Earth's Interior Drags And Traps Huge Amount Of Sea Water, Says Study

The Earth is dragging about three times more sea water than previously estimated, according to a seismic study, with major implications for the global water cycle.

The findings showed that loss of sea water is due to the slow-motion collisions of tectonic plates under Mariana Trench -- deepest ocean trench in the world. 

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.

"People knew that subduction zones could bring down water, but they didn't know how much water," said lead author Chen Cai, from the Washington University in St. Louis.

"This research shows that subduction zones move far more water into Earth's deep interior, many miles below the surface than previously thought," added Candace Major, a programme director in the National Science Foundation's Division of Ocean Sciences. 

For the study, published in the journal Nature, the team 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. 

They found that 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 -- lock the water into the rock in the geologic plate. 

Then, the plate continues to crawl ever deeper into the Earth's mantle, bringing the water along with it.

The seismic images show that the area of hydrated rock at the Mariana Trench extends almost 20 miles or 32.2 km beneath the seafloor, the study showed.

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

Scientists believe that most of the water that goes down at the trench comes back from the Earth into the atmosphere as water vapour when volcanoes erupt hundreds of miles away. 

But with the revised estimates of water, the amount of water going into the earth seems to greatly exceed the amount of water coming out, the researchers noted.