Volcanoes at the Edge of Tectonic Plates

In 1912, a German meteorologist called Alfred Wegener had a theory that at least some of the continents had been joined together in the past. He came to this conculsion when he was comparing the coastlines of Africa and South America. They seemed to fit together reasonably well. When he looked at the edges of the continental shelves of the two continents, the fit was even better. He also spotted that there were rocks and fossils which matched up too.

But our understanding of volcanoes didn't really change for a long time. Geologists ignored his ideas because he was a meteorologist. Those scientists who seriously considered this idea of 'Continental Drift' could not see that there could be any mechanism of moving the continents around. But gradually, geologists found other pieces of evidence that suggested that Wegener was correct and in the middle of the 1960s, the discovery of magnetic stripes on the sea bed confirmed that it was slowly moving on a very long time scale. Wegener's theory became the theory of plate tectonics which is now widely accepted.

Where plates come together or are moving apart, volcanoes appear. At constructive plate boundaries, as the edges of the plates move apart, a type of magma which produces rocks like basalt pushes its way up from the mantle to the surface, forming volcanoes. This type of lava is not very viscous, so it is relatively mobile (runny like warm treacle). The volcanoes are wide and relatively flat.

At destructive plate boundaries, one plate is pushed down into the mantle where it melts. The molten rock from the plate mixes with other types of rock as it rises through the crust. The magma is more viscous and so it does not flow as far meaning that the volcanoes have a taller and narrower shape. Sometimes, the magma is so viscous that the neck of the volcano is blocked. When the pressure builds up enough, the volcano will explode, with disastrous consequences for the living world around the volcano.