ieq-05 plate tectonics notes

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1 Course Title IEQ-05 Earthquake Geology and Geoinformatics by Dr. J. D. Das (Dept. of Earthquake Engineering, IIT Roorkee) Plate Tectonics Brief History of Plate Tectonics Tectonics = study of the deformation of the earth’s surface and plate motions, especially as applied to mountain building. Ideas about and Evidence for Tectonics The ancient Greeks (~200 BC) realized that shells found high up in mountains were actually deposited in old oceans. Leonardo da Vinci (~1500) came to much the same conclusion when he found some fossil sea shells high in the mountains and reasoned that, since the shell layers were discontinuous, the shell layers must have been pushed into mountains rather than the seas being as high as the mountains. James Hutton (~1750) incorporated the idea of uplift and erosion into the rock cycle and his ideas of uniformitarianism (The present is the key to the past). Darwin on the voyage of the H.M.S. Beagle (~1850), realized that tectonic forces, especially earthquakes, raised mountains while erosion lowered them. By the mid 1800’s it was obvious that vertical movements of the earth took place. What remained to be answered is what powered the tectonic forces or what caused the upheaval of the mountains? One of the earliest pieces of evidence that large scale horizontal movement of continents took place was the fit of the continents, especially Africa and South America. Because there was no mechanism explaining how this could happen the idea that continents moved over the earth’s surface was not taken seriously. In 1910 American geologist Frank Taylor, based on the alignment of mountain ranges, proposed that the continents had at one time been connected.

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Page 1: IEQ-05 Plate Tectonics Notes

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Course Title IEQ-05 Earthquake Geology and Geoinformatics

by Dr. J. D. Das

(Dept. of Earthquake Engineering, IIT Roorkee) Plate Tectonics Brief History of Plate Tectonics Tectonics = study of the deformation of the earth’s surface and plate motions, especially as applied to mountain building. Ideas about and Evidence for Tectonics The ancient Greeks (~200 BC) realized that shells found high up in mountains were actually deposited in old oceans. Leonardo da Vinci (~1500) came to much the same conclusion when he found some fossil sea shells high in the mountains and reasoned that, since the shell layers were discontinuous, the shell layers must have been pushed into mountains rather than the seas being as high as the mountains. James Hutton (~1750) incorporated the idea of uplift and erosion into the rock cycle and his ideas of uniformitarianism (The present is the key to the past). Darwin on the voyage of the H.M.S. Beagle (~1850), realized that tectonic forces, especially earthquakes, raised mountains while erosion lowered them. By the mid 1800’s it was obvious that vertical movements of the earth took place. What remained to be answered is what powered the tectonic forces or what caused the upheaval of the mountains? One of the earliest pieces of evidence that large scale horizontal movement of continents took place was the fit of the continents, especially Africa and South America. Because there was no mechanism explaining how this could happen the idea that continents moved over the earth’s surface was not taken seriously. In 1910 American geologist Frank Taylor, based on the alignment of mountain ranges, proposed that the continents had at one time been connected.

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The person who is usually given credit for starting the modern theory of plate tectonics is German meteorologist Alfred Wegener. In 1915 Wegener published a book suggesting that all the continents had been connected in a single large land mass he called Pangaea. Wegener’s proposed continent helped to explain the distribution of certain plants, animals, and fossils. However, the hypothesis was not accepted by European and American geologists, largely because it lacked a mechanism for drifting the continents. Southern hemisphere geologists who were closer to the evidence accepted Wegener’s hypothesis more readily

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 rejoined.

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Present Plate Tectonic set up of the Earth and plate velocities. Wegener's continental drift hypothesis

• Evidence used by Wegener • Fit of South America and Africa • Fossils match across the seas • Rock types and structures match • Ancient climates

• Main objection to Wegener's proposal was its inability to provide a mechanism

New Evidence for Continental Drift and the Development of Plate Tectonics Through much of the 1950’s and early 1960’s northern hemisphere geologists continued to resist the idea of plate tectonics because of the lack of a mechanism. Starting in the mid-1960’s new evidence from the fields of paleomagnetism and seismology was developed that eventually convinced most geologists that continents moved. Paleomagnetism Evidence for the plate tectonics model

• Paleomagnetism • Probably the most persuasive evidence • Ancient magnetism preserved in rocks • Paleomagnetic records show

