The water that cycles through Earth’s oceans, lakes, and atmosphere is just a portion of the water on this planet. A vast amount of water—as much as 2.5 times the mass of all surface reservoirs—is dispersed, molecule by molecule, in the mantle, where it influences mineral material properties and thus geological dynamics (see the article by Marc Hirschmann and David Kohlstedt, Physics Today, March 2012, page 40).

On time scales of millions to billions of years, the water cycle includes exchanges into and out of the mantle, as illustrated in figure 1: Water is released, or degassed, from the mantle through mid-ocean ridges and volcanoes; it’s returned, or regassed, through the subduction of hydrated minerals in tectonic plates. Other volatile species—including compounds of carbon, nitrogen, and sulfur—engage in their own deep cycling.

With volatiles moving in and out of the mantle, the density and makeup of the...

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