Is there an ocean beneath our feet?

Jan 27, 2014
Is there an ocean beneath our feet?
An arc volcano in Chile

(Phys.org) —Scientists at the University of Liverpool have shown that deep sea fault zones could transport much larger amounts of water from the Earth's oceans to the upper mantle than previously thought.

Seismologists at Liverpool have estimated that over the age of the Earth, the Japan subduction zone alone could transport the equivalent of up to three and a half times the water of all the Earth's oceans to its .

Water is carried to the mantle by deep sea which penetrate the oceanic plate as it bends into the subduction zone. Subduction, where an oceanic tectonic plate is forced beneath another plate, causes large earthquakes such as the recent Tohoku earthquake, as well as many earthquakes that occur hundreds of kilometers below the Earth's surface.

Using seismic modelling techniques the researchers analysed earthquakes which occurred more than 100 km below the Earth's surface in the Wadati-Benioff zone, a plane of Earthquakes that occur in the as it sinks deep into the mantle.

Analysis of the seismic waves from these earthquakes shows that they occurred on 1 - 2 km wide fault zones with low seismic velocities. Seismic waves travel slower in these fault zones than in the rest of the subducting plate because the sea water that percolated through the faults reacted with the oceanic rocks to form serpentinite – a mineral that contains water.

Is there an ocean beneath our feet?
Summary of subduction zone structure inferred for waveform modelling of dispersed P-wave arrivals (Garth & Rietbrock, Geology, 2014)

Some of the water carried to the mantle by these hydrated fault zones is released as the tectonic plate heats up. This water causes the mantle material to melt, causing volcanoes above the subduction zone such as those that form the Pacific 'ring of fire'. Some water is transported deeper into the mantle, and is stored in the deep Earth.

"It has been known for a long time that subducting plates carry oceanic water to the mantle," said Tom Garth, a PhD student in the Earthquake Seismology research group led by Professor Rietbrock. "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.

"We found that fault zones that form in the deep oceanic trench offshore Northern Japan persist to depths of up to 150 km. These hydrated fault zones can carry large amounts of water, suggesting that carry much more water from the ocean down to the mantle than has previously been suggested. This supports the theory that there are large amounts of water stored deep in the Earth.

Understanding how much is delivered to the mantle contributes to our knowledge of how the mantle convects and how it melts. This is important to understanding how plate tectonics began and how the continental crust was formed.

Explore further: Seismologists puzzle over largest deep earthquake ever recorded

More information: The research is published in Geology: geology.gsapubs.org/content/early/2014/01/10/G34730.1.abstract

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User comments : 4

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alfie_null
not rated yet Jan 28, 2014
Is there something that balances this process, or rather, how well is this process balanced? There seems to be a decent amount of water here on the surface, and it's been that way for a while.
taka
1 / 5 (2) Jan 28, 2014
That is argument for expanding earth hypothesis, here it is: Before plate tectonic start the solid earth was much smaller and covered by thick atmosphere and water layer. When it starts the solid earth engulf large amounts of water and carbon dioxide and other gases and expand significantly. The remains of its initial crust form tectonic plates that since then drift on its surface; sometimes they come together and sometimes go apart. And of course the water level is balanced. If water pressure becomes higher then less of it can come back to surface throw volcanic arcs.
shavera
not rated yet Jan 28, 2014
The expanding Earth hypothesis is just plain stupid. Like the worst kind of stupid. "oh these pieces fit together like a jigsaw puzzle. Must be that the earth was smaller then, I guess." It doesn't explain anything except coastal shapes, and requires a ton more invention about like... how the planet ever started "growing" magically. Standard plate tectonic theory, though perhaps boring to some people I guess, explains rock strata, explains mountain growth, explains magnetic alignment of rocks, explains all the data we have put together about our Earth. That's why it's the theory of how the Earth is geologically speaking. It doesn't just cherry pick one type of data and then stop.
taka
1 / 5 (1) Feb 03, 2014
Expanding Earth hypotheses explains how enormous amounts of water fit inside earth and who young earth surface does not froze when the young sun was weak, (thick atmosphere!) and where this thick atmosphere disappears. And it explain how plate tectonic starts, it is not alternative to it. If solid earth was all the time as big there would be no room between plates and plates could not drift. That is my interpretation, I know that initially this theory was concurrent to plate tectonic and that was proved wrong. That does not mean it is completely wrong. It is rather common mistake to believe that if something is wrong in some situation then it must be completely wrong. But if theory was strong then it must contain some degree of truth.

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