Franschhoek The Huguenots Museum Information



Continental Drift

From time immemorial, people have thought that the continents had always occupied their present positions and that the sea-bed was older than the continents. Conclusive proof was produced during the 1960's however, to substantiate the fact that the continents move continually over the earth's surface, the sea-bed constantly expands and is consumed beneath some of the continents and the oceanic crust is only a fifth of the age of the continents.

The first plausible theories concerning continental drift and the genesis of new oceans were put forward at the start of the 20th century. In 1915 the German geophysicist Alfred Wegener (1880-1930) put forward his theory of the mother continent PANGEA, according to which all the existing continents were once a solid land-mass surrounded by the mother sea, Panthalassa. Part of the resistance that his theory met with was due to his inability to explain the mechanism by which such drastic shifts occurred.

The South African geologist Alex L du Toit (1878-1948), believed that originally two great continents existed, namely LAURASIA (North America, Greenland, Europe and Asia) and GONDWANALAND (South America, Africa, India, Australia and Antarctica), separated by the long, narrow Tethys Sea.

The early arguments in favour of continental drift were geographical, biological and climatologically.

The geographic arguments were based on complementary coastlines (Africa and South America), identical rock sequences of similar ages on different continents, and prehistoric mountain ranges that appeared to link one continent to another.

The biological arguments rested on identical animal and plant fossils found on widely separate continents, which could only be explained if the continents had originated as one vast land-mass.

The climatologically arguments pointed to past tropical and equatorial climates in temperate Europe, as evidenced by coral reef-derived limestone's in Belgium, and polar climates in tropical Africa and India.

Apart from its inability to provide a detailed mechanism for the drifting, Du Toit's theory met its greatest opposition because it seemed impossible for the continents to "float" on a sea-bed consisting of solid basalt.

Detailed oceanographic explorations in the 1960's provided the first conclusive proof of the theory, together with a mechanism, when it was found that, not only was the ocean floor much younger than the continents, but also that it grew increasingly younger towards what came to be known as "spreading ridges" running centrally through the oceans, where new oceanic crust was being generated at the surface. This part of the mechanism which drives continental drift has come to be known as ocean-floor spreading, and has as a complementary process, sub-duction by which old oceanic crust in the Pacific is consumed (sub-ducted) beneath all the continents that rim the ocean. We now know that the earth's surface is made up of 15 "plates" of varying size, which drift towards, away from and past each other. It is a process that can be measured in millimetres per year but it never stops, and over the billions of years of its existence, has shaped the planet we live on.

The relative movement between plates causes friction, which we experience as earthquakes (e.g. the San Andreas fault in California) and where plates collide mountains such as the Andes and the Himalayas - and even the Cape mountains - are pushed up, as one plate rides up over the other.

This theory of continental drift makes it possible for everyone to better understand why there are no subtropical rain forests left in Franschhoek and the Western Cape, but a Mediterranean climate instead.

It is possible to predict how the mosaic of plates might look in a billion years: some continents will be bigger, though Africa will be smaller after east Africa has broken away from the mainland along the Rift Valley. There will be massive new islands and a host of smaller ones along the spreading - or mid-oceanic - ridges. Exactly how it will be, no-one knows. All we can say for certain though, is it will be very different.

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