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What was Pangaea according to Wegener?
The major plates are as follows :
(i) Antarctica and the surrounding oceanic plate
(ii) North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
(iii) South American (with western Atlantic floor separated from the North American plate along the Caribbean islands) plate
(iv) Pacific plate
(v) India-Australia-New Zealand plate
(vi) Africa with the eastern Atlantic floor plate
(vii) Eurasia and the adjacent oceanic plate.
Which plates are composed of mainly oceanic crust ?
A tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
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The evidences in support of continental drift theory are :
(i)The matching of continents : The shorelines of Africa and Sout America facing each other have a remarkable and unmistakable match.
(ii)Rocks of same age across the oceans :The belt of ancient rocks of 2000 millions years ages from Brazil coast matches with those from western Africa.
(iii)Tillite: These are the sedimentary rocks formed out of deposits of glaciers. The Gondwana system of sediments from India is known to have its counterparts in six different landmasses Southern Hemisphere. The glacial tillites provide unambiguous evidences of palaeoclimates and also of drifting of continents.
(iv)Placer deposits : The occurrence of rich placer deposits of gold in Ghana coast and the complete absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it can be inferred that the gold deposits of Ghana were derived from the Brazil Plateau when the two continents lay side by side.
(v)Distribution of fossils : The similar species of plants and animals are found in different parts of Gondwanaland. The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
(ii)Continental drift theory only considers the horizontal movement.
(iii)Continental drift theory mostly relies on circumstantial evidences of Jig-Saw-Fit, fossils, place deposits etc.
(iv) Continental drift theory relies on the concept of plate tectonics to be validated.
Plate Tectonics :
(i)Plate Tectonics assumes the whole earth’s lithosphere to be divided into different ‘major’ and ‘minor’ plates which are constantly moving.
(ii)Plate tectonics takes into account the subduction of plates also.
(iii)Plate tectonics is based on scientific analysis of the processes inside the earth’s surface.
(iv)Plate tectonic theory validates the drift theory and is not to be validated by the same.
The causes of plate movement are described below:
(ii)Movement of Currents : Hot currents rise and then cool as they reach the surface. At the same time cool currents sink down. The convectional movement moves the crustal plate.
(iii)Floating of Plates : The rigid plates of the lithosphere which floats on more mobile asthenosphere are in constant motion.
(iv)Hot Spots of Volcanic Activity : Small currents of past volcanic activity are often located far from any active plate boundary suggesting the effect of convection currents on the lithosphere. These are called the Hot Spots.
(v)Volcanic Eruptions : The source of magma in the mantle remains fixed in position while the lithospheric plate above it moves steadily. In this way volcanoes are formed over a hot spot but they become extinct. These extinct volcanic form a chain that is a record of plate motion.
C.
Rotation of the earthD.
AntarcticaC.
Distribution of fossils in different continents.1. The matching of the continents : The shore lines of Africa and South America facing each other have a remarkable and unmistakable match.
2. Rocks of the same ages across the oceans : The belt of ancient rocks of 2000 million years from Brazil coast matches with those from western Africa.
3. Placer deposits : Discovery of rich deposits of Gold in the Ghana and the complete absence of source rock in the region has left in scientists wondering. The location of gold bearing veins are in Brazil and make it obvious that the gold deposit of Ghana are derived from Brazil.
4. Distribution of Fossils : The identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers.
5. Tillite : Tillite are sedimentary rocks formed out of deposits of glaciers. Gondwana system of sediments from India is known to have its counterparts in six different land masses of Southern Hemisphere.
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