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[Stockbild] Volcanic rock. Aerial photograph of rhyolitic lava and tuff (both grey) in the valley of Kjos, Vatnajokull, Iceland. The Vatnajokull ice-cap feeds several outlet glaciers, which are the largest glaciers in Europe. Rhyolitic lava has a blocky surface, which is characterized by smooth- faceted blocks of up to 5 metres in diameter. Tuff consists of compacted small to medium-grained pieces of rock that were fragmented in a volcanic eruption. Such fragments are known as pyroclastic material. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER) === ===