappearing.
A COLLISION OF CONTINENTS
Tethys Sea TibetHeavysediments
ALPINE OROGENYBegan in the Cenozoic Era and continues today.This orogeny raised the entire system ofmountain ranges that includes the Pyrenees,the Alps, the Caucasus, and even theHimalayas. It also gave the American Rockiesand the Andes Mountains their current shape.
10 MILLIONYEARS AGO
20 MILLIONYEARS AGO
30 MILLIONYEARS AGO
The composition of rock layers shows the originof the folding, despite the effects of erosion.
The movement of tectonic plates causes distortions and breaks in the Earth'scrust, especially in convergent plate boundaries. Over millions of years,these distortions produce larger features called folds, which become
mountain ranges. Certain characteristic types of terrain give cluesabout the great folding processes in Earth's geological history.
Folding in the Earth's Crust
VOLCANOES AND EARTHQUAKES 21
CompositionBefore mountain ranges were lifted up by the collision of ancientcontinents, constant erosion of the land had deposited large
amounts of sediments along their coasts. These sediments later formedthe rock that makes up the folding seen here. As that rock's shape clearlyshows, tectonic forces compressed the originally horizontal sedimentsuntil they became curved. This phenomenon is seen along Cardigan Bayon the ancient coast of Wales.
20 CONTINUOUS MOVEMENT
he force that forms the mountains also molds the rocks within them. Asthe result of millions of years of pressure, the layers of crust fold intostrange shapes. The Caledonian Orogeny, which began 450 million years
ago, created a long mountain range that joined the Appalachian mountains ofthe United States to the Scandinavian peninsula. All of northern England waslifted up during this process. The ancient Iapetus Ocean once lay betweenthe colliding continents. Sedimentary rocks from the bed of this oceanwere lifted up, and they have kept the same forms they had in the past.
THREE CONTINENTSThe Caledonian orogenywas formed by thecollision of three ancientcontinents: Laurasia,Gondwana, and Baltica.In between them,the Iapetus Oceanfloor containedsediments that nowform the bedrock ofthe coast of Wales.
A MOUNTAIN RANGEThe long Caledonian range is seentoday in the coasts of England,Greenland, and Scandinavia. Since thetectonic movements that created themhave ended, they are being worn awayand sculpted by constant erosion.
The name of the geological periodin which this folding occurred.
WALES, UNITEDKINGDOMLatitude: 51° 30’ N
Longitude: 003° 12’ W
40 miles (64 km)
The great San Andreas fault in thewestern United States is the backbone of
a system of faults. Following the greatearthquake that leveled San Francisco in 1906,this system has been studied more than anyother on Earth. It is basically a horizontaltransform fault that forms the boundarybetween the Pacific and North American tectonic
plates. The system contains many complex lesserfaults, and it has a total length of 800 miles(1,300 km). If both plates were able to slide pasteach other smoothly, no earthquakes wouldresult. However, the borders of the plates are incontact with each other. When the solid rockcannot withstand the growing strain, it breaksand unleashes an earthquake.
Faults are small breaks that are produced along the Earth's crust. Many, such as theSan Andreas fault, which runs through the state of California, can be seen readily.Others, however, are hidden within the crust. When a fault fractures suddenly, an
earthquake results. Sometimes fault lines can allow magma from lower layers to breakthrough to the surface at certain points, forming a volcano.
VOLCANOES AND EARTHQUAKES 2322 CONTINUOUS MOVEMENT
When the Faults Resound Streambeds Divertedby Tectonic Movement
Through friction and surface cracking, atransform fault creates transverse faults
and, at the same time, alters them with itsmovement. Rivers and streams distorted by theSan Andreas fault have three characteristic forms:streambeds with tectonic displacement, divertedstreambeds, and streambeds with an orientationthat is nearly oblique to the fault.
Fault borders do not usually form straightlines or right angles; their direction along
the surface changes. The angle of verticalinclination is called “dip.” The classification of afault depends on how the fault was formed andon the relative movement of the two plates that
form it. When tectonic forces compress the crusthorizontally, a break causes one section of theground to push above the other. In contrast,when the two sides of the fault are under tension(pulled apart), one side of the fault will slip downthe slope formed by the other side of the fault.
Relative Movement Along Fault Lines
350 miles(566 km) The distance that the opposite sidesof the fault have slipped past eachother, throughout their history.
The average interval between majorruptures that have taken place alongthe fault. The interval can varybetween 20 and 300 years.
Juan deFuca Plate
East Pacific Ridge
Queen Charlotte Fault
Diverted StreambedThe stream changes courseas a result of the break.
Displaced StreambedThe streambed looks“broken” along its fault line.
1Normal FaultThis fault is the product ofhorizontal tension. Themovement is mostly vertical,with an overlying block (thehanging wall) movingdownward relative to anunderlying block (thefootwall). The fault planetypically has an angle of 60degrees from the horizontal.
2Reverse FaultThis fault is caused by a horizontalforce that compresses the ground. Afracture causes one portion of thecrust (the hanging wall) to slide overthe other (the footwall). Thrust faults(see pages 18-19), are a common formof reverse fault that can extend up tohundreds of miles. However, reversefaults with a dip greater than 45° areusually only a few yards long.
3Oblique-SlipFaultThis fault has horizontal as well as verticalmovements. Thus, the relative displacementbetween the edges of the fault can bediagonal. In the oldest faults, erosionusually smoothes the differences in thesurrounding terrain, but in more recentfaults, cliffs are formed. Transform faultsthat displace mid-ocean ridges are aspecific example of oblique-slip faults.
Strike-Slip FaultIn this fault the relative movement of the platesis mainly horizontal, along the Earth's surface,parallel to the direction of the fracture but notparallel to the fault plane. Transform faultsbetween plates are usually of this type. Ratherthan a single fracture, they are generally made upof a system of smaller fractures, slanted from acenterline and more or less parallel to each other.The system can be several miles wide.
OPPOSITEDIRECTIONSThe northwestwardmovement of thePacific Plate and thesoutheastward movementof the North American Platecause folds and fissuresthroughout the region.
PAST AND FUTURESome 30 million years ago, the Peninsulaof California was west of the presentcoast of Mexico. Thirty million yearsfrom now, it is possible that it may besome distance off the coast of Canada.
Maximum width of fault
Length of California
Length of fault
770 miles (1,240 km)
800 miles (1,300 km)
60 miles (100 km)
20 feet (6 m)
AFTERMATH OF FURY 36-37
JETS OF WATER 38-39
RINGS OF CORAL 40-41
FROZEN FLAME 42-43
Mount Etna has always beenan active volcano, as seenfrom the references to itsactivity that have beenmade throughout history. It
could be said that the volcano has notgiven the beautiful island of Sicily amoment's rest. The Greek philosopherPlato was the first to study Mount Etna.He traveled to Italy especially to see it
up close, and he subsequently describedhow the lava cooled. Today Etna'speriodic eruptions continue to drawhundreds of thousands of tourists, whoenjoy the spectacular fireworks
produced by its red-hot explosions. Thisphenomenon is visible from the entireeast coast of Sicily because of theregion's favorable weather conditionsand the constant strong winds.
FLAMING FURNACE 26-27
FLASH OF FIRE 30-31
MOUNT ST. HELENS 32-33
MOUNT ETNAWith a height of 10,810 feet(3,295 m), Etna is the largest andmost active volcano in Europe.
26 VOLCANOES VOLCANOES AND EARTHQUAKES 27
Volcanoes are among the most powerful manifestations ofour planet's dynamic interior. The magma they release atthe Earth's surface can cause phenomena that devastate
surrounding areas: explosions, enormous flows of molten rock,fire and ash that rain from the sky, floods, and mudslides.Since ancient times, human beings have feared volcanoes,even seeing their smoking craters as an entrance to theunderworld. Every volcano has a life cycle, during whichit can modify the topography and the climate andafter which it becomes extinct.
Explosiveeruptions canexpel hugequantities of lava,gas, and rock.
LIFE AND DEATH OF A VOLCANO:THE FORMATION OF A CALDERA
STREAMS OF LAVAflow down the flanksof the volcano.
CRATER Depression or hollowfrom which eruptionsexpel magmaticmaterials (lava, gas,steam, ash, etc.)
VOLCANIC CONE Made of layers ofigneous rock, formedfrom previous eruptions.Each lava flow adds anew layer.
MAIN CONDUIT The pipe throughwhich magma rises.It connects themagma chamberwith the surface.
MAGMA CHAMBERMass of molten rock attemperatures that may exceed
2,000° F(1,100° C)
In an active volcano, magmain the chamber is in constantmotion because offluctuations oftemperature and pressure(convection currents).
Magma can reach thesurface, or it can staybelow ground and exertpressure between thelayers of rock. Theseseepages of magmahave various names.
UNDER THE VOLCANO In its ascent to the surface, the magmamay be blocked in various chambers atdifferent levels of the lithosphere.
A void is left in the conduitand in theinternal chamber.
The cone breaks upinto concentricrings and sinks intothe chamber.
A depression, or caldera, formswhere the crater had been, and itmay fill up with rainwater.4.
Ocean crustSCALE IN MILES(KM)
1 When two platesconverge, one movesunder the other(subduction).
2 The rock melts andforms new magma.Great pressure builds upbetween the plates.
Many volcanoes are caused by phenomena occurring insubduction zones along convergent plate boundaries.
SILL Layer of magma formsbetween rock layers.
DIKE Vertical Channelof Magma.
PLUG OF ANEXTINCTVOLCANO
INTRUSION OF MAGMA
Composite volcaniccones have morethan one crater.
3 The heat and pressure in the crust force themagma to seep through cracks in the rock andrise to the surface, causing volcanic eruptions.
CINDER CONECone-shaped, circularmounds up to 980 feet(300 m) high. They areformed when falling debrisor ash accumulates nearthe crater. These volcaniccones have gently slopingsides, with an anglebetween 30° and 40°.
VOLCANOES AND EARTHQUAKES 2928 VOLCANOES
o two volcanoes on Earth are exactly alike, although they havecharacteristics that permit them to be studied according to six basictypes: shield volcanoes, cinder cones, stratovolcanoes, lava cones,
fissure volcanoes, and calderas. A volcano's shape depends on its origin,how the eruption began, processes that accompany the volcanic activity,and the degree of danger the volcano poses to life in surrounding areas.
LAVA DOMEThe sides are formed bythe accumulation of “hard”lava, made viscous by itshigh silicon content.Instead of flowing, itquickly hardens in place.
STRATOVOLCANO(COMPOSITE VOLCANO) Nearly symmetrical in appearance,formed by layers of fragmentedmaterial (ash and pyroclasts)between lava flows. A stratovolcanois structured around a main conduit,although it may also have severalbranch pipes. This is usually the mostviolent type of volcano.
SHIELD VOLCANO The diameter of thesevolcanoes is muchgreater than theirheight. They are formedby the accumulation ofhighly fluid lava flows, sothey are low, with gentlysloping sides, and theyare nearly flat on top.
Caldera that containsa lake
Plug of extinctvolcano
FORMATION OF THE VOLCANIC PLUG
Erosion ofthe cone
The plugis notaffected. The surrounding
terrain is flat.
The volcanicneck remains.
CALDERA VOLCANOLarge basins, similar to craters but greater than0.8 mile (1 km) across, are called calderas. Theyare found at the summit of extinct or inactivevolcanoes, and they are typically filled with deeplakes. Some calderas were formed aftercataclysmic explosions that completely destroyedthe volcano. Others were formed when, aftersuccessive eruptions, the empty cone could nolonger hold up the walls, which then collapsed.
THE MOST COMMONStratovolcanoes, or composite cones,are strung along the edges of thePacific Plate in the region known as the “Ring of Fire.”
FISSURE VOLCANOESLong, narrow openings foundmainly in mid-ocean ridges. Theyemit enormous amounts ofhighly fluid material and formwide slopes of stratified basalticstone. Some, such as that of theDeccan Plateau in India, covermore than 380,000 square miles(1 million sq km).
CHAPEL OF ST. MICHAEL
Built in Le Puy, France, on topof a volcanic neck of hardrock that once sealed theconduit of a volcano. The
volcano's cone has longsince been worn away
by erosion; the lavaplug remains.
IGNEOUS INTRUSIONS: A PECULIAR PROFILE
1 2 3
Compositevolcano 12,400feet (3,776 m)high, the highestin Japan. Itslast eruptionwas in 1707.
Fissure volcano,about 5 miles (8 km)from the top ofKilauea (Hawaii). Thisis one of the mostactive volcanoes inthe central Pacific.
Located onLanzarote, CanaryIslands, in thefissure zone knownas the Montañasde Fuego (FireMountains).
Shield volcanoin Hawaii. Oneof the mostactive shieldvolcanoes onEarth.
Cinder cone located45 miles (65 km)west of the capital ofEl Salvador. Its lastrecorded eruptionwas in October 2005.
FEET (80 M)The height ofthe plug, frombase to peak.
VOLCANOES AND EARTHQUAKES 3130 VOLCANOES
Flash of FireA
volcanic eruption is aprocess that can last froma few hours to several
decades. Some are devastating,but others are mild. The severityof the eruption depends on thedynamics between the magma,dissolved gas, and rocks within thevolcano. The most potentexplosions often result fromthousands of years ofaccumulation of magma and gas,as pressure builds up inside thechamber. Other volcanoes, such asStromboli and Etna, reach anexplosive point every few monthsand have frequent emissions.u
When the mountingpressure of the magmabecomes greater than thematerials between themagma and the floor of thevolcano's crater can bear,these materials are ejected.
IN THE CONDUIT
A solid layer of fragmentedmaterials blocks the magmathat contains the volatilegases. As the magma risesand mixes with volatilegases and water vapor, thepockets of gases and steamthat form give the magmaits explosive power.
Plume of ash
Cloud of burningmaterial from about330 to 3,300 feet(100-1,000 m) high
The column canreach a height of49,000 feet(15 km)
Cloud can reachabove 82,000 feet (25 km).
Dome Low, like ashield volcano, witha single opening
Highly fluid,of basalticcomposition.
In mid-oceanridges and onvolcanic islands.
LavaSeeps out slowly
Burning cloudmoving downthe slope
(11.5 Km) HIGH
Lava flowsViscous anddome-shapedlava
IN THE CHAMBER
There is a level at whichliquefaction takes place andat which rising magma,under pressure, mixes withgases in the ground. Therising currents of magmaincrease the pressure,hastening the mixing.
EXPLOSIVE ACTIVITYTYPES OF EXPLOSIVEERUPTION
LAVA FLOW MT. KILAUEA, HAWAII LAKE OF LAVA MAKA-O-PUHL, HAWAII COOLED LAVA (PAHOEHOE) MT. KILAUEA, HAWAII
TYPES OF EFFUSIVE ERUPTION FROM OUTER SPACEA photo of the eruption of Mt.Augustine in Alaska, taken by theLandsat 5 satellite hours afterthe March 27, 1986, eruption.
