- Continents covers only 29 % of the total surface of the earth and rest of the 71 % is covered by ocean.
- If the coastline of Atlantic Ocean is observed we will notice that there is symmetry of the coastlines on either side of the ocean. Many scientists have observed this fact and considered the possibility of today’s North America, South America, Europe and Africa to be joined together as a single continent. Many scientists such as Abraham Ortelius, Antonio Pellegrini has drawn map showing such possibility.
- In 1912 a German meteorologist “Alfred Wegener” put forth a comprehensive argument supporting above fact in the form of “the continental drift theory”.
- According to Wegener about 250 million years ago all the continents which exists today was once a part of the single continental mass called super continent called Pangaea surrounded by one super ocean called Panthalasa.
- Wegener argued that around 200 million years ago, Pangaea began to split and in its first phase of split it broke into two large continental masses as Angaraland (Laurasia) and Gondwanaland. Laurasia started floating northward whereas Gondwanaland in southward direction.
- Gap which formed between these two continents was filled up with a narrow and shallow water body known as Tethys Sea.
- Laurasia contains present North America, Greenland and Eurasia (without India and Arabia) while Gondwanaland contained Africa with Arabia attached, South America, Australia, Antarctica and India represented the southern part of Pangaea.
- In the next phase of split further breaking of continents happens.
- Africa and India broke from Gondwanaland and started moving in north direction.
- The northward movement was caused by gravitational forces i.e., because of intense gravitational pull of the poles.
- Northward movement of India and Africa caused the sediments deposited in the Tethys to be folded and thus Alpine and Himalayan mountain ranges were formed.
- Westward movement was caused by tidal forces. Westward movement of American continent led to the formation of Andes and Rockies mountain chain and the Caribbean Islands Arcs.
- Evidence in support of the Continental drift:-
- Matching of Continents (Jig Saw Fit):- Map produced by computer programme to find the best fit of the Atlantic Margin proved to be perfect. The shorelines of African and South American continent between atlantic ocean have a remarkable match.
- Rocks of same age across the ocean:- Through the radiometric dating method scientists observed that rocks from Brazil coasts matches with those from Western Africa.
- Tillite: - Tillites are sedimentary rocks which are formed from sediments deposited by glaciers. At the base, Gondwana system has thick tillite deposition. Gondwana system of sediments have found its counterparts in different land masses like India, Africa, Falkland Island, Madagascar, Antarctica and Australia. Similarity of sediments among landmasses that are distance apart shows that initially they have similar histories.
- Placer Deposits:- A rich placer deposits of gold are found on the Ghana Coast in African Continent however there source were not found. The gold bearing veins were found in Brazil in South America hence it is obvious that these two places were initially, joint together.
- Distribution of Fossils:- Fossil of similar plants and animals have found in different landmasses which are separated by vast ocean shows that these land masses are initially joint.
- Force for drifting: - According to Wegener drifting of continents were caused by two forces. They are:
- Pole Fleeing Force.
- Tidal Force.
- Pole Fleeing Force:- This force causes the drift of continents equator wards or north ward and was caused by gravitational force i.e., gravitational pull of the poles. This force relates to the rotation of the earth as centrifugal force is generated during earth’s rotation. At the equator centrifugal force is greatest. According to Wegener this greatest centrifugal force at the equator has led to the generation of pole fleeing force.
- Tidal Force: - This force causes the drift of continents westwards. Wegener suggests that earth rotates from west to east so tidal force is towards east to west. According to Wegener this force is generated due to the gravitational attraction of the sun and the moon which also generates tides in the ocean.
- Post Drift Studies: -
- World War II has led to a number of discoveries related to oceans and continents by soldiers who were fighting from continents as well as ocean.
- This discoveries has improves the geological information which were not available during the time Wegener given continental drift theory.
- Convectional current theory, mapping of ocean floor, ocean floor configuration, distribution of earthquakes and volcanoes, concept of sea floor spreading, Plate Tectonics theory are the outcomes of later studies.
- Convectional Current Theory: -
- This was an attempt to address the issue of force, behind continental drift.
- On the basis of convectional current theory contemporary scientists discarded the continental drift theory.
- Convectional current theory was proposed by Arthur Holmes in 1930. According to him force behind the movement of continents is the convectional currents operating in the mantle of the earth.
- This convectional current is generated by the thermal differences in the mantle.
- And thermal differences are caused due to the radioactive energy produced by the radioactive elements.
- Concept of Sea Floor Spreading: -
- This theory was proposed by Harry Hess in 1961 after considering some facts which were revealed after mapping of ocean floor and Palaeomagnetic studies of rocks from oceanic region.
