Chapter 1 - Introduction to Geology.pdf

7/27/2019 Chapter 1 - Introduction to Geology.pdf

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Geologi Kejuruteraan - BFC 3013 Introduction to Geology

UNIVERSITI TUN HUSSEIN ONN MALAYSIA1

Chapter 1Introduction to Geology

INTRODUCTION

What is Geology and Engineering Geology?

Geology is the study of this planet Earth, its origin, history, composition, structureand dynamics of how it changes. The word geology is derived from Greek word(geo - earth; logos - discourse). Geology is an event formed during geologicaltime which involves interpretation and also observation of the event that occurredand is still occurring at present in our earth; Geological processes that takes

place during the very large span of geological time, left their record in the rocks.One of the unique features of Earth is that the Earth is not a static body but is inconstant motion and changes continually.

In sciences study, engineering geology is the application of the geologicalprincipal in civil engineering (and as a subdivision of the mining engineering).Engineering geology in practice are responsible in civil engineering projects thatinvolve the earth or earth materials which include (1) The identification andevaluation of the physical environment of the site and (2) The analysis of theimpact of the geologic processes on the proposed project. As a result, it isimportant to the civil engineers to understand about history, nature and the

variety behavior of the soil and rock. The knowledge of the applicationengineering is also important for the geologist who works together with theengineer.

Engineering geology is a subfield of geological study concerning about thegeological inputs and the uses of the information to solve the engineeringproblems. It exists solely to serve art and science of engineering throughdescription of the structure and attributes of rocks connected with engineeringworks (Goodman, 1993). Some engineering works that needunderstandings/related to geological aspect are construction of dam, landslide,rock as aggregates and construction material, hydro geological and Etc.


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1.1 The Universe and Solar System

Our solar system consists of an average star we call the Sun, the planetsMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Itincludes the satellites of the planets such as numerous comets, asteroids, andmeteoroids and the interplanetary medium. The Sun is the richest source ofelectromagnetic energy (mostly in the form of heat and light) in the solar system.The nine major planets including our earth and their moons are revolving aroundthe Sun.

Figure 1.1 The solar system

Figure 1.2 The composite above shows Figure 1.3 Planetsthe Sun and the 5 largest planets at a scaleof 3200 km/pixel. (Earth is the tiny spotbetween Jupiter and the Sun)


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1.1.1 The Terrestrial and Jovian Planets

The terrestrial planets are the four innermost planets in the solar system,Mercury, Venus, Earth and Mars. They are called terrestrial because they have acompact, rocky surface like the Earth's. The planets, Venus, Earth, and Marshave significant atmospheres while Mercury has almost none.

Figure 1.4 The Terrestrial Planets

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like)planets. They are called jovian because they are all gigantic compared with

Earth, and they have a gaseous nature like Jupiter's.

Figure 1.5 The Jovian Planets


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1.1.2 Satellites, Asteroids, Comets and Meteors

Satellites or Moons: are those celestial bodies, each of which is revolving aroundany of these nine planets. Asteroids are the minor planets generally situatedbetween orbits of Mars and Jupiter. Comets are the heavenly bodies havingalong tail pointing approximately away from the sun and a brighter head section(coma) that contains a small bright nucleus. Meteors are smaller solid bodiesmoving through the space, and getting illuminated while entering earthsatmosphere.

Figure 1.6 Asteroids

Figure 1.7 Comet


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Table 1.1 The characteristics for the entire planet

1.2 Earth

The largest of four planets of inner group solar system i.e. Mercury, Venus, Earthand Mars and third closest to the sun.

Shape - spherical

Polar radius - 21 km shorter than equatorial radius

Average radius - 6378 km (3965 miles)

Surface Area - 510 x 106 km2 (29% is land)

Overall Density - 5.5 g/cm3 Mount Everest is 8.8 km above sea level

Ocean floor is an average 3.7 km below sea level

Average height above sea level is 7 km

Distance

(AU)

Radius

(Earth's)

Mass

(Earth's)

Rotation

(Earth's) # Moons

Orbital

Inclination

Orbital

Eccentricity Obliquity

Density

(g/cm3)

