Lecture2 Plate Tectonics Part 1

8/12/2019 Lecture2 Plate Tectonics Part 1

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PLATE TECTONICS

Part -I

Lecture-2

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Theory of Plate tectonics

The theory of Plate tectonics was proposed in 1960s

based on the theory of continental drift.

This is the Unifying theory that explains the

formation and deformation of the Earths surface.

According to this theory, continents are carried along

on huge slabs (plates) on the Earths outermost layer

(Lithosphere).

Earths outermost layer is divided into 12 major

Tectonic Plates (~80 km deep). These plates move

relative to each other a few centimeters per year.

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Tectonic plates of Earth

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Source: http://eqseis.geosc.psu.edu


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Evidence for plate tectonic movements

Wegeners theory of continental drift was not accepted initially

because Wegener could not propose a mechanism which could

explain the motion of continents.

Today plate tectonics and continental drift are accepted as facts

because of following evidences.

Matching coastlines of the continents

Matching mountain ranges and rock types and age of

opposite shorelines

Matching glacier deposits and fossils of opposite shores

Ocean floor spreading

Geodetic measurements through satellites

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Evidence for plate tectonic movements

Matching shapes, rock types, rock ages, mountains, glacier

deposits and fossils along the shorelines of continents

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Source: wikipedia


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Ocean floor spreading

Discovered in oceans by shipsdragging magnetometers (1940s

and 1950s)

Extensive mapping of magnetic

stripes is carried out since then.

A series of under-water mountains

called mid-ocean ridges is found

throughout the world. These

mountains are formed as new seafloor is created from magma that

rises up from the mantle below.

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Source: wikipedia


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Evidence for ocean floor spreading

When a magnetometer is suspended across the ocean, itindicates that there are alternating zones of rock with either

normal or reversed polarity. Study of these magnetic

anomalies is called paleomagnetism.

Earths magnetic field flip-flops on average about every250,000 years. North becomes south and south becomes

north. The rocks of the sea floor record these reversals. These

reversals can be seen as alternating bands on the sea floor,

proving that the sea floor was spreading apart as the earths

magnetic field reversed itself over long periods of geologic

time

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Earths magnetic field

Magnetic field of Earth

reverses on semi-regular

basis. Minerals act like

compass needles and pointtowards magnetic north.

Hot rocks record the

direction of the magnetic

field as they cool.

8Source: USGS public domain


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Evidence for plate tectonic movements:

Geodetic measurements

The Global Positioning System (GPS) is a constellation of 24

satellites which is used for precise geodetic position

measurements. Laser geodynamic satellites orbit the earth at an

altitude of 3,700 miles. Laser beams are bounced from one point

on the earth, off the satellite, to a second point on the ground.Scientists can then measure the distance between the two points

with great accuracy. Horizontal velocities, mostly due to motion of

the Earth's tectonic plates and deformation in plate boundary

zones, are recorded and maps are prepared with arrows

representing the movement of plates. These systems show

conclusively that the continents are still drifting at a rate of a few

centimeters a year.

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Movement of global plate boundaries

10Source: wikipedia


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Types of plate boundaries

Divergent plate boundaries: where plates

move apart

Convergent Plate boundaries: where plates

come together

Transform plate boundaries: where plates

slide past each other

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Types of plate boundaries

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Examples of plate boundaries

DIVERGENT- Midatlantic Ridge, Iceland, Gulf OfCalifornia, East African Rift

CONVERGENT- Pacific Northwest (Cascadia),

Alaska/Aleutians, Japan, Mexico, India

TRANSFORM- San Andreas, Dead sea, Turkey

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Divergent Plate Boundary

Plates move away from

each other (tension)

New lithosphere is

formed Causes volcanism

Not very explosive



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Convergent Plate Boundary

Plates move toward

each other

(compression)

Lithosphere isconsumed

Mountain building

Explosive volcanism



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Ocean-continent

plates collide

Ocean plate subducts

below continent Forms a subduction

zone

Earthquakes andvolcanoes

Ocean- Continent convergent margin

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Source: USGS public domain


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Ocean-ocean convergent margin

2 oceanic plates collide

One plate dives

(subducts) beneath other

Forms subduction zone

Earthquakes and

volcanoes

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Continent-continent convergent margin

Two continentalplates collide

Neither plate wants to

subduct Collision zone forms

high mountains

Earthquakes, no

volcanoes

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Himalayas: Continent continent


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Millions of years ago India and an ancient

ocean called the Tethys were sat on a tectonic

plate. This plate was moving northwards

towards Asia at a rate of 10 centimeters per

year. The Tethys oceanic crust was being

subducted under the Asian Continent. Theocean got progressively smaller until about 55

milion years ago when India 'hit' Asia. Because

both these continental landmasses have about

the same rock density, one plate could not be

subducted under the other. The pressure ofthe impinging plates was relieved by the

formation of Himalayas

Himalayas: Continent-continent

convergent margin

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Himalayas: Continent-continent

convergent margin

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Transform plate boundary

Two plates slide past

each other

Lithosphere is neither

consumed nordestroyed.

Earthquakes, no

volcanoes

Responsible for most of

the earthquakes



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What drives plate movement?For many years, it was believed that mantle convection is the

main driving force for plate movement.

hot, less dense material rises along mid-ocean ridges, cools,

and subsides at subduction zones, and the plates "ride" these

convection cells

Recent modeling suggests that the force of convection is not

enough to push enormous lithospheric plates (e g. North

American plate). Geologists suggest that gravity is the main

driving force

cold, dense oceanic crust sinks at subduction zones, pulling

the rest of the plate with it. Magmatic intrusions at spreading

ridges are passive - the magma merely fills a hole created by

pulling two plates apart.22


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Earths internal heat

The Earth convects like a pot of water on a stove trying to

dissipate heat.

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Kramer, S.L. (1996) Geotechnical Earthquake Engineering, Prentice Hall.

Udias, A. (1999): Principles of Seismology, Cambridge University Press,

Cambridge.

Shearer, P. M. (1999): Introduction to Seismology, Cambridge University Press,

Cambridge.

Ben Menahem, A. and Singh, S. J. (1980): Seismic Waves and Sources,

Springer-Verlag, New York.

Cox, A. and Hart, R.B. (1986): Plate Tectonics - How it Works, Palo Alto,

California, Blackwell Scientific Publications, 392 p.

References

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