Continental Drift Theory
Theories that explains various Earth's movement -
Continental drift theory
It was proposed by Alfred Wegner, a German meteorologist (Book: The origin of Continent and Oceans) in 1912. It was proposed to explain the major variations in the Earth's climatic conditions. He observed that some of the regions in temperate region had undergone some kind of polar event (glacial deposit) as well as it also undergone very hot climate over its geological time history.
The cause of such variation can be -
- Either the continents remained stationary and the climatic zones shifted. or
- Climatic zones remains stationary and the continents shifted.
As he did not find any evidence of change of climatic zones (as it depends on latitude, Earth's rotation and revolution & tilt in the axis), he proposed Continental drift as the reason for this.
Assumption of the theory
- He proposed three different layers in the Earth's interior - Outer SiAl, Intermediate SiMa and Inner NiFe.
- SiAl was the continental mass where as SiMa was the oceanic crust.
- SiAl masses were assumed to be floating over SiMa layer without any resistance.
Theory -
- During the Carboniferous period, i.e., before 225 Million years ago, there was only 1 super continent known as 'Pangaea' with 1 super ocean known as 'Panthalassa'. Pangaea consisted all the present continents.
- This super continent started to break during the Carboniferous period and got separated into Northern Angaraland (Laurasia) and Southern Gondwanaland.
- The Northern Angaraland consisted of North America, Green land and Eurasia without India and Arabia. Gondwanaland consisted of South America, Africa with Arabia attached, India, Antarctica and Australia. Both Angaraland and Gondwanaland were separated by Tethys Sea.
- Around 105 Million years ago, India started to move towards the North.
- A North-South rift separated North America from Eurasia and South America from Africa which started to move towards west.
- Africa started to move towards North and Antarctica towards South, Australia towards East.
- Around 20 Million years ago, Arabia got separated from Africa and merged with Asia.
- Equatorward movement is caused by gravitational and differential force and forces of buoyance to adjust the center of gravity and centre of mass.
- Westward movement is caused by the tidal forces of sun and moon.
Evidences -
- Jigsaw fit of continents, i.e., similarities in coastline on opposite sides of Atlantic ocean.
- Structural evidence - The mountain belt of Brazil terminates on South American east coast and the same type of mountain appears on the west coast of Africa.
- Stratigraphic evidence - The eastern coast of Brazil has same type of rock formation observed on North-Western African coast.
- Fossil evidence - Mesosauras an aquatic reptile whose fossils are found only in eastern South America and Southern Africa. Has he be able to swim across the ocean, it should have been widely distributed.
- Fossil of Glossopteris, a fern grown only in sub-polar climate are now found in even warm climate regions separated by wide oceans.
- Glacial deposits - layers of Tillite are found in even warm tropical regions of South America, Africa and Australia.
Limitations/Criticisms of Continental drift theory -
- The geographical mechanism, i.e., forces envisaged by Alfred Wegener for continental drift were not considered adequate.
- Rocks of continental and oceanic crust are very rigid and would not permit free drift of continents over oceanic crust.
- Continental crust is not above the Oceanic crust. They both are adjacent to each other (except Pacific plate which is made up of only Oceanic crust).
- The theory does not describe the conditions of pre-carboniferous period.
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Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own.
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