Internal Earth Processes Lecture Notes

Introductory Engineering Geology

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7. INTERNAL EARTH PROCESSES

7.1 Igneous Processes

Igneous processes involve the melting of rock to form magma and its solidification into rock.

Igneous products form where magma crystallizes below Earths surface or erupts as lava and

other materials onto Earths surface.

There are two primary classes of igneous rocks. The two primary classes are: volcalnic

(extrusive rocks) and plutonic (intrusive) rocks

Volcanic (extrusive) rocks: These are formed from flowing lava at the surface, or from

exploding/ejected materials that tend to form glass.

Plutonic (intrusive) rock: These are formed from magma that cools below Earths surface.

The crystals are big enough to see with the unaided eye. They are large igneous bodies

formed at depth in the Earths crust.

Types of Plutons include, batholiths, dike, sill, stock etc. They are as shown in Figure 7.1.

Laccoliths: Inverted lense-shaped igneous intrusive bodies (convex side up), analogous to a

sill, but much larger and result in upwarping pre-existing strata or rock layers (e.g. Black

Hills of South Dakota).

Batholiths: Large intrusive bodies of igneous rock that are greater than 40,000 square km in

diameter, in reality magma chambers that have cooled and solidified beneath the earth's

surface (e.g. Sierra Nevada Moutains of California)

Stocks: Smaller scale versions of solidified magma chambers less than 10 km.

Dike: Planar bodies of igneous intrusive rock that resulted from the injection of magma

across strata or layers of rock, i.e. tabular, discordant sheet-like intrusive bodies.

Sill: Planar bodies of igneous intrusive rock that resulted from the injection of magma

parallel to strata or layers of rock, i.e. tabular, concordant sheet-like intrusive bodies.


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Figure 7.1: Plutonic (intrusive) rocks

7.1.1 Volcanoes

A volcano is a vent or an opening in the earths crust through which molten rock material,

rock fragments, ash, steam and other hot gases are emitted slowly or forcefully in the course

of an eruption. These materials are thrown out from the hot interior of the earth to its surface.

Such vents or openings occur in those parts of the earths crust where rock strata are

relatively weak.

Volcanoes are evidence of the presence of the intense heat and pressure existing within the

earth. Hot molten rock material beneath the solid outer crust is known as magma. When this

magma is thrown out from the magma chamber to the earths surface it is known as lava

(Figure 7.2). The magma and the gases stored within the earths surface keep trying to come

out to the surface through a line of weakness anywhere in the crust. The tremendous force

created by magma and its gases creates a hole in the crust and the lava spreads out on the

surface along with ash and fragmented rock material. The process by which solid liquid and

gaseous materials escape from the earths interior to the surface of the earth is called

volcanism.


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Figure 7.2: Volcanoes

The volcanic materials accumulate around the opening or hole taking the form of a cone. The

top of the cone has a funnel shaped depression which is called its crater.

7.1.1.1 Types of volcanoes

Volcanoes are classified on the basis of the nature of volcanism. The basis includes the

frequency of eruption, mode of eruption or fluidity and the manner in which volcanic

material escapes to the surface of the earth.

On the basis of the frequency of eruption, volcanoes are of three types:

(i) Active

(ii) Dormant and

(iii) Extinct.

Active volcanoes: The volcanoes which erupt frequently or have erupted recently or are in

action currently are called active volcanoes. Important among these include Stromboli in

Mediterranean, Krakatoa in Indonesia, Mayon in Philippines, Mauna loa in Hawai Islands

and Barren Island in India.

Dormant volcanoes: The volcanoes which have not erupted in recent times are known as

dormant volcano. They are as such the sleeping volcanoes. Important among these are

Vesuvious of Italy, Cotopaxi in South America.

Extinct volcanoes: The volcanoes which have not erupted in historical times are called

extinct volcanoes. Mount Popa of Myanmar (Burma) and Kilimanjaro of Tanzania are

important extinct volcanoes. It is not, always very simple to categorise a volcano as dormant

or extinct. For example the Vesuvious and Krakatoa became suddenly active after lying

dormant for hundreds of years.