- Some of the facts observed are mentioned below:-
- Mid Oceanic Ridges are most active in terms of volcanic eruptions and hence huge amounts of lava comes out to the surface in that area through volcanic eruption.
- Volcanic eruptions are common in mid oceanic ridges. Huge amount of lava come out to the surface in these area.
- Significant similarities between rocks which are equidistant on either side of crest of Ridges are found.
- Rocks which are closer to oceanic Ridges are younger than those which are farther away from Ridges.
- Rocks of oceanic crust are much younger than the rock from continental crust.
- Hess argued that due to volcanic eruptions oceanic crusts ruptures and new lava wedges into it, pushing away the oceanic crust on either side, which causes the spread of oceanic crust. However two facts made the Hess to think again.
- Rocks from the oceanic crust were having younger age.
- Spreading of crust of one ocean has no effect on the area of another ocean.
- Because of the above observation Hess rethinks and finally maintains that due to volcanic eruption oceanic crusts rupture and lava wedges into it. Lava coming out from the mantle pushes the crust away to either direction and this pushed away oceanic crusts sinks down at the oceanic trenches and gets consumed keeping area of sea floor constant.
- Plate Tectonics: -
- This theory was proposed by Harry Hess however it was scientifically explained by McKenzie, Parker, Morgan and Holmes.
- Plate tectonics is a modern version of Continental drift theory.
- This theory explains the large scale motions of Earth’s lithosphere.
- Lithosphere includes the crust and upper mantle. Thickness of lithospheres varies between 5-100 km in ocean and about 200 km in continents.
- According to this theory whole of earth’s lithosphere is divided into small and big parts called plates. These plates are floating and moving horizontally over the semi-liquid Asthenosphere.
- This theory says that the earth’s lithosphere is divided into seven major and several minor plates.
- A plate is called continental or oceanic plate depending on which among the two occupies a major portion of the plate. Generally a plate is composed of both continental and oceanic lithosphere. Pacific plate is also largest among all the major seven plates and is largely an oceanic plate and Eurasian plate is a continental plate.
- Eurasian, Antarctica, North American, South American, Pacific, African and Indian Plate.
- Minor Plates: -
- Cocos plate: - It Lies between Central American plate and Pacific Plate.
- Nazca plate: - It Lies between South American plate and Pacific plate.
- Arabian plate: - It lies mostly on the Saudi Arabian landmass.
- Philippine plate: - It Lies Between the Asiatic and Pacific plate.
- Caroline plate: - It Lies Between the Philippine and Indian plate, North of New Guinea.
- Fuji plate: - It lies to the North-east of Australia.
- Variation in Continental drift theory:-
- Continents are parts of plates and these plates, not the continents moves over the globe.
- According to Wegener initially all the continents are joint together forming a super continent called Pangaea. However later discoveries says that continents which are parts of plates are moving throughout the geological period and Pangaea was a result of converging of all continents which are parts of one or other plates. Positions of continents are changing throughout the geological period. Scientists use paleomagnetic data to locate the position of different continental mass in different geological period.
- For determining the position of Indian sub continent during different geological period, rocks from Nagpur area were analysed.
- Types of plate boundaries: -Three types of plate boundaries were identified which are based on the movement of adjacent plates:-
- Divergent boundaries
- Convergent boundaries
- Transform boundaries
- Divergent Boundaries: -
- When the plates are moving away from each other in opposite direction divergent boundaries were formed.
- When the plates are moving in opposite direction cracks develops in lithosphere from which lava come out from mantle. This lava wedges into the cracks and hence forms new crust.
- Since divergent boundaries are forming new crust they are also called as constructive plate margins.
- Divergence of plates results in :
- Formation of new crust.
- Formation of mountains or ridges underwater in ocean.
- Convergent boundaries: -
- When two plates moves towards each other they collides and as a result subduction of plates happens in which denser plates dives under the lighter one.
- The location where sinking of plates occurs are called “Benioff Zone”.
- Since in convergent boundaries consumption of plates takes place, this plate margin is also called as destructive plate margin.
- There are three ways in which convergence of plates happens:
- Convergence between oceanic and continental plates.
- Convergence between two oceanic plates.
- Convergence between two continental plates.
- Transform boundaries: - These are the boundaries where two plates slide each other horizontally. These boundaries neither form nor destroy crust. Sandreas fault is a best example.
- Movement of Indian Plate: -
- Indian plate was initially associated with Australian plate south of the equator.
- Tethys Sea lies between India and Asia about 225 million years ago.
- About 200 million years ago India started her northward journey after broking from Pangaea and crossed the equator.
- In her northward journey after crossing equator India collided with Asia and Tethys sea filled up by the deposition of sediments.
- When India collided with Asian plate sediments which are filled in Tethys sea got scrambled up and the Himalayan Mountain forms.