Alfred Wegener caused one of the biggest uproars in twentieth century science in 1912. Wegener, a German meteorologist, suggested that the continents on the Earth's crust were not immobile, but could move. Geologists scorned the idea at first for two main reasons. One was that Wegener could not give an explanation of how the continents could move the sort of distances suggested. The other reason was that Wegener was a meteorologist (A scientist who studies the weather) and not a professional geologist. However, as more and more evidence came to light supporting Wegener's idea, the scientific community began to accept the idea of continental drift during the 1960's.
Wegener's idea was that the continents were all joined in a massive super-continent with Wegener called Pangaea in 1915, as shown in the above diagram. Around 200 million years ago, Pangaea broke up into two smaller continents, the southern continent called Gondwanaland, and the northern continent called Laurasia. Over time, these two land masses began to split up into smaller continents, which moved in different directions. The continents moved with speeds ranging from 2.5 centimetres per year to 3 centimetres a year. The continents are not fixed in their present positions, and appear to be still moving. The following animation gives an idea of how the continents moved into their current positions.
The continents are resting on top of plates on the Earth's crust (Refer to The Earth's Interior for a more detailed description of this). When the edge of one plate meets another, the edge of one plate is either pulled down into the mantle, or is pushed up, causing mountains. For example, the Himalayan mountain range in Asia is believed to have been caused by contact between the Indian and Eurasian continental plates. Earthquakes and tsunamis often occur along contact points between continent plates.
What could be causing this motion of such large land masses? This question was the central sticking point of continental drift's acceptance, and a satisfactory answer was only given during the 1960's. The answer involved the idea by a Scottish geologist Arthur Holmes that hot rock rose up from the deeper levels of the Earth's mantle. The hot rock cooled as it neared the crust, and so, like hot and cold air, the cooler rock would sink back deeper into the mantle. This are called convention currents, and could be the cause of continental drift.
Oceanographers also gave more support to the continent drift idea, with their theory of sea floor spreading. They noted that volcanoes and undersea earthquakes often occurred along oceanic trenches. Oceanographers suggested at these trenches could be the edges of continental plates. They used the idea of convention currents in the Earth's mantle to suggest these currents would carry rock up to the underwater ridges between continental plates. As the rock hardens, it would push the plates apart, causing seismic activity and expanding the sea floor.
While many scientists accept the basic idea of continent drift, there is still much debate over the various details. However, continent drift has given a clear and consistent explanation for many of the happenings which occur in the Earth's crust.
(All diagrams of the Earth on this page are from the Serif ArtGallery CD-ROM. The animation was put together by David Douglas using the ArtGallery images.)