On the basis of mode of eruption, volcanoes are divided into two types:


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(i) Central type volcanoes and

(ii) Fissure type volcanoes

Central type volcanoes: When the eruption in a volcano takes place from a vent or a hole, it

is called a central type of volcano. Different types of domes or conical hills are formed by

this type of eruption depending on the nature of erupted materials. Majority of volcanic

eruptions in the world are of this type. The other characteristic of this mode of eruption is that

it is marked by violent explosion due to sudden escape of gases and molten rocks through the

hole. Visuvious and Fuji-yama belong to this group of volcanoes.

Fissure type volcanoes: When magma flows quietly onto the surface from deep elongated

cracks that developed due to earthquakes or faulting, it is called fissure type of eruption. This

eruption helps in the formation of thick horizontal sheets of lava or a low dome shaped

volcano with broad base. It may also form what are identified as lava plateaus, and lava

shields.

On the basis of the fluidity of lava there are two types of volcanoes:

(i) Volcanoes of basic lava and

(ii) Volcanoes of acid lava.

Basic lava has greater fluidity because it is rich in metallic minerals and has a low melting

point. In this type of eruption, lava flows far and wide quietly with greater speed and spreads

out in thin sheets over a large area. Thus, it leads to the formation of shields and lava domes.

The shield volcano of Hawaian Island in Pacific ocean is one of these volcanoes.

Contrary to basic lava, acid lava is rich in silica and has a relatively high melting point.

Therefore, it is highly viscous and solidifies quickly. Hence, the, acid lava volcanoes cause

the formation of usually higher land features with steeper slopes. Acid lava cones are of

steeper slopes than basic lava shields.

Types of Volcanoes based on morphology of Volcano

Cinder Cone Volcanoes: When eruption of gas-rich magma takes place, eruptive products

often are spewed into the air explosively as large chunks. These large pyroclastic materials

may pile up near the exit hole, or vent. When the primary eruptive products are large

fragments of solid material, cinder cone volcanoes form. They tend to be small, with most

cones having heights in the hundreds of meters range. When cinder cones occur on the flanks

of larger volcanoes, they are called parasitic cones. An example of a volcano with parasitic

cones is Mount Kilimanjaro in the African rift valley.


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Shield Volcanoes: Because they form from high-temperature, fluid, basaltic lava, shield

volcanoes erupt with abundant lava flows that can move for kilometres over Earths surface

before stopping. Shield volcanoes are broad, flat structures made up of layer upon layer of

lava. Volcanism in Hawaii produces shield volcanoes.

Composite Volcanoes: When volcanoes occur along convergent plate boundaries, they tend

to have magmas that are richer in silica content than those formed at hot spots or divergent

boundaries. This is because as subduction takes place, water and sediment are forced down to

regions of higher temperature. Partial melting of materials, in which the silica-rich portion of

rock and sediment melts first, produces viscous magma. This produces volcanoes formed

from alternating explosive events that produce pyroclastic materials, and lava flows. These

composite volcanoes, composed of alternating layers, are large, often thousands of meters

high and tens of kilometres across the base.

7.2 Metamorphism

Metamorphism is the process leading to changes in mineralogy and/or texture and often in

chemical composition in a rock. It is the process of mineralogical and structural (textural)

changes of rocks in the solid state in response to physical and chemical conditions which

differ from those under which they originated.

The phase change allows new metamorphic minerals to be formed due to a chemical reaction,

while with textural change, new textures such as alignment of platy minerals, or progressive

coarsening or fining of pre-existing igneous or sedimentary minerals.

