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Introductory Engineering Geology


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

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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. 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
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 (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 20oC to 50oC 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 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 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
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)
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
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).




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).




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.
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


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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.

(a) Monocline

(b) Anticline

(c) Syncline

(d) Overturned anticline and syncline

Figure 7.8: Major types of folds

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


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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
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
earths crust. But, these earthquakes, are limited to the areas of volcanic activity. One
Mauna Loa volcano of Hawaii Island in 1868.
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.
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

Rarely felt by people.
Felt by resting people indoors; some hanging objects may swing.
Felt indoors by several. Vibration like passing of a light truck.
Felt indoors by many. Vibration like passing of a heavy truck. Standing autos
rock. Windows, dishes and doors rattle. Walls and frames may creak.
Felt by nearly everyone indoors and outdoors. Small unstable objects upset. Some
dishes and glassware broken. Swaying of tall objects noticed.
Felt by all. Walking is unsteady, many run outdoors. Windows, dishes, and
glassware broken.
Furniture overturned and plaster may crack.
Difficult to stand. Noticed by drivers of autos. Furniture and chimneys broken.
Well built buildings hardly damaged. Poor structures considerable damage.
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.
General panic. Damage great in substantial buildings. Some houses thrown off
foundations. Underground pipes broken. Serious ground cracks.
Most masonry and frame structures destroyed. Serious damage to dams, dikes,
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
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.


Introductory Engineering Geology

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