I Str:t tcw7 for I !sing This l*cx tbook

Who Needs Geology? Imagine yourself as a student at California State University, Northridge (CSUN) in greater Los Angeles. At 4:30 AM. on January 17, 1994, you are joIted awake when your apartment +ns to sway violently. Dishes, bookcases, and ceiling plaster crash to the floor. The noise and shaking are terrifying as you struggie to stand up. In less rhan a minute the shaking stops and silence momentarily returns. You realize you survived an earthquake, but you are st i l l scared and disoriented.

This seismic event took place because of a sudden shifi of Mrodc eighteen kilometers beneath Northridge. Shock waves spread in all directions with damaging effects in many parts of Los Angeles. Northridge and vicinity suffered the heaviest damage. You feel fortunate, you are safe and your apartment building was not destroyed. Others nearby were not so fortu- nate. Two University students were among the sheen killed h--

one of the forty apartment buildings that collapsed (figure 1.1). (Emergency crews &we rlght past one apartment build- ing because it a p p d undamaged; but it had collapsed straight downward, compIetely crushing the first floor but leaving the upper floors standing.)

After the earthquake you and your neighbors do not have electricity (power was lost, temporarily, as far away as Edmon- ton, Canada). Water is in short supply because of broken water mains. Leaving the area is not a g o d option. Eleven major highway structures have been destroyed, induding a segment of the Canm Monica Freeway, the nation's busiest highway. Despite the deprivations and the jitters aftephocks, y&u are comforted by how p

shared advefs*. ea& &er. A tkmendous sense of community gmws from the

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quake and its damage. But your earthquake jitters continue even though the aftershocks now are infrequent and barely perceptible.

Sixty people were killed in the Northridge earthquake. The monetary cast was estimated at between $20 and $40 billion-financially the most costly natural disaster in North America's history. The loss in lives could have been far worse. For instance, 8,000 people died in the 1985 Mexico City earthquake and 5,000 were killed in 1995 at Kobe, Japan. The Northridge death toll was low due mainly to good planning.

Preparing for major earthquakes has been an ongoing process in California for decades. The quake was expected because of what geologists have learned about how the earth works. Skyscrapers in the Los Angeles area survived because they were built to meet high standards for seismic- resistancethe fruits of many decades of engineering stud-

, +' 5. ies and design. The apartment buildings thit collapsed in Northridge probably would have survived if, during their 1 &-

construction, building codes had been enforced. It is likely that thousands of lives were saved because of adherence to building codes. Pillars for freeway bridges and underpasses were known to be vulnerable. The Sanra Monica and other freeways were scheduled to have their pillars reinforced later in 1994, but the quake came first.

Luck also played a role in keeping the casualties down. Los Angeles' normally densely packed freeways were almost empty, because of the early hour and it being a holidav, Martin Luther King's birthday. b e San Francisco kay area &s e q d y lucky when, four years earlier, the Loma Prieta earthquake took place as the area's 'two baseball teams were about ro begin a World Series game; the normally heavy rush hour d c G& the light- est in anyone's memory when a freeway and part of a brdge coIIapsed.)

The Northridge earthquake was a reminder h t our solid ' 8 1 earth does not stand still. s Figure I .I According to the theory of plate tectonics, the earth's rigid

Damage from the Nonhridge earthqua*. (A) The Northridge outer shell is broken into a series ofphtes. Adjoining plates may Meadows apartments where 16 people or roughly one-third of all slide past, move away from, or collide with one another. Plates thme killed in the quake died when the first fbor was crushed by generally move from 1 to 18 centimeters a year. But the motion the weight of the top two floors. (4 The parking garage at may not be smooth and continuous. Plates may be "lockedn Califwnla State University, Northridge that was destroyed by M @t one another fbr many years and move suddenly.

, earthquake. Sudden motion dong a hdt caused the Northridge Photo A Q Michael EdwardsRos Angeles nmes Syndicate; Photo B by earthquake (figure 1.2). The mountain ranges that rise above hank M. ~anna Los Angela are another product of relentless plate motion.

