Chapter 1 - Introduction To Geology 622j60

ENGINEERING GEOLOGY (BFC 21303) Aziman Madun North Tower FKAAS Level 6

Hp:013-7657034 Department of Geotechnical and Transportation Engineering Faculty of Civil and Environmental Engineering Tun Hussein Onn University Of Malaysia

Syllabus: 1. INTRODUCTION TO GEOLOGY 2. MINERALS 3. THE STUDY OF ROCKS – IGNEOUS, SEDIMENTARY AND METAMORPHIC

4. WEATHERING 5. GEOLOGIC AGENTS 6. GEOLOGICAL STRUCTURES 7. GEOLOGICAL SITE INVESTIGATION & GEOPHYSICS 8. ROCK TESTING 9. ROCK MASS

CHAPTER 1 INTRODUCTION TO GEOLOGY • Geology – study of planet earth, its origin, history, composition, structure and dynamics of how it changes. • The term ‘Geology’ has been derived from the Greek words Geo + logos where Geo means Earth and logos means discourse (study or science). • Geology – event formed during geological time involving interpretation and observation of event occurred and still occurring at present in earth. • Geological process occurred during very large span of geological time left their record in rocks.

• Engineering geology is a subfield of geological study concerning about the geological inputs and the uses of the information to solve the engineering problems. • It exists solely to serve art and science of engineering through description of the structure and attributes of rocks connected with engineering works. (Goodman,1993).

• The geologist presents the geological data and interpretations for use by the civil engineers. • The role of engineering geology begins during the planning stage, where the geologic data are required in order to achieve accuracy.

Some engineering works that need understandings/related to geological aspect: • Construction of dam • Landslide - Slope

• Rock as aggregates & construction material • Hydro geological • Etc s.

THE UNIVERSE AND SOLAR SYSTEM • Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. • It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. • The Sun is the richest source of electromagnetic 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 around the Sun.

The Terrestrial 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 a compact, rocky surface like the Earth's. •The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The Jovian Planets:-

• Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's.

• Satellites or Moons: are those celestial bodies, each of which are revolving around any of these nine planets. • Asteroids are the minor planets generally situated between orbits of Mars and Jupiter. • Comets are the heavenly bodies having along tail pointing approximately away from the sun and a brighter head section (coma) that contains a small bright nucleus.

• Meteors are smaller solid bodies moving through the space, and getting illuminated while entering earth’s atmosphere.

There are 17 bodies in the solar system whose radius is greater than 1000 km.

The composite above shows the Sun and the 5 largest planets at a scale of 3200 km/pixel. (Earth is the tiny spot between Jupiter and the Sun).

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

The origin of the Earth • Based on observational facts cosmologist have developed classes of hypothesis which try to explain the origin of the earth. One of them are: • The Big Bang Theory 13.7 billion years ago, the entirety of our universe was compressed into the confines of an atomic nucleus known as a singularity, this is the moment before creation when space and time did not exist. According to the prevailing cosmological models that explain our universe, an ineffable explosion, trillions of degrees in temperature on any measurement scale, that was infinitely dense, created not only fundamental subatomic particles and thus matter and energy but space and time itself. Cosmology theorists combined with the observations of their astronomy colleagues have been able to reconstruct the primordial chronology of events known as the big bang.

CREATION OF A COSMOLOGY:

The Big Bang Theory

Earth • The largest of four planets of inner group solar system i.e. Mercury, Venus, Earth and 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

Principal Division of Earth • Consist of three:1) Atmosphere – Troposphere and Stratosphere 2) Hydrosphere

3) Lithosphere

(1) Atmosphere: • Gaseous portion of the Earth extending upwards for hundreds of miles above sea level.

• It is a mixture of 78% nitrogen, 21% oxygen, carbon dioxide, water vapor and minor amount of other gases. The atmosphere is divided into two parts which is Troposphere and Stratosphere. • Troposphere - the closest to the Earth ~ 13 km. It contains almost all of the water vapor, clouds and storms. • Stratosphere - the overlying layer ~ 55 km above the surface, contains the ozone layer. • The atmosphere is an important geologic agent and is responsible for the processes of weathering which are continually at work on the Earth's surface.

(2) Hydrosphere: • Total mass of water or the surface of our planet (about 98% of water in the oceans and 2% in lakes, rivers as well as ground water which exist 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 of geologic important. • Hydrosphere is in constant motion - evaporating through atmosphere, precipitating as rain and returning to Earth.

