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Hidden inside the Earth, within the first hundreds of miles below the crust, is another ocean. It is most likely the largest ocean in the world. This water does not flow into a large pool. No fish sinks to its depths. In fact, this ocean is only water in the weakest sense: broken in its hydrogen and oxygen compounds and chemically bound to the surrounding rock, this ocean is stored. Or, most are.
Denis Andrault and Nathalie Bolfan-Casanova, geoscientists at the University of Clermont Auvergne in France, have developed a new model that shows that more of this water is in transit than previously thought. When the solid rock in the mantle, the layer of the planet between the crust and the core, is saturated with chemically dissociated water, it can be transformed into a molten mash rich in water. When it does, it seeps back into the bark. Researchers call this mantle rain.
In the same way that the water cycle between the atmosphere, glaciers, lakes, rivers, aquifers and the ocean affects sea level, the abundance of rainfall and the frequency of drought, the The exchange of water between the mantle and the surface also determines the habitability of the Earth. Scientists already know that water can crawl into the mantle by subduing tectonic plates and returning to the surface by things like volcanic eruptions, hydrothermal vents, and the creation of new crust in ocean expansion centers. If this cycle of deep water between the mantle and the surface is in equilibrium, the sea level of the Earth remains stable. Otherwise, our planet could exist like anything from a singular global ocean to a dried up world.
Earth’s habitability has benefited greatly from the fact that Earth’s sea levels have remained relatively stable for billions of years. According to previous mantle studies, however, it could have been very different. Estimates based on previously understood mechanics of the deep water cycle suggest that almost twice as much water is transported to the mantle than is released to the surface.
“There’s a layer about 410 miles below the surface that can hold a lot of water,” Andrault says. The predominant understanding is that water must remain there forever, he says. If this were the case, the surface water of the Earth would have been slowly decreasing, closed in the mantle.
But this is where the mantle rain comes in.
In their study, Andrault and Bolfan-Casanova show that mantle rain could be enough to keep the deep water cycle in balance.
To find out the mantle rain, the researchers looked at what happens when a subducting rock slab and rock-bound water sinks deeper into the mantle. They found that as it went down, rising temperatures and pressures caused the rocks to melt, releasing water.
“Casting is like slurry,” Andrault says. “Imagine a mixture of grains of sand stuck together with mud in between: mud is the rain of the mantle.”
As more rocks melt and more water is released from the rock, this melting eventually becomes light enough to begin to rise. As it does, the water binds to the minerals in the upper mantle and reduces its melting points, causing more melting which releases more water, and the cycle continues.
Andrault and Bolfan-Casanova’s mantle rain pattern, says Yoshinori Miyazaki, a terrestrial and planetary scientist at the California Institute of Technology who did not participate in the study, “shows that there could be another way. to carry water to the surface in addition to the global-scale convection of the mantle itself. “
“In general, water doesn’t like being in the rock phase,” Miyazaki says. “It will happily escape the fusion phase and filter up.” Andrault says more work is needed to understand the extent to which water escapes in this way.
The mantle rain pattern also suggests that there is currently an oceanic mass in the upper mantle. “Along with the ocean on the surface,” says Andrault, “this guarantees that there will always be water on the Earth’s surface.”
“We still have a lot to learn about the deep water cycle,” Miyazaki says. “But the fact is that it has worked amazingly well to keep the Earth’s average sea level relatively constant for the last 500 million years, and probably longer, to maintain a habitable environment for life to continue.” .