The presence of liquid water on the surface is what makes our 'blue planet' habitable, and scientists have long been trying to figure out just how much water may be cycling between Earth's surface and interior reservoirs through plate tectonics. They now believe reservoirs 400 miles under the surface hold the key.
* Ingredients for water are bound up in rock deep in the Earth's mantle

* Scientists claim the find may represent the planet's largest water reservoir

* It is believed to cover most of the U.S - 400 miles below the surface

* Researchers will now carry out further tests to discover if the water wraps around the entire planet

The Earth's largest expanse of water isn't on the surface of the planet, but instead is buried deep within its mantle, researchers have found.

They claim the ingredients for water are bound up in deep rocks, and scientists believe the discovery may represent the planet's largest water reservoir.

Deep pockets of magma were found using seismic waves beneath North America, and this magma is said to be a clear sign of water.

So far, researchers claim they only have evidence the water sits beneath the U.S.

But they plan to carry out further tests to see if it wraps around the entire planet.

The presence of liquid water on the surface is what makes our 'blue planet' habitable, and scientists have long been trying to figure out just how much water may be cycling between Earth's surface and interior reservoirs through plate tectonics.

Northwestern geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt say the findings, to be published June 13 in the journal Science, will aid scientists in understanding how the Earth formed, what its current composition and inner workings are and how much water is trapped in mantle rock.

'Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight,' said Jacobsen, a co-author of the paper.

'I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet.

'Scientists have been looking for this missing deep water for decades.'

Scientists have long speculated that water is trapped in a rocky layer of the Earth's mantle located between the lower mantle and upper mantle, at depths between 250 miles and 410 miles.

Jacobsen and Schmandt are the first to provide direct evidence that there may be water in this area of the mantle, known as the 'transition zone,' on a regional scale.

The region extends across most of the interior of the United States.

A diamond-anvil cell (approximately 5 mm across). Between the tips of these gem diamonds, heating experiments on deep-mantle minerals were carried out at conditions similar to 660 km depth, or about 400 miles below the surface.
Schmandt, an assistant professor of geophysics at the University of New Mexico, uses seismic waves from earthquakes to investigate the structure of the deep crust and mantle. Jacobsen, an associate professor of Earth and planetary sciences at Northwestern's Weinberg College of Arts and Sciences, used observations in the laboratory to make predictions about geophysical processes occurring far beyond our direct observation.

The study combined Jacobsen's lab experiments in which he studies mantle rock under the simulated high pressures of 400 miles below the Earth's surface with Schmandt's observations using vast amounts of seismic data from the USArray, a dense network of more than 2,000 seismometers across the United States.

This image shows a high-pressure diamond cell of the mineral hydrous ringwoodite, or blue crystal. When heated with a laser, a dehydration reaction occured and ringwoodite was transformed to higher pressure minerals. That dehydration reaction was observed beneath North America by using seismic waves
'Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles,' said Schmandt, a co-author of the paper.

'If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found.'

If just one percent of the weight of mantle rock located in the transition zone is H2O, that would be equivalent to nearly three times the amount of water in our oceans, the researchers said.

Schmandt and Jacobsen's findings build on a discovery reported in March in the journal Nature in which scientists discovered a piece of the mineral ringwoodite inside a diamond brought up from a depth of 400 miles by a volcano in Brazil.

That tiny piece of ringwoodite - the only sample in existence from within the Earth - contained a surprising amount of water bound in solid form in the mineral.

Fragments of the blue-colored mineral called ringwoodite, synthesized in the laboratory. This mineral is thought to exist in the mantle at depths between about 500 and 700 km depth, and act act as a sponge for water
'Whether or not this unique sample is representative of the Earth's interior composition is not known, however,' Jacobsen said.

'Now we have found evidence for extensive melting beneath North America at the same depths corresponding to the dehydration of ringwoodite, which is exactly what has been happening in my experiments.'

For the study reported in Science, Jacobsen subjected his synthesized ringwoodite to conditions around 400 miles below the Earth's surface and found it forms small amounts of partial melt when pushed to these conditions.

'When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit,' Schmandt said.

'This is called dehydration melting.'

'Once the water is released, much of it may become trapped there in the transition zone,' Jacobsen added.

Source: Science