Vast Amounts Of Our Oceans Are Being Sucked Into Earth’s Interior

ROV Deep Discoverer explores an eruptive vent in the Mariana Trench. Image courtesy of NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas.

The deepest ocean trench in the world, the Mariana Trench in the western Pacific, is an otherworldly environment of alien-like creatures and pitch blackness. It turns out, this strange place is also the site of colossal ocean-sucking forces too.

For the first time, a seismic study has taken a deep look at the Mariana Trench in order to find out the scale of its water-snorting abilities. They discovered that the slow-motion collision of tectonic plates – in this case the Pacific plate, which is thrust under the Mariana plate – under the seabed is pushing vast quantities of water, three or four times the amount previously thought, inside the Earth’s deep interior.

The study led by Washington University was published in the journal Nature this week. 

“This research shows that subduction zones move far more water into Earth’s deep interior — many miles below the surface — than previously thought,” Candace Major, a program director in the National Science Foundation’s Division of Ocean Sciences, explained in a statement.

So, you might be wondering, how does this really work exactly? Well, water can actually enter rock in a number of ways. We know, for example, that the subducting plate of the Mariana Trench holds water in so-called “wet rocks.”

Water runs down into the Earth’s crust and upper mantle along the fault lines where the tectonic plates meet. Here, it becomes trapped and subjected to intense pressure and temperature conditions that force the water into a non-liquid form, such as hydrous minerals, aka wet rocks.

Researchers came to these conclusions by looking at a year’s worth of data from a network of passive, ocean-bottom seismographs deployed across the Mariana Trench. This provides vats of data about the three-dimensional structure of the Pacific plate trench and the different types of rock that dwell within. This allowed the researchers to go deep than ever before, revealing that the Trench region holds up to four times more water, namely in the form of wet rocks, than previously calculated.   

The Mariana Trench, the deepest natural point in the world, is 2,550 kilometers (1,580 miles) long, 69 kilometers (43 miles) wide, with maximum depths around 10,994 meters (36,070 feet). Google Maps

Curiously, even though the trench is absorbing all this water, sea levels have remained relatively constant over geologic time, suggesting that water must be also coming back out, not just continuously saturating the Earth. The process of how this happens, however, is a mystery.

This work could have some big implications for our understanding of the global water cycle. Of course, it’s not just the Mariana Trench that absorbs water into the rocky interior. The same applies to subduction zones, where Earth’s tectonic plates meet, around the world.

“Does the amount of water vary substantially from one subduction zone to another, based on the kind of faulting that you have when the plate bends?” asked study author Douglas A Wiens. “There’s been suggestions of that in Alaska and in Central America. But nobody has looked at the deeper structure yet like we were able to do in the Mariana Trench.”

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