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Part of Hall of Planet Earth.
Driven by heat from molten rock, the entire ocean circulates every one to ten million years through the crust along the mid-ocean ridges. About 80 percent of the Earth’s internal heat escapes in this way. This vast circulation — the underwater hydrothermal system — influences the composition of the ocean. The circulating hot seawater deposits some elements, such as magnesium, in the crust, and dissolves other elements, particularly metals, adding them to the ocean.