• Polar wandering (evidence that continents moved) • Earth's magnetic field reversals

• Recorded in rocks as they form at oceanic ridges The following types of paleomagnetic information is preserved in rocks: 1. Declination – direction that the compass needle points. In the northern hemisphere the needle points toward the north pole. This is an important feature for apparent polar wandering. 2. Inclination – dip of the compass needle that is related to latitude 3. Polarity – N vs. S seeking More Evidence for Plate tectonics Evidence for the plate tectonics model

• Earthquake patterns • Associated with plate boundaries • Deep-focus earthquakes along trenches provide a method for

tracking the plate's descent • Ocean drilling

• Deep Sea Drilling Project (ship: Glomar Challenger) confirms age of oceanic crust

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Hot Spots Evidence for the plate tectonics model

• Hot spots • Rising plumes of mantle material • Volcanoes can form over them

• e.g., Hawaiian Island chain • Chains of volcanoes mark plate movement

Inside the Earth

Our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Himalayas, however, the base of the crust can be as deep as 100 km.

Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. The Earth's core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field.

Averaging at least 80 km in thickness over much of the Earth, the lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Scientists believe that below the lithosphere is a relatively narrow, mobile zone in the

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mantle called the asthenosphere (from asthenes, Greek for weak). This zone is composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere.

Layering of the Earth’s interior Layered structure:

Crust

• Continental (about 35 km thick; 60 km in mountain ranges) Granitic composition (Si, Al, Fe, Mg, Ca, Na, K, H, Rb, Sr, Ba, U, Th, Ni)

• Oceanic (about 5 km thick) Basaltic composition (above elements but with higher percentage of heavier elements)

The crust is divided into continental plates which drift slowly (only a few centimeters each year) atop the less rigid mantle. The crust is thinner under the oceans (6-11 km thick); this is where new crust is formed. Continental crust is about 25-90 km thick. The lithosphere is defined as the crust and the upper mantle, a rigid layer about 100-200 km thick. The Mohorovicic discontinuity is the separation between the crust and the upper mantle. Mantle (2885 km thick) Composition: peridotite (Mg Fe silicates), kimberlite (diamonds), eclogite Solid that flows (rheid); plastic behavior Mantle: Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle. The mantle is about 1,700 miles (2,750 km) thick. The mantle gets warmer with depth; the top of the mantle is about 1,600° F (870° C); towards the bottom of the mantle, the temperature is about 4,000-6,700° F (2,200-3,700° C). The mantle contains most of the mass of the Earth. The Gutenberg discontinuity separates the outer core and the mantle. Outer core (2270 km thick) Molten Fe with some Ni Inner core (1216 km radius) Solid Fe with some Ni

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Core: The Earth has a iron-nickel core that is about 2,100 miles in radius. The inner core may have a temperature up to about 13,000°F (7,200°C = 7,500 K), which is hotter than the surface of the Sun. The inner core (which has a radius of about 750 miles (1,228 km) is solid. The outer core is in a liquid state and is about 1,400 miles (2,260 km) thick.

How do we know what the Earth's Interior is like?

1. Volcanic activity Materials are brought up from below. Xenoliths = foreign rock (pieces of the mantle in lava) example: coarse-grained olivine (peridotite) xenoliths in basaltic lava Only useful to depth of about 200 km

2. High pressure laboratory experiments 3. Samples of the solar system (meteorites) 4. Study of seismic waves generated by earthquakes and nuclear explosions

(More below)

Probing the Earth's Interior with Seismic Waves

P and S wave travel times depend on properties of rock materials that they pass through. Search for differences in travel times which will correspond to differences in rock properties.

Major layers of the Earth were detected before 1950. Fine details were delineated in 1960's during nuclear testing.

Wave velocity depends on density and elasticity of rock. Seismic waves travel faster in denser rock. Speed of seismic waves increases with depth (pressure and density increase downward).

Isostasy

Isostasy is a term used in Geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. It is invoked to explain how different topographic heights can exist at the Earth's surface. When a certain area of lithosphere reaches the state of isostasy, it is said to be in isostatic equilibrium. It is important to note that isostasy is not a process that upsets equilibrium, but rather one which restores it. It is generally accepted that the earth is a dynamic system that responds to loads in many different ways, however isostasy provides an important 'view' of the processes that are actually happening. Nevertheless, certain areas (such as the Himalayas) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalayas, by proposing that their elevation is being "propped-up" by the force of the impacting Indian plate).