Comes from the combination of high levels of gas withrelatively viscous lava, which can produce pyroclasts andbuild up great pressure. Different types of explosions aredistinguished based on their size and volume. The greatestexplosions can raise ash into a column several miles high.
EFFUSIVE ACTIVITYMild eruptions with a low frequency of explosions. Thelava has a low gas content, and it flows out of openingsand fissures.
HOW IT HAPPENS
In addition to lava, an eruption caneject solid materials calledpyroclasts. Volcanic ash consists ofpyroclastic material less than 0.08inch (2 mm) in size. An explosion caneven expel granite blocks.
Along themargins ofcontinents andisland chains.
2.5 inches (64 mm) and up
0.08 to 2.5 inches (2 mmto 64 mm)
Up to 0.08 inch (2 mm)
On the volcanic island ofHawaii, nonerupting flowsof lava abound. Local termsfor lava include “aa,”viscous lava flows thatsweep away sediments, and“pahoehoe,” more fluid lavathat solidifies in soft waves.
The volcano Stromboli inSicily, Italy, gave itsname to these high-frequency eruptions. Therelatively low volume ofexpelled pyroclastsallows these eruptionsto occur approximatelyevery five years.
Volcanoes such as MaunaLoa and Kilauea expel largeamounts of basaltic lavawith a low gas content, sotheir eruptions are very mild.They sometimes emitvertical streams of brightlava (“fountains of fire”) thatcan reach up to 330 feet(100 m) in height.
Typical in ocean rift zones,fissures are also found on thesides of composite cones suchas Etna (Italy) or near shieldvolcanoes (Hawaii). Thegreatest eruption of this typewas that of Laki, Iceland, in1783: 2.9 cubic miles (12 cukm) of lava was expelled froma crack 16 miles (25 km) long.
Named after Vulcano inSicily. As eruptions ejectmore material and becomemore explosive, theybecome less frequent. The1985 eruption of Nevadodel Ruiz expelled tens ofthousands of cubic yardsof lava and ash.
Also called Plinian, themost violentexplosions raisecolumns of smoke andash that can reach intothe stratosphere andlast up to two years,as in the case ofKrakatoa (1883).
A plug of lava blocks thecrater and diverts thecolumn to one side after alarge explosion. As with Mt.Pelée in 1902, the pyroclasticflow and lava are violentlyexpelled down the slope in aburning cloud that sweepsaway everything in its path.
VOLCANOES AND EARTHQUAKES 3332 VOLCANOES
Mount St. Helens
Warning SignsTwo months before the greatexplosion, Mount St. Helens
gave several warning signs: a series ofseismic movements, small explosions,and a swelling of the mountain's northslope, caused by magma rising towardthe surface. Finally on May 18, anearthquake caused a landslide thatcarried away the top of the volcano.Later, several collapses at the base ofthe column caused numerouspyroclastic flows with temperatures ofnearly 1,300° F (700° C).
Having noescape route,the magmaexerts pressuresideways andbreaks throughthe north slope.
The side block gaveway, causing a powerfulpyroclastic flow.
A verticalcolumn ofsmoke and ashrose 12 miles(19 km) high.
In the eruption MountSt. Helens lost its conicalstratovolcano shape andbecame a caldera.
Pulverized and incineratedby the force of the lavaand the pyroclastic flow.Temperatures rose above1,110° F (600° C).
8 miles13 km
Range of the shock wave from thepyroclastic flow. The heat and ash leftacres of forest completely destroyed.
15 miles24 km
600 sq km
The uninterrupted flow of magma towardthe volcano's surface caused the northslope of the mountain to swell, and latercollapse in an avalanche.
SURFACE DESTRUCTIONThe effects were devastating:250 houses, 47 bridges, raillines, and 190 miles (300 km)of highway were lost.
BEFORE THE ERUPTIONThe symmetrical cone, surroundedby forest and prairies, was admiredas the American Fuji. The eruptionleft a horseshoe-shaped caldera,surrounded by devastation.
DURING THE EXPLOSION
The energy released was theequivalent of 500 nuclearbombs. The top of themountain flew off like the capof a shaken bottle of soda.
Cut TopLike the cork in a bottleof champagne, the topof the mountain burstoff because of pressurefrom the magma.
The ForestBurned trees coveredwith ash, several milesfrom the volcano
Type of Volcano
Size of Base
Type of Activity
Type of Eruption
Most Recent Eruptions
5.9 mi (9.5 km)
1980, 1998, 2004
PRESSURE ON THE NORTH SLOPE
The swelling of the mountain was nodoubt caused by the first eruption,almost two months before the finalexplosion.
2. INITIAL ERUPTIONS
The north slope gave way to the greatpressure of the magma in an explosiveeruption. The lava traveled 16 miles(25 km) at 246 feet (75 m) per second.
3. EXPLOSION AND VERTICAL COLLAPSE
At the foot of the volcano, a valley 640feet (195 m) deep was buried in volcanicmaterial. Over 10 million trees weredestroyed.
Within the territory of the United States, active volcanoesare not limited to exotic regions such as Alaska orHawaii. One of the most explosive volcanoes in
North America is in Washington state. Mount St. Helens,after a long period of calm, had an eruption of ashand vapor on May 18, 1980. The effects weredevastating: 57 people were killed, and lavaflows destroyed trees over an area of232 square miles (600 sq km). Thelake overflowed, causingmudslides that destroyedhouses and roads. Thearea will need acentury torecover.
34 VOLCANOES VOLCANOES AND EARTHQUAKES 35
BEFOREIn May the volcano began showingsigns in the form of small quakes andspouting vapor, smoke, and ash. Noneof this served to warn of the terribleexplosion to come, and some eventook trips to see the volcano's“pyrotechnics.”
AFTERA crater nearly 4 miles (6.4 km) in diameter was left where the volcanohad been. About 1927, new volcanic activity was observed in the area.In 1930, a cone emerged. Anak Krakatoa (“daughter of Krakatoa”)appeared in 1952; it grows at a rate of nearly 15 feet (4.5 m) per year.
DURINGAt 5:30 a.m. the islandburst from theaccumulated pressure,opening a crater 820 feet(250 m) deep. Waterimmediately rushed in,causing a gigantic tsunami. MEGATONS
The energy released,equivalent to 25,000atomic bombs suchas the one droppedon Hiroshima.
Crater's edgeIn early 1883, Krakatoa was just one ofmany volcanic islands on Earth. It was locatedin the Straits of Sundra, between Java and
Sumatra in the Dutch East Indies, now known asIndonesia. It had an area of 10.8 square miles (28 sq km) anda central peak with a height of 2,690 feet (820 m). In August1883, the volcano exploded, and the island was shattered inthe largest natural explosion in history.
Krakatoa was near the subductionzone between the Indo-Australian
and Eurasian plates. The island's inhabitantswere unconcerned about the volcanobecause the most recent previous eruptionhad been in 1681. Some even thought the
The Island That Exploded
AftereffectsThe ash released into the atmosphereleft enough particles suspended in the
air to give the Moon a blue tinge for yearsafterward. The Earth's average temperaturealso decreased.
volcano was extinct. On the morning ofAug. 27, 1883, the island exploded. Theexplosion was heard as far away asMadagascar. The sky was darkened, andthe tsunamis that followed the explosionwere up to 130 feet (40 m) high.
The height of thecolumn of ash.
FRACTIONTwo thirds of theisland wasdestroyed, and onlya part of Rakatasurvived theexplosion.
PYROCLASTICSThe pyroclastic flows wereso violent that, accordingto the descriptionsof sailors, theyreached up to 37miles (80 km)from the island.
AtmosphereThe ash expelledby the explosionlingered for years
PRESSURE WAVEThe atmospheric pressurewave went around theworld seven times.
WATER LEVELThe water levelfluctuated as far awayas the English Channel.
The height of the tsunamiwaves, which traveled at 700miles per hour (1,120 km/h).
130 feet(40 m)
KRAKATOALatitude 6° 06´ S
Longitude 105° 25´ E
Remaining Surface Area
Range of the Explosion
Range of Debris
10.8 square miles (28 sq km)
3 square miles (8 sq km)
2,900 miles (4,600 km)
1,550 miles (2,500 km)
1 Lighter particlesseparate fromheavier ones and riseupward, forming ablanket-shaped cloud.
2 Ahead of theburning cloud,a wave of hotair destroysthe forest.
LAVA FLOWSIn volcanoes with calderas, low-viscosity lavacan flow without erupting, as with the Lakifissures in 1783. Low-viscosity lava drips withthe consistency of clear honey. Viscous lava isthick and sticky, like crystallized honey.
PYROCLASTIC FLOWIncandescent masses of ash, gas,and rock fragments that comefrom sudden explosive eruptionsflow downhill at high temperature,burning and sweeping awayeverything in their path.
MUDSLIDES OR LAHARSRain mixed with snow andmelted by the heat, alongwith tremors and overflowinglakes, can cause mudslidescalled “lahars.” These can beeven more destructive thanthe eruption itself, destroyingeverything in their path asthey flow downhill. Theyoccur frequently on highvolcanoes that have glacierson their summit.
CINDER CONE MOLDS OF TREES LAVA TUBES
Cone withwalls ofhardenedlava.
When a volcano becomes active and explodes, it sets inmotion a chain of events beyond the mere danger of theburning lava that flows down its slopes. Gas and ash are
expelled into the atmosphere and affect the local climate. Attimes they interfere with the global climate, with moredevastating effects. The overflow of lakes can also causemudslides called lahars, which bury whole cities. Incoastal areas, lahars can cause tsunamis.
Aftermath of Fury
LAVA IN VOLCANO NATIONAL PARK, HAWAII
RESCUE IN ARMERO, COLOMBIA
Mudslide after theeruption of the volcanoNevado del Ruiz. Arescue worker helps aboy trapped in a lahar.
ARMERO FROM ABOVE
On Nov. 13, 1985, the city ofArmero, Colombia, was devastatedby mudslides from the eruption ofthe volcano Nevado del Ruiz.
61-132miles per hour(100-200 km/h)
930-1830° F(500-1000° C)
30-61miles per hour(50-100 km/h)In rhyolitic eruptions.
The underground actionof magma and gascreates pressure that, in turn, causesmovement in the Earth'scrust. The quakes canbe warning signs of animpending eruption.
Aerial photo of a smallfishing village on SanVicente Island, coveredin volcanic ash. Thiseruption had no victims.
Particles of volcanic ash intensifyyellow and red colors. After
the eruption of Tambora inIndonesia in 1815,
unusually colorfulsunrises were seen
A bird caught in the eruption ofMount St. Helens, whichdevastated forests up to adistance of about 8 miles (13 km).The heat and ash left many acrescompletely destroyed.
VOLCANOES AND EARTHQUAKES 37
The petrifiedmold forms aminivolcano.
Inside,the lavastays hotand fluid.
Burned treeunderneathcooled lava.
As the lavaflows upward,the coneexplodes.
Jets of Water
38 VOLCANOES VOLCANOES AND EARTHQUAKES 39
Geysers are intermittent spurts of hotwater that can shoot up dozens of yardsinto the sky. Geysers form in the few
regions of the planet with favorablehydrogeology, where the energy of pastvolcanic activity has left water trapped insubterranean rocks. Days or weeks may passbetween eruptions. Most of these spectacularphenomena are found in Yellowstone NationalPark (U.S.) and in northern New Zealand.
CONVECTION FORCESThis is a phenomenon equivalent to boiling water.
The heat of a magma chamber warms water in thecavity, the chamber fills, and the water rises to thesurface. The pressure in the cavity is released, and thewater suddenly boils and spurts out.
MORPHOLOGY OF THE CHAMBERS
Water cools and sinksback to the interior,where it is reheated.
Mud, clay,mineraldeposits,and water
Bubbles of hot gasrise to the surface andgive off their heat.
The eruptive cycle
5.7,900gallons(30,000 l) OF WATER
1,450 ft(442 m)
The average height reachedof the spurt of water is about
148 feet(45 m)
Temperatures up to
THE CYCLE REPEATS
When the water pressurein the chambers is relieved,the spurt of water abates,and the cycle repeats.Water builds up again incracks of the rock and inpermeable layers.
RECORD HEIGHTIn 1904, New Zealand's Waimangu geyser (nowinactive) emitted a record-setting spurt of water.In 1903, four tourists lost their lives when theyunknowingly came too close to the geyser.
FUMAROLEThis is a place where there is a constant emissionof water vapor because the temperature of themagma is above 212° F (100° C).
SOLFATARAThe thermal layers emit sulfur and sulfurousanhydride.
MUD BASINThese basins produce their own mud; sulfuricacid corrodes the rocks on the surface andcreates a mud-filled hollow.
HEAT SOURCEMagmabetween 2 and6 miles (3-10km) deep, at930-1,110° F(500-600° C).
TERRACESThese are shallow,quickly dryingpools with stair-step sides.
MINERAL SPRINGSTheir water contains many minerals,known since antiquity for their curativeproperties. Among other substances, theyinclude sodium, potassium, calcium,magnesium, silicon oxide, chlorine, sulfates(SO4), and carbonates (HCO3). They arevery helpful for rheumatic illnesses.
Steam EnergyIn Iceland,geothermic steam isused not only inthermal spas butalso to powerturbines thatgenerate most of thecountry's electricity.
4. SPURTING SPRAY The water spurts out ofthe cone at irregularintervals. The lapsebetween spurts dependson the time it takes forthe chambers to fill upwith water, come to aboil, and produce steam.
3. BURSTING FORTH
The water rises byconvection and spurtsout the main vent to thechimney or cone. Thedeepest water becomessteam and explodesoutward.
2. MOUNTING PRESSURE
The underground chambersfill with water, steam, andgas at high temperatures,and these are then expelledthrough secondary conduitsto the main vent.
1. HEATED WATER
Thousands of years after theeruption of a volcano, thearea beneath it is still hot. Theheat rising from the magmachambers warms water thatfilters down from the soil. Inthe subsoil, the water canreach temperatures of up to518° F (270° C), but pressurefrom cooler water abovekeeps it from boiling.
This spring, in Yellowstone NationalPark, is the largest hot spring in theUnited States and the third largestin the world. It measures 246 by377 feet (75 by 115 m), and itemits about 530 gallons (2,000 l)of water per minute. It has aunique color: red mixed withyellow and green.
1,500 ft(457 m)
There are some1,000 geysersworldwide, and50 percent are inYellowstoneNational Park(U.S.).
Great Geysir(Iceland) Kamchatka
North Island(New Zealand)
377 feet (115 m)
PRINCIPAL GEOTHERMAL FIELDS
Streams ofwater and
GRAND PRISMATIC SPRING In the middle ofthe spring, themineral water is200° F (93° C),and it coolsgradually towardthe edges.
On average, a geysercan expel up to
530 gallons(2,000 l)OF WATER PER MINUTE
Scale in miles (km)
VOLCANOES AND EARTHQUAKES 4140 VOLCANOES
Rings of CoralI
n the middle of the ocean, in the tropics, there are round, ring-shaped islands called atolls. Theyare formed from coral reefs that grew along the sides of ancient volcanoes that are nowsubmerged. As the coral grows, it forms a barrier of reefs that surround the island like a fort.
