Continental Drift

I

Introduction

Continental Drift, movement of continents relative to each other across the surface of the Earth.

In 1620, the English philosopher and statesman Francis Bacon drew attention to the remarkable similarity in shape between the west coast of Africa and the east coast of South America, although he never actually suggested that the two continents had once been joined together. The proposal that continents could move with respect to each other was first made in 1858 by Antonio Snider, an American living in Paris, but it was left to the German meteorologist Alfred Wegener to develop the idea in detail, in a book (The Origin of Continents and Oceans) published in 1915. Wegener is therefore often regarded as the originator of the continental drift theory.

II

Wegener's Theory

What Wegener sought to prove was that all the Earth's continents had once been joined together in a single “supercontinent”, which he named Pangaea; that Pangaea had at some time broken up into fragments of land; and that these great fragments—the continents we now recognize—had gradually drifted to their present positions. Most geologists of the time, and of the next 50 years, refused to believe it. For the previous hundred years or so, geologists had assumed that the continents were fixed in position, and they could not accept that they had been wrong. Besides, they argued, how could solid continents plough their way across solid ocean floors to get to where they are now? There was no force on Earth big enough.

III

Evidence for the Theory

By the mid-1960s, however, those Earth scientists who did not accept Wegener's idea were in a very small minority. The reason for its eventual, wide acceptance was largely the phenomenon of palaeomagnetism. Many rocks, when they form, acquire magnetism in the direction of the Earth's magnetic field at the time and place of their formation. By the late 1950s it had become possible to measure this very weak ancient magnetism (palaeomagnetism) using highly sensitive instruments and, by analysing it, to deduce where the continents had been when the rocks were produced. Thus it was shown that all the continents were, indeed, once joined.

Other evidence then fell into place. For example, palaeontologists had long been puzzled about the fact that certain flora and fauna could be found on more than one continent. These plants and animals could not possibly have spread from continent to continent by crossing vast oceans, but they could easily have spread when the continents were joined together. Some geologists had also wondered why certain rock formations of exactly the same type and age occur in both western Africa and eastern South America. When the continents are moved back together again, as in Pangaea, it's clear to see that the formations were once just one, but split when the continents broke apart.

Pangaea began to break up about 200 million years ago, first into two lesser supercontinents—Gondwanaland to the south (comprising what were to become South America, Africa, Australia, Antarctica, and India) and Laurasia to the north (North America, Europe, and most of Asia)—and then into the individual continents, which dispersed. This episode of continental drift is sometimes called “Wegenarian drift”, after the theory's originator.

It is now known that there were also earlier periods of drift. Pangaea had lasted only a few hundred million years, having been formed in the first place by the combination, a few hundred million years earlier, of a set of land masses different in shape from the continents of today. Those land masses, in turn, were fragments of another supercontinent which had itself been formed by the coming together of yet different land masses some hundreds of millions of years before that. The break-up, dispersal, and reconstruction of supercontinents is evidently a repetitive process.

And what about the difficulty of continents ploughing their way through solid ocean floors? By the mid-1960s it had become clear that this was not a problem, as the ocean floors were themselves moving and, in effect, pushing the continents along (see Plate Tectonics).

The process continues: the continents are still drifting, typically at rates of a few centimetres a year. The present arrangement of continents is therefore not permanent. For example, the Atlantic Ocean is gradually widening as the Americas and Africa drift apart, but to compensate for this the Pacific Ocean becomes ever smaller. The Mediterranean Sea is also getting narrower (and will ultimately disappear completely) as Africa drives northwards relative to Europe.

The movement of the Indian land mass demonstrates another phenomenon. When Pangaea split into Gondwanaland and Laurasia, India formed part of Gondwanaland. Later, however, it broke away and moved rapidly northwards at uncharacteristically high speeds of 17 cm (6.6 in) a year until it collided with, and joined, the main part of Asia. The pressure of India moving against Asia caused the crust to bend upwards into the Himalaya, a process that continues today. It is thought that the joining, or suturing, of land masses will occur again and again in the future until at some time all the continents will have come together once again as a new supercontinent.