• As water moves over the Earth's surface it erodes, transports and deposits weathered rock material, constantly modifying the Earth's landscape.

(3) Lithosphere: • Lithos means rock.

• The solid portion of the Earth composed of crust and upper mantle. • It is a layer of rocks about 70 km thick, that rests upon soft weak material that broken into about 12 major plates which is slowly moved by the flow of material in a layer that directly underlies the lithosphere called the asthenosphere. • There are three basic types of rock: Igneous, Sedimentary and Metamorphic.

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.

Major Structural Units of Earth • The constituents of Earth are separated and segregated into layers according to density. • The denser materials - concentrated near the center, the less dense - near the surface.

• The internal layers are recognized on the basis of composition and physical properties.

• The internal layers based of composition are:1. Crust – Continental Crust and Oceanic Crust 2. Mantle – Earth mass, which compose of composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000°C, the mantle is solid but can deform slowly in a plastic manner. 3. Core (inner & outer) - composed mostly of iron (Fe) and is so hot that cause the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid.

• The internal layers based of are physical properties are:1. Lithosphere 2. Asthenosphere 3. Mesosphere 4. Core

Layers of the earth based on composition

Outer Crust

Mantle Outer core Inner core

Outer layer of the Earth, extending from solid surface down to the first major discontinuity in seismic wave velocity in the lithosphere. Thickness of crust varies from 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 of rock they contained where each with its own general composition, thickness and density. (a) Continent Crust: 35 - 60 km thick relatively low density granitic rock average density: 2.8 g/cm3 (b) Oceanic Crust : thickness rarely exceed 5 km denser material basaltic composition average density: 2.9 g/cm3 The next major compositional layer of the Earth which covers the core and this zone constitute 82% of its volume and 68% of mass of the Earth. The mantle is composed of iron and magnesium silicate rock, and it goes down to about 2900 km from surface of Earth. The earth's mantle is a heavy layer of rock which lies under the earth's crust. In places where the crust is broken, part of the upper mantle can melt and form a substance called magma, or molten rock. Magma can be forced through the earths crust. It flows out of volcanoes as lava. Average density: 4.5 g/cm3 It is speculated that the thickness is about 2250 km and it is made of molten iron and nickel. The outer core is so hot that the metals in it are all in the liquid state. Average density: 10.7 g/cm3 The thickness' is about 1300 km and probably consists of mostly iron and nickel. The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!! Average density: 17.0 g/cm3

Internal layers of the Earth based on Physical Properties Lithosphere (rock sphere)

The top of the asthenosphere is about 100 km below the surface. Above the asthenosphere, the material is solid, strong and rigid. This layer is called lithosphere. Contains the continental crust of the uppermost part of the mantle.

Asthenosphere (weak sphere)

A major zone within the upper mantle where temperature and pressure are just the right balance so that part of the material melts. The rocks lose much of their strength and become soft plastic and easily deformed. The thickness is about 200 km.

Mesosphere

The rock below the asthenosphere is stronger and more rigid than the asthenosphere because the high pressure at this depth offsets the effect of high temperature. The region between the asthenosphere and the core-mantle boundary is called the mesosphere.

Core

The core of the Earth marks a change in both physical properties and composition. It is composed mostly of iron and is therefore distinctly different from the silicate (rocky) material above. On the basis of physical properties, the core has two distinct parts - a solid inner core and liquid outer core. Heat loss from the core and the rotation of the Earth probably causes the liquid outer core to circulate and generate the Earth's magnetic field.

Composition of earth

Composition of earth

The internal structure of Earth

Model for Structure of Earth Here is a simplified Drawing

•The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):0 - 40 Crust 40 - 400 Upper mantle 400 - 650 Transition region 650 - 2700 Lower mantle 2700 - 2890 D'' layer 2890 - 5150 Outer core 5150 - 6378 Inner core

The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates.

Because of this, the continents ride on the denser oceanic plates, the crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphalt-like consistency called the Asthenosphere.

The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.