7.2.1 Factors that Drive Metamorphism

The factors that drive metamorphism are: temperature; pressure; fluids and initial

composition of the parent rock

Temperature

Temperature (heat) is the most important factor in metamorphism. Temperature drives the

chemical changes that result in the recrystallization of existing minerals or the creating of

new minerals. Earths internal heat comes from energy being released by radioactive decay

and thermal energy left over from the formation of the planet. Temperature increases as you

go deeper into the Earths crust (Geothermal gradient). The geothermal gradient averages


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30oC per kilometre increase with depth (but it can vary from 20

oC to 50

oC per kilometre of

depth). Metamorphic changes can occur with increasing or decreasing temperature:

Prograde refers to mineral changes that take place during an increase in temperature, while

Retrograde refers to mineral changes that take place during a decrease in temperature. Heat

greatly affect a rocks texture and mineralogy. It breaks chemical bonds and alters the crystal

structure. Atoms and ions re-crystallize into new mineral assemblages. Many new crystals

will grow larger than they were in the parent rock.

Pressure

Pressure changes a rocks mineralogy and texture in a predictable manner. Pressure increases

with depth as you go deeper in the earth crust. Directed pressure guides the shape and

orientation of the new metamorphic minerals. Metamorphic minerals can be compressed,

elongated and/or rotated by being forced into preferred orientations and can form spectacular

and erratic banding.

At low pressures, rocks are brittle and tend to fracture when subjected to differential stress.

At high pressures, rocks are ductile and flow like plastic. Under ductile conditions, mineral

grains tend to flatten and elongate when subject to differential stress

The two major types of pressure are confining pressure and differential stress:

Confining pressure: This kind of pressure is applied from all directions (see Figure 7.3).

Confining pressure causes the spaces between mineral grains to close, producing a more

compact rock with a greater density, but does not fold or deform rocks.

Figure 7.3: Confining pressure


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Differential stress: This kind of pressure comes from a particular direction (such as from the

collision of two tectonic plates) (see Figure 7.4). Differential stress is a pressure that is

applied from a direction (rather than all directions). Rocks subjected to differential stress are

preferentially shortened in the direction that pressure is applied and lengthened in the

direction perpendicular to that pressure

Figure 7.4: Differential stress

Fluids

Fluids composed of water and other volatile components, such as carbon dioxide, play an

important role in metamorphism. Metamorphism can add or remove chemical components

that dissolve in water. Water acts as a catalyst during metamorphism. Water aids in the

exchange of ions between growing crystals. Clay minerals can contained up to 60% water.

Water is part of the crystal structure in many minerals, such as mica and amphibole. When

subject to low to medium temperatures, water molecules can be removed from minerals.

Once expelled, the water moves along the individual mineral grains and is available to

transport ions. At higher metamorphic temperatures, the water and fluids are driven from the

rock.

Composition of parent rocks

Most metamorphic rocks have the same overall composition as the parent rock from which

they formed. Except for the possible loss or accumulation of volatiles such as water and

carbon dioxide

7.2.2 Metamorphic Environments


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Contact (or thermal)

Hydrothermal

Burial

Regional

Shock (impact)

Fault Zone

Contact Metamorphism

Contact or thermal metamorphism is limited to small areas. It occurs when an intrusive

magma heats the surrounding country (or host) rock and changes the mineralogy and texture.

The zone where the rocks are subject to metamorphism is called the metamorphic aureole.

The sedimentary rocks are turned into metamorphic rock by contact metamorphism. Even

small dykes can form aureole of metamorphic rock a few centimetres thick.

Hydrothermal Metamorphism

Hydrothermal fluids can carry dissolved calcium dioxide, sodium, silica, copper and zinc.

Ascending hydrothermal fluids can react with overlying rock, creating new minerals (which

may have great economic value). The most widespread occurrence of hydrothermal

metamorphism is along the mid-oceanic ridges. As seawater percolates through the newly

created crust, it is heated and chemically reacts with the mafic (Fe and Mg rich) basalt. The

ferromagnesian igneous minerals, such as olivine and pyroxene, are changed into

metamorphic minerals such as serpentine, chlorite and talc. Calcium-rich plagioclase

feldspars become more sodium-rich as the sea salt (NaCl) exchanges calcium for sodium.