It's as if a giant vise is slowly dosing, forcing bedrock Weeks later, the spring semester begins at CSUN. You upward into mountains. The Sanm Susana Mountains that

are among the 27,000 students that must cope with the border Northridge grew higher by 38 centimeters during the fact that all the buildings on campus were damaged and earthquake. some were destroyed. But at least the most severely man- The awesome energy released by an earthquake is a prod- gled building was a large, prefabricated parking structure uct of fb- within the earth that move firm rock Earthquakes (figure 1.1B). Repairing your campus will take time and an are only one conseqtlcnce of the ongoing changing of the estimated $35 0 million. Meanwhile, some of your classes earth. Ocean basins open and dose. Mountain ranges rise and are held outdoors, others in hastily erected temporary are worn down to plains through slow, but very effective, buildings. As the weeks pass, you share the pride of the processes. Studying how the earth works can be as exciting as university community and are determined that the quality watchmg a p t theatrid performance. Understyding the of your education shall not be undermined by the earth- changes that mke place in and on the earth, and the reawns for

Introduction to P&al G e o b 5

Compression (indicated by large arrows) in the Los area causes movement along a fau# and the

January 17,1994 earthquake.

those changes, is the ~~g objettive of plogy, the sci- entific study of the d.

PhysiiEal p l o g y is the division of geology concerned with d materids, changes in the su&w and interior of the earth, and the dynamic forces that muse those changes. Put another way, physid ge01ogy is about how the earth works.

Earthquakes and other aspects of geology are interesting, but how do= geoiog bend: you, as an inhabitant of this planet? Some of the ways are discussed next.

Avoiding Geologic Hazards Geology can have a direct application in ensuring people's safety and well-being. For example, if you were building a house in an earthquake-prone arm, you would want to know how to minimize danger to yourself and your home. You would want to b d d the house on a type of ground not likely to be shaken apart by an earthquake. You would want the house designed and built to absorb the kind of vibrations given off by &quakes.

Paople instinctively regard volcanoes as dangerous. They are, but the hazards are not immediately apparent to the non- geologist. (One is not likely to get killed by a lava flow or by a boulder ejected from a volcano.) The 1985 eruption of Nevado dd Ruiz in Colombia is a tragic example. N o one died from the eruption itself, but 23,000 people were killed from the indirect &. The hot rocks blasted out of the volcano ~ ~ d p a r c of the ice and snow capping the peak to melt. The

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Figure 1.3 Armero, Colombia, after the 1985 mudftow. The buildings are the small portion of the town that survived the mudflow. Photo by U.S. Geolcgical Survey

water mixed with loose rock on the flank of the mountain and flowed down stream channels as a mluifiw. At the base of the volcano, the mudflow ,overwhelmed the town of Armero, killing most of its inhabitants (figure 1.3). Colombian ge~lo-

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the earth. The United States economy in particular is geared to petroleum as a cheap source of energy. In a few decades Americans have used up most of the country's known petroleum reserves, which took nature hundreds of millions of years to store in the earth. Americans are now heavily dependent on imported oil. (The Gulf War of 1991 was at least partially fbught because of the industrialized nations' petroleum requirements.) To find more of chis diminishing resource will require more money and increas- ingly sophisticated knowledge of geology. AIthough many people are not aware of it, we face similar ~roblems with diminishing resources of other materials, notably metals such as iron, alu- minum, wpper, and tin, each ofwhich has been concentrated in a o articular environment by the action of the a r t h ' s geologic fbrces.

Protecting the Environment Our demands for more energy and metals have, in the past, led us to extract them with little regard for effects on the

balance of nature within the earth, and therefore on us, earth's residents. Mining of coal, if done carelessly, for example, can release acids into water supplies. Understand- ing geology can help us lessen or prevent damage to the environment-just as it can be used to find the resources in the first place.

The environment is further threatened because these are nonrenewable resources. Petroleum and metal deposits do not grow back after being harvested. As demands for these commodities increase, so does the pressure to disregard the ecological damage caused by the extraction of the remaining deposits.

I'roblems involving petroleum illustrate this. Oil compa- nies employ gmlogists to discover new oil fields, while the pub- lic and government depend on other geologists to assess the potential environmend impact of petroleum's removal from the ground, the transportation of petroleum (see Box 1.2), and disposal of any toxic wastes from petroleum products.

Chapter 1

expected every Few years in the ~ t 6 ~ u a k e bhlts crossed by the pidine- A,n mrhquake could rupture a pipeline- mpzcidly a conventiond pipe as in the origin4 design. However, when the Alaska pipeline was built, in weid pkces sections were speFidly jointed to allow the pipe, LO

shift as much as 6 meten without rupturing. f i e origrnaI estimated cost of the pipdine was $900 mi/-

lion, but the final cost was $7.7 billion, &g ic the ms'tli. est privately band c~wrmcdon project in history. The redesjgni~g an$ zanstructjon that minimized the potential for an environmental disaster were among ehe reasons f~ the increased cost There have been some minor spills from r h pipeline. For inmce , in January 198 1, 5,000 barrels of oil were lost when a &e nrptud.

