BBC NEWS | Science/Nature | Sea floor records ancient Earth: "Today, oceanic crust is created at plate boundaries known as mid-ocean ridges, where magma rises from the asthenospehere through cracks in the ocean floor, cools and spreads away.
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As it moves away from the spreading centre towards the edges of the oceans it becomes cooler, denser and eventually starts to sink back into the mantle to be recycled.
'Sea floor is not normally preserved for more than 200 million years,' said Professor Rosing.
Most is destroyed at subduction zones, such as those found along the edge of the Pacific Ocean, where oceanic crust plunges under the buoyant and long-lived continental crust."
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