Seismic study reveals huge amount of water dragged into Earth’s interior

Slow-motion collisions of tectonic plates under the ocean drag about three times more water down into the deep Earth than previously estimated, according to a first-of-its-kind seismic study that spans the Mariana Trench, a crescent-shaped trench in the Western Pacific that measures 1,500 miles long and is the deepest ocean trench in the world.

Deployment of an ocean bottom seismograph from the research vessel Thompson. Credit: Douglas Weins/Washington University

Researchers listened to more than a year's worth of Earth's rumblings, from ambient noise to actual earthquakes, using a network of 19 ocean-bottom seismographs deployed across the Mariana Trench, along with seven island-based seismographs.

The Mariana Trench is where the western Pacific Ocean plate slides beneath the Mariana Plate and sinks deep into the Earth's mantle as the plates slowly converge.

The new seismic observations paint a more detailed picture of the Pacific Plate bending into the trench, resolving its 3D structure and tracking the relative speeds of types of rock that have different capabilities for holding water.

Rock can grab and hold onto water in a variety of ways. Ocean water atop the tectonic plate runs down into the Earth's crust and upper mantle along fault lines that lace the area where the plates collide and bend. Then it gets trapped.

Under certain temperatures and pressure conditions, chemical reactions force the water into a non-liquid form -- hydrous minerals (wet rocks) -- locking the water into the plate. Then the plate continues to crawl ever deeper into the Earth's mantle, ferrying the water with it.

Previous studies at subduction zones like the Mariana Trench have noted that the subducting plate could hold water. But they didn't determine how much water it held or how deep it went.

The seismic images show that the area of hydrated rock at the Mariana Trench extends almost 20 miles beneath the seafloor -- much deeper than previously thought.

The amount of water in this block of hydrated rock is considerable, the scientists said.

For the Mariana Trench region alone, four times more water subducts than previously calculated. These results can be extrapolated to predict the conditions in other ocean trenches worldwide.

For water in the Earth, what goes down must come up. All the water going into the Earth at subduction zones must be coming back up somehow, not continuously piling up inside the Earth.

Scientists believe that most of the water that goes down at a trench comes back into the atmosphere as water vapor when volcanoes erupt. But with the revised estimates of water from the new study, the amount of water going in seems to greatly exceed the amount of water coming out. (National Science Foundation)

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