Evidence in Support of the Continental Drift

The strongest single piece of evidence for continental drift is the way the continents themselves fit together. Look at the facing shorelines of Africa and South America — the match is remarkable and unmistakable. This is the classic jig-saw-fit argument. In 1964, Bullard used a computer programme to find the best possible fit of the Atlantic margins, and the result proved to be quite perfect. Importantly, the fit was tested not at the present-day shoreline but at the 1,000-fathom line — the line marking the true edge of the continental shelf, where the continent actually ends. That is why the match works so well: coastlines have been reshaped by erosion and sea-level change, but the deeper continental margin preserves the original break.

The second line of evidence comes from the rocks themselves. Radiometric dating methods, developed in recent decades, have made it possible to correlate rock formations on different continents across vast oceans. The belt of ancient rocks about 2,000 million years old along the coast of Brazil matches perfectly with rocks of the same age in western Africa. Even more telling is the age of the earliest marine deposits along the coastlines of South America and Africa — they are of Jurassic age. This suggests that the Atlantic Ocean did not exist before that time; the two continents must have been joined until the Jurassic.

Then there is tillite, a sedimentary rock formed from the deposits of glaciers. The Gondwana system of sediments in India is known to have its counterparts in six different landmasses of the Southern Hemisphere. At its base, this system has a thick layer of tillite, indicating extensive and prolonged glaciation. Counterparts of this entire succession are found in Africa, the Falkland Islands, Madagascar, Antarctica and Australia. The overall resemblance of these Gondwana-type sediments clearly demonstrates that these landmasses had remarkably similar histories. The glacial tillite provides unambiguous evidence of past climates — palaeoclimates — and also of the drifting of the continents themselves.

Placer deposits offer a particularly striking clue. Rich placer deposits of gold occur along the coast of Ghana, yet there is an absolute absence of source rock in the region. The gold-bearing veins are in Brazil. It is obvious that the Ghana gold deposits were derived from the Brazilian plateau when the two continents lay side by side. The gold was carried down and deposited as placers on the adjoining coast, and when the continents split apart, the deposits were left behind on opposite shores. …