Physical geography of India and World
Economic and Human Geography of India and World

Distribution of Ocean and Continents

Continental Drift Theory – Alfred Wegener
Evidence of Continental Drift Theory
Distribution of Oceans and Continents
Continental Drift Theory | Plate Tectonics | Sea Floor Spreading

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.

Continental Drift

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.

  • He argued that, around 200 million years ago, the super continent, Pangaea, began to split.
  • Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively.
  • Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.

A variety of evidence was offered in support of the continental drift.

Evidence in Support of the Continental Drift

  • The Matching of Continents (Jig-Saw-Fit)
  • Rocks of Same Age Across the Oceans
  • Tillite
  • Placer Deposits
  • Distribution of Fossils

Reasons of Continental Drift

Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.

Pole Fleeing 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

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.

Distribution of Earthquakes and Volcanoes

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.

Plate Tectonics

  • A tectonic plate (lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
  • Plates move horizontally over the asthenosphere as rigid units.
  • The lithosphere includes the crust and top mantle with its thickness range varying between 5 and100 km in oceanic parts and about 200 km in the continental areas.
  • A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
  • Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate.

The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.

  1. Antarctica and the surrounding oceanic plate
  2. North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
  3. South American (with western Atlantic floor separated from the North American plate along the Caribbean islands) plate
  4. Pacific plate
  5. India-Australia-New Zealand plate
  6. Africa with the eastern Atlantic floor plate
  7. Eurasia and the adjacent oceanic plate.
Major and minor plates of the world
  • These plates have been constantly moving over the globe throughout the history of the earth.
  • It is not the continent that moves as believed by Wegener.
  • Continents are part of a plate and what moves is the plate. Moreover, it may be noted that all the plates, without exception, have moved in the geological past, and shall continue to move in the future as well.
  • Wegener had thought of all the continents to have initially existed as a super continent in the form of Pangaea.
  • However, later discoveries reveal that the continental masses, resting on the plates, have been wandering all through the geological period, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.
  • Scientists using the palaeomagnetic data have determined the positions held by each of the present continental landmass in different geological periods.
  • Position of the Indian sub-continent (mostly Peninsular India) is traced with the help of the rocks analysed from the Nagpur area.

Rates of Plate Movement

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).

Movement of the Indian Plate

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.