Continental Drift Theory

During the successive episodes of geological time, there were large-scale, split of a big landmass (known as Pangea or Supercontinent) into several small pieces and subsequent slow horizontal movements of these pieces of landmass relative to one another and to the ocean basins, studied as Continental Drift Theory.

What does Exactly Continental Drift Theory Explain?

The continental drift theory is a postulate that explains the movements and process of movements of the Earth’s different landmass (more commonly known as continents). It explains, how the different continents have moved and have been still moving from the millions of years. According to the theory, the continents are not stationary but rather move around on the surface of the Earth. This movement is believed to be caused by forces within the Earth, known as endogenic forces. Under the endogenic forces, the crust (solid upper part of the earth) floats over the molten material in the mantle (the molten middle layer of the earth).

According to the continental drift theory, in the beginning, the continents were once part of a single landmass called Pangaea. Over time, Pangaea split into different landmasses (known as the continents), which started moving into different directions and reached to their present locations. However, this process is still ongoing, and the continents continue to move today.

Historic Background

In 1596, Abraham Ortelius, a Dutch cartographer, noticed that the coastlines of the continents seemed to fit together while assembling the maps, similar to reassembling broken glass. He reasoned out that the continents were torn apart by large-scale earthquakes and floods. Similarly, in the early 19th century, Alexander von Humboldt, a German naturalist, stated that the bight of Africa and the bulge of eastern South America had once been joined.

In the year 1858, a French mineralogist named Antonio Snider opined that the presence of identical fossils in the carboniferous coal seams in North America and Europe could only be possible if both the continents had formerly been connected. With the growing popularity of the study of Continental Drift, numerous scientists around the world have displayed their ideas, of which the most significant findings are discussed below.

Frank Bursley Taylor

He postulated the concept of "Horizontal Displacement of the Continents" in the year 1908. The American geologist invoked the idea of continental collision to address the origin of the fold mountain systems of the tertiary period (recent). He assumed that the oceanic crust is made of denser SIMA (Silicon and Magnesium) and the continental crust is of lighter SIAL (Silicon and Aluminum), and thus SIAL is floating over SIMA.

According to F. B. Taylor, during the Cretaceous period (150–66 million years ago), the northern land mass "Laurasia" and the southern land mass "Gondwana," due to the tidal force of the moon, started drifting equatorward, and the western parts of both the land masses broke apart and drifted westward. During the tertiary period, the Laurasia and Gondwana land masses collided with each other, resulting in East-West running fold mountains, notably the Alps, Atlas, Taurus, and the Great Himalayan Mountain ranges. Simultaneously, due to friction between the west-moving broken SIAL and SIMA, the western part of the land forms a north-south running fold mountain range, majorly the Rockies and Andes Mountains.

Criticism

Through the theory attempted to explain the process nicely, but failed to answer certain questions as −

• The events that happened before the Cretaceous period are left untold.

• If the tidal force of the moon is strong enough to pull the continents, then the gravitational pull of the moon could stop the earth's rotation.

• The friction between SIAL and SIMA was discussed only with respect to the westward drift of the continents, ignoring the equatorward drift.

Alfred Wegener

Alfred Wegener, a German meteorologist, proposed the first detailed and comprehensive theory of continental drift in his most important work, "The Origin of Continents and Oceans," in 1912. Initially, his objective was to explain the major variations in climate in the past. Fixing the continents to be stationary and climate to be shifting spatially is more of an unlikely event, hence he decided to proceed his study by keeping the climate to be stationary and continents to be drifting.

On the similar lines of F. B. Taylor, in addition to SIAL and SIMA, he included another layer of denser NIFE (Nickel and Iron) below the continental crust. He also assumed that SIAL was floating on SIMA without resistance.

During the Carboniferous period (350–300 million years ago), there existed a huge land mass called Pangea (a super continent) and a mega ocean surrounding the super continent called Panthalassa. Eventually, Pangea got bifurcated into Laurasia and Gondwana, which moved towards the respective poles due to the action of a polar fleeing force that resulted in the opening of the Tethys Sea. Then the Laurasia and Gondwana land masses moved towards the equator due to the gravitational differential force and force of buoyancy. Due to the tidal forces of the sun and the moon acting on Earth, the western parts of both land masses broke apart and moved westward, giving rise to the Atlantic Ocean. In due course of time, Laurasia and Gondwana land masses collided with each other, resulting in East-West running fold mountains, notably the Alps, Atlas, Taurus, and the Great Himalayan Mountain ranges.

Due to the resistance between the rocks of SIMA and SIAL, North-South running fold mountain ranges, mostly the Rockies and Andes Mountains, are formed. The front edge of the moving continent is pulled by tidal forces greatly when compared to the trailing edge (Differential movement of continents). This leads to the formation of island arcs, as observed in the north-eastern part of North America.

Evidence

• Jig-saw fits on the opposite coasts of the Atlantic Ocean.

• Fossils and vegetation on the eastern coast of South America match those on the western coast of Africa.

• Geodetic evidence shows the drifting of Greenland towards the Northwest at a rate of 20 cm/year. On reconstruction, Greenland would possibly once within Europe.

• When North America, Greenland, and Europe were once a single large landmass, on which lemmings, a land animal, moved westward from Northern Scandinavia in a circumpolar manner. Meanwhile, due to the effect of the continental drift of Greenland and North America from Eurasia, the lemmings fell into the Norwegian sea. It is evident from the fact that the Norwegian Sea is the only location in the world where fossils of lemmings have been found.

• Ice age evidence during the carboniferous period is found in parts of South America, Africa, India, Australia, and the whole Antarctica, which proves that once they were held together.

• Floral and faunal fossil evidence in Gondwana (present India, Africa, South America, Antarctica, and Australia).

Criticism

The theory has following drawbacks −

• The events that happened before the carboniferous period are left untold.

• The deep geological structures of Africa and South America cannot be brought together due to the existence of the continental shelf and slopes.

• His own assumptions were contradicted by stating resistance as the source of the formation of the Rockies and Andes.

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

• Similar vegetation is found in unrelated parts of the world like Afghanistan, Iran, and Siberia. This shows that identical vegetation is not a reliable criterion.

Conclusion

Though the theory of continental drift technically and scientifically explained movements of continent with reasonable reasons; however, failed to answer certain questions. So, next a few decades these theories were intensely and extensively argued and as a result of this, it laid the foundation of more authentic and reliable hypotheses namely plate tectonics theory and sea floor spreading theory.

Frequently Asked Questions

What is Continental Drift hypothesis?

The Continental Drift hypothesis refers to the theory which describes the mechanism of the splitting up of one large landmass into their current positions as various continents in the world today.

How did the East – West running mountains form according to Taylor?

According to F. B. Taylor, during the Cretaceous period (150–66 million years ago), the northern land mass "Laurasia" and the southern land mass "Gondwana," due to the tidal force of the moon, started drifting equatorward, and the western parts of both the land masses broke apart and drifted westward. During the tertiary period, the Laurasia and Gondwana land masses collided with each other, resulting in East-West running fold mountains, notably the Alps, Atlas, Taurus, and the Great Himalayan Mountain ranges.

What is Pangea and Panthalassa according to Wegener?

According to Alfred Wegener, during the Carboniferous period (350–300 million years ago), there existed a huge land mass called Pangea (a super continent) and a mega ocean surrounding the super continent called Panthalassa.