Distribution Of Oceans And Continents Class 11 Notes Geography Chapter 4 - CBSE

Chapter : 4

What Are Distribution Of Oceans And Continents ?

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Earth is a blue planet because it consists of 71% water and 29% land.
There were many theories related to the current shape and position of continents and oceans. But, the most popular and reliable one is given by Alfred Wegener called as The Continental Drift Theory.
This theory states that the all the current continents were part of single super-continent called as ‘Pangaea’ and the water mass was also united called ‘Panthalasa’.
Gradually the landmass started drifting apart. Initially it got split into two parts Laurasia and Gondwanaland forming the northern and southern components respectively.

Five Evidences In Support Of The Theory

Following are the statements in support of Wegener’s theory:

The Matching of Continents (Jigsaw Fit): The current shape of continents resembles a big jigsaw. For example, the shorelines of Africa and South America facing each other have a remarkable and unmistakable match.
Rocks of Same Age Across the Oceans: There is a connection between ancient rocks of Brazil coast and those from western Africa. Moreover, the coastlines of South America and Africa have similar marine deposits of the Jurassic age. This suggests that the landmass of these continents were united.
Tillite: The Gondwana system of sediments from India is known to have its counterparts in six different
landmasses of the Southern Hemisphere.
Placer Deposits: The source of rich placer deposits of gold in the Ghana coast is derived from the Brazil plateau, when the two continents lay side by side.
Distribution of Fossils: The observations that Lemurs occurred in India, Madagascar, and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.

According to Wegener, the movement responsible for the drifting of the continents were polar-fleeing force
and tidal force.
Due to the temperature variation in earth’s mantle, the convection current develops.
Detailed picture of the ocean relief indicated the existence of submerged mountain ranges as well as deep trenches, mostly located closer to the continental margins.

Ocean Floor Configuration

The study of ocean floor configuration helps in understanding the distribution of continents and oceans.

Continental Margins are the links between continental shores and deep-sea basins. They include continental shelf, continental slope, continental rise, and deep oceanic trenches.
Abyssal Plains are the depositional plains fund between continental margins and mid-oceanic ridge.
Mid Oceanic Ridge are the interconnected chains of mountain system within the ocean. At the centre there is a rift system which is a zone of volcanism.

Seismic Region

Some major regions of seismic activity are: The central parts of the Atlantic Ocean, Alpine-Himalayan system and the rim of the Pacific Ocean.
The foci of the earthquakes vary from shallow depth at mid-oceanic ridges to deep-seated along the Alpine-Himalayan belt as well as the rim of the Pacific Ocean.

Sea Floor Spreading

In 1920, American Geophysicist Harry H. Hess stated that the constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. Thus, the ocean floor spreads.

The revival and addition to Wegener’s theory occurred in 1967, when McKenzie and Parker and also Morgan evolved the theory of ‘Plate Tectonic’.
A tectonic plate is a solid mass of lithosphere which lies on unstable aesthenosphere. This means tectonic
plates are unstable too.
These plates can be continental or oceanic and can be categorized as major and minor plates.

Plates Boundaries

There are three types of plate boundaries:

Divergent Boundaries: In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. The best example of divergent boundaries is the mid-Atlantic Ridge.
Convergent Boundaries: In plate tectonics, a convergent boundary, also known as a convergent plate boundary or a destructive plate boundary, is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move towards one another and collide.
Transform Boundaries: It refers to the boundary where the crust is neither produced nor destroyed as the plates slide horizontally past each other. Transform faults are the planes of separation generally perpendicular to the mid-oceanic ridges.

The movement of tectonic plates ranges from 2.5 cm/year of Arctic ridge to 15 cm/year of South Pacific.
The unstable Aesthenosphere (upper mantle) results in the instability of tectonic plates.
Indian landmass, part of Indo-Australian plate, broke from Pangea some 200 million years ago.
The disappearance of Tethys Sea, formation of Deccan Trap after outpouring of lava happened after Indian
plate got split from Gondwanaland.
India remained an island for millions of years before its journey was stopped by Eurasia.
About 40 million years ago the Indian plate converged with the Eurasia plate, consequently, the subduction zone along the Himalayas formed the northern plate boundary in the form of continent—continent convergence.