To make room for the continual addition of new ocean crust, all the Earth's plates move.

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In December 1995 the space probe Galileo showed that Europa probably has an ocean beneath its icy crust, with tidal heating providing a likely energy source.

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Pictures of the icy satellite Enceladus suggest its surface splits and spreads apart - just like the ocean floor on our planet splits to create new crust.

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Although continental crust is more or less permanent, the ocean floor is constantly recycled by the tectonic processes that bring about continental drift.

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"At the moment the Seychelles is a piece of granite, or continental crust, which is sitting practically in the middle of the Indian Ocean, " explained Prof Torsvik.

BBC: Fragments of ancient continent buried under Indian Ocean