Hidden inside the Earth-within several hundred kilometers below the crust-there is another ocean which is most likely the largest ocean in the world. However, this ocean is only water in the loosest sense: broken into its composite hydrogen and oxygen atoms and chemically bound to the surrounding rock.
Denis Andrault and Nathalie Bolfan-Casanova, geoscientists at the University of Clermont Auvergne in France, have developed a model that shows more of this water is in transit than previously thought.
When the solid rock in the mantle becomes saturated with chemically dissociated water, it can transform into a water-rich molten slurry which then seeps back up toward the crust. The researchers named this mantle rain.
Just as the cycling of water between the atmosphere, lakes, glaciers, rivers, aquifers and the ocean affects the level of the sea, the abundance of rain, and the frequency of drought, the exchange of water between the mantle and the surface also dictates the habitability of the Earth. Scientists already know that water can be dragged down to the mantle by subducting tectonic plates and brought back to the surface by events like volcanic eruptions, hydrothermal vents and the creation of new crust at oceanic spreading centers. If this water cycle between the mantle and the surface is in balance, Earth’s sea level remains stable.
To discover mantle rain, the researchers examined what happens when a subducting slab of rock and rock-bound water sinks deeper into the mantle. They found that as it descends, increasing pressures and temperatures causes the rocks to melt, releasing the water.
“The melt is like a slurry,” Andrault says. “Imagine a mushy mix of sand grains glued to each other with mud in between—the mud is the mantle rain.”
As more rocks melt, and as more water is released from the rock, this melt eventually becomes light enough that it begins to rise. As it does, the water bonds to minerals in the upper mantle and lowers their melting points, causing more melting that releases more water-and the cycle repeats.
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