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In the simplest example, isostasy is the principle of Buoyancy observed by Archimedes in his bath, where he saw that when an object was immersed, an amount of water equal in volume to that of the object was displaced. On a geological scale, isostasy can be observed where the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over geological time flows laterally such that the load of the lithosphere is accommodated by height adjustments.

Isostatic models Three principal models of isostasy are used:

• The Airy-Heiskanen Model

- where different topographic heights are accommodated by changes in crustal thickness.

• The Pratt-Hayford Model

- where different topographic heights are accommodated by lateral changes in rock density.

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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 oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; 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 (for example, the interior parts of North and South America).

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 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. The variations in plate thickness are nature's way of partly compensating for the imbalance in the weight and density of the two types of crust. Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations. Tectonic plates probably developed very early in the Earth's 4.6-billion-year history, and they have been drifting about on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating.

Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continental plate, and eventually disappear completely.

What drives the plates?

Models for the driving forces The rates of plate motion through time have not been constant. What forces cause the plates to move, and why does the rate vary through time? It can be described through

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two principal categories of model for the driving forces of plate tectonics: the mantle plume model, and models based upon force balance calculations. The mantle plume model holds that an important sources of convective heat transfer from the lower to the upper mantle is plumes, which are rising columns of hot material a few hundred kilometers in diameter that spread out into the asthenosphere like a thunderhead beneath the lithospheric plates. The complementary return flow would involve a uniform sinking of the entire mantle below the asthenosphere, in addition to the more localized downflow associated with subduction zones. The lateral spreading of material in the asthenosphere away from a plume produces a radial shear stress on the bottom of the overlying lithosphere. If a number of plumes are aligned, then the dominant asthenospheric flow would be laterally away from the line of plumes, and the shear stress would act to pull the lithosphere apart, creating a spreading center along a line of plumes. Plume model does not consider all the possible forces that can act on a lithospheric plate.

From seismic and other geophysical evidence and laboratory experiments, scientists generally agree that the plate-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. The circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body. Below the lithospheric plates, at some depth the mantle is partially molten and can flow, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth's interior over millions of years.

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Conceptual drawing of assumed convection cells in the mantle (see text). Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form.

Sketch showing convection cells commonly seen in boiling water or soup. This analogy, however, does not take into account the huge differences in the size and the flow rates of these cells.

The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow.

Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat. Radioactive decay, a spontaneous process that is the basis of "isotopic clocks" used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The radioactive decay of naturally occurring chemical elements -- most notably uranium, thorium, and potassium -- releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth -- 4.6 billion years ago -- by the "falling together" and compression of cosmic debris. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery.

Force-Balance Models To develop a more complete understanding of the driving forces of plate tectonics, it is necessary to account for all the possible forces on a plate. Models incorporating these factors are called force-balance models.

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Diagram of plate boundaries with forces used in force-balance model.

The ridge-push force (FRP) represents a push from a divergent plate margin. It

originates from the topographic slope of the ocean bottom created by isostatic uplift at a spreading center. At a given depth in the mantle above the level of isostatic compensation, the pressure P1 at a depth below point 1 close to the spreading center is higher than the pressure P2 at a point below point 2 farther away, because the weight of the overlying column of rock and water is larger. The resulting horizontal pressure gradient provides a force that tends to drive the lithospheric plates apart at the spreading center. This force therefore depends on the average topographic slope of the ridge flank.

Cross section of a ridge-push force.

The mantle-drag force (FDF) is the shear force exerted on the base of the plate by the relative motion of the underlying mantle. It reflects the viscous coupling between the asthenosphere and the lithosphere. This force could either drive the plate forward or resist its movement, depending on the direction of the mantle’s relative velocity with respect to the lithosphere. In the plume model, The slab-pull force (FSP) is caused by the tendency of the colder and denser lithosphere to sink into the underlying mantle. This force should be greater for older

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lithosphere than the younger, because older lithosphere is colder and therefore more dense. As the slab sinks, it tends to pull the surficial part of the plate behind it. Of course, the stress in the slab is not an absolute tensile stress, but the minimum principal stress is less than the lithospheric pressure and is oriented parallel to the slab.