Sun 0 109 332,800 25-36* 9 --- --- --- 1.410

Mercury 0.39 0.38 0.05 58.8 0 7 0.2056 0.1 5.43

Venus 0.72 0.95 0.89 244 0 3.394 0.0068 177.4 5.25

Earth 1.0 1.00 1.00 1.00 1 0.000 0.0167 23.45 5.52

Mars 1.5 0.53 0.11 1.029 2 1.850 0.0934 25.19 3.95

Jupiter 5.2 11 318 0.411 16 1.308 0.0483 3.12 1.33

Saturn 9.5 9 95 0.428 18 2.488 0.0560 26.73 0.69

Uranus 19.2 4 17 0.748 15 0.774 0.0461 97.86 1.29

Neptune 30.1 4 17 0.802 8 1.774 0.0097 29.56 1.64

Pluto 39.5 0.18 0.002 0.267 1 17.15 0.2482 119.6 2.03


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1.2.1 The Origin of the Earth

Based on observational facts cosmologist have developed classes of hypothesiswhich try to explain the origin of the earth. One of them is The Big Bang Theory.At 13.7 billion years ago, the entirety of our universe was compressed into theconfines of an atomic nucleus. Known as a singularity, this is the moment beforecreation when space and time did not exist. According to the prevailingcosmological models that explain our universe, an ineffable explosion, trillions ofdegrees in temperature on any measurement scale, that was infinitely dense,created not only fundamental subatomic particles and thus matter and energy butspace and time itself. Cosmology theorists combined with the observations oftheir astronomy colleagues have been able to reconstruct the primordialchronology of events known as the big bang.

Figure 1.8 The Big Bang Theory


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Figure 1.9 Earth

Figure 1.10 Earth with other Planets


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1.2.2 Principal Division of Earth

(a) Atmosphere

Gaseous portion of the Earth extending upwards for hundreds of miles above sealevel. It is a mixture of 98% nitrogen, 21% oxygen, carbon dioxide, eater vapourand minor amount of other gases. The atmosphere is divided into two parts.

Troposhereis the closest to the Earth ~ 13 km. It contains almost all of the watervapour, clouds and storms.

Stratosphere is the overlying layer ~ 55 km above the surface, contains theozone layer. It acts as an insulating agent protecting us from the heat and ultra

violet radiation of the sun and makes possible the evaporation and precipitationof moisture and is thus of crucial important to organisms on surface of Earth. Theatmosphere is an important geologic agent and is responsible for the processesof weathering which are continually at work on the Earth's surface.

(b) Hydrosphere

Total mass of water or the surface of our planet. The hydrosphere includes about98% of water in the oceans and 2% in lakes, rivers as well as ground water whichexist in the pores and crevices of the crustal rocks and soils. 71% of Earth

covered by oceans to average depth of 4 km. Water is essential to man and ofgeologic important.

All of Earth's weather patterns, climate, rainfall and the extremely importantcarbon dioxide content of atmosphere are influenced by the seas and oceans.Hydrosphere is in constant motion - evaporating through atmosphere,precipitating as rain and returning to Earth. As water moves over the Earth'ssurface it erodes, transports and deposits weathered rock material, constantlymodifying the Earth's landscape.

(c) Lithosphere

Lithos means rock. The solid portion of the Earth composed of crust and uppermantle. It is a layer of rocks about 70 km thick, that rests upon soft weak materialand is broken into about 12 major plates which is slowly moved by the flow ofmaterial in a layer that directly underlies the lithosphere called theasthenosphere. There are three basic types of rock: Igneous, Sedimentary andMetamorphic.


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1.2.3 Physical Features of the Earth

Major Concepts

Earth is segregated and concentrated into layers according to density.

The major internal layers based on physical properties are:

(a) lithosphere (b) asthenosphere(c) mesosphere (d) core

Material within each of these units is in motion, making Earth a changing

dynamic planet.

Continents and ocean basins are the principal surface features of Earth.

1.2.3.1 Major Structural Units of Earth

The constituents of Earth are separated and segregated into layers according todensity. The denser materials are concentrated near the center, the less densenear the surface.

The internal layers are recognized on the basis of composition and physicalproperties.

