Continental Drift Theory of Wegener | Geography

In this article we will discuss about:- 1. Aim of the Continental Drift Theory 2. Basic Premise of the Continental Drift Theory 3. Evidences 4. Process 5. Evaluation.

Contents:

1. Aim of the Continental Drift Theory
2. Basic Premise of the Continental Drift Theory
3. Evidences in Support of the Continental Drift Theory
4. Process of the Continental Drift Theory
5. Evaluation of the Continental Drift Theory

1. Aim of the Continental Drift Theory:

Professor Alfred Wegener of Germany was primarily a meteorologist. He propounded his concept on continental drift in the year 1912 but it could not come in light till 1922 when he elaborated his concept in a book entitled ‘Die Entstechung der Kontinente and Ozeane’ and his book was translated in English in 1924.

Wegener’s displacement hypothesis was based on the works and findings of a host of scientists such as geologists, palaeo-climatologists, palaeontologists, geophysicists and others.

The main problem before Wegener, which needed explanation, was related to climatic changes. It may be pointed out that there are ample evidences which indicate widespread climatic changes throughout the past history of the earth. In fact, the continental drift theory of Wegener ‘grew out of the need of explaining the major variations of climate in the past’.

The climatic changes which have occurred on the globe may be explained in two ways:

(1) If the continents remained stationary at their places throughout geological history of the earth, the climatic zones might have shifted from one region to another region and thus a particular region might have experienced varying climatic conditions from time to time.

(2) If the climatic zones remained stationary the land masses might have been displaced and drifted.

Wegener opted for the second alternative as he rejected the view of the permanency of continents and ocean basins. Thus, the main objective of Wegener behind his ‘displacement hypothesis’ was to explain the global climatic changes which are reported to have taken place during the past earth history.

2. Basic Premise of the Continental Drift Theory:

Following Edward Suess, Wegener believed in three layers system of the earth e.g., outer layer of ‘sial’, intermediate layer of ‘sima’ and the lower layer of ‘nife’. According to Wegener sial was considered to be limited to the continental masses alone whereas the ocean crust was represented by upper part of sima. Continents or sialic masses were floating on sima without any resistance offered by sima.

He assumed, on the basis of evidences of palaeo-climatology, palae­ontology, paleobotany, geology and geophysics, that all the landmasses were united together in the form of one landmass, which he named Pangaea, in carbonifer­ous period. There were several smaller inland seas scattered over the Pangaea which was surrounded by a huge water body, which was named by Wegener as ‘Panthalasa’, representing primeval Pacific Ocean.

Lauratia consisting of present North America, Europe and Asia formed northern part of the Pangaea while Gondwanaland consisting of South America, Africa Madagascar (now Malagasy), Peninsular India, Aus­tralia and Antarctica represented the southern part of the Pangaea. South Pole was located near present Durban (near Natal in southern Africa) during carbon­iferous period.

Thus, Wegener’s theory of continental drift begins from carboniferous period, he does not describe the conditions during pre-carboniferous times but the postulation of a carboniferous Pangaea does not mean that he disbelieves in pre-carboniferous drift: events before this time are known with much less certainty, and the distribution of plants and animals can largely be explained by movements which have taken place since the carboniferous’.

The Pangaea was disrupted during subsequent periods and broken landmasses drifted away from each other and thus the present position of the continents and ocean basins became possible.

3. Evidences in Support of the Continental Drift Theory:

Wegener has successfully attempted to prove the unification of all landmasses in the form of a single Iandmass, the Pangaea, during carboniferous period, on the basis of evidences gathered from geological, climatic and floral records. He claimed that all the present-day continents could be joined to form Pangaea.

The following evidences support the concept of the existence of Pangaea during carboniferous period:

(1) According to Wegener there is geographical similarity along both the coasts of the Atlantic Ocean. Both the opposing coasts of the Atlantic can be fitted together in the same way as two cut off pieces of wood can be refitted (jig-saw fit) (fig. 5.3).

(2) Geological evidences denote that the Cal­edonian and Hercynian mountain systems of the west­ern and eastern coastal areas of the Atlantic are similar and identical (fig. 5.4). The Applachians of the north­eastern regions of North America are compatible with the mountain systems of Ireland, Wales and north­western Europe.

(3) Geologically, both the coasts of the Atlantic are also identical. Du Toit, after detailed study of the eastern coasts of South America and western coast of Africa, has said that the geological structures of both the coasts are more or less similar. According to Du Toit both the landmasses (i.e., South America and Africa) cannot be actually brought together but near to each other because a gap of 400-800 km would sepa­rate them due to the existence of continental shelves and slopes of these two landmasses.

(4) There is marked similarity in the fossils and vegetation remains found on the eastern coast of South America and the western coast of Africa.

(5) It has been reported from geodetic evidences that Greenland is drifting westward at the rate of 20 cm per year. The evidences of seafloor spreading after 1960 have confirmed the movement of landmasses with respect to each other.

