GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Ocean crust. Cut-away illustration of ocean geology. At top, molten rock rises from the mantle, forming new ocean plate and causing volcanic eruptions. Cooling lava builds up a mid- ocean ridge. Below this, two possible types of ridge are shown. In the Pacific, new plate is formed quickly, leading to a crested ridge (left). Atlantic ridges are slow-spreading with a median valley (right). Below is an enlarged view of the valley, showing layers of rock built up by volcanoes and cooling mantle material. At bottom left is a black smoker, a hydrothermal vent found along all mid-ocean ridges. At bottom right, sedimentary rock forms over the sea floor.
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