Water is fundamental for processes that occur at the Earth's surface, but also plays a critical role in many geological processes occurring deep inside of it that shape its evolution. Small amounts of water incorporated into the structure of minerals have a major effect on their stability, behavior and phase equilibria. Global processes such as mantle convection, plate tectonics and naturally occurring catastrophic events such as earthquakes and volcanic eruptions are strongly influenced by the activity of this water.
Water is reintroduced into the Earth’s interior by hydrated tectonic (oceanic) plates that return into the mantle in subduction zones, and released when hydrous minerals/phases are decomposed due to the high pressure and temperature of the Earth’s interior. Much of this water returns to the surface by volcanism, but a large fraction of is retained in newly formed high pressure hydrous phases that are stable at much higher depths, opening the possibility for water to recirculate deeper into the mantle beyond 400 km depth. However, the exact amount of water stored in the solid Earth, and how (and how much) of this water is recycled back to the surface, remains obscure.
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