About

• By Alfred Wegener—a German meteorologist 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.

Evidence in Support of the Continental Drift

1. The Matching of Continents (Jig-Saw-Fit)

• The shorelines of Africa and South America facing each other have a remarkable and unmistakable match.
• It may be noted that a map produced using a computer programme to find the best fit of the Atlantic margin was presented by Bullard in 1964.
• It proved to be quite perfect.
• The match was tried at 1,000- fathom line instead of the present shoreline.

2. Rocks of Same Age Across the Oceans

• The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.
• The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa.
• The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age.
• This suggests that the ocean did not exist prior to that time.

3. Tillite

• It is the sedimentary rock formed out of deposits of glaciers.
• The Gondwana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere.
• At the base the system has thick tillite indicating extensive and prolonged glaciation.
• Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India.
• Overall resemblance of the Gondwana type sediments clearly demonstrates that these landmasses had remarkably similar histories.
• The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.

4. Placer Deposits

• The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact.
• The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.

5. Distribution of Fossils

• When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution.
• The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
• Mesosaurus was a small reptile adapted to shallow brackish water.
• The skeletons of these are found only in two localities: the Southern Cape province of South Africa and Iraver formations of Brazil.
• The two localities presently are 4,800 km apart with an ocean in between them.

Force for Drifting

• 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.
• 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 second force that was suggested by Wegener—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.

Post-Drift Studies

• It is interesting to note that for continental drift, most of the evidence was collected from the continental areas in the form of distribution of flora and fauna or deposits like tillite.
• A number of discoveries during the post-war period added new information to geological literature.
• Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.

Convectional Current Theory

• Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion.
• These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
• Holmes argued that there exists a system of such currents in the entire mantle portion.
• This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.

Mapping of the Ocean Floor

• Detailed research of the ocean configuration that the ocean floor is not just a vast plain but it is full of relief.
• Expeditions to map the oceanic floor in the post-war period provided a detailed picture of the ocean relief and indicated the existence of submerged mountain ranges as well as deep trenches, mostly located closer to the continent margins.
• The mid-oceanic ridges were found to be most active in terms of volcanic eruptions.
• The dating of the rocks from the oceanic crust revealed the fact that they are much younger than the continental areas.
• Rocks on either side of the crest of oceanic ridges and having equidistant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age.

Divisions of the Ocean Floors

(i) the Continental Shelf;  (ii) the Continental Slope;  (iii) the Deep Sea Plain; (iv) the Oceanic Deeps.

Besides these divisions there are also major and minor relief features in the ocean floors like ridges, hills, sea mounts, guyots, trenches, canyons, etc.

Continental Shelf

• The continental shelf is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less.
• The shelf typically ends at a very steep slope, called the shelf break.
• The average width of continental shelves is about 80 km.
• The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width.
• The depth of the shelves also varies.
• It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m.
• Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.

Continental Slope

• The continental slope connects the continental shelf and the ocean basins.
• It begins where the bottom of the continental shelf sharply drops off into a steep slope.
• The gradient of the slope region varies between 2-5°.
• The depth of the slope region varies between 200 and 3,000 m. The slope boundary indicates the end of the continents.
• Canyons and trenches are observed in this region

Deep Sea Plain

• Deep sea plains are gently sloping areas of the ocean basins.
• These are the flattest and smoothest regions of the world.
• The depths vary between 3,000 and 6,000m.
• These plains are covered with fine-grained sediments like clay and silt.

Oceanic Deeps or Trenches

• These areas are the deepest parts of the oceans.
• The trenches are relatively steep sided, narrow basins.
• They are some 3-5 km deeper than the surrounding ocean floor.
• They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes.
• That is why they are very significant in the study of plate movements.
• As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean.

Minor Relief Features

Apart from the above mentioned major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans.

Ocean Floor Configuration

• The ocean floor may be segmented into three major divisions based on the depth as well as the forms of relief.
• These divisions are continental margins, deep-sea basins and mid-ocean ridges.

Continental Margins

• These form the transition between continental shores and deep-sea basins.
• They include continental shelf, continental slope, continental rise and deep-oceanic trenches.
• The deep-oceanic trenches are the areas which are of considerable interest in so far as the distribution of oceans and continents is concerned

Abyssal Plains

• These are extensive plains that lie between the continental margins and mid-oceanic ridges.
• The abyssal plains are the areas where the continental sediments that move beyond the margins get deposited.

Mid-Oceanic Ridges

• A mid-oceanic ridge is composed of two chains of mountains separated by a large depression.
• The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface.
• Iceland, a part of the mid- Atlantic Ridge, is an example
• This forms an interconnected chain of mountain system within the ocean.
• It is the longest mountain-chain on the surface of the earth though submerged under the oceanic waters.
• It is characterised by a central rift system at the crest, a fractionated plateau and flank zone all along its length.
• The rift system at the crest is the zone of intense volcanic activity.

Submarine Canyons

• These are deep valleys, some comparable to the Grand Canyon of the Colorado river.
• They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers.
• The Hudson Canyon is the best known submarine canyon in the world.

Guyots

• It is a flat topped seamount.
• They show evidences of gradual subsidence through stages to become flat topped submerged mountains.
• It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.

Atoll

• These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression.
• It may be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water.