Thus fa, the pipeline company's claim char there is vir- tually no chanrx: of a major spill from a pipeline rupture seam vindicated. However, the risks of marine cransporta-

Tkg Alaska pipsline. tion remain. In hindsight, a pipeline through Canada t~

P ~ Q @ S m McCJaheorVAlaska Ptctcw~al Semce the American Midwar might have been krter in the iang ?run (as advocated by geologists who prepared the environ- mental impact sodternat). ALthough tonget it would trai verse less hazardous terrain, would avoid rhe long'rnarlne

J I I ~ ~ ~ ~ ~ fW b a t ~ l o g i s f i W @ d as ser i~m hazards leg, and would bring the oil to refineries in she renull part the S ~ U ~ U E . ~ ~ i l d i n g mfing on 6- ground cream of th,e United Stares. A jainr venture with Caoada would problems. For exam~k* the mad in I was builr ' have benefited both countries, parricuhly since Canada hri% exploration of A1ak& Nor& Slape &rcwcr& malor peroleurn rc&~df& in its Arcric tar- When rhearoad was bqing built, the protective vegetation rirory. , . wa scraped off Du.ing the a J p c r thaw ihe road b m e . a q'ua&ire. & thawing wnctnud, the 0 o ~ d e d tm"dcks *

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Understanding Our surroundings Understanding geology leads to a greater apprecia- tion of your surroundings. If, for instance, you were traveling through the Canadian Rockies, you might see the scene in figure 1.4 and wonder how the landscape came to be.

You rmght wonder: (1) why there are layers in the rock exposed in the cliffs; (2) why the peaks are so jagged; (3) why there is a glacier in a valley w e d into the mountain; (4) why this is part of a mountain belt that extends northward and south- ward fbr thousands of kilometers; (5) why there are mountain ranges here and not in the central part of the continent. After completing a course in physi- cal geology, you should be able to answer these questions as d l as understand how other kinds of landsapes formed.

People who have completed a physial geology course find that their undersrandq of geoiogid p m m& travel much more interesting. Fire 1.5 is a map showing the landforms of the conrip- ws United States and southernmost Canada. Major features U or depicd in this book a ~ e named with bladr kttering. (In addition, on the inside, front cmer is a p e d d gwlbgic map of EJorth Arne& &at will be a useful reference as you progress th+ this book.) In your htlvc air or & travel, you may want to take alnng your book with these maps to enhance your appreciation of the scenery.

An Overview of Physical Geology-Impportant Concepts The remainder of this chapter is an overview of physical gaolo%y that should provide a framework for most of the material in this book. Although the conepm probably are totally new to you, it is important that you comprehend what follows. You may want to reread portions of this chapter while studying later chapters when you need to expand or

reinforce your comprehension of this basic material. The earth can be visualid as a giant machine driven by

N ~ O engines, one internal and the other mternd. Both are kcat engines, devices that converc heat energy into mechanical

Figum 1.5 ! Raisz physiographic map ol the 48 contiguous I - states and southernmost Canada.

1 Q Erwin Raisz: Cwrtesy of Rarsz Landform Maps, PO Box 2254, Jamaica Plain, MA 02130. (Large copies may be purchased from them.)

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t t t Heat-rgy g

Figure 1.6 Two examples of simple heat engines. (A) A 'lava lampm Blobs are heated from below and rise. Bbbs cool off at top of the lamp and sink. (B) A p i n w e l held over steam. Heat energy is converted to mechanical energy.

energy, Two simple heat engines ate shown in figure Id. An automobile is powered by a heat engine. When p h e is ignited in the cylinders, the resulting hot gases expand, dciving pistons m the far end of cylinders. In this way, h e hear energy of the e q a d q gas has been converted to the medmnid energy of the moving pistons, then transferred to the wheds, where the energy is put to work moving the car.

The i d h a engine ofthe tarth is @ by heat mov- ing from the hot inmior of the mward cooler e r . Moving plates and euthqudus ptuduas of this heat engine.

The e a d s Extmrrrl heat engine is driven by solar power and gravity. Hmt from the s u n prcwids the energy fbr dmrlat- ing the atrnosphae and oceans. Water, qeci+iy from the

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1.7 Movement of wax due to d e w d i m caused by heating and cooling.

oceans, is evaporad due to solar heating When moist air mls, rain or snow fslls.

Over long periods of time, miscureatrhe eardis surface hdpsrodc&~te.~washingELownhillsi&aMlflow- ingin~loclsensandc-arriesawaytfie&partides. Inthis way mountains o+dly iaised by d~ d s i n t d form are worn away by p m driven by the e z r t d heat engine.