How does the process work? Gradually, volcanic islands sink, and the reefs around them form abarrier. Finally, the volcano is completely submerged; no longer visible, it is replaced by an atoll.
FORMATION OFAN ATOLL
WHAT ARECORALS? FORMATION OF A VOLCANIC ISLAND
ATOLLS AND VOLCANICISLANDS AROUND THE WORLD
Corals are formed from theexoskeletons of a group ofCnidarian species. Thesemarine invertebrates havea sexual phase, called amedusa, and an asexualphase, called a polyp. Thepolyps secrete an outerskeleton composed ofcalcium carbonate, andthey live in symbiosis withone-celled algae.
68° and 82° F(20-28° C).
1.THE BEGINNING OF AN ATOLL. The undersea flanksof an extinct volcanoare colonized bycorals, whichcontinue to grow.
2.THE CORALS GAIN GROUND. As the surroundingreef settles andcontinues to expand,it becomes a barrierreef that surroundsthe summit of theancient volcano, nowinactive.
Eventually the islandwill be completelycovered and will sinkbelow the water,leaving a ring ofgrowing coral with ashallow lagoon in themiddle.
Polyps onthe Ends ofBranches
Polyp Forming Branches
Original polypformation (dead)
Layer oflive polyps
M A R A K E I
The coralreef formsa ring.
Volcanoes form whenmagma rises from deepwithin the Earth. Thousandsof volcanoes form on theseafloor, and many emergefrom the sea and form thebase of islands.
Coral reefs are foundin the world'soceans, usuallybetween the Tropicof Cancer and theTropic of Capricorn.
CRESTBarrier that protects theshore from waves. Deepgrooves and tunnels letseawater inside the reef.
FACE Branching corals grow here,though colonies can breakloose because of the steepslope.
AWhen a plate of the crustmoves over a hot spot, avolcano begins to eruptand an island is born.
Hawaii13,799 ft(4,206 m)
Maui10,023 ft(3,055 m)
Lanai3,369 ft(1,027 m)
Kohoolave3,369 ft(1,027 m)
Molokai1,476 ft (450 m)
Coral is mainly found in thephotic zone (less than 165 feet[50 m] deep), where sunlightreaches the bottom andprovides sufficient energy. Forreefs to grow, the watertemperature should be between
Republic of Kiribati
10.8 square miles (28 sq km)
6.9 ft (2.1 m)
Frozen FlameI t is known as the land of ice and fire. Under
Iceland's frozen surface there smolders avolcanic fire that at times breaks free and
causes disasters. The island is located over ahot spot on the Central Atlantic Ridge. Inthis area the ocean bed is expanding,and large quantities of lava flowfrom vents, fissures, and craters.
VOLCANOES AND EARTHQUAKES 43
Split Down the MiddlePart of Iceland rests on the North American Plate, which isdrifting westward. The rest of Iceland is on the Eurasian
Plate, drifting eastward. As tectonic forces pull on the plates, theisland is slowly splitting in two and forming a fault. The edges ofthe two plates are marked by gorges and cliffs. Thus, the oceanbed is growing at the surface.
The magma thatemerges at the surfacecomes from a series ofcentral volcanoesseparated by fissures.
Birth of an IslandOn Nov. 15, 1963, an undersea volcanic eruption offthe southern coast of Iceland gave rise to the island
of Surtsey, the newest landmass on the planet. Theeruption began with a large column of ash and smoke.Later, heat and pressure deep within the Earth pushedpart of the Mid-Atlantic Ridge to the surface. The islandkept growing for several months, and today it has asurface area of 1.0 square mile (2.6 sq km). The island wasnamed after Surtur, a fire giant from Icelandic mythology.
Its largest eruptionwas in 1104.
The largest eruptionof lava in historyoccurred in 1783: theashes reached China.
ERUPTION UNDERTHE ICEIn 1996 a fissure opened up between Grimsvötn andBardarbunga. The lava made a hole 590 feet (180 m)deep in the ice and released a column of ash andsteam. The eruption lasted 13 days.
ICELANDLatitude 64° 6’ N
Longitude -21° 54’ E
1/5of all the lava thathas emerged on the
Earth's surfacesince 1500 has
come from Iceland.
GLACIAL CAP OFVATNAJÖKULL
REYKJAVIKThe capital ofIceland is thenorthernmostcapital in the world.
The first eruptions were caused bythe interaction of magma andwater. The explosions wereinfrequent, and rocks were thrownonly a few yards from the volcano.
1 Repeated eruptions expelled vaporand ash into the air, forming acolumn over 6 miles (10 km) high.The island was formed fromvolcanic blocks and masses of lava.
2 The entire process lasted three-and-a-half years. Over 0.25 cubic mile (1cu km) of lava and ash was expelled,with only 9 percent of it appearingabove sea level.
Area of lakes
39,768 sq miles (103,000 sq km)
1 per sq mile (2.8 per sq km )
1,064 sq miles (2,757 sq km)
4,603 sq miles (11,922 sq km)
Lake Viti (Hell in Icelandic);Krafla Volcano
Crater of 1,640 feet (500 m).The caldera measures
6 miles (10 km) across.
If the rift zone that crosses theisland from southwest to northwere cut in two, different agesof the Earth would be revealedaccording to the segment beinganalyzed. For example, the rock60 miles (100 km) from the riftis six million years old.
The islanders usegeothermal (steam)energy from volcanoes andgeysers for heat, hotwater, and electric energy.
Average Annual Expansion:
0.4 inch (1 cm)
KAFLA VOLCANIC CRATERThis volcano has been very active
throughout history. Of its 29 activeperiods, the most
recent was in1984.
Large eruptions often givewarning signs months inadvance. These signs consist ofany observable manifestationon the exterior of the Earth's
crust. They may include emissions ofsteam, gases, or ash and risingtemperatures in the lake thattypically forms in the crater. This iswhy volcanic seismology is
considered one of the most usefultools for protecting nearby towns.Several seismic recording stations aretypically placed around the cone of anactive volcano. Among other things,
the readings scientists get give them aclear view of the varying depths of thevolcano's tremors-extremely importantdata for estimating the probability of amajor eruption.
Study and Prevention INCANDESCENT ROCKA river of lava from Mt.Kilauea flows constantly,forming surface wrinkles thatdeform under the lightest step.
BURIED IN A DAY 52-55
ERUPTIONS THROUGH TIME 56-57
LATENT DANGER 46-47
LEARN MORE 48-49
PREPARATIONS FOR DISASTER 50-51
46 STUDY AND PREVENTION VOLCANOES AND EARTHQUAKES 47
ome locations have a greater propensity forvolcanic activity. Most of these areas arefound where tectonic plates meet,
whether they are approaching or movingaway from each other. The largestconcentration of volcanoes isfound in a region of thePacific known as the“Ring of Fire.” Volcanoesare also found in theMediterranean Sea, inAfrica, and in theAtlantic Ocean.
PACIFIC PLATEAUSTRALIAN PLATE
NORTH AMERICAN PLATE
MOUNT ST. HELENSWashington, U.S.It had anunexpected, violenteruption in 1980.
KILAUEAHawaii, U.S.The most active shieldvolcano, its lava flowshave covered morethan 40 square miles(100 sq km) since1983.
EAST EPIVanuatuThis is anundersea calderawith sloweruptions lastingfor months.
TAMBORAIndonesiaIn 1815 it produced35 cubic miles (150 cu km) of ash.It was the largestrecorded eruptionin human history.
NOVARUPTAAlaska, U.S.It is in the Valleyof Ten ThousandSmokes.
VESUVIUSItalyErupted twiceduring the20th century.
ETNAItaly10,990 feet (3,350m) high; has been active forthousands of years.
ELDFELLIcelandDuring one eruption,it expelled 3,500cubic feet (100 cu m)of lava per second.
MT. PELÉEMartiniqueIts eruption completely
destroyed the city ofSaint-Pierre and itsport in 1902.
OJOS DELSALADOChile/ArgentinaThe tallest volcanoin the world, itslast eruption wasin 1956.
MAUNA LOAHawaii, U.S.The largest activevolcano on Earthis rooted in theocean floor andtakes up nearlyhalf of the island.
PINATUBOPhilippinesIn 1991 it had thesecond most violenteruption of the20th century.
SubductionMost volcanoes in thewestern United States
were formed by subductionof the Pacific Plate.
The Antilles The Lesser Antilles is a
volcanically active region.
50 VolcanoesIndonesia has the highestconcentration of volcanoes
in the world. Java alonehas 50 active volcanoes.
IcelandThe western half of
Iceland lies on the NorthAmerican Plate, but the
eastern half is on theEurasian Plate.
The most dangerous volcanoes arethose located near denselypopulated areas, such as in
Indonesia, the Philippines, Japan,Mexico, and Central America.
DangerThe “top five” listchanges when thevolcanoes are measuredfrom the base ratherthan from their altitudeabove sea level.
On May 5, near the summit, the calderaEtang Sec ruptured, releasing the waterthat it contained. A large lahar formed.
On May 8, Saint-Pierre was destroyedby a burning cloud that devastated anarea of 22 square miles (58 sq km),killing all 28,000 inhabitants.
On May 2, the first rain of ash fell onSaint-Pierre. The sky around theisland was darkened for several days.
OJOS DEL SALADOChile/Argentina22,595 ft (6,887 m)
LLULLAILLACOChile/Argentina22,110 ft (6,739 m)
TIPASArgentina21,850 ft (6,660 m)
INCAHUASIChile/Argentina21,720 ft (6,621 m)
SAJAMABolivia21,460 ft(6,542 m)
The tallestThese are found in themiddle of the Andes
range, which forms part ofthe Pacific Ring of Fire. Theywere most active 10,000years ago, and many arenow extinct or dampened by fumarolic action.
Shield volcano13,680 feet
(4,170 m) abovesea level.
AVACHINSKYRussiaThis is a young, activecone inside an oldcaldera, on theKamchatka peninsula.
FUJIYAMAJapanThis sacred mountainis the country'slargest volcano.
The Pacific “Ring of Fire”
Formed by the edges of the Pacific tectonic plate,where most of the world's volcanoes are found.
O C E A N I A
A F R I C A
E U R O P E
A S I A
N O R T HA M E R I C A
C E N T R A LA M E R I C A
S O U T HA M E R I C A
48 STUDY AND PREVENTION
Increasing KnowledgeV olcanology is the scientific study of volcanoes. Volcanologists study eruptions
from airplanes and satellites, and they film volcanic activity from far off.However, to study the inner workings of a volcano up close, they must scale
near-vertical cliffs and face the dangers of lava, gas, and mudslides. Only then canthey take samples and set up equipment to detect tremors and sounds.
VOLCANOES AND EARTHQUAKES 49
Field MeasurementsMonitoring a volcano includes gathering andanalyzing samples and measuring various
phenomena. Seismic movements, varyingcompositions of gases, deformations in therock, and changes in electromagneticfields induced by the movement ofunderground magma can all provideclues to predict volcanic activity.
SAMPLING OFVOLCANIC GASESGas and water vapor dissolved inmagma provides the energy thatpowers eruptions. Visibleemissions, such as sulfur andsteam, are measured, as areinvisible gases. Analyzing thegases' composition makes itpossible to predict the beginningand end of an eruption.
TEMBLORS OREARTHQUAKESPortable seismographs are used to detect movements in the groundwithin 6 miles (10 km) of thevolcano that is being studied. Thesetremors can give clues about themovements of the magma.
GLOBAL POSITIONINGSYSTEM (GPS)Movements in the magma causehundreds of cracks in the cone. AGPS system records imagescontinuously and analyzes thedeformation over a period of time.
LAVA COLLECTIONThe study of lava candetermine its mineralcomposition and its origin. Lava deposits are also analyzedbecause the history of avolcano's eruptions can giveclues about a future eruption.
LAVA TEMPERATUREis measured with a thermometercalled a Thermopar; glassthermometers would melt fromthe heat. Temperatures of waterand of nearby rocks are othervariables to take into account.
TILTOMETERSThese are placed on the slopesof a volcano to record soilchanges that precede aneruption. Points are determined(A-B) to monitor how pressurefrom the magma deforms thesurface between them.
Volcanologists Taking GasSamples from a Fumarole onLipari Island, Italy
Takingmeasurementsof the crater
HYDROLOGICALMONITORINGMudslides, or lahars, can burylarge areas. Monitoring thevolume of water in the area makesit possible to alert and evacuatethe population when the amountof water passes critical points.
GAS ESCAPINGFROM CRACK
THE SIZE OF THE CRATERThe widening of the cratercaused by volcanic activity andthe growth of the solid lavadomes are measured. Thisgrowth implies certain risks forthe proximity of an eruption.
50 STUDY AND PREVENTION
Volcanic eruptions are dangerous to surrounding populations for two basic reasons.One danger is posed by the volcanic material that flows down the sides of thevolcano (lava flows and mudslides), and the other danger is from the volcano's
pyroclastic material, especially ash. Ash fallout can bury entire cities. Experts havedeveloped an effective series of prevention and safety measures for people living involcanic areas. These measures greatly reduce the highest risks.
VOLCANOES AND EARTHQUAKES 51
Preparations for Disaster
Before an EruptionIt is best to get informed about safetymeasures, evacuation routes, safe areas,
and alarm systems before a volcanic eruption.Other safety measures include stocking up onnonperishable food, obtaining gas masks andpotable water, and checking the load-bearingcapacity of roofs.
Evacuation of Nearby Areas
In the immediate area (within 12 miles[20 km]) of the volcano, evacuation is
the only possible safety measure. Returninghome will be possible only when permission isgiven. Keep in mind that it takes a long timefor life to return to normal after an evacuation.
Areas of Falling AshMost of the population lives outsidethe volcano's range, but ash from an
eruption can become highly volatile andfall over wide areas. Wind can carry ash toother areas, so the best preventive efforts are focused onwarning people about what to do in case of falling ash.
12 miles(20 km)
60 miles(100 km)
12 miles(20 km)
Considered to be the critical distance froma volcano in emergency relief efforts.
HIGHER ELEVATIONSThese are the preferred sites forevacuations from volcanic eruptions.High ground is safe from lahars andlava flows, and if there is shelterthere, it is also safe from rains of ash.
AT HOMEIt is best to stay indoors during an ashfall.One of the main precautions is to providefor potable drinking water, because theusual water supply will beinterrupted because of pollutionrisks, especially if the watersupply comes from lakes orrivers in the area.
WIND AND RAINWind is a risk factor thatspreads volatile ash over a largearea so that settlements at adistance greater than 60 miles(100 km) can be affected. Thegreatest danger posed by fallingash is that it can mix with rainfalling on the roofs of housesand form a heavy mass that willcollapse the buildings.