Burial Metamorphism

Burial metamorphism occurs when thick accumulations of sedimentary strata on the ocean

floor are subducted beneath another plate. This is a low grade metamorphism that typically

begins when the subducted sediments reach a depth of 6-10 kilometres (3-6 miles) or when

the temperature reaches about 200oC

Regional Metamorphism

It involves large scale recrystallization. It occurs where high temperature and pressure occur

over a large region (plate tectonics). Most metamorphic rocks are created during the process


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of regional metamorphism associated with mountain building. During these dynamic events,

large segments of the Earths crust are intensely deformed along convergent plate boundaries.

The mountain building applies differential stress literally over a wide regional area.

Sediments and crustal rock lifted up from the ocean floor are folded and faulted

Metamorphism of all grades, from low to high occurs. The Swiss and Austrian Alps in

Europe are other famous examples where extensive regional metamorphism has occurred.

Impact Metamorphism

In a fraction of a second, the energy of the rapidly moving object is transferred into heat

Impact metamorphism occurs when an asteroid or comet impacts the Earths surface. These

objects can be moving as fast as 100,000 miles per hour (~28 miles per second) In a fraction

of a second, the energy of the rapidly moving object is transferred into heat energy and shock

waves as it smashes into the Earth. The impacting asteroid or comet is vaporized. The

impacted rock is shattered, pulverized and sometimes even melted. Minerals in the rock are

instantly subjected to both high temperature and high pressure. Rare and unusual

metamorphic minerals such as coesite, which are normally never found on the Earths

surface, are nearly instantly formed. Staggering quantities of matter are blown into the

atmosphere. Fortunately for life on Earth, this is a rare event, but these impacts have

repeatedly caused mass extinctions

Fault Zone Metamorphism

Near the surface, rock behaves like a brittle solid. So near the surface, movement along a

fault zone fractures and pulverizes the rock, creating what is called fault breccias. In contrast,

at depth under higher heat and pressure, rock is ductile and flows like plastic. At depth along

a fault zone, the mineral structures are deformed by the ductile flow, giving the metamorphic

rock a foliated or lineated appearance.

7.3 Principles of rock deformation

Rocks deform in response to differential stress. The resulting structure depends on the stress

orientation. At high temperatures, ductile flow of rocks occurs. At low temperatures, brittle

fractures form. The folds and faults exposed in canyon walls and mountain ranges show that

crustal rocks can be deformed on large scales and in dramatic ways.


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Force applied to an area is stress. Stress is the same thing as pressure and is a measure of the

intensity of the force or of how concentrated the force is. Rocks deform in response to the

forces applied to them.

All of Earths rocks are under some type of stress, but in many situations the stress is equal in

all directions and the rocks are not deformed. In many tectonic settings, however, the

magnitude of stress is not the same in all directions and rocks experience differential stress.

As a result, the rocks yield to the unequal stress and deform by changing shape or position.

Geologists call the change in shape strain. In other words, differential stress causes strain.

Under some conditions, rock bodies change shape by breaking to form continuous fractures

and they lose cohesion; this is brittle deformation (Figure 7.5). On the other hand, ductile

deformation occurs when a rock body deforms permanently without fracturing or losing

cohesion. The most obvious type of ductile deformation is the viscous flow of fluids, such as

molten magma, but solids can also deform ductilely. Rocks can flow in a solid state, under

the right conditions. This type of solid state flow is usually called plastic flow and is

accomplished by slow internal creep, gliding on imperfections in crystals, and

recrystallization.

Depending on the temperature or pressure of the surroundings and the rate at which stress is

applied, most types of rocks can deform by brittle fracture or ductile flow. Low pressures,

low temperatures, and rapid deformation rates favour brittle deformation (Figure 7.5B). As a

result, brittle structures are most common in the shallow crust. We use the term shear to

describe slippage of one block past another on a fracture. High confining pressures, high

temperatures, and low rates of deformation all favour ductile behaviour (Figure 7.5C).