Composition layers are:

Crust

Mantle

Core

Layers based on physical properties are:

Lithosphere

Asthenosphere

Mesosphere

Core


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Table 1.2 Layers of the Earth Based on Composition

OuterCrust

Outer layer of the Earth, extending from solid surface down to the fi rst majordiscontinuity in seismic wave velocity in the lithosphere. Thickness of crust variesfrom about 8 km under the oceans to about 35 km under the continents.There are two kinds of earth crust classified according to two different kinds ofrock they contained where each with its own general composition, thickness anddensity.(a) Continent Crust: 35 - 60 km thick

relatively low densitygranitic rockaverage density: 2.8 g/cm3

(b) Oceanic Crust : thickness rarely exceed 5 kmdenser material

basaltic compositionaverage density: 2.9 g/cm3

Mantle

The next major compositional layer of the Earth which covers the core and thiszone constitute 82% of its volume and 68% of mass of the Earth (Earth largestlayer).The mantle has a property called "plasticity" (where a solid has the ability to flowlike a liquid). You might call the mantle "partially molten". Remember that thetemperature of the mantle increases the deeper you go. This difference intemperature causes CONVECTION CURRENTS to form. This type of currentforms when hot things rise and cooler things sink. These convection currentstumble throughout the mantle. They cause the Lithospheric plates floating on the

mantle to move around. These currents cause our continents and oceans tochange location slightly each year. The currents are the driving force for PlateTectonics or Continental Drift, which we will discuss in more detail in a latersection.The forces which drive continental drift seem to come from the mantle. The hotrock, which boils up at mid-ocean ridges, comes from the upper mantle. This rockspreads out forming new oceanic plates.When these meet the continents they plunge back down into the mantle,sometimes going down as far as the outer core.In addition there are hot spots, which start at the outer core and rise up throughthe mantle to form islands such as Hawaii or Iceland.

The mantle is composed of iron and magnesium silicate rock, and it goes down toabout 2900 km from surface of Earth.Average density: 4.5 g/cm3

OuterCore

It is speculated that the thickness is about 2250 km and it is made of molten ironand nickel.Average density: 10.7 g/cm3

InnerCore

The thickness' is about 1300 km and probably consists of mostly iron and nickel.Average density: 17.0 g/cm3


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Figure 1.11 The Lithosphere

Figure 1.12 The layer of Lithosphere and Asthenosphere


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Figure 1.13 The Convection Currents in the Mantle

Figure 1.14 The Outer Core and The Inner Core


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Table 1.3 Internal layers of the Earth based on Physical Properties

Lithosphere(rock sphere)

The top of the asthenosphere is about 100 km below thesurface. Above the asthenosphere, the material is solid, strongand rigid. This layer is called lithosphere. Contains thecontinental crust of the uppermost part of the mantle.

Asthenosphere(weak sphere)

A major zone within the upper mantle where temperature andpressure are just the right balance so that part of the materialmelts. The rocks lose much of their strength and become softplastic and easily deformed. The asthenosphere is the part ofthe mantle that flows and moves the plates of the Earth.The thickness is about 200 km.

Mesosphere

The rock below the asthenosphere is stronger and more rigidthan the asthenosphere because the high pressure at thisdepth offsets the effect of high temperature. The regionbetween the asthenosphere and the core-mantle boundary iscalled the mesosphere.

Core

The core of the Earth marks a change in both physicalproperties and composition. It is composed mostly of iron andis therefore distinctly different from the silicate (rocky) materialabove. On the basis of physical properties, the core has twodistinct parts - a solid inner core and liquid outer core. Heatloss from the core and the rotation of the Earth probablycauses the liquid outer core to circulate and generate theEarth's magnetic field.


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(a)

(b)


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(c)

Figure 1.15 (a), (b) and (c) Composition and Physical Properties Layers of theEarth


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Figure 1.16 The Compositional and Mechanical Layers of Earth

Figure 1.17 The Internal Structure of Earth


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1.2.3.2 The Structure of Earth

Continents and ocean basins are the principle surface features of the Earth. Bothare distinctly different in composition, density, rock type, structure and origin.

(a) Continental Masses

This part of the earth covers about 29% of the earths surface and has anaverage elevation of about 5 km above the floors of the ocean basins and about1 km above sea level. It composed largely of rocks known as granite. Thecontinents rise above the ocean basins as large platforms. The highest mountainon the continental surface is Mount Everest which is 29000 feet above sea level

but the deepest part of the ocean is about 35000 feet below sea level at PacificOcean.

