Ch 5 Continental Drift Theory

Structure

1. Objectives 2. Introduction 3. Continental Drift 4. Evidences 5. Forces for drifting 6. What continental drift theory explained 7. Evaluation 8. Summary 9. Prelims Questions 10. Mains Questions 11. Trend Analysis

Objectives

After completing this chapter, you will be able to

Understand Continental Drift Theory

Summarize the evidences in its support

Explain the forces of continental drift theory

Evaluate the theory

Introduction

Continents cover 29 per cent of the surface of the earth and the remainder 71 percent is under oceanic waters. The positions of the continents and the ocean bodies, as we see them in the map, have not been the same in the past. Moreover, it is now a well-accepted fact that oceans and continents will not remain in their present positions in times to come.

Different views, concepts, hypotheses and theories regarding the origin and position of the continents and oceans basins have been put forth by the scientists from time to time. One of them which challenged permanency of Oceans and Continents was Continental Drift Theory by Alfred Wegner.

CONTINENTAL DRIFT

Alfred Wegener, a German meteorologist 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.

Continental Drift is the hypothesis that the earth’s continent ahs moved over geologic time relative to each other, thus appearing to have drifted across the ocean bed.

According to Wegener, there was The super continent was named PANGAEA surrounded by the mega- ocean called PANTHALASSA 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. Bullard in 1964 produced a map using a computer programme to find the best fit of the African and South American coast line proved to be quite perfect. The match was tried at 1,000- fathom line instead of the present shoreline.

1 fathom = a unit of length equal to six feet (1.83 meters) used especially for measuring the depth of water

2. Rocks of Same Age Across the Oceans

The belt of ancient rocks of 2 billion 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 Atlantic ocean did not exist between them prior to that time.

3. Tillite

The Gondawana system of sediments from India is known to have its counterparts in Africa, Falkland Island, Madagascar, Antarctica and Australia. Counterparts of this succession are found in Overall resemblance of the Gondawana-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.

Paleoclimatology is the study of past climates. Since it is not possible to go back in time to see what climates were like, scientists use imprints created during past climate, known as proxies, to interpret paleoclimate. Organisms, such as diatoms, forams, and coral serve as useful climate proxies.

Tillite is a sedimentary rock that consists of consolidated masses of unweathered blocks (large, angular, detached rock bodies) and glacial till (unsorted and unstratified rock material deposited by glacial ice) in a rock flour (matrix or paste of unweathered rock).

4. Placer Deposits

The occurrence of rich placer deposits of gold in the Ghana coast and the presence of the gold bearing veins in Brazil. This obviously shows 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 are presently 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. You are aware of the fact that 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.

What Continental Drift Theory also explained?

It explained mountain building at the front edges of continental blocks. According to Wegener while moving North-westwards American continent crumpled and folded against resistance of rocks on seafloor thus led to the formation of Rockies and Andes mountains.

It also explained origin of island arc at the tail edges of continental block due to differential rate of continental drifting.

Evaluation of Continental Drift Theory

The forces for drifting given by Wegener were not enough to move the continents. Wegener did not elaborate the direction of continental drifting.

The resistance of sea floor by continents that led to formation of Mountains was not well thought argument.

Although there were some drawbacks in the theory but it was watershed moment in the geography in terms of shifting its attention from theory of permanency of oceans and continents towards their movement. And later this theory developed in the well explained Plate tectonics theory.

Summary

1. Continents cover 29 per cent of the surface of the earth and the remainder 71 percent is under oceanic waters. The positions of the continents and the ocean bodies, as we see them in the map, have not been the same in the past. 2. Alfred Wegener, a German meteorologist put forth a comprehensive argument in the form of “the continental drift theory. 3. Continental Drift is the hypothesis that the earth’s continent ahs moved over geologic time relative to each other, thus appearing to have drifted across the ocean bed. 4. Evidences in its support are The Matching of Continents (Jig-Saw-Fit), Rocks of Same Age Across the Oceans, Tillite, Placer Deposits, Distribution of Fossils. 5. Drifting of the continents was caused by pole-fleeing force and tidal force.

Previous Years Prelims Questions

Which of the following phenomena might have influenced the evolution of organisms? (2014)

1. Continental drift

2. Glacial cycles

Select the correct answer using the code given below.

A. 1 only

B. 2 only

C. Both 1 and 2

D. Neither 1 nor 2

Answer 1(c)

Previous Years Mains Questions

Q. What do you understand by continental drift? Discuss the prominent evidences in its support? (2013)

Approach: This is the direct question.

Introduction: Talk about Alfred Wegener and Continental Drift Theory.

Body: Discuss evidences Conclude with Evaluation.

Trend Analysis

This is an important chapter along with the next two chapter on sea floor spreading and continental drift theory to understand the various landforms that developed on the earth’s surface. UPSC has asked few direct questions and many indirect questions from this chapter.