(a) (b) (c)

Figure 7.5: Brittle versus ductile behaviour (a) initial shape (b) low confining pressure

(c) high confining pressure


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Ductile deformation is more common in the mantle and deeper parts of the crust. The flow of

rocks in a solid state to form folds in metamorphic rocks is a good example of ductile

behaviour. The formation of fractures, joints, and faults are common expressions of

deformation in the brittle regime.

To visualize this, imagine how two adjacent blocks can interact. They can move away from

one another (extension), move toward one another (contraction), or slip horizontally past

one another (lateral-slip). Obviously, the orientation of the stresses acting on the blocks

determines which of the three cases is dominant. In the simplest case, the stress orientations

are directly related to plate tectonic settings. Extension is caused when the differential

stresses point away from one another (Figure 7.6a). This type of deformation results in

lengthening and is common at divergent boundaries. In brittle rocks it is expressed by

fracturing and faulting, and in ductile rocks by stretching and thinning. Contraction is caused

by horizontal compression when the differential stresses are directed toward one another

(Figure 7.6b). Contraction is common at convergent boundaries and causes shortening and

thickening of rock bodies, expressed as faults in brittle rocks and folds in ductile rocks.

Lateral-slip is the kind of shear that occurs when rocks slide horizontally past one another

along nearly vertical fractures and dominates at transform plate boundaries (Figure 7.6c).

(a) (b) (c)

Figure 7.6: (a) extension (b) contraction and (c) lateral slip

7.3.1 Joints

Joints are tension fractures in brittle rocks along which no shear has occurred. They form at

low pressure and are found in almost every exposure. The simplest and most common

structural features of rocks at Earths surface are cracks or fractures, known as joints, along

which little displacement (or slip) has occurred. Their most important feature is the absence

of shear; no movement occurs parallel to the fracture surface. Joints form by the brittle failure


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of rocks at low pressure as stress accumulates and exceeds the rocks strength. They do not

occur at random but are usually perpendicular to the direction of tension. Multiple sets of

joints that intersect at angles ranging from 45 to 90 are very common. They divide rock

bodies into large, roughly rectangular blocks. These joint systems can form remarkably

persistent patterns extending over hundreds of square kilometres. Each set probably formed at

a different time and under a different stress orientation.

Joints have great economic importance. They can be paths for groundwater migration and for

the movement and accumulation of petroleum. Analysis of joint patterns has been important

in exploration and the development of these resources. Joints also control the deposition of

copper, lead, zinc, mercury, silver, gold, and tungsten ores. Hot aqueous solutions associated

with igneous intrusions migrate along joint systems and minerals crystallize along the joint

walls, forming mineral veins. Modern prospecting techniques therefore include detailed

analysis of fractures. Major construction projects are especially affected by joint systems

within rocks, and allowances must be made for them in project planning. For example, dams

must be designed so that the stresses caused by water storage tend to close any fractures in

the bedrock foundation. Joint systems can be either an asset or an obstacle to quarrying

operations. Closely spaced joints severely limit the sizes of blocks that can be removed. If a

quarry follows the orientation of intersecting joints, however, the expense of removing

building blocks is greatly reduced, and waste is held to a minimum.

7.3.2 Faults

Faults are fractures in Earths crust along which displacement has occurred. Three basic types

of faults are recognized: (1) normal faults, (2) reverse faults, and (3) strike-slip faults.

Normal faults are usually the result of extension, thrust faults the result of horizontal

compression, and strike-slip faults the result of lateral slip.

Slippage (or shear) along brittle fractures in Earths crust creates faults (Figure 7.7). Like

other deformation features, they form by the application of differential stress.

Displacement along faults ranges from a few centimetres to hundreds of kilometres. Faults

grow by a series of small movements, which occur as stress built up in the crust is suddenly

released in earthquakes. Displacement can also occur by an almost imperceptibly slow

movement called tectonic creep.