(b) Ocean Basins

The greatest part of the hydrosphere is the ocean basin which covers about 70%of the earth's surface. The ocean floors are also as irregular and posses manydeep trenches and mountain ranges as the continental masses. The rocks of theocean are rather dense, dark basaltic rock.

Figure 1.18 A graph of the Elevation of the Continents and Ocean Basins


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1.2.3.3 The Geologic Processes that Change the Earth's Structure

Geologic Forces: Earth has undergone great changes over million of years.Generally processes of gradation, tectonism and volcanism.

(a) Gradation

Degradation: Erosion results from wearing of rocks by water, air and ice.

Aggradation: Deposition results in accumulation of sediment and ultimate buildingup of rock strata.

(b) Tectonism

Plate tectonics is a dynamic process of the lithospheric plate which moves over aweak plastic layer in the upper mantle known as asthenosphere. These platesinteract with one another along their boundaries. Indicative of crustal instability,produce faulting (fracture and displacement), folding, subsidence and uplift ofrock formation. Responsible for formation of mountain ranges.

Earths lithosphere is composed of seven large plates (Figure 1.19) withthickness ranging from 75 to 125 km.

Table 1.4 Earths Lithosphere Plates

(c) Volcanism

A volcano is a vent in the earth's crust through which molten rock materials within

the earth, lavas, ashes, steam and gas are ejected and responsible for theformation of plutonic rocks, once solidified at great depth. Majority of volcanoesare located along the margins of tectonic plates.

Pacific Plate Eurasian PlateAntartic Plate North America PlateIndian Plate South American PlateAfrician Plate 20 other small plates in between


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Figure 1.19 Major Plates of the Lithosphere

1.2.3.4 Geologic Time Scale

The Earth's crust is known to be at least 40 million centuries old. The time spanof the earth is called eras and subdivided into periods (Table 1.5). Rocks havebeen created and destroy throughout geologic time. Rocks which are createdduring that particular period for example Cambrian are said to belong to theCambrian system. The nature of rocks created or formed during various eras canactually reveal about its strength and condition, for example rocks from thePrecambrian era are known to be very hard, crystalline materials but often withmany fractures and microstructures, whereas sandstone formed from Piloceneseries tends to be porous as soil and easily excavated without blasting.


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Table 1.5 Geologic Time Span

Era Period Absolute (million yearsCenozoic

Tertiary

Mesozoic

Upper Paleozoic

Lower Paleozoic

Proterozoic and

Archaeozoic

HolocenePleistocene

PiloceneMiloceneOligocene

EocenePalaeocene

CretaceousJurassicTriassic

PermianCarboniferous

Devonian

SilurianOrdovicianCambrian

0.0122

726385465

135195225

280345395

440500570

4600

1.3 Plate Tectonic

1.3.1 What is a Tectonic Plate?

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped

slab of solid rock, generally composed of both continental and oceaniclithosphere.

Plate size can vary greatly, from a few hundred to thousands of kilometersacross; the Pacific and Antarctic Plates are among the largest. Plate thicknessalso varies greatly, ranging from less than 15 km for young oceanic lithosphere toabout 200 km or more for ancient continental lithosphere.

How do these massive slabs of solid rock float despite their tremendous weight?The answer lies in the composition of the rocks. Continental crust is composed of


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granitic rocks which are made up of relatively lightweight minerals such as quartzand feldspar. By contrast, oceanic crust is composed of basaltic rocks, which aremuch denser and heavier.

Most of the boundaries between individual plates cannot be seen, because theyare hidden beneath the oceans. Yet oceanic plate boundaries can be mappedaccurately from outer space by measurements from GEOSAT satellites.Earthquake and volcanic activity is concentrated near these boundaries.

1.3.2 The Theory of Tectonic Plate

The theory of tectonic plate states that the Earth's outermost layer is fragmentedinto a dozen or more large and small plates that are moving relative to oneanother as they ride atop hotter, more mobile material.

The present is the key to the past, the geologic forces and processes - gradualas well as catastrophic - acting on the Earth today are the same as those thathave acted in the geologic past.

Continental Drift - introduced by a 32 year old German meteorologist namedAlfred Lothar Wegener. He contended that, around 200 million years ago, the

supercontinent Pangaea began to split apart.

Alexander Du Toit, Professor of Geology at Johannesburg University, proposedthat Pangaea first broke into two large continental landmasses, Laurasia in thenorthern hemisphere and Gondwanaland in the southern hemisphere. Laurasiaand Gondwanaland then continued to break apart into the various smallercontinents that exist today.