LAHARS AND PYROCLASTIC FLOWSLahars (mudflows) can form fromrainwater or melting snow.Volcanic danger zones often havestrategies to divert rivers andreduce the volume of water in
dams and reservoirs.
ASH ON THE ROOFAsh should be removedimmediately (before itrains) so the roof doesnot collapse.
DO NOT WASH WITH WATER. After the ashfall,washing with water will form a sticky andheavy paste that will be very hard to remove.
WATER TANKRoof-mounted watertanks should bedisconnected and covereduntil the roof has beencleared of ashes.
DOORS ANDWINDOWSIt is best to always leavedoors and windows shuttightly, as airtight aspossible, for as long as theashfall continues.
MASKSUse masks and specialash-protective clothingwhen outdoors.
AVOID DRIVINGIf you must drive, doso slowly and turn onyour headlights. It isbest to leave the carparked in an enclosedspace or under cover.
CHILDRENIf children are atschool, do not go topick them up: theywill be safe there.
STAY CALMTo breathe, coveryour face with ahandkerchiefsoaked in waterand vinegar. INFORMATION
Listen to the radioat all times.
AIRCONDITIONINGAir conditioners andlarge clothes dryersshould not be usedduring an eruption.
RIVERS AND STREAMSLarge volumes of waterpose a threat of mudslides.Avoid these areas.
BRIDGESWhen possible, do notuse for your evacuationroute because theymight collapse.
MAIN ROUTESThese usually cross low-lyingareas. They can be a potentialpath for flows of lava or mud.
ALTERNATIVEROUTESRoads running throughhigher elevations arepreferred because theycannot be reached bylava and mudflows.
44 pounds(20 kg)OF PROVISIONS.
Do not carry more than
MEDICALPRECAUTIONSKeep a first-aid kitand essentialmedications on hand,and keep vaccinationsup to date.
PROVISIONSWater and food areindispensable,especially if youevacuate the area onyour own.
SHUT OFF UTILITIES Before leaving a house, shut off theelectricity, gas, andwater. Tape doors andwindows shut.
CIVIL DEFENSE Follow allrecommendations, bealert to officialinformation, and donot spread rumors.
VOLCANOES AND EARTHQUAKES 5352 STUDY AND PREVENTION
Mount Vesuvius had been inactive for more than 800 years, until the pressure that had accumulated inside produced its
explosion in the year 79. Most of the deaths during this tragedywere originally blamed on the ash that buried parts of theneighboring settlements (Herculaneum and Stabiae, as well asPompeii). Now, though, the eruption is believed to have producedthe typical “burning clouds” of a Plinian eruption: Flames ofincandescent ash and gases were expelled at high speeds by theeruptive pressure. Suspended moist particles charged the air withelectricity, causing an intense electric storm, whose flashes oflightning would have been the only source of light under the ashfall.Since then Vesuvius has had a dozen other important eruptions. Theworst killed 4,000 people in 1631. The first volcanologyobservatory in the world was installed at Vesuvius in 1841.
The Violent Awakening
At noon on Aug. 24, AD 79, Mount Vesuvius erupted near the coast ofNaples Bay. The Roman cities of Pompeii and Herculaneum werecompletely buried in ashes and pyroclasts, in what would become one of
the worst natural tragedies of ancient times. Many details from that day havereached us thanks to the narrative of Pliny the Younger. His well-known descriptionof the eruption column as “shaped like a pine” caused this type of eruption to benamed after him: a “Plinian eruption.”
Buried in One Day
POMPEII,ITALYLatitude 40° 49’ N
Longitude 14° 26’ E
Distance from Vesuvius
Population in the year AD 79
Ash dispersion (79)
Last Eruption of Vesuvius
6 miles (10 km)
60 miles(100 km) (SE)
1 AN ALMOST NORMAL DAYTremors and earthquakes had been felt inthe city for four days. Hanging lampsswayed, furniture moved, and some doorframes had even cracked. Because thesethings happened about once a year
without any consequences, theinhabitants of Pompeii continued withtheir normal lives. The public forum wasfilled with people. The festivities of Isiswere celebrated in the temple of Apollo.
THE ERUPTION.Suddenly Vesuvius spewed out a hugecolumn of smoke, lava, and ash thatformed a pyroclastic flow movingtoward Pompeii. People ran inall directions seekingrefuge in houses.The roughnessof the seamade escapeby waterimpossible.
POMPEII'S FORUMThis was the political, religious,and commercial heart of thecity. Every day the forum wasalive with Pompeii's citizens, as
it was on August 24.
SEQUENCE OF THE ERUPTIONFor more than 20 hours (the time theeruption lasted), the ash column roseand then fell on the surrounding area.
1 After the first explosion, thecolumn of smoke began itsvertical climb. The wind blew ittoward the southeast.
2 The cloud spread nearly 60miles (100 km) from side toside, and ash fell on the cityfor a whole day.
3 By 7:30 A.M. on August 25, thepyroclastic flows reached Pompeii.These flows are estimated to have reached temperatures of1,022° F (550° C).
The maximumdepth of the ashes.
(7 m)23 feet
RICHESSeveral preciousobjects such as thisgold bracelet have
RAIN OF STONESMoments after the eruption,incandescent pumice stones fell from the sky.
9 P.M.A TWO-DAY NIGHTThe tongues of lava from the volcanowere seen better at night. The nextmorning the Sun's light could not beseen through the ash cloud. Pliny'snarrative mentions a constant rain of
pyroclasts, continuing on the followingmorning, and emissions of sulfuric gasesthat killed many people. Many soughtshelter on the beaches. Only on August26 did the ashfall begin to disperse.
54 STUDY AND PREVENTION VOLCANOES AND EARTHQUAKES 55
2 RECONSTRUCTION The work of Fiorelli was tofill these natural “tombs”(ash molds) with plaster.When the plaster hardened,the surrounding layers ofash were removed, leavingthe outlines or molds of thebodies.
There were frescoes on the walls showing obsceneimages and pictures ofdrunken customers.
The funnel-shapedroofs were used tocollect rainwater.
Wine was served in small cups called“carafes.”
Slaves worked inthe kitchens, andthere wereutensils similar tothose we usetoday.
Clients seekingprotection or favorswere received in theatrium or central patio.
Romans were more than fond of feasts. A dinnerfor the whole family, which normally began at
four in the afternoon, could last for more than fourhours. Meals were sumptuous affairs, and no one leftuntil completely satisfied. Pompeii's wine was famousthroughout Rome. Kept in pitchers, it was alwaysserved watered down. The Romans sometimes addedflavorings, commonly including honey and pepper.
Good Eating and Drinking
THETHERMOPOLIUMThe typical bar had along marble tabletopwith embeddedcontainers in which foodcould be kept warm.
Objects and human bodies were found underPompeii's ashes, preserved in the position in
which the disaster surprised them. These valuabletestimonies to the past have made possible thereconstruction of daily life in ancient Rome.The House of the Faun was one of the mostluxurious villas in Pompeii.
In the House of the Faun
3 THE FORM, UNTOUCHED Making plaster casts allowed precise reconstructions of thepeople's postures at the time of the disaster, and we havebeen able to learn details such as the hairstyles and dress ofthese people. Animal forms and other organic objects havealso been reconstructed. Today the use of resins and siliconesmakes it possible to obtain even greater detail.
In 1709, some of Pompeii's artifacts were found buriedunder volcanic ash, and that started a treasure hunt. It
was not until 1864, though, that reconstruction and conservationof materials began with the work of Giuseppe Fiorelli. Theexhibits that are the most fascinating to people who visitPompeii's ruins today (about two million people every year) arehis reconstructions of the bodies.
As in That Moment
1 THE CATASTROPHE Several corpses werecovered by volcanic ash,which had accumulated inlayers and later hardened.The bodies haddecomposed, but theirforms were molded in thevolcanic rock.
There were several types of food and drinkestablishments in Pompeii, from food vendors
in the streets to luxury services. These food placesserved many different socialpurposes but acted primarily asplaces for businessmen to meet.They were run mostly by slaves,men as well as women.
In a Pompeii Bar
BRONZE FAUN The name given to the
house is from thisstatue found in the
villa's atrium. The faunwas considered a wild
deity, with the ability topredict the future.
Some of thesehouses wereanterooms ofbrothels.
Tiles decoratedwith the floraand fauna ofthe Nile.
This merchant home wasthe largest in Pompeii,with 32,290 square feet(3,000 sq m).
Slave couplescould meet onlyin the gardens.
was theapproximatenumber of places ofthis type in the city.
Volume of ejected ashin cubic feet (cu m)
End of a cycle
MOUNT VESUVIUSNaples, Italy
MOUNT VESUVIUSNaples, Italy
Volume of ejected ashin cubic feet (cu m)
VOLCANOES AND EARTHQUAKES 57
The cities of Pompeii and Herculaneumwere destroyed in AD 79 when MountVesuvius erupted. Until that day, it wasnot known that the mountain was avolcano because it had been inactive forover 300 years. This was one of the firsteruptions to be recorded: Pliny theYounger stated in one of his manuscriptsthat he had seen how the mountainexploded. He described the gas and ashcloud rising above Vesuvius and howthick, hot lava fell. Many people diedbecause they inhaled the poisonous gases.
370 miles (600 km) away from the epicenterof the eruption, and it was so thick that ithid the Sun for two days. The ashfall coveredan area of 193,051 square miles (500,000 sqkm). It is considered to be the mostdestructive volcanic explosion that ever tookplace. More than 10,000 people died duringthe eruption, and 82,000 died of illness andstarvation after the eruption.
Volume of ejected ashin cubic feet (cu m)
490 billion (14 billion)
LAKI VOLCANO Iceland
In spite of the fact that the eruptionsare related to conic forms, most of thevolcanic material comes out throughfractures in the crust, called “fissures.”The fissure eruptions of Laki were thegreatest in Iceland; they created morethan 20 vents in a distance of 15 miles(25 km). The gases ruined grasslands andkilled livestock. The subsequent faminetook the lives of 10,000 people.
Volume of ejected ashin cubic feet (cu m)
100 billion (3 billion)
Volume of ejected ashin cubic feet (cu m)
670 billion (19 billion)
KRAKATOA VOLCANOJava, Indonesia
1883Volume of ejected ashin cubic feet (cu m)
MOUNT PELÉEMartinique, Antilles
1902Volume of ejected ashin cubic feet (cu m)
656 feet (200 m)
ELDFELL VOLCANOHeimaey Islands, Iceland
Volume of ejected ashin cubic feet (cu m)
35 billion (1 billion)
MOUNT ST. HELENS State of Washington, U.S.
Volume of ejected ashin cubic feet (cu m)
EL CHICHÓN VOLCANO Mexico
On Sunday, March 28, after 100 years ofinactivity, this volcano became activeagain and unleashed an eruption on April4. The eruption caused the deaths ofabout 2,000 people who lived in thesurrounding area, and it destroyed ninesettlements. It was the worst volcanicdisaster in Mexico's history.
Also known as the Mount Fuji of theAmerican continent. During the 1980explosion, 1,315 feet (401 m) of themountain's top gave way through afault on its side. A few minutes afterthe volcano began its eruption, rivers of lava flowed down its sides, carryingaway the trees, houses, and bridges intheir path. The eruption destroyedwhole forests, and the volcanic debrisdevastated entire communities.
Even though Krakatoa began toannounce its forthcoming eruption withclouds of vapor and smoke, these signs,instead of preventing a disaster, becamea tourist attraction. When the explosiontook place, it destroyed two thirds ofthe island. Stones shot from the volcanoreached a height of 34 miles (55 km)-beyond the stratosphere. A crater 4miles (6.4 km) in diameter opened achasm 820 feet (250 m) deep. Land andislands were swept bare.
The lava advanced, and it appeared that itwould take everything in its path.Volcanologists decided that HeimaeyIsland, south of Iceland, should beevacuated. But a physics professorproposed watering the lava with seawaterto solidify or harden it. Forty-seven pumpswere used, and, after three months and6.5 million tons (6 million metric tons) ofwater, the lava was stopped, and the portwas saved. The eruption began onJanuary 23 and ended on June 28.
A burning cloud and a thick mass of ashand hot lava were shot from this smallvolcano that completely destroyed theport city of Saint-Pierre. Most strikingis the fact that this destruction tookplace in only a few minutes. The energyreleased was so great that trees wereuprooted. Almost the entire populationdied, and only three people survived,one of them because he was trapped inthe city jail.
After giving off fumes for seven months,Tambora erupted, and the ensuingcatastrophe was felt around the globe.The ash cloud expanded to more than
the previous one in 1906, caused severematerial damage. The eruptions wereresponsible for more than 2,000 deathsfrom avalanches and lava bombs.Additionally, the 1944 eruption tookplace during World War II and causedas much damage as the eruption at thebeginning of the 20th century had,because it flooded Somma, Atrio deCevallo, Massa, and San Sebastiano.
With this last activity, the Vesuviusvolcano ended the cycle of eruptions itbegan in 1631. This explosion, along with
The lava falls and flows, sweeping away everything in its path. This happens in a slow, uninterruptedway, and the lava destroys entire cities, towns, and forests and claims thousands of human lives.One of the most famous examples was the eruption of Mount Vesuvius in AD 79, which wiped out
two cities and two cultures, those of Pompeii and Herculaneum. In the 20th century, the eruption of MountPelée destroyed the city of Saint-Pierre in Martinique in a few minutes and instantly killed almost its entirepopulation. Volcanic activity also seems to be closely related to changes in climate.
There is a strongly supported theorythat relates climate changes to volcanic
eruptions. The idea of linking the twophenomena is based on the fact thatexplosive eruptions spew huge amounts ofgases and fine particles high into thestratosphere, where they spread around theEarth and remain for years. The volcanic
material blocks a portion of solar radiation,reducing air temperatures around the world.Perhaps the most notable cold period related tovolcanic activity was the one that followed theeruption of Tambora in 1815. Some areas ofNorth America and Europe had an especiallyharsh winter.
Volcanoes and Climate
56 STUDY AND PREVENTION
KALAPANA. After the Kilaueavolcano (Hawaii) erupted in 1991,a lava flow advanced on the city,covering everything in its path.
VIOLENT SEAS 68-69
AFTER THE CATASTROPHE 70-71
CAUSE AND EFFECT 72-73Earthquakes
Earthquakes shake the groundin all directions, even thoughthe effects of a quake dependon the magnitude, depth, anddistance from its point of
origin. Often the waves are so strongthat the Earth buckles, causing thecollapse of houses and buildings, ashappened in Loma Prieta. Inmountainous regions earthquakes
can be followed by landslides andmudslides, whereas in the oceans,tsunamis may form; these walls ofwater strike the coast withenough force to destroy whole
cities, as occurred in Indonesia inDecember 2004. Thailand recordedthe highest number of tourist deaths,and 80 percent of tourist areas weredestroyed.