Normal (Extensional) Faults. Along normal faults, movement is mainly vertical, and the

rocks above the fault plane (the hanging wall) move downward in relation to those beneath


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the fault plane (the footwall) (Figure 7.7a). Most normal faults are steeply inclined, usually

between 65 and 90, but some dip at low angles.Their predominantly vertical movement

commonly produces a cliff, or scarp, at the surface. Normal faults are rarely isolated features.

A group of parallel normal faults may develop a series of fault-bounded blocks. A narrow

block dropped down between two normal faults is a graben (German, trough or ditch),

and an upraised block is a horst. A graben typically forms a conspicuous fault valley or basin

marked by relatively straight, parallel walls. Horsts form plateaus bounded by faults. Normal

faults are common because rocks are weaker during extension than during compression. This

type of extensional stress occurs on a global scale along divergent plate margins.

Consequently, normal faults are the dominant structures along the oceanic ridge, in

continental rift systems, and along rifted continental margins.

Reverse (Contractional) Faults. Faults in which the hanging wall has moved up and over the

footwall are reverse faults (Figure 7.7b). Thrust faults are low-angle reverse faults and dip

at angles less than 45. Movement on a thrust fault is predominantly horizontal, and

displacement can be more than 50 km. Thrust faults result from horizontal compression with

the maximum stress perpendicular to the trend of the fault. This shortens and thickens the

crust. In contrast to normal faults, thrust faults usually place older over younger strata and

instead of omitting layers, units are repeated in a vertical section.

Strike-Slip Faults. Strike-slip faults are high-angle fractures in which slip is horizontal,

parallel to the strike of the fault plane. Ideally, there is little or no vertical movement, so high

cliffs do not usually form along strike-slip faults. Instead, these faults are expressed

topographically by a straight valley or by a series of low ridges and commonly mark

discontinuities in the drainage and types of landscape. No crustal thinning or thickening is

produced, except at bends in the fault where extension or contraction can occur. Strike-slip

faults result from horizontal shear along nearly vertical faults. They commonly are produced

by lateral-slip where one tectonic plate slides past another at a transform fault boundary.


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Figure 7.7: Major types of fault (a) normal fault (b) thrust fault and (c) strike-slip fault

7.3.3 Folds

Folds are warps in rock strata during ductile deformation. They are three dimensional

structures ranging in size from microscopic crinkles to large domes and basins that are

hundreds of kilometers across. Most folds develop by horizontal compression at convergent

plate boundaries where the crust is shortened and thickened.

Broad, open folds form in the stable interiors of continents, where the rocks are only mildly

warped. Almost every exposure of sedimentary rock shows some evidence that the strata

have been deformed. In some areas, the rocks are slightly tilted; in others the strata are folded

like wrinkles in a rug. Small flexures are abundant in sedimentary rocks and can be seen in

mountainsides and road cuts and even in hand specimens. These warps in the strata are called

folds and are a manifestation of ductile deformation in response to horizontal compression.

This kind of deformation is also called contraction. Large folds cover thousands of square

kilometres, and they can best be recognized from aerial or space photographs or from

geologic mapping.

Like faults, folds form slowly over millions of years, as rock layers gradually yield to

differential stress and bend.

Folds are of great economic importance because they commonly form traps for oil and gas

and may control localization of ore deposits.

Three general types of folds are anticline, monocline and syncline. They are illustrated in

Figure 7.8.

An anticline, in its simplest form, is up-arched strata, with the two limbs (sides) of the fold

dipping away from the crest. Rocks in an eroded anticline are progressively older toward the

(a) (b)

(c)


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interior of the fold. Synclines, in their simplest form, are downfolds, or troughs, with the

limbs dipping toward the centre (Figure 7.8).

Rocks in an eroded syncline are progressively younger toward the centre of the fold.

Monoclines are folds that have only one limb; horizontal or gently dipping beds are modified

by simple steplike bends.

