Chapter - 4
Distribution of Oceans and Continents
️ Formation of continents and oceans :-
Since the origin of the Earth, continents and oceans were formed about 3.8 billion years ago , but these continents and oceans were not in the form they are today. Many scientists from time to time tried to prove that all the continents were together in the early stages of formation.
Continental Displacement Theory: -
German scholar Alfred Wegener , in this sequence, presented the continental drift theory in 1912 . Wegener assumed that in the Carboniferous era, all the land parts were connected to each other as one large site . Wegener named this vast terrestrial part as Pangea .
Wegener thought that some parts of Pangea began to move towards the equator. This process started in the last Carboniferous era, about 300 million years ago . In the Pleistocene epoch about 5-6 million years ago, the continents had assumed a shape almost similar to the present situation.
️ Pangea :-
All the continents of today were part of a single landmass called Pangea.
Two large continental bodies came into existence from the split of Pangea .
- Laurasia (Northern Lands)
- Gondwana Land (Southern Plot)
️ Panthalassa :-
The vast ocean around Pangea was called Panthalassa.
Evidence in favor of displacement of continents :-
Equivalence in Continents: - If we look carefully at the shape of the continents, then we will find that the coast lines opposite to each other shows amazing similarity.
Similarity in age of rocks across oceans: - There is a similarity in the age of rocks of two continents which are currently far from each other, for example, a strip of 200 million years old rock groups, Brazil coast (South America) and South America. . Found on the coast of Africa, it shows that the two continents were together in ancient times.
Tillite:- These are sedimentary rocks formed by glacial deposition. Patterns of such deposits are found in six different site blocks of the southern hemisphere, which are evidence of their association in ancient times.
Placer deposits: - Veins containing gold are found in Brazil while placer deposits are found in Ghana, this proves that the . America and Africa were once one place.
Distribution of Fossils:- On some continents remains of such organisms are found which are not presently found at that place.
Which forces were responsible for continental displacement by Wegener :-
According to Wegener, there are two causes of continental displacement .
Polar Flying Force:- Due to the rotation of the earth, the continents moved from their place. ,
Tidal force: - Tidal force is related to the attraction of the Sun and the Moon, due to this force of attraction, continental segments can be displaced.
️ Reasons for the origin and formation of currents in the mantle :-
Convection currents in the mantle arise from the temperature variation produced by radioactive elements . A system of such currents exists in the entire mantle. Convection currents are due to radioactive elements.
️ Mid-Oceanic Ridge :-
The mid-ocean ridge is a chain of interconnected mountains from north to south in the middle of the Atlantic Ocean, which is submerged in ocean water.
️ Plate tectonic theory :-
The biggest challenge in accepting the theory of continental drift in the early part of the twentieth century was that scholars did not understand how the continents formed by Sial floated and displaced along the boundary.
At that time, it was the view of scholars that the oceanic crust is an extension of the basaltic layer. Arthur Holmes in 1928 AD. It was told that due to the difference in temperature in the earth, there are conductive currents which give motion to the plates. In this way the plates are always in motion and cause displacement in the continents.
Meaning of plate in plate tectonics theory :-
Consisting of continental and oceanic landforms, a large land block of solid and irregular shape which is in the form of a rigid unit. called plate.
According to the 'Plate Tectonics Theory', there are seven main and some minor plates :-
Main plates :-
- 1) Antarctic Plate
- 2) North American plate.
- 3) South American plate.
- 4) Pacific Ocean Plate.
- 5) Indo-Australian plate.
- 6) Africa plate.
- 7) Eurasian plate.
Small plates :-
- 1 ) Cocos Plate
- 2 ) Najka plate
- 3) Arabian Plate
- 4) Philippine plate
- 5 ) Carolyn Plate
- 6) Fuji plate
️ Plato's Movement :-
Divergent limit :-
- In this, two plates move apart from each other in the opposite direction.
- In this a new crust is formed.
- It is also called the spreading space.
- An example of this is the Mid-Atlantic Ridge.
Convergence Limit :-
- In this two plates come close to each other.
- One plate sinks under another plate and there the crust is destroyed.
- It is also called the Subduction zone.
- Examples of this are the Pacific Ocean Plate and the American Plate.
Conversion Limit :-
When two tectonic plates move horizontally along with each other, but neither the formation nor destruction of new crust takes place, such a boundary is called a metamorphic boundary.
️ Ring of Fire :-
A chain of active volcanoes is found on the edge of the Pacific Ocean, which is called Ring of Fire or Ring of Fire.
The difference between Wegener's Continental Drift Theory and Plate Tectonics Theory :-
Wegener's concept only tells the continents to move. Whereas the continent is part of a lithospheric plate and this entire plate is in motion .
According to Wegener, initially Pangea was an organized form of all the continents . Whereas later discoveries proved that the continental fragments that lie on top of the plate were in motion throughout geological time, and Pangea was formed by the convergence of different continental fragments and this process continues in the plates.
Wegener's theory could not explain the novelty of ocean floor rocks and the presence of mid-oceanic ridges. While it can be explained by plate tectonics.
Wegener's theory could not explain the novelty of ocean floor rocks and the very antiquity of continental rocks.
Wegener's theory considered the polar flying force and tidal force responsible for the movement of the continents. While both these forces were unable to govern the continents. The reason for the movement of plates is the convective currents running in the weakness zone. Due to which the plates keep moving.
️ How the equality was proved in the continents :-
In 1964, Bullard prepared a map connecting the Atlantic coasts with the help of a computer program, in which the balance of the coasts proved to be perfect.
Continental similarity: -
A zig-saw-fit is visible in the boundaries of the continents . If North America and South America are combined with the borders of Europe and Africa, then there will be a lot of uniformity in these boundaries.
️ Reasons for the initiation and persistence of convection currents in the mantle :-
Convection currents in the mantle arise from the temperature variation produced by radioactive elements. The system of such currents exists in the entire mantle. Convection currents are due to radioactive elements.
️ Hypothesis of development of ocean floor :-
The hypothesis of the development of the ocean floor was presented by Henry Hess in 1961. He said this on the basis of analysis of the magnetic properties of rocks on both sides of the mid-oceanic ridges.
According to Hess, continuous, volcanic eruptions at the top of the oceanic ridges led to the infiltration of the oceanic crust and new lava is filling this crack and pushing the oceanic crust to both sides. In this way the ocean floor is expanding.
Relatively latest of the oceanic crust, as well as the expansion in one ocean to the other, not shrinking, Hess spoke of the deposition of the oceanic crust. According to him, if a new crust is formed by a volcanic eruption in the mid-oceanic ridge, then on the other hand the crust is destroyed in the ocean trenches.
The main three belts of earthquake and volcano :-
First region: - In the central part of the Atlantic Ocean, there is a chain of earthquakes and volcanoes parallel to the coastline, which goes further to the Indian Ocean.
Second region: - Alpine to Himalayan ranges and are similar to the Pacific Ocean coast.
Third region: - It is in the form of a ring on the edge of the Pacific Ocean, which is also called (Ring of Fire).