DEEP RUPTURE 60-61
ELASTIC WAVES 62-63
BURSTS OF ENERGY 64-65
MEASURING AN EARTHQUAKE 66-67
LOMA PRIETA On Oct. 18, 1989, an earthquakemeasuring 7.0 on the Richter scale, withits epicenter in Loma Prieta, 52 miles(85 km) south of San Francisco, causedgreat damage, including the collapse ofa section of the Bay Bridge.
arthquakes take place because tectonic plates are in constant motion, and thereforethey collide with, slide past, and in some cases even slip on top of each other. TheEarth's crust does not give outward signs of all the movement within it. Rather energy
builds up from these movements within its rocks until the tension is more than the rock canbear. At this point the energy is released at the weakest parts of the crust. This causes theground to move suddenly, unleashing an earthquake.
HYPOCENTER OR FOCUSPoint of rupture, where thedisturbance originates. Canbe up to 435 miles (700 km)below the surface.
30 SecondsThe time lapsebetween eachtremor of theEarth's crust
1Tension Is GeneratedThe plates move in oppositedirections, sliding along thefault line. At a certain pointalong the fault, they catchon each other. Tensionbegins to increase betweenthe plates.
2Tension Versus ResistanceBecause the force ofdisplacement is still activeeven when the plates arenot moving, the tensiongrows. Rock layers nearthe boundary are distortedand crack.
3EarthquakeWhen the rock'sresistance is overcome,it breaks and suddenlyshifts, causing anearthquake typical of a transform-faultboundary.
SEISMIC WAVEStransmit the force ofthe earthquake overgreat distances in acharacteristic back-and-forth movement. Theirintensity decreases withdistance.
ORIGIN OF AN EARTHQUAKE
EARTHQUAKEThe main movement or tremorlasts a few seconds, afterwhich some alterationsbecome visible in the terrainnear the epicenter.
FORESHOCKSmall tremor that cananticipate an earthquakeby days or even years. Itcould be strong enough tomove a parked car.
EPICENTERPoint on the Earth's surfacelocated directly above the focus.
FAULT PLANEUsually curves ratherthan following astraight line. Thisirregularity causes thetectonic plates tocollide, which leads toearthquakes as theplates move.
8 or Greater
7 to 7.9
6 to 6.9
5 to 5.9
4 to 4.9
3 to 3.9
AFTERSHOCKNew seismic movement thatcan take place after anearthquake. At times it canbe even more destructivethan the earthquake itself.
FOLDSThese result from tension thataccumulates between tectonicplates. Earthquakes releasepart of the tension energygenerated by orogenic folds.
Average depth of the Earth's crustbelow the island.
15 miles (25 km)
NEWZEALANDLatitude 42° S
Longitude 174° E
Earthquakes per year (>4.0)
Total earthquakes per year
103,737 square miles(268,680 sq km)
35 people per square mile (13.63 people per sq km)
To the west thereis a plain that hastraveled nearly310 miles (500km) to the northin the past 20million years.
ALPINE FAULT IN NEW ZEALANDAs seen in the cross-section, South Island is divided by a largefault that changes the direction of subduction, depending onthe area. To the north the Pacific Plate is sinking under theIndo-Australian Plate at an average rate of 1.7 inches (4.4 cm)per year. To the south, the Indo-Australian Plate is sinking 1.4inches (3.8 cm) per year under the Pacific Plate.
S O U T H I S L A N DALPINEFAULT
SOUTHERN ALPS1 32
VOLCANOES AND EARTHQUAKES 61
FUTURE DEFORMATION OF THE ISLAND
Earthquakes originate at pointsbetween 3 and 430 miles (5 and 700km) underground. Ninety percentoriginate in the first 62 miles (100km). Those originating between 43and 190 miles (70 and 300 km) are considered intermediate.Superficial earthquakes(often of highermagnitude) occur abovethat level, and deep-focus earthquakesoccur below it.
BASED ON FOCUS DEPTH
High-speed waves that travel in straight lines, compressingand stretching solids and liquids they pass through.
The seismic stationregisters both waves.
SPEED IN DIFFERENT MATERIALS
TrepidatoryLocated near theepicenter, where thevertical component of themovement is greater thanthe horizontal.
OscillatoryWhen a wave reachessoft soil, the horizontalmovement is amplified,and the movement issaid to be oscillating.
Types of EarthquakesAlthough earthquakes generally cause all typesof waves, some kinds of waves may
predominate. This fact leads to a classification thatdepends on whether vertical or horizontal vibrationcauses the most movement. The depth of theepicenter can also affect its destructiveness.
TRAJECTORY OF P AND S WAVESThe Earth's outer core acts as abarrier to S waves, blockingthem from reaching any pointthat forms an angle greaterthan 105° from the epicenter. Pwaves are transmitted fartherthrough the core, but they maybe diverted later on. Thus theyare detected at points that forman angle of greater than 140°from the epicenter.
The ground is compressed and stretchedby turns along the path ofwave propagation.
Speed of surface waves inthe same medium.
1.9 miles per second(3.2 km/s)
appear on the surface after the P and S waves reach theepicenter. Having a lower frequency, surface waves have a greatereffect on solids, which makes them more destructive.
Direction ofseismic waves
Vibration of rockparticles
BASED ON TYPE OF MOVEMENT
S waves are 1.7 times asslow as P waves.
2.2 miles per second(3.6 km/s)
190 miles (300 km)Primary (P) Waves
Secondary (S) Waves
The ground is moved in anelliptical pattern.
The soil is moved toboth sides,perpendicular to thewave's path of motion.
P waves travel through alltypes of material, and thewaves themselves move inthe direction of travel.
Typical Speed of P Wavesin the Crust.
3.7 miles per second(6 km/s)
They travel only through solids.They cause splitting motionsthat do not affect liquids. Theirdirection of travel isperpendicular to the directionof travel.
These waves travelonly along thesurface, at 90percent of the speedof S waves.
Body waves that shake the rock up and down and side toside as they move.
Seismic energy is a wave phenomenon, similar to the effectof a stone dropped into a pool of water. Seismic wavesradiate out in all directions from the earthquake's
hypocenter, or focus. The waves travel faster through hard rockand more slowly through loose sediment and through water. Theforces produced by these waves can be broken down intosimpler wave types to study their effects.
Different Types of Waves
There are basically two types of waves: bodywaves and surface waves. The body waves
travel inside the Earth and transmit foreshocksthat have little destructive power. They are dividedinto primary (P) waves and secondary (S) waves.Surface waves travel only along the Earth'ssurface, but, because of the tremors they producein all directions, they cause the most destruction.
RAYLEIGH WAVESThese waves spread with anup-and-down motion, similar toocean waves, causing fracturesperpendicular to their travel bystretching the ground.
LOVE WAVESThese move like horizontal Swaves, trapped at the surface,but they are somewhat slowerand make cuts parallel to theirdirection.
The soil ismoved toboth sides.
Wave speed in feetper second (m/s)
43 miles (70 km)
430 miles (700 km)
Magnitude Energy released by TNT
The energy released by an earthquakeof magnitude 4 on the Richter scale isequal to the energy released by alow-power atomic bomb.
4 = 1,000 tonsMagnitude Energy released by TNT
The energy released by an earthquake ofmagnitude 7 on the Richter scale, such as the1995 earthquake in Hyogo-Ken Nanbu, Japan,is equal to the energy released by a high-powered thermonuclear bomb (32 megatons).
1 The soil is compact,even though itcontains water.
3 Solid structuressink, and waterrises.
DIRECT ANDINDIRECT EFFECTS
Direct effects are felt in thefault zone and are rarely seenat the surface. Indirecteffects stem from the spreadof seismic waves. In the Kobeearthquake, the fault causeda fissure in the island of Awajiup to 10 feet (3 m) deep. Theindirect effects had to dowith liquefaction.
2 During the earthquake,the water causes thesolid particles to shake.
OAKLAND,CALIFORNIALatitude 37° 46’ N
Longitude 122° 13’ W
Surface area ofstate
Range of Damages
Earthquakes peryear (>4.0)
Magnitude onRichter scale
156,100 square miles(404,298 sq km)
68 miles (110 km)
Seismic tremors apply a force to muddy or water-saturated soils, filling the emptyspaces between grains of sand. Solid particles become suspended in the liquid, thesoil loses its load-bearing capacity, and buildings sink as if the ground werequicksand. That displaces some of the water, which rises to the surface.
FACTORS THAT DETERMINE THEEFFECTS OF AN EARTHQUAKE
SOCIALQuality of constructionPreparedness of the populationTime of day
7 = 32 million tonsMagnitude Energy released by TNT
The energy released by a hypotheticalearthquake of magnitude 12 on the Richterscale (the greatest known earthquake was 9.5)would be equal to the energy released if theEarth were to split in half.
12 = 160 quadrillion tons
Each soil type respondsdifferently to an earthquake.The figure shows how thesame quake can producewaves of different strengthsin different soils of differentcomposition. The 0.86-mile(1.4-km) collapsed section ofInterstate 880 was built onthe mud of San Francisco Bay.
0 10 20 30
VOLCANOES AND EARTHQUAKES 65
Bursts of tension
Seismic energy can even be compared to the powerof nuclear bombs. In addition, the interactionbetween seismic waves and soil materials also
causes a series of physical phenomena that can intensifyits destructive capacity. An example of such an effect
occurred when a section of an interstate highwayplummeted to the ground during the 1989 earthquake inLoma Prieta, California.
66 EARTHQUAKES VOLCANOES AND EARTHQUAKES 67
Earthquakes can be measured in terms of force, duration, and location.Many scientific instruments and comparative scales have beendeveloped to take these measurements. Seismographs measure all
three parameters. The Richter scale describes the force or intensity of anearthquake. Naturally, the destruction caused by earthquakes can bemeasured in many other ways: numbers of people left injured, dead, orhomeless, damage and reconstruction costs, government and businessexpenditures, insurance costs, school days lost, and in many more ways.
Measuring an Earthquake EMS 98 SCALEIn use since 1998 throughout the European Union andother countries that use the protocol, including thoseof northern Africa. This scale describes the intensityof earthquakes in European contexts, where the mostmodern construction may be found side by side withancient buildings. Earthquakes there can have widelyvarying effects. The scale has 12 points that combinemagnitude readings with levels of destruction.
Richter ScaleIn 1935, seismologist Charles Richterdesigned a scale to measure the amplitude of
the largest waves registered by seismographs. Animportant feature of this scale is that the levels increaseexponentially. Each point on the scale represents 10 timesthe movement and 30 times the energy of the point below it.Temblors of magnitude 2 or less are not perceptible to humans.This scale is the most widely used in the world because it can beused to compare the strength of earthquakes apart from their effects.
CHARLES RICHTERAmerican seismologist(1900-85) who developedthe scale of magnitude thatbears his name.
GIUSEPPE MERCALLIItalian volcanologist (1850-1914) who developed thefirst scale for measuring theintensity of an earthquake.
3.5 5.5 6.0 6.5 7.5 8.04.0 7.0 9.0
Very greatearthquake. Totaldestruction.
Major earthquake.Causes extensivedamage.
May cause heavydamage inpopulated areas.
Registered only byseismographs.
Some buildingsare lightlydamaged.
The tremor isfelt. Only minordamages.
Most peopleperceive thequake.
Very fewpeople feelthe tremor.
The energy released in a seismic event.
The wholeinterior of abuilding vibrates.
Parked carsrock backand forth.
Walls popout of theirframes.
Wide cracksform in theground.
The groundsplits openand sinks.
Drivers losecontrol of
The shaking isperceptible toeverybody.
Everyone is awareof the earthquake.People flee outside.
Buildings aredamaged.Cracks form inthe ground.
Total destruction.Waves are visibleon the ground.
VIII XIII II III IV V VI VII IX X
The shaking is feltby people inside.
Animalsbecome upsetand anxious.
Mounds ofsand and mudwell up.
Richter and Mercalli EMS
USE OF SCALES WORLDWIDE
No structureis leftstanding.
Concept of the destruction caused byan earthquake.
Modified Mercalli ScaleBetween 1883 and 1902, this Italian volcanologistdeveloped a scale to measure the intensity of
earthquakes. It originally had 10 points based on theobservation of the effects of seismic activity; it was latermodified to 12. The first few levels consist of barely perceptiblesensations. The highest levels apply to the destruction ofbuildings. This scale is widely used to compare levels ofdamage among different regions and socioeconomic conditions.
90%Movement oftectonic plates
The displaced water tends to level out,generating the force that causes waves.
Only earthquakes above thismagnitude on the Richter scalecan produce a tsunami strongenough to cause damage.
Typical height a major tsunamican reach.
BPRDetectsvariations in thecolumn of water
Sensor BPR: Registers pressureon the ocean floor.
The tsunami passes abovethe BPR and activates thenotification procedure.
The buoy sends thesatellite encodedinformation.Signal
The word tsunamicomes from Japanese
Water level rises Water level drops
2THE WAVES ARE FORMEDAs this mass of water drops, the waterbegins to vibrate. The waves, however,are barely 1.5 feet (0.5 m) high, and aboat may cross over them without thecrew even noticing.
Alarge earthquake or volcanic eruption can cause a tsunami, whichmeans “wave in the harbor” in Japanese. Tsunamis travel very fast,up to 500 miles per hour (800 km/h). On reaching shallow water,
they decrease in speed but increase in height. A tsunami can become a wallof water more than 33 feet (10 m) high on approaching the shore. Theheight depends partly on the shape of the beach and the depth of coastalwaters. If the wave reaches dry land, it can inundate vast areas andcause considerable damage. A 1960 earthquake off the coast ofChile caused a tsunami that swept away communities along 500miles (800 km) of the coast of South America. Twenty-twohours later the waves reached the coast of Japan, wherethey damaged coastal towns.
1THEEARTHQUAKEA movement of theocean floor displacesan enormous mass ofwater upward.
Displacement ofthe plate.
Detection device.Located at a depthof 16,000 feet(5,000 m).
FloatSystem ofglass spheres.
How It HappensA tremor that generates vibrationson the ocean water's surface can be
caused by seismic movement on theseafloor. Most of the time the tremor iscaused by the upward or downwardmovement of a block of oceanic crust thatmoves a mass of ocean water. A volcaniceruption, meteorite impact, or nuclearexplosion can also cause a tsunami.
COMPARISON OF THE SIZE OF THE WAVE
VOLCANOES AND EARTHQUAKES 69
Buildings onthe coast maybe damaged ordestroyed.
Between 5 and 30minutes before thetsunami arrives, the sealevel suddenly drops.
LENGTH OF THE WAVEFrom 62 to 430 miles(100 to 700 km) on theopen sea, measured fromcrest to crest.
31 miles per hour(50 km/h)Speed of the Tsunami
18,000 feet(5,500 m)
3,000 feet(900 m)
65 feet(20 m)
518 miles/hour(835 km/h)Speed of the tsunami
210 miles/hour(340 km/h)Speed of the tsunami
33 feet(10 m)
25 feet(8 m)
9 feet (3 m)
6 feet(1.8 m)
B The giant wave forms.At its highest, the wavemay become nearlyvertical.