W e will h k at haw the earth's hmt engines work and show how some of the major topics of physid geology are relamd m the rrqFcial (on tfie earchs d c e ) and in& pnnxsscs pawered by the heat engines.

How the M's Internal H a t w e Works The earth's i n 4 heat engine works becaw hot, buoyant marerial deep within the earth moves slowly upward coward h e cool & and cold, denser material mwes downward. Visualize a vat of hot wax, heated from MQW (figure 1.7). As the wax immdiady above the fire gets hotter, it atpands, becomes less dense (that is, a given volume of the material will weigh less) and wiU rise. Wax at the top of the vat loses heat to the air, cools, contracts, b m e s denser, and wil sink. A simila~ process takes place in the earth's interior. Rock that is deep within the earth and is very hot rises slowIy toward the s u r k while rock that has cooled near the surface is denser and sinks downward, Instinctively, we don't want to beiiwe that rock can flow Like hot wur. However, experiments have shown that, under the right wndiaons, rocks are capable of being molded (Like wax or putty). The hotter a rock and the p t c r the pressure on the rock, the more 8kely a rock is to debrm. Pressure and temperature, even a few kilometers beneath the 4 s surface, can be high enough so that rock ~ f l o w v e l y ~ y .

~ u u s t Continental crust

Fpo- 1.8 C m s section through the earth, Expanded sectlon shtms the relatlonshlp between the two types of mst, the Ihtmpbm and the asthenosphere, and the mantle. The crust ranges from five to ~ - f h r e kilometers thldr. -byNASA

The Earth's Interior ner. It is made of rock that is somewhat densu than the rock that underlies the continents.

The lower parts of the crust and the entire mantle are inac- ~ b l e to dkct o h t i o n . No mine or oil well has pene- tnoad the mist , so our concept of the d s interior is based on i n k evidence (the topic of chapter 2).

The uuse and the uppermost part of the mantle are rela- tivcly rigid. CoUcccivcly they make up the fithoaphcro. (To hdp you mnembtr terms, the meanings of commonly used p& and suffixcr are given in Appendix G . For example, litb means "rock" in Greek. You will find litk to be part of many geologic terms.) The up^ mantle underlying the lithosphere, d d the &cnoephcr~, is soft and therefore flows. It provib a lubricatingm layer over which the litho- sphere mwcs (&I means uwcakID in Greek). Where hot mantle material web upward, it will uplift the lithosphere. Where the lithosphere is coldest and densat, it will sink down through the asthenosphere and into the deeper mantle

, h k flowage k believed w dce place in part of the interior of I the earth in the mne known as the mantle, the largest, by vd- ; ume, of the tarch's &rce major concentric zones (see f i p 1 1.8). The mantle is solid (exetpt in a few spa) and probably ! composed of rock not very different from some kind, of rodr

found at the earth's surf$ce. The other two zones are the w a d the The aust

of the earth is anaiopus to the skin on an apple. Thc thickna of the crust is insipiiant mmpared to the whole d. We have direct aacss to only the crust, and not m& of the crust at that. We arc like microbes crawling on an apple, without thc

. ability to penetrate its skin. &cam it is our home and we I depend on it for mm, we am eoncmd mom with the ! : crust than with che M b 1 e mulde and arc The aust

varies in thickna. Two major t y p of crust arc & rrvrt and contimtal emit The crust under the I much thin-

Flgw I .% Hot mantle travels upward. Cold crust and mantle sink.

forces strong enough to outdo gravitational f~rces. (Mount Everest, the world's highest peak, is made of rock that formed beneath an ancient sea.) Mountain ranges are built over extended periods, as portions of the earth's crust are squeezed, stretched, and raised.

Most tectonic forces arc m e h i d hrces. Some of the energy from these forces is put to work defbrming rock, bend- ing and breaking it, and raising mountain ranges. The mechanical energy may be stored (an earthquake is a sudden release of stored mechanid enefgy) or converted to heat energy (rock may melt, resulting in voIcanic eruptions). The working of the machinery of the earth is elegantly demon- strated by plate tectonics.

The Theo y of P h e T-nics From time to time a theory emerges within a &mce that m l u -

(figure 1.9). The effect of this intern4 heat engine on the t i o k that field The pka m n i c theory, currendy ac~epted crust is of great significance to geology. The forces generated by v i r t d y ail plogim, is a unifying theory that aocouna for inside the earch, called m n i c forca, cause deformation of many ssemingIy u n d a d geolo@al phenomena The theory is rock as well as vertical and horizon4 movement of portions as important to geology as the theory of relaavity is to physics, of the earth's crusr. Mountain ranges are evidence of tectonic the uomic theory m chemistry, or evolution is to biolagy.