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Figure 1.20 According to the continental drift theory, the supercontinentPangaea began to break up about 225-200 million years ago, eventuallyfragmenting into the continents as we know them today


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Figure 1.21 Major plates of the lithosphere are broken into a dozen or so rigidslabs that are moving relative to one another


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(a) (b)

Figure 1.22 These two maps showing the American and African continents mayonce have fit together, then later separated. (a) The formerly joined continentsbefore their separation. (b) The continents after the separation.

Wegener's theory was based in part on what appeared to him to be theremarkable fit of the South American and African continents, the matching animalfossils found on coastlines of South America and Africa, and the evidence ofdramatic climate changes on some continents.

For example, the discovery of fossils of tropical plants (in the form of coaldeposits) in Antarctica led to the conclusion that this frozen land previously musthave been situated closer to the equator, in a more temperate climate where

lush, swampy vegetation could grow. Other mismatches of geology and climateincluded distinctive fossil ferns (Glossopteris) discovered in now-polar regions,and the occurrence of glacial deposits in present-day arid Africa, such as theVaal River valley of South Africa.


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Figure 1.23 As noted by Snider-Pellegrini and Wegener, the locations of certainfossil plants and animals on present-day, widely separated continents would formdefinite patterns (shown by the bands of colors), if the continents are rejoined

1.3.3 Developing the Theory of Continental Drift

Four major scientific developments spurred the formulation of the plate-tectonicstheory:

a) The ruggedness and youth of the ocean floor

b) Repeated reversals of the Earth magnetic field in the geologic past

c) Developing of the seafloor-spreading and associated recycling of oceanic crust

d) The world's earthquake and volcanic activity is concentrated along oceanictrenches and submarine mountain ranges.


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1.3.3.1 The Ruggedness and Youth of the Ocean Floor

Ocean floor mapping shows the ruggedness and youth of the ocean floor.

The sediment layer on the floor of the Atlantic was much thinner thanoriginally thought.

Scientists had previously believed that the oceans have existed for at least 4billion years, so therefore the sediment layer should have been very thick.

Why then was there so little accumulation of sedimentary rock and debris onthe ocean floor? The answer to this question, which came after furtherexploration, would prove to be vital to advancing the concept of platetectonics.

The discovery that a great mountain range on the ocean floor virtuallyencircled the Earth. Called the global mid-ocean ridge, this immensesubmarine mountain chain - more than 50,000 kilometers (km) long and, inplaces, more than 800 km across - zig-zags between the continents, windingits way around the globe like the seam on a baseball. Rising an average ofabout 4,500 m above the sea floor.

Figure 1.24 The mid-ocean ridge (shown in red) winds its way between thecontinents much like the seam on a baseball


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1.3.3.2 Repeated Reversals of the Earth Magnetic Field in the Geologic Past

Using magnetic instruments (magnetometers) adapted from airborne devicesto detect submarines, began recognizing odd magnetic variations across theocean floor.

This finding, though unexpected, was not entirely surprising because it wasknown that basalt - the iron-rich, volcanic rock making up the ocean floor -contains a strongly magnetic mineral (magnetite) and can locally distortcompass readings.

Figure 1.25 A theoretical model of the formation of magnetic striping. Newoceanic crust forming continuously at the crest of the mid-ocean ridge cools andbecomes increasingly older as it moves away from the ridge crest with seafloorspreading (see text): a. the spreading ridge about 5 million years ago; b. about 2to 3 million years ago; and c. present-day.

Rocks generally belong to two groups according to their magnetic properties.One group has so-called normal polarity, characterized by the magnetic

minerals in the rock having the same polarity as that of the Earth's presentmagnetic field. This would result in the north end of the rock's "compassneedle" pointing toward magnetic north. The other group, however, hasreversed polarity, indicated by a polarity alignment opposite to that of theEarth's present magnetic field. In this case, the north end of the rock'scompass needle would point south.

How could this be? This answer lies in the magnetite in volcanic rock. Grainsof magnetite - behaving like little magnets - can align themselves with theorientation of the Earth's magnetic field. When magma (molten rockcontaining minerals and gases) cools to form solid volcanic rock, the


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alignment of the magnetite grains is "locked in," recording the Earth'smagnetic orientation or polarity (normal or reversed) at the time of cooling.