(6) The lemmings (small sized animals) of the northern part of Scandinavia have a tendency to run westward when their population is enormously in­creased but they are foundered in the sea water due to absence of any land beyond Norwagian coast. This behaviour of lemmings proves the fact that the land- masses were united in the ancient times and the ani­mals used to migrate to far off places in the western direction.

(7) The distribution of glossopteris flora in India, South Africa, Australia, Antarctica, Falkland islands etc. proves the fact that all the landmasses were previously united and contiguous in the form of Pangaea.

(8) The evidences of carboniferous glaciation of Brazil, Falkland, South Africa, Peninsular India, Aus­tralia and Antarctica further prove the unification of all landmasses in one landmass (Pangaea) during carbon­iferous period.

4. Process of the Continental Drift Theory:

As stated earlier the main aim of Wegener behind the postulation of his ‘drift theory’ was to explain major climatic changes which are reported to have taken place in the past geological history of the earth, such as carboniferous glaciation of major parts of the Gondwanaland. Besides, Wegener also attempted to solve other problems of the earth e.g., origin of mountains, island arcs and festoons, origin and evolu­tion of continents and ocean basins etc.

(1) Force responsible for the drift:

According to Wegener the continents after breaking away from the Panagaea moved (drifted) in two directions e.g.:

(i) Equatorward movement and

(ii) Westward movement.

The equatorward movement of sialic blocks (continen­tal blocks) was caused by gravitational differential force and force of buoyancy. As already stated the continental blocks, according to Wegener, were formed of lighter sialic materials (silica and aluminium) and were floating without any friction on relatively denser ‘sima’.

Thus, the equatorward movement of the sialic blocks (continental blocks) would depend on the rela­tion of the centre of gravity and the center of buoyancy of the floating continental mass. Generally, these two types of forces operate in opposite directions. ‘But because of the ellipsoidal form of the earth, these forces are not in direct opposition, but are so related that, if the buoyancy point lies under the centre of gravity, the resultant (force) is directed toward the equator’.

The westward movement of the continents was caused by the tidal force of the sun and the moon. According to Wegener the attractional force of the sun and the moon, which was maximum when the moon was nearest to the earth, dragged the outer sialic curst (continental blocks) over the interior of the earth, towards the west. It may be pointed out that in any drift theory the weakest point and the most difficult prob­lem is related to the competent force responsible for the movement of the continents.

‘Such a force (tidal force/attractional force of the sun and the moon) is extraor­dinarily small, but, as in the case of other forces, the question of time is all important: given sufficient time, it is claimed that even these very small forces are able to cause movements’.

(2) Actual drifting of the continents:

The disrup­tion, rifting and ultimately drifting of the continental blocks began in carboniferous period. The movement of the continental blocks away from the poles was dramatically called by Wegener as ‘the flight from the poles’. Pangaea was broken into two parts due to differential gravitational force and the force of buoy­ancy. The northern part became Lauratia (Angaraland) while the southern part was called by Wegener as Gondwanaland.

The intervening space between these two giant continental blocks was filled up with water and the resultant water body was called Tethys Sea. This phase of the disruption of Pangaea is called ‘opening of tethys’. Gondwanaland was disrupted dur­ing cretaceous period and Indian peninsula, Madagas­car, Australia and Antarctica broke away from Pangaea and drifted apart under the impact of tidal force of the sun and the moon. North America broke away from Angaraland and drifted westward due to tidal force.

Similarly, South America broke away from Africa and moved westward under the impact of tidal force. Due to northward movement of Indian Peninsula Indian Ocean was formed while the Atlantic Ocean was formed due to westward movement of two Americas.

It may be mentioned that North and South Americas were drifting westward at different rates and hence ‘S’ shape of the Atlantic Ocean could be possible Arctic and North Sea were formed due to flight of the continental blocks from north pole.

The size of the panthalasa (primitive Pacific Ocean) was remarkably reduced because of the movement of continental blocks from all sides towards Panthalasa. Thus, the remaining portion of Panthalasa became the Pacific Ocean. It may be mentioned that disruption, rifting and displacement (drifting) of continental blocks continued from car­boniferous period to pliocene period when the present pattern and arrangement of the continents and ocean basins was attained (fig. 5.5). There have been fre­quent changes in the positions of the equator and the poles as given in table 5.1.

Equator was located at the most northerly location during Silurian period as it passed north of Norway. It passed through London during carbonif­erous period and through present locations of the European Alpine mountains during tertiary period (fig. 5.6.). ‘The south pole and equator obviously moved into accordant positions. The prevailing westward and equator ward movement must be referred to these positions’.

(3) Mountain building:

A.G. Wegener also at­tempted to solve the problem of the origin of folded mountains of tertiary period on the basis of his conti­nental drift theory. The frontal edges of westward drifting continental blocks of North and South Ameri­cas were crumpled and folded against the resistance of the rocks of the sea-floor (sima) and thus the western Cordilleras of the two Americas (e.g. Rockies and Andes and other mountain chains associated with them) were formed.

