Plate Tectonics theory: Geography UPSC

The term plate tectonics was first used by Tuzo Wilson, of the University of Toronto but the Plate Tectonics theory was first published by W.J Morgan of the Princeton University in 1962.

This theory is based on the concept of seafloor spreading’ advocated by Hess.

It is an improvement over the Wegener’s continental drift theory and has been considered as the most
sophisticated and comprehensive theory about the drift of continents and the expansion of sea floors.

According to this theory –

the lithosphere is believed to have been broken into fragments that are floating on a ductile layer called asthenosphere (upper part of the mantle).
The movement of these plates is attributed to the convention currents being generated in the upper mantle.
Plates move horizontally over the asthenosphere as rigid units.
The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
The oceanic plates contain mainly the Simatic crust and are relatively thinner, while the continental plates contain Sialic material and are relatively thicker.
Lithospheric plates (tectonic plates) vary from minor plates to major plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), sometimes a combination of both continental and oceanic plates (Indo-Australian plate).
The movement of these crustal plates (due to convection currents in the mantle) 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.
Tectonics is derived from the world tektonikos (greek), meaning building or construction, refers
to deformation of the earth’s crust as a result of internal forces.

Lithospheric Plates

A plate is a broad segment of the lithosphere (crust + rigid upper mantle), 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
North American plate
South American plate
Pacific plate
India-Australia-New Zealand plate
Africa with the eastern Atlantic floor plate
Eurasia and the adjacent oceanic plate

Minor tectonic plates

Arabian plate: Mostly the Saudi Arabian landmass
Bismark plate
Caribbean plate
Carolina plate
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
China plate
Fiji plate

Most of the plates include both continental and oceanic crusts. The area of the places is fairly large in comparison to their depth and thickness. It has also been established that the depth of the plates is even less under the oceanic crust.

Three types of motion are possible between the plates:

Separation or divergent or constructive plate margins
Closing together or convergent or destructive plate margins
Transform or conservative plate margin

Plate boundary			Plate movement		seafloor		Events observed	examples
Divergent plate boundaries	Ocean -Ocean		apart		Forms by seafloor spreading		The ridge forms a spreading center. Plate area increases. Many small volcanoes and earthquakes	Mid Atlantic Ridge, east pacific rise
	Continent -Continent				New ocean basin may form as the continent split		Continent drifts apart, the ocean may intrude. Formation of rift valleys and block mountains	East African rift.
Convergent plate boundary	Ocean -Continent		together		Destroyed at subduction zones		The dense oceanic lithosphere plunges beneath less dense continental crust. Earthquake traces the path of the down moving plate as it descends into the asthenosphere. A trench is formed. Subducted plate partially melts and magma rises to form continental volcanoes.	Western South America
	Ocean -Ocean				Destroyed at the subduction zone		Denser crust plunges into lighter crust and is subducted forming a curved trench and a volcanic arc.	Aleutians
	Continent- Continent				NA		A collision between masses of gigantic continental lithosphere. Neither mass is subducted. Plate edges are compressed, folded and uplifted	The Himalayas, alps
Transform plate boundary		Past each other		Neither created nor destroyed		A transform fault is formed where plates move past each other. Strong earthquakes along the fault			San Andreas fault.

Continental rift Valleys

Divergent boundaries can also develop within a continent resulting in a continental rift valley such as the Great East African Rift Valley that extends from Ethiopia southward through Mozambique. The Red Sea is also the outcome of spreading taking place within a continent—in this case, the spreading has been great enough to form a “proto-ocean.”

Convergent Boundaries

At a convergent boundary, plates collide and as such are sometimes called “destructive” boundaries because they result in removal or compression of the surface crust. Convergent plate boundaries are responsible for some of the most massive and spectacular of earthly landforms: major mountain ranges, volcanoes, and oceanic trenches. The three types of convergent boundaries are oceanic–continental convergence, oceanic–oceanic convergence,
and continental–continental convergence.

Oceanic–Continental Convergence

Because the oceanic lithosphere includes dense basaltic crust, it is denser than the continental lithosphere, and so oceanic lithosphere always under-rides the continental lithosphere when the two collide.
The dense oceanic plate slowly and inexorably sinks into the asthenosphere in the process of subduction. The subducting slab pulls on the rest of the plate—such “slab pull” is probably the main cause of most plate movement, pulling the rest of the plate in after itself, as it were.

Oceanic–Oceanic Convergence

If the convergent boundary is between two oceanic plates, subduction also takes place. As one of the oceanic plates subducts beneath the other, an oceanic trench is formed, shallow- and deep-focus earthquakes occur and volcanic activity is initiated with volcanoes forming on the ocean floor.
With time, a volcanic island arc (such as the Aleutian Islands and the Mariana Islands) develops; such an arc may eventually become a more mature island arc system (such as Japan and the islands of Sumatra and Java in Indonesia are today).

Continental-Continental Convergence

Where there is a convergent boundary between two continental plates, no subduction takes place because continental crust is too buoyant to subduct. Instead, huge mountain ranges, such as the Alps, are built up. The most dramatic present-day example of the continental collision has resulted in the formation of the Himalayas.

The significance of Plate Tectonics

Almost all major landforms formed are due to plate tectonics.
New minerals are thrown up from the core with the magmatic eruptions.
Economically valuable minerals like copper and uranium are found near the plate boundaries.
From present knowledge of crustal plate movement, the shape of landmasses in the future can be predicted.
For instance, if the present trends continue, North and South America will separate. A piece of land will separate from the east coast of Africa. Australia will move closer to Asia.

Comparison: Continental Drift & See Floor Spreading & Plate Tectonics

Continental Drift	Seafloor Spreading	Plate Tectonics
Explained by	Put forward by Alfred Wegener in 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	The apparent affinity of physical features, botanical evidence, fossil evidence, Tillite deposits, placer deposits, rocks of the 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.