Cross section of slabs illustrating dominant first-motion solutions for earthquakes in subducted slabs. Open circles indicate down-dip contraction. Closed circles indicate down-dip extension. Opposing arrows indicate thrust fault solution. A. Slabs extending to shallow depths show down-dip extension. B. Slabs extending to intermediate depths show down-dip extension in the shallower part of the slab, and down-dip contraction in the deeper part. C. Slabs extending to great depths are dominated by down-dip contraction. D. Segment slabs show down-dip extension in the shallow part and down-dip contraction in the deep part. The slab-drag force (FSD) arises from the resistance of the mantle to the slab as it sinks. This force should be less in the asthenosphere, where the mantle viscosity is relatively low, than below the asthenosphere, where the viscosity increases. First motions showing down-dip contraction are characteristic of many deep earthquakes and of earthquakes through-out slabs that are continuous to depths of 600 to 700 km. The transform-resistance force (FTR) is a resistance to strike-slip motion along a transform fault. The presence of seismic activity along transform faults is evidence of friction on the fault that resists the fault motion. At deeper levels, the resistance is associated with ductile shearing. The subduction-resistance force (FSR) results from the shearing between an overriding plate and a downgoing plate. This force gives rise to the many shallow earthquakes that occur along subducting plate margins. The magnitude of the force presumably depends on the effective coefficient of friction between the plates. The trench-suction force (FSU) tends to draw the overriding plate toward the trench. Its importance may be indicated by the observations that all circum-Pacific trenches are moving towards the ocean basin, and the Pacific basin is getting smaller. The implication of this observation is that, above the subducting slab, there is a flow of the mantle and lithosphere toward the trench. Such a flow could be set up as a result of the entrainment of the mantle by the downward motion of the subducted slab, which would cause a corner flow in the overlying mantle wedge.

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Present Day Plate tectonics Evidence for the plate tectonics model

Earthquake patterns • Associated with plate boundaries • Deep-focus earthquakes along trenches provide a method for

tracking the plate's descent Ocean drilling

• Deep Sea Drilling Project (ship: Glomar Challenger) confirms age of oceanic crust

Measuring plate motion

By using hot spot “tracks” like those of the Hawaiian Island - Emperor Seamount chain

Using space-age technology to directly measure the relative motion of plates

• Very Long Baseline Interferometry (VLBI) • Global Positioning System (GPS)

Plate tectonics: the new paradigm Plate boundaries

• Types of plate boundaries • Divergent or Constructive • Convergent or Destructive • Shear or Transform

Divergent Boundaries Plate boundaries

• Types of plate boundaries • Divergent plate boundaries (constructive margins)

• Two plates move apart • Mantle material upwells to create new seafloor • Ocean ridges and seafloor spreading

• Oceanic ridges develop along well-developed boundaries

• Along ridges, seafloor spreading creates new seafloor

Present active divergent boundary on continental part exists in the eastern part of the Africa.

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Convergent (or destructive), Divergent (or constructive) and Transform or Shear type plate boundaries. At divergent boundaries new crust is created as the plates pull away from each other. Oceans are born and grow wider where plates diverge or pull apart. As seen below, when a diverging boundary occurs on land a 'rift', or separation will arise and over time that mass of land will break apart into distinct land masses and the surrounding water will fill the space between them. Iceland offers scientists a natural laboratory for studying - on land - the processes that occur along submerged parts of a divergent boundary. Iceland is splitting along the Mid-Atlantic Ridge - a divergent boundary between the North American and Eurasian Plates. As North America moves westward and Eurasia eastward, new crust is created on both sides of the diverging boundary. While the creation of new crust adds mass to Iceland on both sides of the boundary, it also creates a rift along the boundary. Iceland will inevitably break apart into two separate land masses at some point in the future, as the Atlantic waters eventually rush in to fill the widening and deepening space between.

