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Volcanic crater. Layers of soft tephra forming the crater of the Kawah Ijen volcano in Java, Indonesia. Tephra is formed from pyroclastic material, which was thrown up in the air by a volcanic eruption. It can include 'bombs' of molten rock (lava), ash, volcanic glass and solid rocks, for example pumice. Layers of these materials are deposited on the sides of a volcano and the lighter materials can be carried many kilometres by the wind. Soft tephra is eroded easily by rain water, which forms gullies in the surface. Photographed in 2002. (KEYSTONE/SCIENCE PHOTO LIBRARY/Str) === ===