Continental Drift: Theory, Evidences and Explanation

In this article we will discuss about Continental Drift:- 1. Continental Drift Theory 2. Peninsular India 3. Paleontological Evidence of Continental Drift 4. Physical Evidence of Continental Drift 5. Palaeoclimate 6. Explanation of the Continental Drift Theory in the Light of Vertebrate Distribution.

Contents:

1. Continental Drift Theory
2. Peninsular India
3. Paleontological Evidence of Continental Drift
4. Physical Evidence of Continental Drift
5. Palaeoclimate
6. Explanation of the Continental Drift Theory in the Light of Vertebrate Distribution

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1. Continental Drift Theory:

Most palaeontologists are in favour that in early palaeozoic, the six major land masses were coalesced and formed a single super land mass, called Pangaea (Fig. 1,6A), first named by a German meteorologist, Alfred Wegner in 1912.

A Paper, “The Origin of Continents and Oceans” (Die Enstehung der Kontenente) was published by him in German in 1912 and in his paper he postulated a theo­ry that the different continents coalesced in the early Palaeozoic period and subsequently broke in early or mid-Mesozoic and drifted apart to occupy the present positions.

This the­ory was not accepted in those days but widely accepted since 1960 with the studies of Palaeomagnetism of the ocean floor. From the mid-60s, the concept of Continental Drift has been incorporated into the view of tectonic plates.

According to recent findings, Wegner’s Continental Drift Theory has gained a wide acceptance among different scientists.

The exact period of coalescence and separation of different land masses is still controversial. At present different scientists have presented different opinions on the basis of analysis of palaeomagnetic data. There were two land masses throughout the Devonian period, of which is Laurasia (Angara), (Fig. 1,6B) a northern land mass that included St. Lawrence area of North America, Europe and parts of Asia.

Another southern land mass is called Gondwanaland (Fig. 1,6B) (named after a South Indian tribe, Gond) that is formed by the union of South America, Africa, Arabia, India, Madagascar, Antarctica and Australia. In the middle Carboniferous period, the two land-masses, converged to form a single super land-mass, is called Pangaea.

Pangaea lasted about 100 million of years. In the late Triassic or in the early Jurassic, the tectonic plate movement and volcanic activity forced to rupture the Pangaea into smaller present land masses (Fig. 1.6C).

In the Jurassic, the Gondwana rotated anticlockwise, and Europe and N. America separated from it and moved north­wards so that in the Jurassic, the Tethys Sea (named after the Greek ‘mother of seas’, the daughter of Oceanus) created in between Africa and Eurasia, and in coarse of time, it became the Mediterranean sea.

The breakup of Pangaea produced a series of Orogeny (mountain building) that continues up to date. During the Cretaceous period the formation of Rocky mountains and the Andes along North America and Western South America took place. Also in the N. America, the late Palaeozoic and Mesozoic era is marked by the Appalachian Revolution, in which, mountain formation took place by the compression of the tecto­nic forces.

In between middle to late Cretaceous peri­od, South America, Antarctica and Australia separated from Africa, and the Tethys Sea, opened to the Atlantic. In the Eocene, Australia separated from the Antarctica and India drifted northwards and joined with the Asia about 25 million years ago. The conti­nents are still drifting today. North America recedes westward and Australia northward at approximately 4 cm per year.

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2. Peninsular India:

Peninsular India was an elevated land mass for the past 150 million years. The par­ting of Gondwana that started in the late Jurassic and in this division of lands, South America, Antarctica and Australia were inclu­ded in one land mass, and Africa, Madagascar and India in another mass.

India rotated away from Madagascar and began to move northwards about 100 Ma ago, and finally joined with Asian part of Laurasia about 25 Ma ago when the great Oligocene and Miocene oogenesis of the Himalayan-Alpine belt were uplifted.

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3. Paleontological Evidence of Continental Drift:

The Tethys Sea, which extended as a shal­low epicontinental sea across southern Asia and north-Africa also washed the ancient northern coastline of India. The Jurassic fauna of Tethys Sea is known from Europe in the west and in the east it is in the Himalayan region.

There are still marine sediments which mark the northern coastline of India that ran from the Cutch in the North-west, eastward along the present course of the Narmada valley to Baroda.

