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The Earth's surface is divided into large, rocky plates, which move around on the more fluid material beneath. Near Sumatra, two of these plates are colliding. The Australia plate is moving northwards, hitting the Asia plate, and then sliding under it. The energies released in this process drive a multitude of volcanoes, including Krakatoa, which exploded in 1883, with disastrous consequences. Sometimes the plates jam, and the huge forces pushing them compress and deform the rocks, storing enormous amounts of energy. Finally something snaps, the stresses relax catastrophically and there is an earthquake. Events like this result in the sudden movement of billions of tonnes of rock. To visualize what happens then, think of a spinning ice skater.
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