1.3.3.3 Seafloor Spreading and Recycling of Oceanic Crust

Why there is so little sediment accumulation on the ocean floor, and whyoceanic rocks are much younger than continental rocks?

At or near the crest of the ridge, the rocks are very young, and they becomeprogressively older away from the ridge crest.

The youngest rocks at the ridge crest always have present-day (normal)polarity.

Stripes of rock parallel to the ridge crest alternated in magnetic polarity(normal-reversed-normal, etc.), suggesting that the Earth's magnetic field hasflip-flopped many times.

When the ages of the samples were determined by paleontologic and isotopicdating studies, they provided the clinching evidence that proved the seafloorspreading hypothesis.

1.3.3.4 Concentration of Earthquakes and Volcano Activity

During the 20th century, improvements in seismic instrumentation and greateruse of earthquake-recording instruments (seismographs) worldwide enabledscientists to learn that earthquakes tend to be concentrated in certain areas,most notably along the oceanic trenches and spreading ridges.

By the late 1920s, seismologists were beginning to identify several prominentearthquake zones parallel to the trenches that typically were inclined 40 - 60from the horizontal and extended several hundred kilometers into the Earth.These zones later became known as Wadati-Benioff zones, or simply Benioffzones.


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Figure 1.26 As early as the 1920s, scientists noted that earthquakes areconcentrated in very specific narrow zones. In 1954, French seismologist J.P.Roth published this map showing the concentration of earthquakes along thezones indicated by dots and cross-hatched areas


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1.3.4 Understanding Plate Motions

Scientists now have a fairly good understanding of how the plates move and howsuch movements relate to earthquake activity. Most movement occurs alongnarrow zones between plates where the results of plate-tectonic forces are mostevident. There are four types of plate boundaries:

1. Divergent boundaries - where new crust is generated as the plates pullaway from each other.

2. Convergent boundaries - where crust is destroyed as one plate divesunder another. It can divide into three:

(1) Oceanic - continental convergence

(2) Oceanic - oceanic convergence(3) Continental - continental convergence

3. Transform boundaries - where crust is neither produced nor destroyed asthe plates slide horizontally past each other.

4. Plate boundary zones - broad belts in which boundaries are not welldefined and the effects of plate interaction are unclear.

Figure 1.27 An illustrating the main types of plate boundaries; East African RiftZone is a good example of a continental rift zone


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1.3.4.1 Divergent Boundaries

Divergent boundaries occur along spreadingcenters where plates are moving apart and newcrust is created by magma pushing up from themantle. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimetersper year (cm/yr), or 25 km in a million years.

Figure 1.28 The Mid-Atlantic Ridge, which splitsnearly the entire Atlantic Ocean north to south, isprobably the best-known and most-studiedexample of a divergent-plate boundary

In East Africa, spreading processes have alreadytorn Saudi Arabia away from the rest of theAfrican continent, forming the Red Sea. Theactively splitting African Plate and the ArabianPlate meet in what geologists call a triple

junction, where the Red Sea meets the Gulf ofAden.

Figure 1.29 Map of East Africa showing some of the historically activevolcanoes (red triangles) and the Afar Triangle (shaded, center) - a so-calledtriple junction (or triple point), where three plates are pulling away from oneanother: the Arabian Plate, and the two parts of the African Plate (the Nubian andthe Somalian) splitting along the East African Rift Zone


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1.3.4.2 Convergent Boundaries

The Earth's unchanging size implies that the crust must be destroyed at aboutthe same rate as it is being created. Such destruction (recycling) of crust takesplace along convergent boundaries where plates are moving toward each other,and sometimes one plate sinks (is subducted) under another. The location wheresinking of a plate occurs is called a subduction zone.

The type of convergence - called by some a very slow "collision" - that takesplace between plates depends on the kind of lithosphere involved. Convergencecan occur between an oceanic and a largely continental plate, or between twolargely oceanic plates, or between two largely continental plates.

Figure 1.30 Subduction Zone and Mid-ocean Ridges


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1.3.4.2.1 Oceanic - Continental Convergence

Oceanic-continental convergence (Figure 1.31) also sustains many of the Earth'sactive volcanoes, such as those in the Andes and the Cascade Range in thePacific Northwest. The eruptive activity is clearly associated with subduction.

