“We tend to think of the water cycle as a relatively short-term phenomenon; the average molecule of water stays in the atmosphere for about nine days; the residence time of water even in the largest lakes, like Superior, is a century or two; deep groundwater may be stored for a millennium. But there is a 100 million-year water cycle that involves the interior of the Earth, and adding water to the mantle is in fact the critical step in the recipe for continental crust.”