•The mantle is approximately 2900 kilometers thick, making it Earth's largest layer. •The mantle has a property called "plasticity" (where a solid has the ability to flow like a liquid). You might call the mantle "partially molten". • that the temperature of the mantle increases the deeper you go. •This difference in temperature causes CONVECTION CURRENTS to form. This type of current forms when hot things rise and cooler things sink. •These convection currents tumble throughout the mantle. •They cause the Lithosphere plates floating on the mantle to move around. •These currents cause our continents and oceans to change location slightly each year. •The currents are the driving force for Plate Tectonics or Continental Drift. The forces which drive continental drift seem to come from the mantle. •The hot rock, which boils up at mid-ocean ridges, comes from the upper mantle. •This rock spreads 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 through the mantle to form islands such as Hawaii or Iceland.

Convection Currents

Convection Currents - Large convection systems in the mantle may carry along the plates of the lithosphere like a conveyor belt

Outer Core & Inner core

• Inner Core – The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid.

– The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. – The temperatures may reach 9000 degrees F and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!

Geologic Processes that Change the Earth's Structure •

Geologic Forces: Earth has undergone great changes over million of years. Generally this come form the 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 building up of rock strata.

(b) Tectonism • Plate tectonics - a dynamic process of the lithosphere plate which moves over a weak plastic layer in the upper mantle. • These plates interact with one another along their boundaries. • Produce faulting (fracture and displacement), folding, subsidence and uplift of rock formation. • Responsible for formation of mountain ranges.

Earth’s lithosphere is composed of seven large plates with thickness ranging from 75 to 125 km.

Pacific Plate

Eurasian Plate

Antarctic Plate

North America Plate

Indian Plate

South American Plate

African Plate

20 other small plates in between

Major Plates of the Lithosphere

Major Plates of the Lithosphere

(c) Volcanism • Volcano - a vent in the earth's crust through which molten rock materials within the earth, lavas, ashes, steam and gas are ejected. • Responsible for the formation of plutonic rocks, once solidified at great depth. • Majority of volcanoes are located along the margins of tectonic plates.

Geologic Time Scale • The Earth's crust is known to be at least 40 million centuries old. • The time span of the earth is called eras and subdivided into periods (see Table 1.2). • Rocks have been created and destroy throughout geologic time. • Rocks which are created during that particular period for example Cambrian are said to belong to the Cambrian system. • The nature of rocks created or formed during various eras can actually reveal about its strength and condition. • For example rocks from the Precambrian era are known to be very hard, crystalline materials but often with many fractures and microstructures, whereas sandstone formed from Pliocene series tends to be porous as soil and easily excavated without blasting.

GEOLOGIC TIME Era

Period

Cenozoic

Holocene Pleistocene

0.012 2

Tertiary

Pliocene Miocene Oligocene Eocene Paleocene

7 26 38 54 65

Mesozoic

Cretaceous Jurassic Triassic

135 195 225

Upper Paleozoic

Permian Carboniferous Devonian

280 345 395

Lower Paleozoic

Silurian Ordovician Cambrian

440 500 570

Proterozoic and Archaeozoic

Absolute (million years)

4600

Tectonic Plate • What is a tectonic plate?

• A tectonic plate (also called lithosphere plate) - a massive, irregularly shaped slab of solid rock, • Generally composed of both continental and oceanic lithosphere. • Plate size can vary greatly - from a few hundred to thousands of kilometers across. (e.g. The Pacific and Antarctic Plates are among the largest) • Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere.

• This massive slabs of solid rock can float despite their tremendous weight because Continental crust is composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar. • By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier.

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

The theory of tectonics plate • The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another.

• The present is the key to the past, the geologic forces and processes - gradual as well as catastrophic - acting on the Earth today are the same as those that have acted in the geologic past. • Continental Drift - introduced by a German meteorologist named Alfred 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, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. • Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today.

According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million years ago, eventually fragmenting into the continents as we know them today

PERMIAN

TRIASSIC

225 MILLION YEARS AGO

200 MILLION YEARS AGO

JURASSIC

CRETACEOUS

135 MILLION YEARS AGO

65 MILLION YEARS AGO

PRESENT DAY

Major plates of the lithosphere are broken into a dozen or so rigid slabs that are moving relative to one another

Major plates of the lithosphere

Cont’d

• Wegener's theory - based in part on remarkable fit of the South American and African continents. • For example, the matching animal fossils found on coastlines of South America and Africa, and the evidence of dramatic climate changes on some continents. • For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. • Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in nowpolar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa.

