Earth Is Sucking Down Way More Water Than We Thought, And No One's Sure Where It's Going

Earth Is Sucking Down Way More Water Than We Thought, And No One's Sure Where It's Going. Slow-motion collisions of tectonic plates under the ocean drag about three times more water down into the deep Earth than previously believed, according to a seismic study that spans the Mariana Trench.


Slow-motion collisions of tectonic plates under the ocean drag about three times more water down into the deep Earth than previously believed, according to a seismic study that spans the Mariana Trench.

The observations from the deepest ocean trench in the world have important implications for the global water cycle, researchers say.

Chen Cai, who recently completed his doctoral studies at Washington University in St. Louis said, "People knew that subduction zones could bring down water, but they didn't know how much water." He is first author of the paper, which appears in Nature.

Candace Major, a program director in the National Science Foundation's Division of Ocean Sciences, said, "This research shows that subduction zones move far more water into Earth's deep interior—many miles below the surface—than previously thought. The results highlight the important role of subduction zones in Earth's water cycle."
To conduct the study, researchers listened to more than one year's worth of Earth's rumblings—from ambient noise to actual earthquakes—using a network of 19 passive, ocean-bottom seismographs deployed across the Mariana Trench, along with seven island-based seismographs.
The 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 nuanced picture of the Pacific plate bending into the trench—resolving its three-dimensional 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 plate runs down into the Earth's crust and upper mantle along the fault lines that lace the area where plates collide and bend. Then it gets trapped.
Under certain temperature and pressure conditions, chemical reactions force the water into a non-liquid form as hydrous minerals—wet rocks—locking the water into the rock in the geologic plate.
All the while, the plate continues to crawl ever deeper into the Earth's mantle, bringing the water along with it.
Previous studies at subduction zones like the Mariana Trench have noted that the subducting plate could hold water. But they could not determine how much water it held and how deep it went.
"Previous conventions were based on active source studies, which can only show the top 3-4 miles into the incoming plate," Cai says, referring to a type of seismic study that uses sound waves created with the blast of an air gun from aboard an ocean research vessel to create an image of the subsurface rock structure.
He said, "They could not be very precise about how thick it is, or how hydrated it is. Our study tried to constrain that. If water can penetrate deeper into the plate, it can stay there and be brought down to deeper depths."
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 that can be held in this block of hydrated rock is considerable.
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Scien-Tech News: Earth Is Sucking Down Way More Water Than We Thought, And No One's Sure Where It's Going
Earth Is Sucking Down Way More Water Than We Thought, And No One's Sure Where It's Going
Earth Is Sucking Down Way More Water Than We Thought, And No One's Sure Where It's Going. Slow-motion collisions of tectonic plates under the ocean drag about three times more water down into the deep Earth than previously believed, according to a seismic study that spans the Mariana Trench.
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