Figure 1.31 Oceanic - Continental Convergence

Figure 1.32 Volcanic arcs and oceanic trenches partly encircling the PacificBasin form the so-called Ring of Fire, a zone of frequent earthquakes andvolcanic eruptions


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1.3.4.2.2 Oceanic - Oceanic Convergence

The Marianas Trench (paralleling the Mariana Islands), for example, markswhere the fast - moving Pacific Plate converges against the slower movingPhilippine Plate. The Challenger Deep, at the southern end of the MarianasTrench, plunges deeper into the Earth's interior (nearly 11,000 m) than MountEverest, the world's tallest mountain, rises above sea level (about 8,854 m).

Figure 1.33 Oceanic Oceanic Convergence

Subduction processes in oceanic-oceanic plate convergence also result in theformation of volcanoes. Over millions of years, the erupted lava and volcanicdebris pile up on the ocean floor until a submarine volcano rises above sea levelto form an island volcano. Such volcanoes are typically strung out in chainscalled island arcs.


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1.3.4.2.3 Continental - Continental Convergence

The Himalayan mountain range dramatically demonstrates one of the mostvisible and spectacular consequences of plate tectonics. When two continentsmeet head-on, neither is subducted because the continental rocks are relativelylight and, like two colliding icebergs, resist downward motion.

Figure 1.34 Continental Continental Convergence

1.3.4.3 Transform Boundaries

The zone between two plates sliding horizontally past one another is called atransform-fault boundary, or simply a transform boundary.Most transform faults

are found on the ocean floor. They commonly offset the active spreading ridges,producing zig - zag plate margins, and are generally defined by shallowearthquakes. However, a few occur on land, for example the San Andreas faultzone in California. This transform fault connects the East Pacific Rise, adivergent boundary to the south, with the South Gorda - Juan de Fuca - ExplorerRidge, another divergent boundary to the north.


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Figure 1.35 Aerial view of the San Andreas Fault slicing through the CarrizoPlain in the Temblor Range east of the city of San Luis Obispo. (Photograph byRobert E. Wallace, USGS)

Figure 1.36 The Blanco, Mendocino, Murray, and Molokai fracture zones aresome of the many fracture zones (transform faults) that scar the ocean floor andoffset ridges. The San Andreas is one of the few transform faults exposed onland.


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1.3.4.4 Plate - Boundary Zones

Not all plate boundaries are as simple as the main types discussed above. Insome regions, the boundaries are not well defined because the plate-movementdeformation occurring there extends over a broad belt (called a plate-boundaryzone). Because plate-boundary zones involve at least two large plates and oneor more microplates caught up between them they tend to have complicatedgeological structures and earthquake patterns.

SUMMARY

1. The major structural units of the Earth, based on composition are (a) crust, (b)mantle and (c) core.

2. The internal layers of Earth based on physical properties are lithosphere,asthenosphere, mesosphere and core.

3. The two major topographic features of Earth are (a) the continents and (b)ocean basins.

4. Geologic process that change the Earth's structure are gradation, tectonism

and volcanism.

ASSIGNMENT

Importance of geology and it's relation to civil engineering works.

During the early nineteenth century, civil engineers were also geologists..,Assignment not more than 300 words.

What isa mohorovicic discontinuities?

Do you know that Himalayan mountains were formedas a result of the collisionand convergence of the Indian and Eurasian plates?


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REVIEW QUESTIONS

1. Draw a diagram of the internal structure of Earth and briefly describe the core,mantle, asthenosphere and lithosphere.

2. What are the major differences between continents and the ocean basins?

True (T) /False (F) Questions1. Continents and ocean basins do not differ markedly in rock type, density or

chemical composition. [ ]

2. The difference in elevation of continents and ocean basins represents afundamental difference in rock density. Continental rocks are less dense thanthe rocks of the ocean basins. [ ]

3. The equatorial radius is shorter by 21 km than the polar radius. [ ]

4. The three broad categories of rocks are formed at the lithosphere. [ ]

5. The core is the Earth's rigid outer layer. [ ]

6. The continental crust has a granitic layer underneath a basaltic layer. [ ]

7.Earth is the largest of four planets of inner solar group and the third closest tothe sun. [ ]

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Chapter 1 - Introduction to Geology.pdf

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