Continental Drift Theory

 Introduction

In 1908, American geologist Frank B.Taylor proposed that the continents were once joined together in a single landmass and then drifted apart to their current positions. Taylor’s analysis of tectonic features foreshadowed modern plate collision theory. In particular, he suggested that the bow-shaped mountain belts of Asia and Europe were the result of the continents creeping towards the equator.

Continental Drift Theory was put forward by the German scientist Alfred Wegner in 1912.

According to the Continental Drift Theory, part of the crust are capable of horizontal movement round the globe causing the continents to slowly change their positions in relation to one another.

The fact that South America is a mirror image of Africa is presented as a proof of the continental drift theory.

For hundreds of millions of years, all the land of Earth was joined together in one large mass or super continent. Scientists call it Pangaea (meaning “all lands” in Greek). Then about 200 million years ago the land began to drift apart. It broke into two pieces, and scientists have called the continent in the north Laurasia and the continent in the south Gondwanaland  (named by Eduard Suess, an Austrian geologist).The two large continents continued to break apart into the smaller continents that exist today. Scientists call this movement ‘continental drift’.

 

Assumptions of Continental Drift Theory by Alfred Wegner

 

Forces responsible for drifting of continents (According to Alfred Wegner)

According to Wegener, the drift was in two directions:

1.     Towards the equator due to the interaction of forces of gravity, pole-fleeing force (due to centrifugal force caused by earth’s rotation) and buoyancy (ship floats in water due to buoyant force offered by water)

2.     Westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener).

• Wegener suggested that tidal force (gravitational pull of the moon and to a lesser extent, the sun) also played a major role.
• The polar-fleeing force relates to the rotation of the earth. Earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth (greater centrifugal force at the equator).
• Centrifugal force increases as we move from poles towards the equator. This increase in centrifugal force has led to pole fleeing, according to Wegener.
• Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters (tides explained in detail in oceanography).
• According to Wegener, these forces would become effective when applied over many million years, and the drift is continuing.

The evidences in support of the continental drift theory:

Jigsaw Fit:

The similarity in outline of the coastlines of eastern South America and West Africa had been noted for some time. The best fit is obtained if the coastlines are matched at a depth of 1,000 meters below current sea level.

Example:- The coastlines of many continents fit together like puzzle pieces, such as the coastlines of South America and Africa.

 Geological Fit:

When the geology of eastern South America and West Africa was mapped it revealed that ancient rock outcrops (cratons) over 2,000 million years old were continuous from one continent to the other.

Example:- resources of gold

 Tectonic Fit:

• Fragments of an old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today.
• Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia. When these land masses are re-assembled the mountain, belt forms a continuous linear feature.

Glacial Deposits:

• Today, glacial deposits formed during the Permo-Carboniferous glaciation (about 300 million years ago) are found in Antarctica, Africa, South America, India and Australia.
• If the continents haven’t moved, then this would suggest an ice sheet extended from the South Pole to the equator at this time – which is unlikely as the UK at this time was also close to the equator and has extensive coal and limestone deposits.
• If the continents of the southern hemisphere are re-assembled near the South Pole, then the Permo-Carboniferous ice sheet assumes a much more reasonable size

Fossil Evidence:

• There are many examples of fossils found on separate continents and nowhere else, suggesting the continents were once joined. If Continental Drift had not occurred, the alternative explanations would be:
• The species evolved independently on separate continents – contradicting Darwin’s theory of evolution.
• They swam to the other continent/s in breeding pairs to establish a second population. 

Example:- Fossils of the same species have been found on opposite sides of oceans. 

Criticism faced by Continental Drift Theory: 

• Wegener failed to explain why the drift began only in Mesozoic era and not before.

• The theory doesn’t consider oceans.

• Proofs heavily depend on assumptions that are generalist.

• Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents.

• Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift.

Continental Drift Theory 

 Continental drift theory was proposed by Alfred Wegener in 1912. 

 The theory deals with the distribution of the oceans and the continents. 

 According to Wegener’s Continental drift theory, all the continents were one single continental mass (called a Super Continent) – Pangaea and a Mega Ocean surrounded this supercontinent. The mega ocean is known by the name Panthalassa. 

 The supercontinent was named Pangaea and the Mega-ocean was called Panthalassa. 

 According to this theory, the supercontinent, Pangaea, began to split some two hundred million years back. 

 Pangaea first split into 2 big continental masses known as Gondwanaland and Laurasia forming the southern and northern modules respectively. 

 Later, Gondwanaland and Laurasia continued to break into several smaller continents that exist today. 

Evidence supporting the Continental Drift Theory 

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

 The coastlines of South America and Africa fronting each other have a remarkable and unique match. 

 In 1964, Bullard created a map using a computer program to find the right fit of the Atlantic margin and it proved to be quiet. 

2. Rocks of the Same Age across the Oceans 

 The radiometric dating methods have helped in correlating the formation of rocks present in different continents across the ocean. 

 The ancient rocks belts in the coast of Brazil match with those found in Western Africa. 

 The old marine deposits found in the coasts of South America and Africa belong to the Jurassic Age. This implies that the ocean never existed before that time.

3. Tillite 

 It is the sedimentary rock made from glacier deposits. 

 The Gondwana system of sediments from India is recognized as having its counterparts in 6 different landmasses in the Southern Hemisphere. 

 Counterparts of this series are found in Madagascar, Africa, Antarctica, Falkland Island, and Australia not to mention India. 

 At the base, the system has thick tillite signifying widespread and sustained glaciation. 

 Generally, the similarity of the Gondwana type sediments evidently shows that these landmasses had exceptionally similar origins. 

 The glacial tillite gives a clear evidence for palaeoclimates and the drifting of continents. 

4. Placer Deposits 

 The presence of abundant placer deposits of gold along the Ghana coast and the complete lack of its source rocks in the area is a phenomenal fact. 

 The gold-bearing veins are present in Brazil and it is evident that the gold deposits of Ghana in Africa are obtained from the Brazil plateau from the time when the two continents were beside each other. 

5. Distribution of Fossils 

 The interpretations that Lemurs occur in India, Africa and Madagascar led to the theory of a landmass named “Lemuria” connecting these 3 landmasses. 

 Mesosaurus was a tiny reptile adapted to shallow brackish water. 

 The skeletons of these creatures are found in the Iraver formations of Brazil and Southern Cape Province of South Africa. 

Force for Drifting 

 Wegener proposed that the movement accountable for the drifting of the continents was instigated by tidal force and pole-fleeing force. 

 The polar-fleeing force relates to the rotation of the earth. 

 The shape of earth 

 The second force that was proposed by Wegener, the tidal force. 

 Though, most of the scholars considered these forces to be totally insufficient.