January 27, 2014
Water Escaping From Earth’s Oceans Into Mantle
Lee Rannals for redOrbit.com - Your Universe Online
University of Liverpool scientists believe that up to three and a half times the water of all the Earth’s oceans could be being transported beneath our feet.
Seismologists reported in the journal Geology that they found deep sea fault zones could transport much larger amounts of water from Earth’s oceans to the upper mantle than previously thought. The team analyzed earthquakes that occurred more than 60 miles below the Earth’s surface in the Wadati-Benioff zone.
Water is carried to the mantle through deep sea fault zones that penetrate the oceanic plate as it bends into the subduction zone. When the oceanic tectonic plate is forced beneath another plate it causes large earthquakes that can cause tsunamis, as well as other quakes that happen much deeper below the earth’s surface.
Seismic waves in the wide fault zones travel slower than in the rest of the subduction plates because the sea water that percolated through the faults reacted with the ocean rocks to form serpentinite, which is a mineral that contains water. Some of the water carried to the mantle by these fault zones is released as the tectonic plates heat up, while other water is transported deeper into the mantle and stored in the deep Earth.
“It has been known for a long time that subducting plates carry oceanic water to the mantle,” Tom Garth, a PhD student in the Earthquake Seismology research group led by Professor Andreas Rietbrock, said in a statement. “This water causes melting in the mantle, which leads to arc releasing some of the water back into the atmosphere. Part of the subducted water however is carried deeper into the mantle and may be stored there.”
The team found that fault zones that form in the deep oceanic trench offshore Northern Japan reaches down to depths of up 100 miles. These hydrated fault zones are able to carry large amounts of water, which suggests that subduction zones carry much more water from the ocean down to the mantle than scientists previously thought.
“This supports the theory that there are large amounts of water stored deep in the Earth,” said Garth.
Having a better understanding of how much water is being transported to the mantle adds to the knowledge of how the mantle convects, how it melts, and how the continental crust was formed.
“These results offer the first direct measure of the lower lithosphere hydration at intermediate depths, and suggest that regassing of the mantle is more vigorous than has previously been proposed,” the team concluded in their study.