\ I r Mid-oceanic ndge and

Diue(ging boundary Converging boundary /Transform boundary diverging boundary \

Continent ' I I

Figum 1 .I 0 (A) Plates of the world. Large arrows indicate direction of plate motion. (4 Plate motion away from a divergent boundary toward a convergent boundaly. After W. Hamilton, U.S. Geological Survey.

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u l h g away from each other, sliding past each toward each orher. plate tectonics, dbqent bundaria exist mwiny a p t b b s t divergent boundaries crests of submarine mouncain mp, called

(figures 1.10Band 1.11). A rnidsceamc ridge is higher than deep ocean floor (fig-

rocks, being hotter there, are less dense. Tensional cradts develop along the ridge crest (figure 1.11B).

zed magma (molten rock) chambers in sphere, and the magma squezm into h e lithosphere). Some magma erupts d the rest solidifies in the fissure. Con-

ridge crest develops new cracks, and by what takes place along p h where the process of fding and cracking continues indefinitely. Thus,

I m d m i o t l ~ Pbicd Grob~ 17

F m 1.42 A canvergeat boundary.

new octanic crust is antintmusly craud at a diverging boundary. Not all of the mande material melts; a solid residue remains undct thc newly c d crust. New crust and underly- ing solid mantle make up the Iithosphcrc that mows away from the ridge crest, traveting like the tap of a mnveyor belt. The rate of motion is gentrally 1 to 18 centimeters per year- slow in human terms but quite f$st by geologic standards.

As the lithosphere mows away from the divergent bound- ary, the m a t e d slowly cools. As it coals it con- and becomes denser. T h e contraaion of the lithosphere and its slow sinking ( b u s e of the i n d density) cause the floors bcncath oceans to d-n away From ridge crcsts.

The top of a plate may bt m m p d d u s i d y of manic uust or indude a eontinat or part of a continent. For cxample, if you live on the North American place, you llie r i d q wcsnvard dative to Europe because the plate's divergent boundary is along the midsccanic rdge in the North Adantic Ocean (iig- ures 1.10 and 1.1 1). The wtcm halfofthe North h t i c sea floor and North America are moving rogether in a d y direction m y from the mid-Atlantic ridge plate boundary

A second type ofboundary, a d r m bonnday, o m where two plates slide past each other. The San Andreas Mt in California is an srample of this typc OF boundary and the earthqualm along the h l t arc a result of place motion.

The third rype of boundary, one resulting in a wide range of geologic activities, is a convqent b o a d q , where plates move toward each other (figure 1.12). If one plate is capped by oceanic crust and the other by continental crust, the l a dense, more buoyant continental plate will override the denser, oceanic plate. The oceanic plate sinks along what is known as a aubddon mne, a zone where an oceanic plate descends into the maotle beneath an overriding plate. In the

region where the top of the subducting plate slides beneath the asthenosphere, melting taka place and magma is created. Magma is lcag denst than &e wcrlying =lid rock. Therefore, the magma ucated dong the subdudon zone works its way upward and either erupts on the d s s u h c e to solidify as mtmshe- r d or wlidihcs within the crust to become i n t m k igneous rock Hot rock, under high prcasure, near the subduction zone that docs not melt may change in the solid state to a new rock--orphic rc&

In addition to containing ipcous and metamorphic rocks, major mountain Mts shaw the effects of squeezing caused by plate convergence (hr -, the ufoldcd sedimentary rocks" shown on fi%urt 1.12). In the pcloc#rs, rock that may haw been below sea level might bt s q u d upward to become part of a mouneain range

Box 1.3 d a r i b s how plate tectonic thtory was developed through the ximtz$c tncthod If you do not have a thorough comprehension of how the scientific method works, be sure to study the box.

