Plate Tectonic Theory & Creation of India
The theory of Continental Drift by Alfred Wegener in 1912 gave the world insights about the evolution of the Earth’s surface. However it was rejected due to its inability to explain the mechanism behind it. Despite this, it paved the way for modern theories that were able to explain the phenomenon such as sea floor spreading and most importantly Plate tectonics. Plate tectonics theory suggests that Earth's outer shell (lithosphere) is divided into several plates that move over the asthenosphere above the soft core (mantle). The plates act like a hard and rigid shell compared to Earth's mantle.
Tectonic plates constantly move to reshape the Earth’s landscape. Plate tectonics has become the unifying theory of geology. It explains the earth’s surface movement, current and past, which has created the tallest mountain ranges and the deepest oceans.
Tectonic Plates
The Earth’s surface comprises a series of crustal plates that are continuously in motion. These plates comprises of two types of crusts:
- Continental crust: It is made up of granite, a type of igneous rock. This rock tends to be rather old, and considered to be felsic. Felsic rocks contain high amounts of silica. It tends to be very light compared to other minerals, especially the minerals that make up the crust upon which the oceans are contained.
- Oceanic crust: oceanic crust is made up of basalt, another type of igneous rock. This rock tends to be younger and mafic. Mafic rocks tend to be made up of heavier iron- and magnesium-rich minerals that are often darker in colour than felsic rocks. This is why continental crust ‘floats’ above oceanic crust: it is much lighter and less dense.
The tectonic plates can be completely continental, completely oceanic or a mix of both. The Earth's surface is divided into Major and Minor plates.
- Major Plates: There are 7 major plates that cover most of the Earth’s surface.
- The Antarctic (and the surrounding oceanic) plate.
- The North American plate (with western Atlantic floor separated from the South American plate along the Caribbean islands)
- The South American plate (with western Atlantic floor separated from the North American plate along the Caribbean islands)
- The Pacific plate
- The India-Australia-New Zealand plate
- The Africa with the eastern Atlantic floor plate
- Eurasia and the adjacent oceanic plate
- Minor Plates: Apart from these plates there are some minor plates as well such as:
- Cocos plate: Between Central America and Pacific plate
- Nazca plate: Between South America and Pacific plate
- Arabian plate: Mostly the Saudi Arabian landmass
- Philippine plate: Between the Asiatic and Pacific plate
- Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
- Fuji plate: North-east of Australia
- Juan De Fuca plate: South-East of North American Plate
There are convection currents within the mantle and several mechanisms that drive plate motions such as:
- Slab pull: where a plate is being subducted (sucked under) another and it ‘pulls’ along the rest of the plate.
Fig 2. forces for plate boundary interaction
- Ridge push: where new material is formed at mid-ocean ridges forming a higher topography and gravity ‘pushes’ on that material inside the Earth.
- Mantle drag: where convection currents within the mantle and under these crustal plates aid in moving these plates in different directions. The source of the convection currents is radioactive heat from deep within the Earth’s mantle.
Convergent Plate Boundaries
‘Convergent’ simply means coming together. Thus at convergent boundaries two or more plates collide with each other. These plates, depending on what type of crust (oceanic or continental) they are made of, can collide in a variety of ways. If two plates made of continental crust are colliding they will crumple to form mountains. One modern example is the Himalayan Mountains, caused by the northward-moving Indian plate colliding with the Eurasian plate.
When a denser oceanic plate crashes into a lighter and less dense continental plate oceanic plate will subduct or sink underneath the continental plate. The oceanic plate will then be sucked into the Earth’s mantle, where it will eventually melt. One major example of subducting oceanic plates beneath a continental plate is in the Pacific Ocean. This area is more commonly known as ‘The Ring of Fire’, where the Pacific Plate is subducting underneath the Australian and North American plates.
