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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
14Source: USGS public domain
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Convergent Plate Boundary
Plates move toward
each other
(compression)
Lithosphere isconsumed
Mountain building
Explosive volcanism
15Source: USGS public domain
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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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Source: USGS public domain
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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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Source: USGS public domain
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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Source: USGS public domain
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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
21Source: USGS public domain
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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
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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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