These two maps showing the American and African continents may once have fit together, then later separated. Left: The formerly ed continents before their separation Right: The continents after the separation

As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are reed

Developing the theory of continental drift Four major scientific developments spurred the formulation of the plate tectonics theory: 1. The ruggedness and youth of the ocean floor 2. Repeated reversals of the Earth magnetic field in the geologic past

3. Developing of the seafloor-spreading and associated recycling of oceanic crust 4. The world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges

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 than originally thought.

• Scientists had previously believed that the oceans have existed for at least 4 billion years, so therefore the sediment layer should have been very thick. • Why then was there so little accumulation of sedimentary rock and debris on the ocean floor? • The answer to this question, which came after further exploration, would prove to be vital to advancing the concept of plate tectonics.

Repeated reversals of the Earth magnetic field in the geologic past • This finding, though unexpected, was not entirely surprising because it was known that basalt - the iron-rich, volcanic rock making up the ocean floor - contains a strongly magnetic mineral (magnetite) and can locally distort com readings.

• 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 present magnetic field. • This would result in the north end of the rock's "com needle" pointing toward magnetic north.

Cont’d

• The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth's present magnetic field.

• In this case, the north end of the rock's com needle would point south. • How could this be? This answer lies in the magnetite in volcanic rock. • Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. • When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.

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

Seafloor spreading and recycling of oceanic crust •

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

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

 The youngest rocks at the ridge crest always have presentday (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 has flip-flopped many times.

 When the ages of the samples were determined by

paleontologists and isotopic dating studies, they provided the clinching evidence that proved the seafloor spreading hypothesis.

Concentration of earthquakes and volcano activity • During the 20th century, improvements in seismic instrumentation and greater use of earthquakerecording instruments (seismographs) worldwide enabled scientists 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 prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones.

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

Understanding Plate Motions • There are four types of plate boundaries:

(1) Divergent boundaries (2) Convergent boundaries: Oceanic - continental convergence  Oceanic - oceanic convergence  Continental - continental convergence (3) Transform boundaries (4) Plate boundary zones

(1) Divergent boundaries  The new crust is generated as the plates pull away from each other.

(2) Convergent boundaries  The crust is destroyed as one plate dives under another. • • •

Oceanic-continental convergence Oceanic-oceanic convergence Continental - continental convergence

(3) Transform boundaries The crust is neither produced nor destroyed as the plates slide horizontally past each other.

(4) Plate boundary zones

Broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.

An illustrating the main types of plate boundaries; East African Rift Zone is a good example of a continental rift zone

Divergent boundaries • Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. • The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years (between South America and Africa continents). • In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea. • The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden.

The Mid-Atlantic Ridge, which splits nearly the entire Atlantic Ocean north to south, is probably the best -known and most-studied example of a divergent-plate boundary

Map of East Africa showing some of the historically active volcanoes (red triangles) and the A far Triangle (shaded, center) -- a so-called triple junction (or triple point), where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone

Convergent boundaries • The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created. • Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. • The location where sinking of a plate occurs is called a subduction zone. • Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.

Subduction zone and Mid Ocean Ridges

Oceanic - continental convergence • Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest.

• The eruptive activity is clearly associated with subduction.

Oceanic – continental convergence

Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the socalled Ring of Fire, a zone of frequent earthquakes and volcanic eruptions

Oceanic - oceanic convergence

Oceanic – oceanic convergence •

The Marianas Trench (paralleling the Mariana Islands), the best example, marks where the fastmoving Pacific Plate converges against the slower moving Philippine Plate.

•

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes.

•

Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano.

•

Such volcanoes are typically strung out in chains called island arcs.

Continental - continental convergence

Continental – continental convergence

• The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. • When two continents meet head-on, neither is subducted because the continental rocks are relatively light.

Transform boundaries • Occur at the zone between two plates sliding horizontally past one another. • Was called a transform-fault boundary, or simply a transform boundary. • Most transform faults are found on the ocean floor. • However, a few occur on land, for example the San Andreas fault zone in California. • They commonly offset the active spreading ridges, producing zig - zag plate margins, and are generally defined by shallow earthquakes.

Transform fault ting segments of the Mid-Atlantic Ridge

Plate-boundary zones • Not all plate boundaries are as simple as the main types discussed above. • In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt (called a plate-boundary zone). • Because plate-boundary zones involve at least two large plates and one or more micro plates caught up between them they tend to have complicated geological structures and earthquake patterns.