• Plate tectonics is a scientific theory that describes the movement of large and small plates of the Earth’s lithosphere.
• The term “plate tectonics” was first used by Tuzo Wilson of the University of Toronto.
• It was first published by W.J. Morgan of Princeton University in 1962.
• Plate tectonics builds on the concept of continental drift.
• Seafloor spreading validated plate tectonic theory in the late 1950s and early 1960s.
• Plate tectonics explains the large-scale motion of 7 large plates and the movements of a larger number of smaller plates over the last hundreds of millions of years.

Plate Tectonics Theory

• The theory explains most of the dynamism of Earth’s crust and features of the endogenetic forces.
• The development of the theory began in the 1960s with extensive seafloor mapping.
• The theory is based on two principal hypotheses – Arthur Holmes’ convection current hypothesis and the concept of seafloor spreading advocated by Hess.
• It is an improvement over Wegener’s continental drift theory and is considered the most sophisticated and comprehensive theory about the drift of continents and the expansion of sea floors.
• The Earth’s lithosphere is broken up into tectonic plates.
• There are seven or eight major plates and many minor plates in the Earth’s lithosphere.
• The relative motion of the plates determines the type of boundary: convergent, divergent, or transform.
• Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries.
• The relative movement of the plates ranges from 0 to 100 mm annually.

Postulates of Plate Tectonic Theory

• The earth’s interiors according to mechanical rigidity can be classified into lithosphere, asthenosphere and mesosphere.
• The theory rejects the ideas of SIAL, SIMA based classification.

According to the theory,

• The lithosphere is broken into fragments that float on a ductile layer called the asthenosphere.
• The movement of plates is due to convection currents in the upper mantle.
• Plates move horizontally over the asthenosphere as rigid units.
• The lithosphere consists of the crust and top mantle, with a thickness range varying from 5-100 km in oceanic parts and about 200 km in continental areas.
• Oceanic plates contain mainly Simatic crust and are relatively thinner, while continental plates contain Sialic material and are relatively thicker.
• Lithospheric plates range from minor plates to major plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), and sometimes a combination of both continental and oceanic plates (Indo-Australian plate).
• Movement of crustal plates causes the formation of various landforms and is the principal cause of all earth movements.
• The margins of the plates are the sites of considerable geologic activity, such as seafloor spreading, volcanic eruptions, crustal deformation, mountain building, and continental drift.

Lithospheric Plates

• A plate is a broad segment of the lithosphere, that floats on the underlying asthenosphere and move independently of the other plates.
• Broadly they can be classified into continental plates and oceanic plates.
• La Pichon divided the earth into seven major and nine minor plates.
• Major Tectonic Plates:
  • Antarctica and the surrounding oceanic plate – (Surrounded by divergent boundaries.)
  • North American plate – (shifting westwards, velocity 4-5 cm/year. It is half oceanic—half continental)
  • South American plate – (shifting westwards, Half continental — half oceanic. 3-4 cm/year)
  • Pacific plate – (Truly oceanic plate. Shifting NW 2- 3cm/year)
  • India-Australia-New Zealand plate
  • Africa with the eastern Atlantic floor plate
  • Eurasia and the adjacent oceanic plate – (mostly continental, shifting eastwards. Velocity -2-3cm/year)
• Minor Tectonic Plates:
  • Arabian plate: Mostly the Saudi Arabian landmass
  • Bismark plate (North Bismarck Plate & South Bismarck Plate)
  • Caribbean plate
  • Carolina plate [straddles the Equator in the eastern hemisphere located north of New Guinea]
  • Cocos Plate
  • Juan de Fuca Plate (between Pacific and North American plates)
  • Nazca plate
  • Philippine plate: Between the Asiatic and Pacific plate
  • Persian Plate
  • Anatolian Plate [or the Turkish Plate is a continental tectonic plate comprising most of the Anatolia (Asia Minor) peninsula (and the country of Turkey)]
  • China plate
  • Fiji plate [located b/w the Pacific Plate and the Indo-Australia Plate.]