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Birth of an Island: On November 14, 1963, in the early morning, several miles off the southern coast of Iceland, fishermen noticed black smoke bubbling from the sea. By evening, a ridge of hardening lava was noticed just below the waves. And by the following morning, a tiny island had emerged (one square mile) above the surface. Rift Valley A new ocean basin is created when a tectonic plate carrying a continent literally splits apart. In this process the heat from underlying magma wells up from deep within the earth, weakening and stretching the overlying continental crust. The brittle crust then fractures on each side of the stressed area, allowing sections to drop. The result is a rugged terrestrial rift valley. This early stage of ocean building is evident in several parts of today’s world, including the Baikal region of southeastern Siberia known as the Basin, and the United States from western Utah to eastern California, an area known to geologists as the Range. But the most dramatic example of an emerging ocean basin in its infancy is the Great Rift Valley of East Africa, stretching between Ethiopia and Tanzania. As the continent of Africa breaks apart along a rift, a new plate (the Somali Plate) is taking shape. In time, the sea will invade the gap created by the separation, thus forming a new ocean basin. The Red Sea is a widening ocean basin located where the Arabian Peninsula was severed from Africa long ago by the pulling apart of the African Plate and Arabian Plate. Africa is literally coming apart at the seams. As a young ocean widens and matures, the undersea rift develops a ridge of lava mountains on the trailing edge of each plate. The Mid-Atlantic Ridge, for example, rises where the American continents are separating from Europe and Africa. Other Mid-Ocean Ridges include the East Pacific Rise, several hundred miles off the western coast of South America, and the Indian Ridge, off the Eastern coast of Africa, south of India. Each of these mid-ocean ridges first appeared as a terrestrial rift valley involved in the break-up of some ancient land mass. East African Rift Africa's Great Rift Valley is a 6,000-mile crack (fissure) in the earth's crust, stretching from Lebanon to Mozambique. One of its most dramatic sections slices through East Africa, dividing Kenya into two segments. Geologists know that the Rift Valley was formed by violent subterranean forces that tore apart the earth's crust. These forces caused huge chunks of the crust to sink between parallel fault lines and force up molten rock in volcanic eruptions. Evidence that this process, called rifting, is still in progress comes from the many active and semi-active volcanoes, located along the Rift. Evidence of volcanic activity along the rift is provided by the presence of numerous boiling hot springs. Widening of the rift is 3.7±0.9 mm/year, and that most of this widening is concentrated in the deepest, most youthful part of the rift zone (2.9±1 mm/year).

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East African rift system also showing triple plate junction

There has been considerable discussion on this over the years. Some have ascribed rifting to up-doming of the crust over a hot-spot; certainly parts of the E African rift system are very elevated, compared with other sectors, suggesting that the doming reflects an underlying hot low-density mantle plume. In other cases, geophysical models suggest the asthenospheric mantle is rising to high levels beneath the rift. However it is also apparent that rifting can take place without extensive uplift; in such cases it may be the convective processes in the underlying asthenosphere which are causing the extension. To rift a continent apart it needs the rifts associated with various possible thermal domes to link together.

As continents drift slowly over hotspots the hotspots weaken the plate - like a blowtorch impinging on the base - and these weakened zones become the sites of continental rifting. Transform or Shear Boundaries Plate boundaries

• Types of plate boundaries • Transform fault boundaries

• Plates slide past one another • No new crust is created • No crust is destroyed

• Transform faults • Most join two segments of a mid-ocean ridge • At the time of formation, they roughly parallel the

direction of plate movement • Aid the movement of oceanic crustal material

Plates on either side of a transform boundary slide past each other without either late being consumed and without a gap opening between the plates.

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Recent analysis of satellite altimeter data has allowed scientists to use slight variations in the elevation of the ocean surface to determine the topography of the seafloor

. Examination of oceanic ridges along the East Pacific Rise or Mid-Atlantic Ridge show offsets along transform boundaries.

Transform or Shear Boundaries

The San Andreas Fault is a transform boundary that separates the North American and Pacific Plates. The smaller Juan de Fuca plate lies between these two plates opposite Oregon, Washington, northern California, and part of British Columbia. The Pacific Plate moves northwest relative to the North American Plate. Los Angeles will migrate toward San Francisco over the next several million years.