C The wave breaks along the coast.The force of the wave is releasedin the impact against the coast.There may be one wave or severalwaves.
D The land is flooded.The water may take severalhours or even days to returnto its normal level.
WHEN THE WAVE HITS THE COAST
3THE WAVES ADVANCEWaves may travel thousands ofmiles without weakening. As thesea becomes shallower near thecoast, the waves become closertogether, but they grow higher. 4
TSUNAMIOn reaching the coast,the waves find their pathblocked. The coast, like aramp, diverts all the forceof the waves upward.
Sea level drops abnormally low.Water is “sucked” away from the coast bythe growing wave.
After a catastrophe
70 EARTHQUAKES VOLCANOES AND EARTHQUAKES 71
The illustration shows a satellite image of Khao Lak, in the coastal province of Phang Ngain southwestern Thailand. The picture was taken three days after the great tsunami ofDec. 26, 2004, that was caused by the colliding Eurasian and Australian plates near
Indonesia. This was the greatest undersea earthquake in 40 years. Below, the area as itlooked two years earlier.
THAILANDLatitude 16° N
Longitude 100° E
Persons missing in tsunami
Surface area 198,114 square miles(513,115 sq km)
289 people per squaremile (112.9 people/sq km)
The tropical forestsalong the beach ofCape Pankarang werewiped out.
River levelsrose after thecatastrophe.
Thailand had thegreatest number oftourist deaths.
Sofitel MagicLagoon Resort
The coastlinewas devastated.
Many beacheslost all theirsand.
Theptaro Lagoon Resort
A N D A M A N S E A Area floodedby the wave 3,300-4,900 feet (1,000-1,500 m)
0 feet (m)1,650 (500)
Deaths caused by thetsunami in this areaalone.
Time when the wavehit the beach.
The hypothetical line shows howfar inland the tsunami reached.Over half a mile (1 km) of thecoast was swept away by thetidal wave. Eighty percent of thesurface in Khao Lak's touristarea was destroyed.
Dec. 29, 2004Everything swept
away by the wavewas left piled up
on the beach.
The river's mouthwas completelyflooded.
EPICENTER OF THE QUAKE
Nearly two years before thetragedy. Wide areas of lushvegetation, fine, white sandbeaches, and many buildingsenhanced the natural attractionsof this tourism center.
Jan. 13, 2003
The tsunami's advance.A seismic station in Australia detected theseismic movement that later caused the great
tsunami that struck the nearest coastlines with wavesmore than 33 feet (10 m) high. An hour and a half later,the tsunami reached Sri Lanka and Thailand. The
tsunami had seven crests, which reached the coastsat 20-minute intervals. By the time the tsunami
arrived at the coast of Africa hours later,the waves had been greatly
Percentage of Earth's surface
Percentage of total volumeof the oceans
Length of plate boundaries (in focus)
Countries affected in 2004
28.3 million square miles (73.4 million sq km)
745 miles (1,200 km)
INDIAPop. 1.065 billion
MYANMAR Pop. 42,720,196
THAILANDPop. 64,865,5238,212 estimated
SRI LANKA Pop. 19,905,165
SOMALIA Pop. 8,863,338
Speed of the first wave
500 miles per hour (800 km/h)
Local time when thetsunami was unleashed(00:58 universal time)
Multiple aftershocks of up tomagnitude 7.3.
0.4 inches per year
(1.0 cm/yr)4 inches p
(1 cm/year) .
EPICENTER3° 18’ N95° 47’ E
30 percent were children
6h Indian Ocean
VOLCANOES AND EARTHQUAKES 73
Undersea earthquakeDisplacement of 50 feet(15 m) along the edge ofthe Indian Plate, 18 miles(30 km) below the seabed.
The wave beginsLarge waves are detected northwest andsoutheast of the epicenter.
First impactA 33-foot-high (10 m)wave destroys BandaAceh, Indonesia, reaching2.5 miles (4 km) inland.
Plate movements atdifferent speeds
Time it took the waveto reach the indicateddotted line.
Movement ofthe wave.
DurationThe tremor lasted between 8 and 10minutes, one of the longest on record.
The waves took six hours to reach Africa, over5,000 miles (8,000 km) away.
The wavereaches land
On Dec. 26, 2004, anearthquake occurred thatmeasured 9.0 on the Richter
scale, the fifth greatest on record.The epicenter was 100 miles (160km) off the west coast ofSumatra, Indonesia. This quakegenerated a tsunami thatpummeled all the coasts of theIndian Ocean. The islands ofSumatra and Sri Lankasuffered the worst effects.India, Thailand, and theMaldives also suffereddamage, and therewere victims as faraway as Kenya,Tanzania, andSomalia, in Africa.
THE VICTIMSOn this map, the number of confirmed deathsand the number of missing persons in eachcountry are added together, giving anestimated total death toll. In addition,1,600,000 persons had to be evacuated.
A F R I C A
A S I A
Study and Prevention
Predicting earthquakes is verydifficult because of a largenumber of variables, because notwo fault systems are alike. Thatis why populations that have
settled in areas with high seismic riskhave developed a number of strategies tohelp everyone know how to act should theearth begin to shake. California and Japanare examples of densely populated
regions whose buildings, now designedaccording to a stable construction model,have saved many lives. There children aretrained periodically at their schools: theydo practice drills, and they know where to
look for cover. Experts have learned manythings about earthquakes in their attempt tounderstand the causes of these tremors, butthey still are not able to predict when anearthquake will take place.
RISK AREAS 76-77
PRECISION INSTRUMENTS 78-79
CONTINUOUS MONITORING 80-81
STABLE BUILDINGS 82-83
ON GUARD 84-85
SAN FRANCISCO IN FLAMES 86-89
HISTORIC EARTHQUAKES 90-91
NORTHRIDGEThe 1994 earthquake in thisCalifornia city measured 6.7 onthe Richter scale and causedapproximately 60 deaths anddamage estimated at a valueof $40,000,000,000.
76 STUDY AND PREVENTION VOLCANOES AND EARTHQUAKES 77
seismic area is found wherever there is an active fault, andthese faults are very numerous throughout the world. Thesefractures are especially common near mountain ranges and
mid-ocean ridges. Unfortunately, many population centers werebuilt up in regions near these dangerous places, and, when anearthquake occurs, they become disaster areas. Where thetectonic plates collide, the risk is even greater.
ANTARCTICPLATE ANTARCTIC PLATE
A F R I C A
E U R O P E
A S I A
N O R T HA M E R I C A
C E N T R A LA M E R I C A
S O U T HA M E R I C A
An d e s
A l p s
Ca u c a
Ur a l
H i m a l a y a
Mou n t a i n s
H i m
y a s
MARIANA TRENCHThe deepest marine trench onthe planet, with a depth of35,872 feet (10,934 m) belowsea level. It is on the westernside of the north Pacific andeast of the Mariana Islands.
AFRICAN AND ARABIANPLATESThe African Plate includespart of the Atlantic,Indian, and Antarcticoceans. To the north itborders with the ArabianPlate. When these twoplates separated, theyformed the Red Sea, whichis still widening.
NEW ZEALAND FAULTA large fault that moveshorizontally, crosses the lithosphere,accommodating the movementbetween two large crust plates; it isa special type of directional platecalled a transforming plate.
Most-vulnerable regionsThey are unpredictable, andamong the most destructive
of natural phenomena. Earthquakesshake the earth. They open andmove it, and, within a few seconds,they can turn a peaceful city intothe worst disaster area, an area in
which seismic activity and a highpopulation density coincide. But in the open country, whereearthquakes have much less effect,we can conclude that it is notearthquakes, but buildings, that kill people.
COCOS ANDCARIBBEAN PLATESContact between thesetwo plates is of theconvergent type: the CocosPlate moves under theCaribbean Plate, aphenomenon known assubduction. This causes agreat number of tremorsand volcanoes.
MID-OCEAN RIDGEA submarine mountain rangeformed by the displacementof tectonic plates, these areactive formations. Thesemountain systems are thelongest in the world.
9.2Alaska, 1964Lasted betweenthree and fiveminutes andcaused a tsunamiresponsible for122 deaths.
The city of Kobeand nearby villages
were destroyed inonly 30 seconds.
8.3San Francisco,1906Major firescontributed tothe devastationof the city.
Portugal, 1755More than
60,000 peopledied, and a
tsunami followedthe earthquake.
8.7Assam, 1897More than 1,600people died innorthwest India.
Destroyed thecity of Spitak and
took more than25,000 lives.
7.5Iran, 1990More than 60,000dead. This was theworst disaster in Iranin the 20th century.
8.1Mexico, 1985Two days laterthere was a 7.6aftershock. Morethan 11,000people died.
9.5Chile, 1960The most powerfulearthquake everregistered: 5,700people died andtwo million wereleft homeless.
9.0Sumatra, 2004Tsunami in AsiaAn earthquake nearthe island of Sumatracreated 33-foot (10-m) waves and ahuman tragedy.
78 STUDY AND PREVENTION VOLCANOES AND EARTHQUAKES 79
The destructive potential of earthquakes gave rise to the need to study, measure,and record them. Earthquake records, called “seismograms,” are produced byinstruments called “seismographs,” which basically capture the oscillations of
a mass and transform them into signals that can be measured and recorded. Anearthquake is usually analyzed by means of three seismographs, each orientedin a unique direction at a given location. In this way one seismographdetects the vibrations produced from north to south, anotherrecords those from east to west, and a third detects verticalvibrations, those that go up and down. With these threeinstruments, a seismic event can be reconstructed.
Precision InstrumentsWILMORE PORTABLESEISMOMETERA sensitive mass vibrates and movesto the rhythm of the seismic energyinside this tube-shaped mechanism.An electromagnet translates thisvibration to electric signals, whichare transmitted to a computer thatrecords the data.
Seismometers in HistoryModern seismometers have digital mechanisms that provide maximum precision. Thesensors are still based on seismic energy moving a mechanical part, however, and
that is essentially the same principle that operated the first instrument used to evaluateearthquakes. It was invented by a Chinese mathematician almost 2,000 years ago.Beginning with his invention, the mechanism has been perfected to what it is today.
JOHN MILNEBritish geologist and engineer(1850-1913), created a needleseismograph, a forerunner ofcurrent seismographs, andrelated earthquakes tovolcanism.
ROBERT MALLETFrom Dublin, Ireland (1810-81). Carried out importantstudies on the speed of thepropagation of earthquakes,even before havingexperienced one.
RICHARD OLDHAMBritish (1858-1936).Published a study in 1906on the transmission ofseismic waves, in which healso proposed the existenceof the Earth's core.
Pioneers of seismologyThe defining principle of modern seismology emerged from relatingearthquakes to the movement of the continents, but that did not take place
until well into the 20th century. Starting in the 19th century, however, manyscholars contributed elements that would be indispensable.
BOSCH-OMORISEISMOMETER Is a horizontal pendulum with apen that makes a markdirectly on a paper roll. Withit, Omori, a Japanesescientist, registered the1906 earthquake inSan Francisco.
SPRING Allows the pivot andmass to movevertically (vibratingmovement).
ROTATING DRUMS Move the roll at a constantand precise speed.
CLOCK ANDRECORDER Take the signal,synchronize it, andconvert it.
PIVOT Supports andmaintains anaxis. It canhave a hinge.
HOW IT WORKS The oscillating massvibrates when an earthquake takes place. The“dragons,” joined to the pendulum by a rigidbar, hold small balls in a delicate equilibrium.
OSCILLATING PENCIL Moves to the rhythm ofthe vibrations amplifiedby the mechanism..
SEISMOGRAM The record of amplitude onthe paper strip.
SUSPENDEDMASS Moves according to thedirection of the waves of theearthquake and in proportion totheir strength.
MOVEMENTSENSORThe floating mass is displaced and moves a partinside an electromagnet.Variations produced in themagnetic field are convertedinto signals.
CONNECTING CABLETransmits the electricsignals that aregenerated.
SUSPENSION The smallvibrations of theground will movethe base more.
HENG'S SEISMOSCOPE The first known seismometer was Chinese. The metallicpendulum mass hung from the cover of a large bronze
jar. The small balls fell from the mouths of thedragons to the mouths of the frogs, depending on
the direction of the seismicmovement. Some of these modelswere 6 feet (2 m) tall.
ZHANG HENGChinese mathematician,astronomer, andgeographer (AD 78-139),also invented the odometer,calculated the number pi asthe square root of 10 (3.16),and corrected the calendar.
PORTABLE SEISMOMETER Their strong structure allowed theseseismometers to be installed in the field. This model translated movement toelectric impulses so the signal could betransmitted over some distance.
How a Seismograph WorksThe Earth's tremors produce movements in the massthat serves as a sensor. If the pivot is hinged, it allows
movements in only one direction: horizontally or vertically,depending on the sensitivity and calibration of the spring.These movements are transformed into electric or digitalsignals to give versatility in processing and recording the data.
ANCHORED BASE The greater the degree ofsuspension, the greater thesensitivity of the mechanism.
80 STUDY AND PREVENTION
umankind has tried throughout history to find a way to predict earthquakes.Today, this is done through the installation of seismic observatories and of variousfield instruments that gather information and compare it to the data sent by
scientists from other locations. Based on these records, it is possible to evaluate thechances that a great earthquake is developing and act accordingly.
SEISMOMETER Registers ground vibrations, theiramplitude, and the direction inwhich they are produced. Aseismometer can detect even thesmallest tremor. Some, such asthose pictured here, are poweredby solar energy.
Placement of a Creep MeterTo measure the relative movement between the ends, two posts arefixed, one at each side of the fault, 6 feet (2 m) under the ground, orover the concrete base, at a fixed angle (but not at a right angle).
Placement of the Seismometer The movement of the sensor mechanism, located under the ground,is converted into electric signals that are transmitted either to therecording module located on the surface or to computers.
Earthquakes cannot be predicted
For a prediction system to be acceptable, itmust be accurate and trustworthy.
Therefore, it must have a small level of uncertaintyregarding location and the timing, and it mustminimize errors and false alarms. The cost ofevacuating thousands of people, of providinglodging for them, and of making up for their loss oftime and work for a false alarm would beunrecoverable. At this time there is no trustworthymethod for predicting earthquakes.
CREEP METER Measures the movement betweenearthquakes or time intervalbetween the two boundaries of afault. It includes a tension systemwith a calibrated mechanism. Anymovement between the ends altersthe magnetic field.
NETWORK OF NETWORKS Findings in an area can haverepercussions at a great distance.The immediate availability of dataallows for linked work.
VOLCANOES AND EARTHQUAKES 81
GPSThe global positioning system(GPS) receiver picks upsignals from the satellitesand transmits them to theobservatories. Because these
signals register thereceivers' exact locations,
a change in theirposition over time
indicates a movementin the crust.