Figure 7.8: Major types of folds

7.4 Earthquakes

An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion

capable of shaking building apart. The earthquake is a form of energy of wave motion

transmitted through the surface layer of the earth. An earthquake can also be defined as the

seismic vibration of Earth caused by the rapid release of energy. Earthquake events can be

either natural or human-caused. Passing trains or large trucks and explosions can cause Earth

to vibrate.

All the earthquakes are not of the same intensity. Some of them are very severe, others are

very mild and still others are not even noticed.

The instrument used for recording the earthquakes is known as seismograph. The point within

the earths crust where an earthquake originates is called the focus. It is also referred as

seismic focus. It generally lies within the depth of 60 kilometres in the earth crust.

(a) Monocline (b) Anticline

(c) Syncline (d) Overturned anticline and syncline


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The point vertically above the focus on the earths surface is known as epicentre. The impact

of the earthquake is carried from the point of its origin by earthquake waves. These

earthquake waves originating from the focus travel in all directions. But their intensity is the

highest at the epicentre. That is why the maximum destruction occurs at and around the

epicentre (Figure 7.9). The intensity of vibrations decreases as one moves away from the

epicentre in all directions.

Figure 7.9: Focus and epicentre of earthquake

7.4.1 Causes of earthquakes

Folding, faulting and displacement of rock strata are the main causes of earthquakes. Some

examples of this type of earthquakes are the San Francisco earthquakes of California in 1906,

the Assam earthquakes of 1951, the Bihar earthquakes of 1935.

The second important cause lies in the phenomenon of volcanic eruption. The violent

volcanic eruptions put even the solid rocks under great stress. It causes vibrations in the

earths crust. But, these earthquakes, are limited to the areas of volcanic activity. One

important example is the earthquake which continued for six days preceding the eruption of

Mauna Loa volcano of Hawaii Island in 1868.

Minor earthquakes often accompany or are the result of landslides, seepage of water causing

the collapse of the rocks of cavern or underground mines and tunnel. These are least

damaging earthquakes.

7.4.2 Effects of earthquakes

Violent earthquakes are generally very disastrous. They may themselves cause land-slides,

damming of river course and occurrence of floods, and sometimes, the depressions leading to

the formation of lakes. An earthquake often forms cracks and fissures in the earths crust. It


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changes the drainage system of an area as was witnessed in Assam after its 1951 earthquake.

Earthquakes also cause vertical and horizontal displacement of rock strata along fault line.

They prove most catastrophic and devastating when they cause fires and seismic sea waves.

Such tidal waves are called Tsunamis. These waves may wash away coastal cities. Buildings

and bridges collapse causing death of the thousands of people. Lines of transport,

communication and of electric transmission get disrupted. The after effect of earthquake is

spread of epidemics like cholera.

7.4.3 Earthquake Measurement

Two measurement schemes that have been used to characterize earthquakes are the Modified

Mercalli intensity scale and the Richter magnitude scale. Intensity is a measure of ground

shaking and the damage that it causes. The Modified Mercalli scale, (see Table 1), ranks

earthquakes in a range from IXII, XII being the worst, and uses eyewitness observations and

post earthquake assessments to assign an intensity value. The Richter magnitude scale uses

the amplitude of the largest earthquake wave. Richter magnitude is intended to give a

measure of the energy released during the earthquake. Figure 7.10 shows a seismogram and

how it is used to determine a Richter value.

Figure 7.10: A seismograph

Table 1: The Mercalli Scale of Earthquake Intensity

Level Description

I Rarely felt by people.

II Felt by resting people indoors; some hanging objects may swing.

III Felt indoors by several. Vibration like passing of a light truck.

IV Felt indoors by many. Vibration like passing of a heavy truck. Standing autos

rock. Windows, dishes and doors rattle. Walls and frames may creak.

V Felt by nearly everyone indoors and outdoors. Small unstable objects upset. Some

dishes and glassware broken. Swaying of tall objects noticed.

VI Felt by all. Walking is unsteady, many run outdoors. Windows, dishes, and

glassware broken.