Plate Boundaries

• There can be three types of plate boundaries defined by the motion between the plates: divergent, convergent and transform.
• 
• Divergent Boundaries:
  • A divergent boundary occurs when two tectonic plates move away from each other.
  • Lava spews from long fissures, and geysers spurt superheated water along these boundaries.
  • Frequent earthquakes strike along the rift.
  • Magma, molten rock, rises from the mantle beneath the rift.
  • Magma oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates.
  • Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor.
  • Oceanic crust, made of basalt, is created at divergent boundaries.
  • Features: Mid-oceanic ridges, rift valleys, fissure volcanoes.
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  • 
• Convergent Boundary:
  • A convergent boundary occurs when two plates come together.
  • The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench.
  • A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench.
  • Powerful earthquakes shake a wide area on both sides of the boundary.
  • If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt.
  • Magma rises into and through the other plate, solidifying into new crust.
  • Magma formed from melting plates solidifies into granite, a light-colored, low-density rock that makes up the continents.
  • Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.
  • Three types of convergent boundaries are there:
    • Oceanic-continental (O-C) convergence – Ocean plate being denser, goes under subduction, sinks beneath the continent into the asthenosphere and ultimately melts.
    • Oceanic-oceanic (O-O) convergence – The cooler, denser plate sinks beneath the warmer, lighter one – releases water from dehydration of hydrous minerals in the oceanic crust. Forms oceanic trenches and volcanic arcs.
    • Continental-continental (C-C) convergence – Sediments gets squeezed and upthrust between plates form fold mountains. Eg: Himalayas
• Transform Boundary:
  • A transform fault or boundary is a fault along a plate boundary where the motion is predominantly horizontal.
  • It is a location where two plates are sliding past each other, with no creation or destruction of landform, but only deformation of the existing landform.
  • The fracture zone that forms a transform plate boundary is known as a transform fault.
  • In oceans, transform faults are generally perpendicular to the mid-oceanic ridges and are planes of separation.
  • The North Anatolian Fault and the San Andreas Fault, which runs along the western coast of the USA, are the best examples of a transcurrent edge on continents.
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Evidences of Plate Tectonic Theory

• Ocean deep drilling- Glomar challenger
• JOIDES- Joint Oceanographic Institutions for Deep Earth Sampling)
• Hot spots
• Paleomagnetism
• Magnetic reversal and seafloor spreading

Significance of Plate Tectonic Theory

• Almost all major landforms are due to plate tectonics.
• Magmatic eruptions bring up new minerals from the core.
• Valuable minerals such as copper and uranium are found near the plate boundaries.
• The future shape of landmasses can be predicted from current knowledge of crustal plate movement.
• Based on current trends, North and South America will separate, a piece of land will separate from the east coast of Africa, and Australia will move closer to Asia.

Comparison: Continental Drift & See Floor Spreading & Plate Tectonics

	Continental Drift	Sea Floor Spreading	Plate Tectonics
Explained by	Put forward by Alfred Wegener in the 1920s	Arthur Holmes explained Convectional Current Theory in the 1930s.Based on convection current theory, Harry Hess explained See Floor Spreading in the 1940s	In 1967, McKenzie and Parker suggested the theory of plate tectonics. Morgan later outlined the theory in 1968
Theory	Explains the Movement of Continents only	Explains the Movement of Oceanic Plates only	Explains the Movement of Lithospheric plates that include both continents and oceans.
Forces for movement	Buoyancy, gravity, pole-fleeing force, tidal currents, tides	Convection currents in the mantle drag crustal plates	Convection currents in the mantle drag crustal plates
Evidence	Apparent affinity of physical features, botanical evidence, fossil evidence, Tillite deposits, placer deposits, rocks of same age across different continents etc.	Ocean bottom relief, Paleomagnetic rocks, distribution of earthquakes and volcanoes etc.	Ocean bottom relief, Paleomagnetic rocks, distribution of earthquakes and volcanoes, gravitational anomalies at trenches, etc.
Drawbacks	Too general with silly and sometimes illogical evidence.	Doesn’t explain the movement of continental plates	———————
Acceptance	Discarded	Not complete	Most widely accepted
Usefulness	Helped in the evolution of convection current theory and seafloor spreading theory	Helped in the evolution of plate tectonics theory	Helped us understand various geographical features.