How the subsurface ocean can affect life on Earth

This article originally appeared on Hakay Magazine Online publication on science and society in coastal ecosystems. Read more stories like this at

Hidden inside the Earth – in the first several hundred kilometers under the crust – there is another ocean. It is probably the largest ocean in the world. This water does not flow into a large pond. No fish anchored in its depths. In fact, this ocean is nothing but water in the most fluid sense: it is broken down into hydrogen and oxygen atoms complex and chemically bonded to the surrounding rocks, and this ocean is stored. Or, most are.

Denis Andreu and Nathalie Boulevin-Casanova, geologists 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 solid rock in the mantle — the planet’s layer between the crust and core — becomes saturated with chemically separated water, it can turn into a molten slurry rich in water. When that happens, it seeps back toward the crust. Researchers call this mantle rain.

As much as the circulation of water between the atmosphere, glaciers, lakes, rivers, aquifers, and the ocean affects sea level, the abundance of rain, the frequency of droughts, and the exchange of water between the mantle and the surface also dictate the land’s habitability. Scientists already know that water can be drawn into the mantle by subduing plate tectonics and bringing it back to the surface through things like volcanic eruptions, hydrothermal vents, and the creation of new crust at oceanic spreading centers. If this deep water cycle between the mantle and the surface is in equilibrium, then sea level remains stable. If not, our planet may exist as anything from a unique global ocean to a dry world.

The habitability of the Earth has benefited greatly from the fact that sea levels on Earth have remained relatively stable over billions of years. According to previous studies on the mantle, it could have been completely different. Estimates based on previously understood mechanics of the deep water cycle suggest that nearly twice as much water transported into the mantle is released back to the surface.

“There is a layer about 410 kilometers below the surface that can hold a lot of water,” Andrault says. The common understanding is that the water should stay there forever. If so, the Earth’s surface water would have slowly decreased, locked away in the mantle.

But this is where the rain cloak comes in.

Andrault and Bolfan-Casanova showed in their study that mantle rain can be sufficient to maintain the balance of the deep water cycle.

To detect mantle rain, researchers looked at what happens when a submerged piece of rock and water bound to the rock sinks deep into the mantle. They found that as it descends, increasing temperatures and pressures cause the rocks to melt, releasing water.

“It melts like mud,” Andrault says. “Imagine a soft mixture of grains of sand sticking together and mud in between — mud is mantle rain.”

As more rock melts, and as more water is released from the rock, this melt eventually becomes light enough that it starts to rise. As it stands, the water binds to the minerals in the upper mantle and lowers their melting points, causing more melts that release more water—and the cycle continues.

Yoshinori Miyazaki, an Earth and planetary scientist at Caltech who was not involved in the study, says Andrault and Pulvan-Casanova’s model of mantle rain “shows the possibility of another way of moving water toward the surface in addition to global-scale convection of the mantle itself.”

“Water generally doesn’t like to be in the rock stage,” Miyazaki says. “You will happily escape to the thaw stage and seep upward.” Andrault says more work is needed to understand the extent of water intrusion in this way.

The mantle rain model also indicates that there is currently only one oceanic mass in the upper mantle. “Together with the ocean at the surface, this ensures that there is always water at the surface of the Earth,” Andrault says.

“We still have a lot to learn about the deep water cycle,” Miyazaki says. “But one fact is certain that it has worked in an astonishing way to keep the average sea level of the Earth relatively constant over the past 500 million years, and possibly longer, to maintain a habitable environment for the continuation of life.”