Observing froma Distance
Seismologists placeinstruments at fault lines
in earthquake-prone areas. Later,at the seismologic observatory,the information taken by fieldinstruments is compiled, and anysignificant change is noted. Ifanything suggests that anearthquake is about to takeplace, emergency services arealerted. Most of theseinstruments are automatic, andthey send digital data throughthe telephone system.
Seismologic NetworksInstalling complex detection systemswould not be of much use if the systems
worked in an isolated manner and were not ableto share the information they generated. Thereare national and international seismologicnetworks that, by means of communicationstechnology, send their observations to otherareas that might be affected.
LABORATORY Networking at the researchcenters allows for the comparisonof data and provides a globalvision that broadens the predictivepower of science.
SATELLITES Some are used by the GPSsystems, but others arecritical because they takephotographs with extremeaccuracy, and they are thusable to record indicators thatcan be communicated quicklyto the base.
MAGNETOMETER The magnetic field of theEarth changes when thetensions between rocks vary.Therefore, a change inmagnetism can indicatetectonic movement. Themagnetometer can distinguishbetween these changes and
those that are moregeneral.
Swaying PrincipleEach floor also has a symmetricstructure. During an earthquake,the ends opposite the eaves actas a counterbalance to achieveequilibrium.
Suspension SystemSo that a building will suffer only smalloscillations during an earthquake, it is
isolated and built in a large trench, separated byspecial devices. In addition, because the higherfloors move more than the lower ones,mechanical dampers are emplaced diagonally soas to be more highly tensioned on top than onthe bottom. This makes the structure as a wholemore flexible, but it also offers resistance tosudden variations.
VOLCANOES AND EARTHQUAKES 8382 STUDY AND PREVENTION
n cities located in seismic areas, buildings must be designed and constructed withan earthquake-resistant structure that can adequately withstand the movementscaused by an earthquake. Foundations are built with damping so that they can
absorb the force of the seismic movement. Other buildings have a large metallicaxis, around which the stories of the building can oscillate without falling.Currently the amount of knowledge on the effect of earthquakes on structures,as well as knowledge on the behavior of different materials, allows for theconstruction of less-vulnerable buildings.
There are many ways to design an earthquake-resistant structure: the distribution of walls, the
joints between beams and columns, and geometricsimplicity. There are also earthquake simulators, largeplatforms that shake a structure to test it. The
simulators are used to test materials and study theforces that act on them. A building's true earthquakeresistance, though, can be proven only when it has beenbuilt and has survived actual earthquakes.
COLUMNSAn example of a buildingthat shows simplicity inits geometric design and,therefore, in its behavior.
898 FEET (274 M)
Why Pagodas in Japan Do Not Fall
Japanese pagodas have survived centuries ofearthquakes. They are five stories tall, and higher sections
of the building are smaller than lower parts. The pagodas areheld up by a central pillar that acts as the only support for thebuilding. During a quake, each floor balances independently,without transmitting the oscillatingforce to the other floors.
SHIJU Fourth floor
THE INTELLIGENT BUILDINGWhen an earthquake is detected,a computer-controlled systemprovides variable compression tothe dampers, which absorb themovements according to theheight of the floor and theintensity of the vibrations.
BASE ISOLATIONA system made of steel disks,interspersed with plates made of a softmaterial, softens the transmission ofseismic movements from the ground tothe building.
DAMPERSAre made of pistonsfilled with oil. Theyreduce the force ofhorizontal movement.
STRUCTURE To avoid imbalances, the upper elements of a structure must belocated over only a few axes, without any isolated vertical segments.
AVOIDANCE OF OFF-CENTER JOINTSIf the beam remains still when the wall moves, the joint breaks.Forces spread out over an axis with flexible material.
ROPPONGIHILLSTOKYO, JAPANLatitude 36° N
Longitude 140° E
275 feet (84 m)
780 feet (238 m)
54 (+6 underground)
4,089,930 squarefeet (380.105 sq m)
ROPPONGI HILLS TOWERLocated in Tokyo, its structureconsists of simple geometricbodies, symmetrically placed,without any irregularities inshape. The tower is formed bya massive central frameworkand a lightweight and flexibleexterior framework.
Conventional earthquake-resistant structure
Plates 0.12 inch (3 mm)thick alternate withelastic rubber layers.
84 STUDY AND PREVENTION
When the earth shakes, nothing can stop it. Disaster seems inevitable,but, though it is inevitable, much can be done to diminish the extentof the catastrophe. Residents of earthquake-prone areas
have incorporated a series of preventive measures to avoidbeing surprised and to help them act appropriately athome, at the office, or outdoors. These are basic rules of behavior that will help you survive.
VOLCANOES AND EARTHQUAKES 85
PreventionIf you live in an earthquake-prone area, familiarize yourself
with the emergency plans for thecommunity where you live, plan howyour family will behave in the event ofan earthquake, know first aid, and knowhow to extinguish fires.
LIGHTSHave emergencylighting, flashlights, atransistor radio, andbatteries on hand.
SECURINGOBJECTSSecure heavy objectssuch as furniture,shelves, and gasappliances to the wallor to the floor.
BREAKERSHave a breakerinstalled, and knowhow to shut off theelectricity and thegas supply.
FIRST AIDLearn first aid, andparticipate incommunityearthquake-responsetraining.
FIRST-AID KITKeep a first-aidkit, and keep yourvaccinations up to date.
AT HOMEIt is essential that the homebe built following regulationsfor earthquake-resistantconstruction and thatsomeone be in charge ofshutting off the electricity andgas supplies.
AT THE OFFICEOffices are usually located in areas conducive to bringing large groups ofpeople together. Thus it is recommended that you remain where you areand not rush to the exits. When people panic, there is a greater probabilityof their being crushed by a crowd than by a building, especially in buildingsthat contain a lot of people.
IN PUBLIC PLACESWhen you are outside, it is important to keep away from tall buildings, light poles, and otherobjects that could fall. The safest course of action is to head to a park or other open space. Ifthe earthquake takes you by surprise while you are driving, stop and remain in the car, but makesure you are not close to any bridges.
Do not drink tapwater because itmight be polluted.
Toxic orflammablematerials mustnot be in dangerof spilling.
Objects that couldfall because ofmovement should beattached to the wall.
Determine safe spotsunder doorframes,next to a pillar, orunder a table.
Do not lightmatches or flames:use flashlights.
If you are near anexit, leave the buildingand walk away. Do notblock doorways.
Know where emergency equipment,such as fire extinguishers, hoses,and axes, is located.
Stay away frombuildings, walls, utilitypoles, and otherobjects that could fall.
Fires are frequently moredangerous than theearthquake itself. Theycan easily get out ofcontrol and spreadthrough the city. If you are in a vehicle, stop in
the safest place possible (awayfrom large buildings, bridges,and utility poles). Do not leaveyour car unless it becomesnecessary to do so.
Head toward open spacessuch as squares and parks,and move away from anytrees to the extent possible.
Stay awayfrom windowsand balconies.
Fixing breaks inwater and gas linesis a priority.
Do not useelevators: theelectricity mightbe cut off.
In case of evacuation,stairs are the safestplace, but they couldbecome filled withpeople.
Mark escape routes andkeep them free ofobstacles.
Follow theinstructions of civildefense officers.
Do not approach the coastline because of thepossibility of a tsunami. Also avoid rivers, whichcould develop strong currents.
TRANSPORTATIONIt is important to keepaccess routes to affectedareas open to ensure entryby emergency teams.
RESCUERSThe first priority afteran earthquake is tosearch for survivors..
DOGSSpecially trained animalswith protective helmetsand masks can search forpeople under the rubble.
It is good to designatea leader who can guideothers. Form a humanchain to preventgetting lost and toprevent accidents.
Do not run on thestreet; doing so willcause panic.
During an earthquakeAs soon as you feel the earth under yourfeet begin to move, look for a safe place,
such as beneath a doorframe or under a table, totake cover. If you happen to be on a street, headto an open space such as a square or park. It isimportant to remain calm and to not beinfluenced by people who panic.
Rescue TasksOnce the earthquake ends, rescue tasks must begin. At this stage it isimperative to determine whether anyone is injured and to apply first aid. Do
not move injured people who have fractures, and do not drink water from opencontainers.
Seek protection undera table or desk toavoid being hurt byfalling objects.
VOLCANOES AND EARTHQUAKES 8786 STUDY AND PREVENTION
Until the earthquake struck, City Hall had been the seat of city governmentand the symbol of the city. Built in the second half of the 19th century, it
represented a time of accelerated growth, powered by the gold riches of the stateof California. Construction began on Feb. 22, 1870, and ended 27 years later, aftermany revisions to architect Auguste Laver's original project. While it stood, CityHall was said to have been constructed with bricks held together with corruption,typical of a time of easy money and weak institutions. The total cost of the workrose to a little more than $6,000,000 of that time, and, according to currentcalculations, it is estimated that it was prepared to withstand an earthquake upto a magnitude of 6.6. Only the dome and the metal structure were left standing.The remnants of the building were demolished in 1909.
History of City Hall
The earthquake that shook San Francisco on April 18, 1906, was a major event: inonly a few seconds, a large part of one of the most vital cities of the United Stateswas reduced to rubble. Suddenly, centuries of pent-up energy was released when
the earthquake, measuring 8.3 on the Richter scale, devastated the city. Though theearthquake destroyed many buildings, the worst damage was caused by the fires thatdestroyed the city in the course of three days, forcing people to flee their homes.
San Francisco in Flames
3 THE GREAT FIRETwo days later, what had begun as alocalized fire had become an inferno thatconsumed the city. There were massevacuations of people to distant areas,while the army dynamited somebuildings. Firefighters had to control theflames using seawater.
4 REBUILDINGThe city reemerged from the ashes, powered by its wealth and economic importance. Losses are estimated to have reached$5,000,000,000 in present-day dollars,and, until Hurricane Katrina struck in2005, the 1906 earthquake was thegreatest natural disaster the UnitedStates had experienced.
April 18 THE FACE OF THEBUILDING COLLAPSED.The facade collapsed completelyon top of the rotunda at its base.CITY HALL
The facade was crowned by adome that was supported bya system of columns on asteel structure. It wasconsidered one of the city'smost beautiful buildings.
UNITED STATESSAN FRANCISCO,CALIFORNIALatitude 42° 40’ N
Longitude 122° 18’ W
Perceptible earthquakes felt annually
Total earthquakes per year
46 square miles (120 sq km)
16,000 people per squaremile (6,200 people/sq km)
100 - 150
GAS LIGHTINGGas lighting was one of the signs ofprogress that gave prominence to the city.
EVERYTHING STARTED LIKE THIS.On April 18, 1906, at 5:12 a.m., the PacificPlate experienced movement ofapproximately 19 feet (6 m) along its 267-mile (430-km) length along the northernSan Andreas fault. The earthquake'sepicenter lay 39 miles (64 km) north of
San Francisco. In seconds, the earth beganto move, and the majority of the city'sbuildings collapsed. The trolleys andcarriages that were moving through thecobblestoned streets of the city werereduced to rubble.
THE EARTHQUAKENot only was the earthquake extremelyviolent, but it oscillated in every directionfor 40 seconds. People left their houses andran down the streets, completely stunnedand blinded by fear. Many buildings split
open, and others became piles of rubble. Apost office employee related that “The wallswere thrown into the middle of the rooms,destroying the furniture and coveringeverything with dust.”
VOLCANOES AND EARTHQUAKES 89
The great fire that followed the earthquakeexpanded quickly. Firefighters, in a desperate
attempt to block the spread of the fire, used explosives tomake firebreaks because there was no water supplyavailable. The army evacuated the area, and people couldnot take anything with them. During the three days whenthe city burned, it is speculated that many homeownersburned their houses that had been partially destroyed bythe earthquake, in order to be able to collect insurancemoney. Other things that contributed to the fire were theintentional explosions that, at first badly implemented,spread the fire. By the fourth day, the center of the citywas reduced to ashes.
Three Days of FireIt is calculated that some 3,000 people diedin the 1906 catastrophe, trapped in their
destroyed homes or burned by the fire theearthquake started. In the following weeks, thearmy, firefighters, and other workers deposited the
rubble in the bay, forming new land, which is todayknown as the Marina District. Little by little, trafficresumed in the major streets, and the trolley systemwas reestablished. Six weeks after the earthquake,banks and stores opened for business.
Clearing the Rubble
88 STUDY AND PREVENTION
WORKERSBy Saturday, April 21, some 300 plumbers had entered the city to reestablishservices, mainly the water system. During the following weeks, thousands ofworkers tore down unstable buildings, prepared the streets for traffic, andcleared the city of rubble. Nearly 15,000 horses were used to haul rubble.
SHORTLY AFTERWARDThis panoramic photographshows the destruction of thecity. Despite the destruction,many buildings were leftstanding.
1 The fire began in theMarket Street area,south of the city in theworker's district, wheremany houses were madeof wood.
2 On the second day, thefire spread west. About300,000 people wereevacuated from the bayin ferries.
3 During the third day, thefire swept throughChinatown and NorthBeach, causing heavydamage to the Victorianhomes on North Beach hill.
4 Once the fire wasextinguished, Russian Hilland Telegraph Hill (shownas white spots) were stillintact, as was the port.
Army tents tohouse refugees.
Workers posewhile they
demolish a house.
The firefighterstried to extinguishthe fire.
18,000 BUILDINGS OF THAT PERIODare still standing, despite the 1906 earthquakeand the tremors that passed through the cityafterward.
28,000 BUILDINGS DEMOLISHEDThe damage calculated to have been caused by thegreat earthquake. Many of these buildings, such asCity Hall, were famous for their lavish architecture.
FIELD LUNCHThe army set up kitchens in the camps. There wasalways food in these field kitchens, and there was evena free ration of tobacco for every person.
REFUGEE CAMPSThe army set up field camps in theparks to house those who had losteverything. Months later, thegovernment built temporary homesfor about 20,000 people.
CHINATOWNWas completelydestroyed by the fire.
MILLS BUILDINGThis building in thefinancial district had beenbuilt in 1890.
MERCHANT EXCHANGEBuilt in 1903, it remained standingand was later refurbished.
90 STUDY AND PREVENTION VOLCANOES AND EARTHQUAKES 91
Historic EarthquakesSAN FRANCISCO, U.S.
The city was swept by the earthquake and bythe fires that followed it. The quake was theresult of the rupture of more than 40 miles ofthe San Andreas fault. It is the greatestearthquake in the history of the United States:300,000 people were left homeless, andproperty losses reached millions in 1906dollars. Buildings collapsed, the fires spread forthree days, and the water lines were destroyed.
An earthquake whose epicenter crossed theisland of Sumatra, Indonesia, took place onDecember 26. This earthquake generated atsunami that affected the entire Indian Ocean,primarily the islands of Sumatra and SriLanka, and reached the coasts of India,Thailand, the Maldives, and even Kenya andSomalia. This was a true human tragedy, andthe economic damages were incalculable.
