Plate Tectonics Theory - Notes | Study Geography for UPSC CSE - UPSC

Introduction

The comprehensive theory tries to explain most of the dynamism of earth crust and features of the endogenetic forces.

The development towards the theory began in the 1960s with extensive seafloor mapping.

This theory is based on the two principle hypothesis – Arthur Holmes convection current hypothesis, and the concept of seafloor spreading’ advocated by Hess.

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

Postulates of Plate tectonics theory

• The earth’s interiors according to mechanical rigidity can be classified into; 
  1. Lithosphere
  2. Asthenosphere
  3. Mesosphere
• The theory rejects the ideas of SIAL, SIMA based classification.

Lithosphere – The crust and part of the upper mantle = lithosphere (100 km thick, and less dense than the material below it so it “floats”)

Asthenosphere – The plastic layer below the lithosphere = asthenosphere (The plates of the lithosphere float on the asthenosphere)

According to Plate tectonics theory –

• the lithosphere is believed to have been broken into fragments 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 various landforms formation and is the principal cause of all earth movements.
• The plates' margins 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 tectonics (greek), meaning building or construction, refers to the deformation of the earth’s crust due to 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

1. Antarctica and the surrounding oceanic plate – (Surrounded by divergent boundaries.)
2. North American plate – (shifting westwards, velocity 4-5 cm/year. It is half oceanic—half continental)
3. South American plate – (shifting westwards, Half continental — half oceanic. 3-4 cm/year)
4. Pacific plate – (Truly oceanic plate. Shifting NW 2- 3cm/year)
5. India-Australia - New Zealand plate
6. Africa with the eastern Atlantic floor plate
7. Eurasia and the adjacent oceanic plate – (mostly continental, shifting eastwards. Velocity -2-3cm/year)

Minor tectonic plates

1. Arabian plate: Mostly the Saudi Arabian landmass
2. Bismark plate
3. Caribbean plate
4. Carolina plate
5. Cocos Plate
6. Juan de Fuca Plate (between Pacific and North American plates)
7. Nazca plate
8. Philippine plate: Between the Asiatic and Pacific plate
9. Persian Plate
10. Anatolian Plate
11. China plate
12. Fiji plate

Most of the plates include both continental and oceanic crusts. The area of the places is relatively 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:

1. Separation or divergent or constructive plate margins
2. Closing together or convergent or destructive plate margins
3. Transform or conservative plate margin

Divergent Boundaries

The boundary between two plates that are moving apart or rifting, Rifting causes Seafloor Spreading.

Features of Divergent Boundaries –

• Mid-ocean ridges
• Rift valleys
• Fissure volcanoes

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 significant enough to form a “proto-ocean.”

Convergent Boundaries

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

The three types of convergent boundaries are –

1. Oceanic–continental convergence,
2. Oceanic–oceanic convergence,
3. 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. 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 Sumatra and Java in Indonesia are today).

Continental-Continental Convergence

• There is a convergent boundary between two continental plates; no subduction occurs 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.

Transform Fault Boundaries

Transform Plate Boundaries are locations where two plates are sliding past each other, and there is 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 the planes of separation generally perpendicular to the mid-oceanic ridges.

San Andreas Fault (Silicon Valley lies dangerously close to the faultline) along the USA's western coast is the best example for a transcurrent edge on continents.

San Andreas fault in California

Evidence

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

The significance of Plate Tectonics

• Almost all significant 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 the 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