Surficial Processes: The Earth's Extemal Heat Engine When tecconic forces shove a portion of the earth's cmt above sea level, roclcs are exposed to the atmosphere. The earth's external heat engine, driven by solar power and gravity, then wmts into play. Our weather patterns are largely a product of this heat engine. For instance, hot air rises near the equator and s i d s in amla mnes to the north and south. !Solar heating of air c ram wind; ocean waws are, in turn, produced by wind When moist air cools, it rains or snows. Rainfill on hiU- sides flows down s l o p and into streams. Streams flow to M w

~ t ~ ~ f i O r X l ~ h u b , m d r h e qpdhd~ lpw ' Asthe h y p o r h a t p m h = ! i r ~ ~ k c h a n a ~ p r s d d ) i ~ n a the m t r t d dge, d c r w t treoldar~fioor (up m h t 2 0 0 ~ y t ~ r s d ~ b f s r J r t s t f i b m t h t * b W 2 1 . ,

This test was only oneofades . V i w other ~ ~ W i r r ~ ~ h t e r i n t l r k k k , t # t d e d ~ & r b c h $ # & d $ ~ ~ ~ ~ b . Somctemdidraotwmrkwte&yIpr#tid. ~ o f r b i s , a n d m m ~ s a r r E y o f & t a , the 0~~ was, d a & m be, tnodified. ~ ~ p ~ h t p w c w a , % ~ y ~ w vatid.

Step C , a l q p d d s bcama r -,Most ~IQ$SS it, the world w n M zhc muha of this d a c h c r - = p o g i t i v ~ , ~ h r b c c o n c e p t i P ~ l y t n r c . ~ t c a n ~ a w b e ~ r h t *-&thcorg..

~ & l n s t f i w ~ , p l a t c w c m n i c t h c o ~ ~ b # n firrJler confifmod % tk 4 bf m y rtery;sceurate aatellitc sump &a i k m h w h m points on sepmu continents arc rdath w one mo&c~ The d w ~ i n c b t c that the con- b e acc lndaad moving d a t k to o m moth#. Europe a n d N o t t h ~ s t c m o v i n g ~ ~ ,

A l h @ it is d h f y that piau twonic damry will be r c p l d by somedhg we Wt: thought dyer, aspem chat mdcr plate tectonics' umb& conrink to be mdyd and mid as n w data b e d a b l c . Far

' iastaace, rhe q d o a of how mapw is p d deep h in a suMu&on zone has beea addrtsaed by several f r y p o t h ~ (end is discuaged in chapter 10). lDots melting take p k in h e k n d i n g phte or i9 the wer&q man- tk&Lswaterarrieddownbyt)KdtsmdtfigpIptcanaid to m c h g ? Similar questions a# r a i d h i n y other

i or seas. Glaciers gtow where there is abundant snowfall at colder, high elevations and flow downhill due to gravity.

! Where moving water, ice, or wind loosens and rem- : material, m i o n is mking place. Strwms flowing tomrd

oceans remove some of the land ovtr which thcy run. Crashing waves carve back a masdine. Glaciers grind and w r y away underlying rock as they move. In each case, rock originally raised by the mth's internal proctsses is worn down by d- cial proctsscs (figure l . 13).

Roch formed at high tcmpemtutt and under high prcssurr deep within the Mh and pushed upwd by tectonic fo- are unstable in their new envimnment. Air and water tend to eausc

: chc once deep-seated rocla to b d down and brm new materi- als. The new materials, scable under conditions at the d s sur- face, are said to be in opailibrium, that is, a d j d to the physid and chemid conditions of their environment so that thcy do not change or alter with timc. For rxampIe, much of an igneous rock (such as granite) that formed at a high ttm- tends to break down chemically to day. Clay is in equilibrium, that is to say it is stable, at the a d s sut fb

The product of the breakdown of rock is *t, loose material. Sediment may be transpotted by an agent of ero- sion, such as running water in P stream. Sediment is deposited when the transporting agent loses its carrying power. For example, when a river slows down as it meets the sea, the sand being transported by the s c m is deposited as a layer of sediment.

In time a layer of d i m c n t deposited on the sa floor becomes buried under anothcr laytr. This p m may eon- tinue burying our or ig id layer p@dy +. The pres- sure from overlying layers compresses the d i t , help Ye consolidate the iwse materia. With the cemenmtion o loose particles, the sediment bccomcs kb#d (ccmcnd or otherwise consolidated) into a &

1

Geologic Time

. . ...' ~~~are;zsof~I.o&yalzddkeeprheartenrionoffunuegen- . . & p l a n e m b h b y a m i s s i k f i d b a - & i p a t s ~ a : ' erations of scientia. a thirdthEoryisrhatasptkignitbdin a h 1 e k m d the

plane q l o d d " Cialy, each "theorf is an bypthesis in

We have mentioned the great amount of time required for geological p r m s e s . As humans, we think in unit. of time related to personal experien-nds, hours, years, a human lifetime. It stretches our imagination to contemplate