VII Furniture overturned and plaster may crack.

VIIII Difficult to stand. Noticed by drivers of autos. Furniture and chimneys broken.

Well built buildings hardly damaged. Poor structures considerable damage.

IX People frightened. Ordinary buildings slightly damaged. Driving of autos


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affected. Tree limbs fractured. Damage to tall objects. Cracks in wet ground.

X General panic. Damage great in substantial buildings. Some houses thrown off

foundations. Underground pipes broken. Serious ground cracks.

XI Most masonry and frame structures destroyed. Serious damage to dams, dikes,

embankments.

XII Water splashed out of rivers, canals, lakes. Rails bent.

7.5 Theory of Plate Tectonics

Plate Tectonics are enormous moving pieces of the earths lithosphere. Tectonic Plates move

in four ways:

Spreading: horizontally moving apart

Subduction: dividing under another plate

Collision: crashing into one another

Transform: sliding past each other (Folds and faults)

Divergent boundary- moving apart

Convergent boundary: coming together

Plates are composed of a rigid layer of uppermost mantle and a layer of either oceanic or

continental crust above. Some plates are composed only of oceanic crust, and some are

composed of part oceanic and part continental crust.

There are three main kinds of plate motions. These are best visualized by considering how

plates interact along plate boundaries, where they meet. Plates can move apart, move

together, or slide past one another. Although often visualized as narrow boundaries, scientists

now consider many boundaries to be wide zones of interaction.

Divergent Plate Boundaries: At a mid-ocean ridge (MOR), magma rises along a faulted rift

valley, spreads, and cools to form new oceanic crust. This spreading apart is what happens at

divergent boundaries. Mid-ocean ridge represents divergence that is well-developed and

that has resulted in the production of major ocean basins. In some locations on Earth today,

divergent boundaries exist as rift valleys, where no mature ocean basins exist yet, such as in

East Africa, shown in Figure 7.11.


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Figure 7.11: Large lakes and volcanic mountains are characteristics of a continental rift

valley

Convergent Plate Boundaries: Where plates collide, they come together to form

convergent boundaries. In some cases, less-dense, thick continental lithosphere moves

toward denser, thin oceanic lithosphere. This results in the oceanic side bending and being

forced downward beneath the continental slab in a process called subduction. Heat along a

subduction zone partially melts rock at depth and produces magma, which rises toward the

surface. This magma feeds a volcanic arc that parallels this zone, shown in Figure 7.12. The

region of collision also has a deep-sea trench that parallels the zone. The Andes mountain

range in South America is an example.

Convergent plate boundaries also exist between two slabs of oceanic lithosphere. In this case,

the oceanic lithosphere that is colder, and therefore denser, subducts. Magma erupted here

produces chains of volcanic islands called island arcs. Japan is an example of an ocean-ocean

convergent boundary, also shown in Figure 7.12. As plates converge, stress builds, which

could be released as tsunami-causing earthquakes. Along some convergent plate boundaries,

two continental slabs of low density collide and tend not to subduct. Because of this

resistance to subduction, the plates collide and buckle upward to form a high range of folded

mountains. Volcanic activity is noticeably absent and there is no trench. The Himalaya of

Asia is an example of folded mountains that occur where continental lithosphere collides.


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Figure 7.12: When plates collide, the more dense plate is subducted. The resulting

features include volcanoes, mountains, and deep trenches.

Transform Plate Boundaries: Some boundaries among plates exist as large faults, or cracks,

along which mostly horizontal movement is taking place, as shown in Figure 7.13. In this

case, no new lithosphere is forming, as along a divergent boundary. In addition, old

lithosphere is not being recycled, as along a subduction zone. The main result of transform

boundaries is horizontal motion of lithosphere.

Figure 7.13: Friction between plates moving side by side causes cracks and breaks in the

edges of the plates. This is the site of brief, but rapid energy release called an

earthquake

Documents

Internal Earth Processes Lecture Notes