IEQ-05 Plate Tectonics Notes

5/19/2018 IEQ-05 Plate Tectonics Notes

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Course Title

IEQ-05 Earthquake Geology and Geoinformaticsby

Dr. J. D. Das

(Dept. of Earthquake Engineering, IIT Roorkee)

Plate Tectonics

Brief History of Plate Tectonics

Tectonics= study of the deformation of the earths 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 wereactually deposited in old oceans.

Leonardo da Vinci(~1500) came to much the same conclusion when he found somefossil 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 ofuniformitarianism (The present is the key to the past).

Darwinon the voyage of the H.M.S. Beagle (~1850), realized that tectonic forces,

especially earthquakes, raised mountains while erosion lowered them.

By the mid 1800s 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 continentstook 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 continentsmoved over the earths 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.Wegeners 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 mechanismfor drifting the continents.Southern hemisphere geologists who were closer to the evidence accepted Wegeners

hypothesis more readily

The locations of certain fossil plants and animals on present-day, widely separatedcontinents 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 1950s and early 1960s northern hemisphere geologists continued

to resist the idea of plate tectonics because of the lack of a mechanism.Starting in the mid-1960s 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 ageof 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 layeredstructure 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 beneathcontinents 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. TheEarth'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 Earths 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 ofheavier 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 isabout 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), eclogiteSolid 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 andis part of the lithosphere(together with the crust). The lower mantle flows

slowly, at a rate of a few centimeters per year. The asthenosphereis 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 thebottom 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 discontinuityseparates 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 13,000F (7,200C = 7,500 K), which is hotter thanthe surface of theSun.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 activityMaterials 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 experiments3. 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 densityand elasticityof rock.Seismic waves travel faster in denser rock.

Speed of seismic waves increases with depth (pressure and density increase downward).

Isostasy

Isostasyis a term used in Geology to refer to the state of gravitational equilibriumbetween the earth's lithosphere andasthenosphere such that the tectonic plates "float" at

an elevation which depends on their thickness and density. It is invoked to explain howdifferent 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 nota process that upsets equilibrium, but rather one

which restores it. It is generally accepted that the earth is a dynamic system that respondsto 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 notin isostatic equilibrium, which has forced researchers to identify other reasons toexplain 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).
http://www.enchantedlearning.com/subjects/astronomy/sun/http://www.enchantedlearning.com/subjects/astronomy/sun/http://www.enchantedlearning.com/subjects/astronomy/sun/http://en.wikipedia.org/wiki/Asthenospherehttp://en.wikipedia.org/wiki/Asthenospherehttp://www.enchantedlearning.com/subjects/astronomy/sun/


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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 beobserved 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 incrustal

thickness.

The Pratt-Hayford Model

- where different topographic heights are accommodated by lateral changes in

rock density.
http://en.wikipedia.org/wiki/Crust_%28geology%29http://en.wikipedia.org/wiki/Crust_%28geology%29http://en.wikipedia.org/wiki/Crust_%28geology%29http://en.wikipedia.org/wiki/Crust_%28geology%29


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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 rocksare 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 causethe 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 likea 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 thebottom 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 mantlethat 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 (theparent) 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. Modelsincorporating 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 isostaticcompensation, the pressure P1at a depth below point 1 close to the spreading center is

higher than the pressure P2at a point below point 2 farther away, because the weight of

the overlying column of rock and water is larger. The resulting horizontal pressuregradient 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 bythe 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 resistits movement, depending on the direction of the mantles 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 denserlithosphere 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 isless than the lithospheric pressure and is oriented parallel to the slab.

Cross section of slabs illustrating dominant first-motion solutions for earthquakes insubducted 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 depthsshow 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-dipcontraction in the deep part.

The slab-drag force (FSD) arises from the resistance of the mantle to the slab as itsinks. 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 resistanceto strike-slip motion along a

transform fault. The presence of seismic activity along transform faults is evidence offriction 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 thetrench. Its importance may be indicated by the observations that all circum-Pacifictrenches 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 themantle 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 wouldcause 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-developedboundaries

Along ridges, seafloor spreading creates new

seafloorPresent 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, whena 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 onboth sides of the boundary, it also creates a rift along the boundary. Iceland willinevitably 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 thefollowing 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 thenfractures 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 todays world, includingthe 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 Africabreaks 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 fromAfrica 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-OceanRidges 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 ofthese mid-ocean ridges first appeared as a terrestrial rift valley involved in the break-upof 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 twosegments.Geologists know that the Rift Valley wasformed 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, calledrifting,is still in progresscomes 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 3.70.9 mm/year, and that most of this widening is

concentrated in the deepest, most youthful part of the rift zone (2.91 mm/year).
http://www.cotf.edu/ete/modules/rift/rvtopography.htmlhttp://www.cotf.edu/ete/modules/rift/rvtopography.htmlhttp://www.cotf.edu/ete/modules/rift/rvtopography.htmlhttp://www.cotf.edu/ete/modules/rift/rvglossary.html#riftinghttp://www.cotf.edu/ete/modules/rift/rvglossary.html#riftinghttp://www.cotf.edu/ete/modules/rift/rvglossary.html#riftinghttp://www.cotf.edu/ete/modules/rift/rvglossary.html#riftinghttp://www.cotf.edu/ete/modules/rift/rvtopography.html


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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 riftingto 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 anunderlying 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 acontinent 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


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 showoffsets along transform boundaries.

Transform or Shear Boundaries

The San Andreas Fault is a transformboundary that separates the North Americanand Pacific Plates. The smaller Juan de Fucaplate lies between these two plates oppositeOregon, Washington, northern California, andpart of British Columbia. The Pacific Platemoves northwest relative to the North

American Plate. Los Angeles will migratetoward San Francisco over the next severalmillion 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 AmericanPlate. 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 millionsof years from now.

Convergent 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


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


Convergent plate boundaries (destructive margins) Oceanic-oceanic convergence

Two oceanic slabs converge and one descendsbeneath 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 continentalmaterial, 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 havereconstructed 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 withAsia (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 50million 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 stretchingout 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 betweensubducting and overlaying plates. This is the type of most tsunamigenicearthquakes.

Slab (intraplate) earthquake, if the location is within the subducting slab. Thisinclude deep earthquakes, although only those occurring less than about 100 kmdepth are tsunamigenic. Crustal earthquake, if it is in the overlaying crust, can betsunamigenic 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 thedeformation 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 largeand create movements in the sea floor.They can be more aptly described as a series of waves of extremely long wavelengthand long period generated in a body of water by an impulsive disturbance that displacesthe water.Wind-generated waves usually have period (time between two successive waves) of fiveto twenty seconds and a wavelength (distance between two successive waves) of about100 to 200 metres (300 to 600 ft).

Tsunamis can have a period in the range of ten minutes to two hours and a wavelengthin 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 littleenergy as it propagates.

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

For example, when the ocean is more than 5000 metres deep, unnoticed tsunami travelabout 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 theybe seen from the air in the open ocean.

IEQ-05 Plate Tectonics Notes

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