Imp-t Note the sien& sense of tbe word. This has 14 ro considerable

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Words by ainrtisrr & nor dmys have -= m a . . fw rmwk~dsrs l b a ~ rdcoa. YOU haw pmh- < ing when used by perd public. A - in paint i-, bly heard the --ion3' 'It's jusr a t h e ~ ~ . " Statements,

umrd "&mqu To mart ppls a 'theoryn is wha J*mtis~ such z, 'Ewbuart is jua a theag = u d to hply that ' regard as ul "fiypa&&." You may n- repom , &&crupport is d The d t y is dur&mrls st& as

4 i a b u t an airliner that exploded offrhore from New York in ' ml~t ia" and fmonia have m h d m i n g l ~ 19%. A typical rutemefir on ion -: 'One hmry is "ified that rheg come d a a p i b k what rden&ts that a h m b in the plane exploded; a second theory is that " p t as b*"~ p d me.

*,

Former earth Sea

Portion r e m m Present earth 94dmml by erosion . e'

. +. . - - . --- I

Layers of sediment cotled on the sea floor and wlll lithrfy to sdmmtary rock

B Flgur 1.13 UpR, erosion, and &positron. (A) Magma has solidified underground to become igneous rodt. (B) bnd is uplmed. Upper portion is eroded. Sediment Is transparted to the sea to become sedimentary rock.

ancient history that involvw 1,000 or 2,000 years. Geology involves vastly greater amounts of time, often referred to as &ep timc

To be sure, mmc goologid processes occur quickly, such as a great landslide or a volcanic eruption. These events occur when stored energy (like the energy stored in a car bat- tery) is suddenly released. Most geologid processes, how-

. ever, arc r lok but rele&s, reflecring the pace at which the heat engines work. It is unlikely that a hilt will visibly change in shape or height during your lifetime (unless through human activity). However, in a geologic time frame, the hill probably is ctoding away quite rapidly. "Rapidlyw to a geologist may mean that within a fm million ycars the hilI will be reduced nearly to a plain. Similarly, in the gaologi- cally "recentn past of several miltion yca~s ago, a sea may have existed where the hill is now. Some proctssts are regarded by geologists as ubt" if they arc begun and com- pIeted within a million years.

The race of plare motion is relatively fast. If new magma erupts and solidifia along a mid-oceanic ridge, we can easily calculate how long it will take that igneous rode to move 1,000 kilomctets away from the spreading center. At the rate of 1 centimeter per year, it will take 100 million years for the presently forming part of the crust to travel che 1,000 kilo- meters.

Although we will dircuss geologic time in detail in chapter 8, table 1.1 shows some reference points to keep in mind. The arth is estimated to k about 4.5 billion years old (4,500,000,000 -1. Fossils in rocks indicate that complex forms of animal I& have existed in abundance on the earth for about the past 545 million ycan. Reptiles became abundant about 230 million yeam ago. Dinosaurs evolved from reptiles and became extinct about 65 million years ago. Humans have k e n h m only h u t the last 3 million years. The eras and periods shawn in table 1.1 comprise a kind of dcndar for geologists into which goorogic events art p l d (as explain4 in the chapter on geologic time).

Not only m thc immense spans of geologic time difficult to comprehend, but very slow processes arc impossible to duplicate. A geologist who wants to mdy a certain process annot r e p t in a h hours a chemical d o n that &s a million years to occur in nature. As Mark Twain wrote in Lrp on dv Miurjpi&i uN~thing hurries geology.*

Geology is the scientific study of the &. d s mantle. Surficial pmccws are driven plates that move relative to each other wcr Geological inwtigations indicate that the by solar cnugy and pviry. Internal b r a s the asthenosphere. The plates are moving e d is changing barnuse of internal and sur- cause the crust of the earth to move. Place uwq h m divergent boundarifi usually fid procam. Internal processes are driven tmonic theory visual- the lithosphere (the located at rhe crests of mid-0-ic - . _ ri&- mostly by temperature differences within the crust and uppermost mantle) as broken into where new crust is being ueated. Plates

g g f J - $ , T . ,;- d B . . &%f

Although tbt cvth is chpngrng con- abtPplaccattbteuth's& ds .&Mh=tis*toa w , t h e n t e s o f b ~ m ~ ~ r o a i r a n d w a r e ~ l o r r r r c i d e v p t i o n w h e i t i s ~ t d ( ~ ~ m - ~ ~ ~ S l O w b y h u m P l ~ .

hypothesis 19 pcicntific mcthd 19 igneous rock 18 sediment 2 1

lithosphere 15 sedimentary rock 2 1

magma 17 subduction zone 1 8

mantle 15 tectonic form 16 metamorphic rock 18 theory 19 m i d h c ridgc 17 transform boundary 1 8

