New evidence further backs up the idea that there are oceans of water deep beneath the Earth's surface.
This water is not in liquid form, but is bound to rocks deep within the Earth's mantle, a Northwestern University news release reported.
Researchers found magma located about 400 miles beneath North America, meaning there is most likely water there as well. This water could have been driven from the surface by plate tectonics.
"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," Northwestern geophysicist Steve Jacobsen and University of New Mexico said in the news release. "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."
The researchers looked at seismic waves from earthquakes to look at the composition of the crust and mantle. Many rocks at this depth contain a mineral called ringwoodite, which is believed to be the "key to the process," the news release reported.
"Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles," seismologist Brandon Schmandt said in the news release. "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."
This mysterious water is trapped inside the molecular structure of these rocks; intense heat and pressure allow it to split into hydroxyl radical (OH), which can be incorporated into crystal structures.
A ringwoodite-containing diamond that was volcanically brought to the surface from a depth of 400 miles helped the researchers make their findings.
"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."
About one-percent of ringwoodite's weight is believed to come from water, it is believed to soak up water "like a sponge."
The findings suggest that dehydration melting is present, which is when rocks in the transition zone can hold a lot of H2O but rocks in the lower mantle cannot. When water is forced out of the ringwoodite in the transition zone it forms a high-pressure non-water-absorbing mineral called silicate perovskite. This can lead to melting in the transition zone and lower mantle.
"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 said.