The southern coastline of Indian peninsula is not known from the Jurassic sedi­ments but is known from the Cretaceous onwards of Tiruchirapalli district of southern India. From the analysis of faunal composition it is presumed that the nature of fauna which inhabited the sea to the south of India presum­ably the forerunner of the Indian Ocean.

After the elevation of the Himalayan- Alpine mountains at the end of Miocene, the realm of the Tethys became restricted to the present northern portion of the Indian Ocean. The palaeontological picture suggests that India has occupied the northern Indian Ocean since the late Jurassic times and its northern peninsular region has lain near the boundaries of both the Tethys and the Indian Ocean.

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4. Physical Evidence of Continental Drift:

On the basis of Palaeomagnetic observa­tions, Adie (1965) and Creer (1966) refer that India formed part of a supercontinent, Gondwanaland located near the geographic South Pole in the early Mesozoic. The superconti­nent started to be disrupted in the mid-Jurassic times and the process of dismemberment ended in the Cretaceous and India drifted at least 60° of latitude northward in the post Jurassic time.

Bullard (1969) states that Palaeomagnetic work show that India has been moving northward for the past 100 Ma. McElhinny (1969) deduces that an Indo-Madagascar-Antarctica block broke away from Africa between 155 and 100 million years ago, opening up the Indian Ocean for the first time.

An India-Madagascar block then separated from Antarctica, and at first drifted southwards before reversing its course to move northward.

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5. Palaeoclimate:

In the early period of Palaeozoic, from Cambrian to Devonian, the climate was more or less constant. Due to extensive glaciation from late Proterozoic, the entire earth became cool during the Cambrian period.

But this is not proved by the geological record. From the Ordovician to Silurian, a uniform tropical and subtropical climate prevailed in the vicinity of equator. Late Carboni­ferous and Permian is marked by the arid and cooler climate.

In the Triassic of Mesozoic, the climate was warm and arid. Jurassic was marked by moist climate. The Cretaceous was warm and moist for the most part. In the beginning of Tertiary, the tropical and sub-tropical regions extended its areas. At the end of Tertiary, the area of tropi­cal zone had gradually receded. Pleistocene was the period of glaciation and inter-glaciation.

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6. Explanation of the Continental Drift Theory in the Light of Vertebrate Distribution:

Some of the enigmatic facts are that the present distribution of animals can be explained on the basis of Continental Drift Theory. It is an established fact that the distri­bution of mammals in the Northern hemi­sphere (Nearctic and Palaearctic region) has more similarity than the distribution of mam­mals in southern regions (Africa, South America and Australia).

It is believed that the animals of the northern region have spread in recent times due to land connections at the Bering Strait and it has been established from the theory of Continental Drift that the Laurasia started splitting and drifted apart at a later date than parts of Gondwana.

The mammals of the Southern hemisphere originated during the breakup of the conti­nents and underwent evolution after the split­ting. Once the mammals are isolated, they may evolve in different ways and, hence, as Gondwana began to split first, it is expected that the mammals of different continents evolved and radiated separately.

Thus the South American mammals like llamas, alpacas, sloths, armadillos are not found in other continents of Southern hemisphere. The giraffe, zebra, hippopotamus, chimpanzee and gorilla of the Ethiopian region and egg-laying mammals and some marsupials of Australian region are not found anywhere except these regions.

Though the distribution of mammals in the Gondwana is different, but, in contrast to that, the distribution of some fish, amphib­ians and reptiles are very similar in relation to the Continental Drift Theory.

The distribution of the lung fishes supports the Continental Drift. The dipnoans which appeared in mid-Devonian, are represented now by three genera Protopterus, Lepidosiren and Neoceratodus, and are found in Africa, South America and Australia respectively.

This distribution reminds us that the Gondwana lands in the mid-Devonian and Africa, South America and Australia were in contact at that period and, with the separation of these continents, the dipnoans are repre­sented by three separate genera at present.

Among amphibians, the distribution of the members does not support the Continen­tal Drift at random. Only the distribution of pipids in between Africa and South America, and salamanders in Nearctic and eastern part of Eurasia support the Continental Drift to some extent.

Among reptiles, the distribution of some members supports the Continental Drift. Some members of the primitive chelonians origina­ted during Permian period and still are con­fined to the Southern hemisphere. The crocodilians such as crocodiles and gharials originated in the Triassic period, now restric­ted to the Central America, Africa, North Australia and in some parts of Oriental region.