~hyJiCa geology G

The l ihpherr is (a) the stmc as the umt (b) the layer beneath

AlJkgrc, C. 1988. The behsubr of tbe - Earth, Boston: Harvad Unimsity Press. Drake, E. T., and W M. Jordan, &. 1985. Gco&# an8 &: A Aisary of N o d American pbo Boulder, Colo.: Gcologieaf h i c r y of America. McPhte, J. 1981. Barinadmnge. New York: Farrar, Straw & Girouw. This is one of =ral outstanding boob h u t geology written for the layman by John M&. Mmm, E. M., 4. 1990. W i m g h d: Tccconici of conditlntcs rwrB mdrrr. New York: W. H. Freeman a d Company. Nuhfer, E. El., R J. P m r , and l? H. Mowr. 1993. T b e c h k w g w d c t o g e o ~ h w d . h a d a , Colorado: American Institute of Pmkiond Gcologis&. Officer, C. B. and J. Page. 1993. Tslcr 4th c d d : Pamysm dndpertu&~&m om b h pdsnet. New York: Oxford University P m . Pirsig, R M. 1974. Zm a d de art of trnotorryele mintenQme. New York: Ban- Bmb (paperback). This book contains an exceptionally g o d exposition of the scientific method as well as considerable insight inm the philosophy of science. Rhoda, E H. T, and R 0. Stone, ads. 198 1. h n g u g e of the cad. New York: krgamon Prss.

- mEsrsb RcwalerL Southern GWornk Consortium. A scrim- of videompcs produced for a televised p ~ d p l o g y course.

Q I-w P h TcrtDnirs by Plumrner/McGeary. I 996.

WCBlMcGm-Hill. hrtp:ilwww.&c.com/cPrthsci/geoI.sy/ plummer This is the dodicad website for this book. You can go to it fbr new and updated information. Thc unived m u r c e lwfors (URL) listed in thk book arc also given as linh on the website, making it easy to go to those webeices without typing in the URL. Links to additional websites can a h be found. W e have added questions to some of the links to allow you to get the most of your exploration of the web. Using the web is an enjoyable way of enhancing your knowledg of geology. htrp://pub.usgs.gwlpublicatiodt~d

dynamic.htm1 This Qmmic fid by the U.S. Geological Sumy is an online, ilIustntcd publication explaining plate tectonics. You may want to go w the section *Understanding plate motion." This will help reinforce what you read about plate tccmnia in this chapter. Howwer, it gmi into plate tectonics in grater depth, covering material that is in chapter 4 of this textbook.

hnp:/lwww.uh.edd~jbutler/monl montripshtd

Krtwl FieM Tkips. The site provides access to geologic sits throughout the world. Many are field trips takm by geology classes. Check the alphabetical listing and see if there are any site near you. Or watch a video dip in one of the Quick Time field trips. One of the well-done trips in the alphabetid listing is the Onconta to the Hudson Riwr field trip in Central N m Yotk. http:llwww.usgs.gov The U.S. &&c Su-S home page. Use this as a gateway to a wide range of geologic information. h a p J / ~ . k e d u l W M S E prwi&s amas to photographs taken from the space shuttle. You may mrim photos by raqutsting a gmlogic k m r t (e.g, vdano) or of a spcific geographic p i o n of the carth.

h c r p J / m v w ~ . g e , d g d Tht Cd&d.%w of G P 4 home page.

I .. -

1. Why are some pam of the lower mantle 3. What prcmmg of p b g k time is encounter if YOU uied to drill a hole to hotter than other parrs? accmnted for by the last anwry? the center of the mrth!

2. According to plate tectonic theory, 4. What would the d be like without where arc crustal 6 crated? Why solar hcadng! dmn't the earth keep getting larger if 5. What are some of the technid rock is m n h d y - difficulciesyou~~ukl~pecr to

Wund y m r Knowle& of thc prp smted in this chapter b~ wing the CD-ROM te answer tkcfiloudng pcstionr.

1. The ah has changad g& over time tkrou$I the procmsts of plate Wtonics. Watch the anlmacions on &e C b p g

! module to find mampks of each I of these plate p m

antinents splitting apart at Transform Fa- then click on divergent bundatics "Earthquake Footage." What were the

ooeans shrinking as the ocevl floor effects of the Lorna %eta earthquake? is consumed at subduction wncs How was this &quake dated to

continents wllidmg at cowergent plate p m ? h&cr

2. Go to the Tm+m module. Afrer the introducdon, click on