Dynamic Surface of the Earth: Continental, Evidences of Drift and Plate Tectonics

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Continental Drift

  • According to Alfred Wegener, the entire landmass of the globe was together about 280 million years ago. It was termed as Pangea, the super continent. The huge water body surrounding the Pangea was known as Panthalassa. From 280 to 150 million years ago, the Pangea was broken latitudinally into northern and southern parts known as Laurasia or Angaraland and Gondwanaland, respectively.

  • Both of them drifted away and in between a shallow sea emerged by filling up the water from Panthalassa. It was known as the Tethys Sea. Later on, Laurasia and Gondwanaland rifted and finally drifted to form the present-day distribution of land and water on the earth.

Continental Drift

Continental Drift

Evidences of Drift

Wegener gave a number of evidences in support of the unification of landmass in the geologic past. They are:

  • Jig-Saw Fit: The eastern coast of South America is identical to the western coast of Africa which fits to a certain depth in the ocean. To a certain extent, the coastal areas and the continental shelves have been modified by oceanic waves through denudation.

  • Geological Similarities: The mountain systems of southern Atlantic coast in South America and Africa show the similarity of the extension in both the continents.

  • Coal and Vegetation Evidences: The distribution of coal and vegetation over South America, Africa, India and Australia proves that they were together in the geological past. The classical glacial deposits during the carboniferous period over these landmasses resemble each other which tells the story of togetherness. Today, they lie in different climatic zones.

Apart from above evidences put forward by Wegener, there are some other evidences, which support the idea of continental drift.

  • Evidences from Palaeomagnetism: Palaeomagnetism is the study of direction of pole through ages. Magnetically susceptible minerals like haematite and pyrrhotite magnetite get aligned with the magnetic pole of the earth and recorded in the solidification of magma during that time. It is found that the periodic changes have occurred, and the poles have wandered which is not possible for the entire earth. Hence, it is the twist and turn of the land block not for the entire earth which has again explained that the continents have shifted their positions.

  • Sea Floor Spreading: Along the Mid-Atlantic Ridge, the magma comes out at the seabed and gets solidified. Thus, a new zone is formed, and this process is continuing since millions of years. It is leading for the diversion of continental block, and hence the size of the Atlantic Ocean is increasing which is termed as the sea floor spreading. It is the classical example of the shifting of continents. The explanation of the continental drift through sea floor spreading and the study of Palaeomagnetism is generally known as Plate Tectonics.

Plate Tectonics

The uppermost outer solid and rigid layer of the earth is known as crust. The thickness of the crust varies considerably. It is about 5 km thick beneath the oceans at some places but under some mountain ranges it extends up to a depth of 70 km. Below the crust denser rocks are found, which is known as the mantle crust. This upper part of mantle up to an average depth of 100 km from the surface is solid. This solid mantle plus the upper crust form a comparatively rigid block termed as the lithosphere. Mantle is partially molten between 100 to 250 km depth. This zone is said to be the asthenosphere, also known as Mohr discontinuity, named after the seismologist Mohorovicic, who discovered it.

The lithosphere is broken into several blocks. These blocks are known as plates which are moving over asthenosphere. There are seven major plates:

  • Eurasian Plate

  • African Plate

  • Indo-Australian Plate

  • Pacific Plate

  • North American Plate

  • South American Plate

  • Antarctic Plate

Apart from these major plates, there are 20 minor plates. A few important among them are:

Arabian Plate, Philippine Plate, Cocos Plate, Nazca Plate, Caribbean Plate, Scotia Plate, etc.

The major and minor plates constitute the whole surface of the earth.

Plates of the Lithosphere

Plates of the Lithosphere

Plate tectonics is a method or way of understanding the land-water distribution of the earth. Tectonics is a sort of movement of plates. Through the movement, the internal forces are explained which are responsible for the distribution of the earth’s crust, formation of mountain chains and the distribution of earthquakes and volcanism.

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