Known as the Great Chilean Earthquake, thiswas the strongest earthquake of the 20thcentury. The surface waves produced were sostrong that they were still being registered byseismometers 60 hours after the earthquake.The earthquake was felt in various parts of theplanet, and a huge tsunami spread through thePacific Ocean, killing more than 60 people inHawaii. One of the most powerful earthquakes inmemory, its aftershocks lasted for more than aweek. More than 5,000 people died, and nearlytwo million people suffered damage and loss.
1995MEXICO CITY, MEXICO
The city shook on September 19. Two dayslater, there was an aftershock measuring 7.6 onthe Richter scale. In addition to 11,000 deaths,there were 30,000 injured, and 95,000 peoplewere left homeless. As the Cocos Plate slidunder the North American Plate, the NorthAmerican Plate fractured, or split, 12 miles (20km) inside the mantle. The vibrations of theocean floor off the southwestern coast ofMexico provoked a tsunami and producedenergy 1,000 times as great as that of anatomic bomb. Strong seismic waves reached asfar east as Mexico City, a distance of 220miles (350 km).
Also known as the Indian SubcontinentEarthquake, the North Pakistan Earthquake,and the South Asian Earthquake. It occurred onOct. 8, 2005, in the Kashmir region betweenIndia and Pakistan. Because schools were insession when the earthquake struck (9:20 a.m.),many of the victims were children, who diedwhen their school buildings collapsed. It wasthe strongest earthquake experienced by theregion for a century. Three million people losttheir homes. The most-heavily affected areaslost all their cattle. Entire fields disappearedunder earth and rock. The epicenter waslocated near Islamabad, in the mountains ofKashmir, in an area governed by Pakistan.
It was the Day of the Dead, and, at 9:20 in themorning, almost the entire population of Lisbonwas at church. While mass was celebrated, theearth quaked, and this earthquake would beone of the most destructive and lethal inhistory. The earthquake unleashed a tsunamithat was felt from Norway to North Americaand that took the lives of those who had soughtshelter in the river.
he Earth is alive. It moves, it shifts, it crashes and quakes, and it has done so since its origin. Earthquakesvary from a soft vibration to violent and terrorizing movements. Many earthquakes have gone down inhistory as the worst natural catastrophes ever survived by humanity. Lisbon, Portugal, 1755; Valdivia,
Chile, 1960; and Kashmir, Pakistan, 2005, are only three examples of the physical, material, and emotionaldevastation in which an earthquake can leave a population.
AN INFERNOThe great earthquake of Hanshinthat occurred on Jan. 17, 1995, inKobe, a Japanese port, left behindmore than 6,000 dead, 38,000injured, and 319,000 people whohad to be housed in more than1,200 emergency shelters. TheNagata District was one of thehardest-hit areas. Almost 80percent of the victims diedbecause the old wooden homescrumbled in the generalized firesthat followed the earthquake.
AFTER THE HORROR. The world was shaken, looking atthe horrendous images of how Kobe, the city by the sea,had been painfully broken to pieces.
92 GLOSSARY VOLCANOES AND EARTHQUAKES 93
AaType of lava flow that presents sharpprojections on its surface when it hardens.
AbrasionModification of rock surfaces by friction andby the impact of other particles transportedby wind, water, and ice.
Active VolcanoVolcano that erupts lava and gas at regularintervals.
AerosolSmall particles and drops of liquid scatteredin the air by volcanic gases.
AftershockSmall temblor or quake produced as rocksettles into place after a major earthquake.
AseismicThe characteristic of a building designed towithstand oscillations, or of areas with noseismic activity.
Aseismic RegionTectonically stable region of the Earth, wherethere are almost no earthquakes. Forexample, the Arctic region is aseismic.
AshfallPhenomenon in which gravity causes ash (orother pyroclastic material) to fall from asmoke column after an eruption. Thedistribution of the ash is a function of winddirection.
AsthenosphereInternal layer of the Earth that forms part ofthe mantle.
AvalancheRapid movement of enormous volumes ofrock and other materials caused by instabilityon the flanks of the volcano. The instabilitycan be caused by the intrusion of magma intothe structure of the volcano, by a largeearthquake, or by the weakening of thevolcano's structure by hydrothermalvariation, for example.
Ballistic (Fragment)A lump of rock expelled forcefully by avolcanic eruption and that follows a ballisticor elliptical trajectory.
BalticOf or pertaining to the Baltic Sea, or to theterritories along it.
BatholithMassive body of magma that results from anintrusion between preexisting layers.
CalderaLarge, round depression left when a volcanocollapses onto its magma chamber.
Convection CurrentsVertical and circular movement of rockmaterial in the mantle but found exclusivelyin the mantle.
Convergent BoundaryBorder between two colliding tectonic plates.
CoreCentral part of the Earth, with an outerboundary 1,800 miles (2,900 km) below theEarth's surface. The core is believed to becomposed of iron and nickel—with a liquidouter layer and a solid inner core.
Epicentral AreaRegion around the epicenter of anearthquake, usually characterized by beingthe area where the shaking is most intenseand the earthquake damage is greatest.
Epicentral DistanceDistance along the Earth's surface from thepoint where an earthquake is observed to theepicenter.
Extinct VolcanoVolcano that shows no signs of activity for along period of time, considered to have a verylow probability of erupting.
Fault DisplacementSlow, gradual movement produced along afault. It is characterized by not generating anearthquake or tremor.
FocusInternal zone of the Earth, where seismicwaves are released, carrying the energy heldby rocks under pressure.
FumaroleEmission of steam and gas, usually at hightemperatures, from fractures or cracks in thesurface of a volcano or from a region withvolcanic activity. Most of the gas emitted issteam, but fumarole emissions can includegases such as CO2, CO, SO2, H2S, CH4, HCl,among others.
Geothermal EnergyNaturally heated steam used to generateenergy.
GeyserSpring that periodically expels hot waterfrom the ground.
GondwanaSouthern portion of Pangea, which at onetime included South America, Africa,Australia, India, and Antarctica.
Hot SpotPoint of concentrated heat in the mantlecapable of producing magma that shoots upto the Earth's surface.
Hydrothermal AlterationChemical change in rocks and minerals,produced by an aqueous solution that is rich involatile chemical elements found at high tem-perature and that rises from a magma body.
Igneous ActivityGeologic activity involving magma andvolcanic activity.
IncandescentA property of metal that has turned red orwhite because of heat.
LaharMudflows produced on the slopes ofvolcanoes when unstable layers of ash anddebris become saturated with water andflow downhill.
LapilliFragments of rock with a diameter between0.06 and 1.3 inches (2 and 32 mm) expelledduring a volcanic eruption.
LavaMagma, or molten rock, that reaches theEarth's surface.
Lava BombsMasses of lava that a volcano expels, whichhave a diameter equal to or greater than 1.2inches (3.2 cm).
Lava FlowRiver of lava that flows out of a volcano andruns along the ground.
LiquefactionTransformation of ground from solid to fluidstate through the action of an earthquake.
LithosphereRigid part of the outer layer of the Earth,formed by the crust and the outer layer ofthe mantle. This is the layer that is destroyedin subduction zones and that grows in mid-ocean ridges.
MagmaMass of molten rock deep below the surface,which includes dissolved gas and crystals.When magma has lost its gases and reachesthe surface, it is called lava. If magma coolswithin the Earth's crust, it forms plutonicrocks.
Magma ChamberSection within a volcano where incandescentmagma is found.
MantleLayer between the Earth's crust and theouter core. Its lower part, the asthenosphere,is partially molten. The more superficial andless-fluid outer part is called the lithosphere.
Mid-Ocean RidgeAn elongated mountain range on the oceanfloor, which varies between 300 and 3,000miles (500 and 5,000 km) in breadth.
NeckColumn of lava that has solidified inside avolcano.
CraterDepression on the peak of a volcano, orproduced by the impact of a meteorite.
CrustOutermost, rigid part of the Earth, made upmostly of basaltic rocks (underneath theoceans) and of rocks with a higher silicatecontent (in the continents).
DensityRatio of a body's mass to its volume. Liquidwater has a density of 62.4 pounds per cubicfoot (1 g/cu cm).
DikeTabular igneous intrusion that crossesthrough layers of surrounding rock.
DomeCup-shaped bulge with very steep sides,formed by the accumulation of viscous lava.Usually a dome is formed by andesitic, dacitic,or rhyolitic lava, and the dome can reach aheight of many hundreds of feet.
Duration of EarthquakeTime during which the shaking or tremor ofan earthquake is perceptible to humans. Thisperiod is always less than that registered by aseismograph.
EarthquakeVibration of the Earth caused by the releaseof energy.
EonThe largest unit of time on the geologic scale,of an order of magnitude greater than an era.
EpicenterPoint on the Earth's surface located directlyabove the focus of an earthquake.
94 GLOSSARY VOLCANOES AND EARTHQUAKES 95
Normal FaultFracture in rock layers where the ground isbeing stretched, which generally causes theupper edge to sink relative to the lower part.
Ocean TrenchLong, narrow, extremely deep area of theocean floor formed where the edge of anoceanic tectonic plate sinks beneath anotherplate.
Pahoehoe LavaLava with a smooth surface that has aropelike form.
Pelean EruptionType of volcanic eruption with a growingdome of viscous lava that may be destroyedwhen it collapses because of gravity or briefexplosions. Pelean eruptions producepyroclastic flows or burning clouds. The termcomes from Mount Pelée in Martinique.
Permeable LayersStrata of the Earth's crust that allow waterto reach deeper layers.
Plate TectonicsTheory that the Earth's outer layer consistsof separate plates that interact in variousways, causing earthquakes and formingvolcanoes, mountains, and the crust itself.
Plinian EruptionExtremely violent and explosive type ofvolcanic eruption that continuously expelslarge quantities of ash and other pyroclasticmaterials into the atmosphere, forming aneruption column typically 5 to 25 miles (8 to40 km) high. The term honors Pliny theYounger, who observed the eruption ofMount Vesuvius (Italy) in AD 79.
PlumeColumn of hot rock that rises from within themantle, inside of which the rock may melt.
Primary (P) WaveSeismic wave that alternately compressesand stretches the ground along its directionof travel.
PumicePale volcanic rock full of holes, which give it alow density. Its composition is usually acidic(rhyolitic). The holes are formed by volcanicgases that expand as volcanic material risesto the surface.
Pyroclastic FlowDense, hot mix of volcanic gas, ash, and rockfragments that flows rapidly down the sidesof a volcano.
Reverse FaultFractures in rock layers where the ground isbeing compressed, which generally causesthe upper edge to rise above the lower partin a plane inclined between 45 and 90degrees from the horizontal.
Richter ScaleMeasures the magnitude of an earthquake orof the energy it releases. The scale islogarithmic, such that an earthquake ofmagnitude 8 releases 10 times as muchenergy as a magnitude 7 quake. Anearthquake's magnitude is estimated basedon measurements taken by seismicinstruments.
Rift ZoneArea where the crust is splitting andstretching, as shown by cracks in the rock.Such areas are produced by the separation oftectonic plates, and their presence causesearthquakes and recurrent volcanic activity.
Scale of IntensityScale used to measure the severity ofmovement of the ground produced by anearthquake. Degrees of intensity are assignedsubjectively depending on how the tremor isperceived and according to the damagecaused to buildings. A widely used scale isthe Mercalli scale.
Secondary (S) WaveTransverse or cross-section wave withmotion perpendicular to the direction of itstravel.
Seismic EventShaking of the ground caused by an abruptand violent movement of a mass of rockalong a fault, or fracture, in the crust. Activevolcanoes cause a wide variety of seismicevents.
Seismic GapFault zone, or zone of a segment at theboundary between tectonic plates, with aknown seismic history and activity, whichrecords a period of prolonged calm, or ofseismic inactivity, during which largeamounts of elastic energy of deformationaccumulate, and that, therefore, presents agreater probability of rupture and occurrenceof a seismic event.
Seismic Hazard CalculationProcess of determining the seismic risk ofvarious sites in order to define areas withsimilar levels of risk.
Seismic RiskThe probability that the economic and socialeffects of a seismic event will exceed certainpreestablished values during a given period,for example, a certain number of victims, anamount of building damage, economic losses,etc. Also defined as the comparative seismichazard of one site relative to another.
Seismic WaveWavelike movement that travels through theEarth as a result of an earthquake or anexplosion.
Seismic ZoneLimited geographic area within a seismicregion, with similar seismic hazard, seismicrisk, and earthquake-resistant designstandards.
SeismographInstrument that registers seismic waves ortremors in the Earth's surface during anearthquake.
SeismologyBranch of geology that studies tremors in theEarth, be they natural or artificial.
Shield VolcanoLarge volcano with gently sloping flanksformed by fluid basaltic lava.
SiliconOne of the most common materials, and acomponent of many minerals.
SubductionProcess by which the oceanic lithosphere sinksinto the mantle along a convergence boundary.The Nazca Plate is undergoing subductionbeneath the South American Plate.
Subduction ZoneLong, narrow region where one plate of thecrust is slipping beneath another.
Surface WaveSeismic wave that travels along the Earth'ssurface. It is perceived after the primary andsecondary waves.
Swarm of EarthquakesSequence of small earthquakes that occur inthe same area within a short time period,with a low magnitude in comparison to otherearthquakes.
SymmetryCorrespondence that exists in an object withrespect to a center, an axis, or a plane thatdivides it into parts of equal proportions.
Tectonic PlatesLarge, rigid sections of the Earth's outer layer.These plates sit on top of a more ductile andplastic layer of the mantle, the asthenosphere,and they drift slowly at an average rate of 1inch (2.4 cm) or more per year.
Thrust FaultA fracture in rock layers that is characterizedby one boundary that slips above another atan angle of less than 45 degrees.
Transform FaultFault in which plate boundaries cause frictionby sliding past each other in oppositedirections.
TremorSeismic event perceived on the Earth'ssurface as a vibration or shaking of theground, without causing damage ordestruction.
TsunamiWord of Japanese origin that denotes a largeocean wave caused by an earthquake.
ViscousMeasure of a material's resistance to flow inresponse to a force acting on it. The higherthe silicon content, the higher the viscosity.
Volcanic GlassNatural glass formed when molten lava coolsrapidly without crystallizing. A solid-likesubstance made of atoms with no regularstructure.
Volcanic RingChain of mountains or islands located nearthe edges of the tectonic plates and that isformed as a result of magma activityassociated with subduction zones.
VolcanoMountain formed by lava, pyroclasticmaterials, or both.
Volcanology(Vulcanology)Branch of geology that studies the form andactivity of volcanoes.
Vulcanian EruptionType of volcanic eruption characterized bythe occurrence of explosive events of briefduration that expel material into theatmosphere to heights of about 49,000 feet(15 km). This type of activity is usually linkedto the interaction of groundwater andmagma (phreatomagmatic eruption).
Water SpringNatural source of water that flows out of thecrust. The water comes from rainwater thatseeps into the ground in one place and comesto the surface in another, usually at a lowerelevation. Because the water is not confinedin waterproof chambers, it can be heated bycontact with igneous rock. This causes it torise to the surface as hot springs.