We know that the Earth’s crust and the top part of the mantle are split into rigid sections, called plates, which are up to 70-100 km (43-62 miles) thick. Beneath the plates, temperatures are high and the rocks are semi-molten. Currents in the molten material are moving the plates about. Plates are moving apart along the ocean ridges. When they move, molten material wells up to fill the gaps, sometimes, lava piles up into volcanoes which may, like Surtsey (off Iceland) reach the surface as islands. So, the ocean rides are one place where volcanoes occur.
Another place is beside ocean trenches. Here one plate is being pushed beneath another. Descending plates are melted, creating magma at temperatures about 1,090°C – 1,200°C (1,994°F – 2,192°F). Some of this magma rises and emerges through volcanoes as lava. Many volcanoes are of this type. For instance, Indonesia – which lies above a descending plate – has 167 volcanoes, 77 of which have erupted in historic times.
A few volcanoes, like those in Hawaii, are far from any plate edge. Scientists think they are fuelled by ‘hot spots’ in the Earth’s mantle.