Similarly, the Alpine ranges of Eurasia were folded due to equator ward movement of Eurasia and Africa together with Peninsular India (equator was passing through Tethys sea at that time). Here, Wegener postulated contrasting viewpoints. According to Wegener sial (continental blocks) was floating upon sima without any friction and resistance but during the latter part of his theory he pointed out that mountains were formed at the frontal edges of floating and drifting continental blocks (sialic crust) due to friction and resistance offered by sima.

How could it be possible? The question remains unanswered. Inspite of this serious flaw in the continental drift theory of Wegener, S.W. Wooldridge and R.S. Morgan have remarked, ‘certainly the problem of mountain building is one in which the hypothesis of continental drift solves more difficulties than it creates.’

(4) Origin of island arcs:

Wegener has related the process of the origin of island arcs and festoons (of eastern Asia, West Indies and the arc of the southern Antilles between Tierra del Fugo and Antarctica) to the differential rates of continental drift. When the Asiatic block (part of Angaraland) was moving westward, the eastern margin of this block could not keep pace with the westward moving major landmass, rather lagged behind, consequently the island arcs and festoons consisting of Sakhalin, Kurile, Japan, Phillippines etc. were formed. Similarly, some portions of North and South Americas, while they were moving westward, were left behind and the island arcs of West Indies and southern Antilles were formed.

(5) Carboniferous glaciation:

There are ample evidences to demonstrate that there was large-scale glaciation during carboniferous period when Brazil, Falkland, Southern Africa, Peninsular India, Australia, Antarctica etc. were extensively glaciated. According to Wegener all the continental blocks were united together in the form of one land mass called as Pangaea.

South Pole was located near the present position of Durban in Natal. Thus, South Pole was located in the middle of Pangaea. Consequently, ice sheets might have spread from South Pole outward at the time of glaciation and the aforesaid land areas, which were closer to South Pole, might have been covered with thick ice sheets.

At much later date, these land areas might have parted away due to disruption of Pangaea and related continental drift. Glossopteris flora might have also been distributed over the aforesaid areas when these were united together.

5. Evaluation of the Continental Drift Theory:

It may be pointed out that Wegener’s continen­tal drift theory widely departed from the contemporary orthodox geological ideas of the nineteenth century and the time-honoured thermal contraction theory of the mountain building and thus it was obvious that the believers of contraction theory should also discard it. ‘It is now widely agreed that he (Wegener) handled his case as an advocate rather than as an impartial scien­tific observer, appearing to ignore evidences unfa­vourable to his ideas and distort other evidences in harmony with the theory’.

The critics of Wegener’s conti­nental drift theory fall in two broad categories e.g.:

(i) The critics and writers who always attempted to search errors and discrepancies in Wegener’s original synthe­sis and

(ii) The scientists who attempted to modify, enlarge and correct the original theory of Wegener while retaining its basic tenet.

The following flaws and defects have been pointed out by different scientists in Wegener’s theory of the continental drift:

(1) The forces applied by Wegener (differential gravitational force and the force of buoyancy and tidal force of the sun and the moon) are not sufficient enough to drift the continents so apart. The tidal force as invoked by Wegener to account for the supposed westerly drift of the continents would need to be 10,000 million times as powerful as it is at present to produce the required effects, and, if it had such a value, it would stop the earth’s rotation completely in a year’.

Similarly, the differential gravitational force and the force of buoyancy are also not adequate to cause equator ward movement of the continents, instead the force, if so enormous, might have caused the concen­tration of the continents near the equator.

(2) Wegener has described several contrasting view-points. Initially, sialic masses (continents) were considered by Wegener as freely floating over ‘sima’ without any friction offered by ‘sima’ but in later part of his theory he has described forceful resistance of offered by ‘sima’ in the free movement of sialic conti­nents to explain the origin of mountains along the frontal edges of floating continents.

Moreover, ‘it is difficult to show how the sial blocks, in their passage through the sima, would crumple at their frontal edges and Produce Mountains’. According to Willis no compression could be possible to form the Rockies and the Andes if the ‘sima’ is more rigid than the ‘sial’. Bowie has maintained that sima has no strength to crumple sial to form mountains.

(3) Both the coasts of the Atlantic Ocean cannot be completely refitted. Thus, the concept of juxtaposi­tion’ or ‘jig-saw fit’ cannot be validated.

(4) Wegener has not elaborated the direction and chronological sequence of the displacement of the continents. He did not describe the situations of pre- carboniferous times. Many questions remain unan­swered such as, what kept Pangaea together till its disruption in mesozoic era? Why did the process of continental drift not start before mesozoic era? Etc.

Some writers argue that ‘it is not a fair criticism to say that any pre-carboniferous mountain building cannot be explained on Wegener’s hypothesis merely because he does not develop his scheme in earlier geological times’.

It may be concluded that ‘even if all the matter of his theory is wrong, geologists and others can but remember that it is largely to him that we owe our more recent views on world tectonics’.

Though most point of Wegener’s theory was rejected but its central theme of horizontal displace­ment was retained. In fact, the postulation of plate tectonic theory after 1960 is the result of this continen­tal drift theory of Wegener. Wegener is, thus, given credit to have started thinking in this precarious field.