And, finally, when two oceanic plates come together, whichever crust is denser, will get subducted. This often related to the ages of the oceanic crust, with older crust being subducted because it has cooled over its lifespan and become denser than newer crust which is warmer and less dense.
And, finally, when two oceanic plates come together, whichever crust is denser, will get subducted. This often related to the ages of the oceanic crust, with older crust being subducted because it has cooled over its lifespan and become denser than newer crust which is warmer and less dense.
Divergent Plate Boundaries
Divergent means to spread apart. At divergent plate boundaries, a new crust is being generated which pushes two plates away from one another. These spreading centres commonly occur on the seafloor, and are areas where new oceanic (heavy, dense, mafic crust) is being brought to the surface of the Earth. These areas are not explosive like many volcanoes on the Earth’s surface, but instead lava comes out of the Earth’s crust. Divergent boundaries are often associated with many minor faults, or breaks, in the Earth’s crust. One of the major spreading centres today occurs in the middle of the Atlantic Ocean, called the Mid Atlantic Ridge. Today the ridge produces about 2.5 centimeters (0.98 inches) of new crust per year. As new plate material is produced, plate material is subducted and reworked into the mantle meaning the Earth is not ‘growing’ or ‘expanding’ but maintaining the total amount of crust at its surface. Also, remember these motions are often slow processes that take immense amounts of time.
Transform Plate Boundaries
In plate tectonic terms, ‘transform’ means sliding past one another. At transform plate boundaries, two plates slide against one another, thus causing shallow earthquakes. At these boundaries, the plates typically do not simply slide past one another through time. Instead, they may stick together through friction until enough energy is built up, and then they move. It is this action that causes intense earthquakes.
Today, the most famous example in the United States of a transform fault is the San Andreas Fault system, where you can easily identify the movement between the two plates by matching up corresponding rock on either side of the boundary. Another famous example is the Alpine Fault that cuts New Zealand in half.
Hotspots
There was one nagging question with the plate tectonics theory: Most volcanoes are found above subduction zones, but some form far away from these plate boundaries. How could this be explained? This question was finally answered in 1963 by a Canadian geologist, John Tuzo Wilson. He proposed that volcanic island chains, like the Hawaiian Islands, are created by fixed “hot spots” in the mantle. At those places, magma forces its way upward through the moving plate of the sea floor. As the plate moves over the hot spot, one volcanic island after another is formed. Wilson’s explanation gave further support to plate tectonics. Today, the theory is almost universally accepted.
Indian Connection to Plate Tectonics
The Indian Plate or India Plate is a minor tectonic plate straddling the Equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, India broke away from the other fragments of Gondwana 100 million years ago and began moving north. Once fused with the adjacent Australian Plate to form a single Indo-Australian Plate, recent studies suggest that India and Australia have been separate plates for at least 3 million years and likely longer. The Indian Plate includes most of South Asia i.e. the Indian subcontinent—and a portion of the basin under the Indian Ocean, including parts of South China and western Indonesia.
In the late Cretaceous, approximately 100 million years ago and subsequent to the splitting off from Gondwana of conjoined Madagascar and India, the Indian Plate split from Madagascar. It began moving north, at about 20 centimeters (7.9 inches) per year, and is believed to have begun colliding with Asia as early as 55 million years ago in the Eocene epoch of the Cenozoic. This led to the formation of Himalayas and the corresponding antecedent drainage as we see today.
The Indian Plate is currently moving north-east at five centimeters (2.0 in) per year, while the Eurasian Plate is moving north at only two centimeters (0.79 in) per year. This is causing the Eurasian Plate to deform, and the Indian Plate to compress at a rate of four millimeters (0.16 in) per year. This is the reason that the Himalayas are still increasing in their height.
The discovery of Plate Tectonics theory was a landmark event that explained the evolution of the Earth’s crust as well as most of the features of the Earth surface. The event was a paradigm shift and scientific revolution as far as the study of geological history of the Earth is concerned.
