As continents cover 29% of the surface of the earth and the remainder is under oceanic waters. The position of the continents and oceanic bodies as we see in the map have not been same in the past. Moreover, it is now a well-accepted fact that oceans and continents will not continue to enjoy their present positions in times to come.
Now, we will understand the concept behind this.
Alfred Wegener—a German meteorologist who put forth a comprehensive argument in the form of “the continental drift theory” in 1912. This was regarding the distribution of the oceans and the continents.
According to Wegener,
All the continents formed a single continental mass and mega ocean surrounded the same.
The super continent was named PANGAEA, which meant all earth.
The mega-ocean was called PANTHALASSA, meaning all water.
A variety of evidence was offered in support of the continental drift.
Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation of the earth. As the earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth.
The tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters.
Wegener believed that these forces would become effective when applied over many million years.
However, most of scholars considered these forces to be totally inadequate.
the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.
Study the maps showing the distribution of seismic activity and volcanoes given in Figure.
You will notice a line of dots in the central parts of the Atlantic Ocean almost parallel to the coastlines. It further extends into the Indian Ocean. It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana. You will notice that this line of dots coincides with the mid-oceanic ridges. The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean.
In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas, along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones. The map of volcanoes also shows a similar pattern. The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area.
The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.
These rates vary considerably. The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
The Indian plate includes Peninsular India and the Australian continental portions. The subduction zone along the Himalayas forms the northern plate boundary in the form of continent— continent convergence.
In the east, it extends through Rakinyoma Mountains of Myanmar towards the island arc along the Java Trench.
The eastern margin is a spreading site lying to the east of Australia in the form of an oceanic ridge in SW Pacific.
The Western margin follows Kirthar Mountain of Pakistan. It further extends along the Makrana coast and joins the spreading site from the Red Sea rift southeastward along the Chagos Archipelago. The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.