San Andreas Fault

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Land on the west side of the San Andreas Fault, including Los Angeles and San Diego, is part of the Pacific Plate. San Francisco lies east of the fault and is on the North American Plate. Western California is being slowly displaced to the northwest relative to the rest of the state. It is not going to drop off into the ocean but it will eventually migrate along the western boundary of the North American Plate, eventually colliding with Alaska millions of years from now. Convergent Boundaries Plate boundaries

• Types of plate boundaries • Convergent plate boundaries (destructive margins)

• Plates collide, an ocean trench forms and lithosphere is subducted into the mantle

• There are 3 types of convergent boundaries: • 1) oceanic-continental, • 2) oceanic-oceanic, • 3) continental-continental

Convergent Boundaries – Oceanic-Continental Plate boundaries

• Types of plate boundaries • Convergent plate boundaries (destructive margins)

• Oceanic-continental convergence • Denser oceanic slab sinks into the asthenosphere • Pockets of magma develop and rise • Continental volcanic arcs form • Examples include the Andes, Cascades, and the

Sierra Nevadan system Convergent Boundaries Oceanic-Oceanic Plate boundaries

• Types of plate boundaries • Convergent plate boundaries (destructive margins)

• Oceanic-oceanic convergence • Two oceanic slabs converge and one descends

beneath the other • Often forms volcanoes on the ocean floor • Volcanic island arcs forms as volcanoes emerge

from the sea • Examples include the Aleutian, Mariana, and Tonga

islands

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Convergent Boundaries – Continental-Continental Plate boundaries

• Types of plate boundaries • Convergent plate boundaries (destructive margins)

• Continental-continental convergence • When subducting plates contain continental

material, two continents collide • Can produce new mountain ranges such as the

Himalayas The collision of India and Asia produced the Himalayas (before)

The collision of India and Asia produced the Himalayas (after)

The Himalayas: Two continents collide

Among the most dramatic and visible creations of plate-tectonic forces are the lofty Himalayas, which stretch 2,500 km (100 to 400 km wide) along the border between India and Tibet. This immense mountain range began to form between 40 and 50 million years ago, when two large landmasses, India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only

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be relieved by thrusting skyward, contorting the collision zone, and forming the jagged Himalayan peaks.

About 225 million years ago, India was a large island still situated off the Australian coast, and a vast ocean (called Tethys Sea) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. By studying the history -- and ultimately the closing-- of the Tethys, scientists have reconstructed India's northward journey. About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.

The 6,000-km-plus journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago (see text). India was once situated well south of the Equator, near the continent of Australia.

The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year -- a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher? Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up, and such stretching would result in some subsidence due to gravity. At present, the movement of India continues to put enormous pressure on the Asian continent, and Tibet in turn presses on the landmass to the north that is hemming it in. The net effect of plate-tectonics forces acting on this geologically complicated region is to squeeze parts of Asia eastward toward the Pacific Ocean. One serious consequence of these processes is a deadly "domino" effect: tremendous stresses build up within the

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Earth's crust, which are relieved periodically by earthquakes along the numerous faults that scar the landscape. Some of the world's most destructive earthquakes in history are related to continuing tectonic processes that began some 50 million years ago when the Indian and Eurasian continents first met.

Tsunamigenic Earthquake Typical interplate earthquakes occur at the seismogenic interface between subducting and overlaying plates. This is the type of most tsunamigenic earthquakes. Slab (intraplate) earthquake, if the location is within the subducting slab. This include deep earthquakes, although only those occurring less than about 100 km depth are tsunamigenic. Crustal earthquake, if it is in the overlaying crust, can be tsunamigenic if the source is beneath water.

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Plate tectonic set up of the Andaman-Sumatra region

Sumatra earthquake and its aftershock distribution

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Formation of a tsunami

Physics of the tsunami Waves are formed as the displaced water mass attempts to regain its equilibrium. And the size of the resultant tsunami waves is determined by the quantum of the deformation of the sea floor. More the vertical displacement, greater will be the size of the waves. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. They can be more aptly described as a series of waves of extremely long wavelength and long period generated in a body of water by an impulsive disturbance that displaces the water. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength (distance between two successive waves) of about 100 to 200 metres (300 to 600 ft).

Tsunamis can have a period in the range of ten minutes to two hours and a wavelength in excess of 500 km.

It is because of their long wavelengths that tsunamis behave as shallow-water waves.

Since tsunamis have a very large wavelength, in excess of 500 km, it will lose little energy as it propagates.

Hence in very deep water, a tsunami will travel at high speeds and travel great distances with limited energy loss.

For example, when the ocean is more than 5000 metres deep, unnoticed tsunami travel about 890 km per hour, the speed of a jet airplane.

As the tsunami crosses the deep ocean, its wavelength — distance from crest to crest — may be hundred kilometres or more and its amplitude — height from crest to trough — will be in the order of a few feet or less. They cannot be felt aboard ships nor can they be seen from the air in the open ocean.