June 19, 2014

Evidence of a vast reservoir of water long thought to exist deep within Earth has been revealed in two recent studies. The reservoir, which may contain as much water as all of Earth’s oceans, is more than 300 miles (480 kilometers) below the surface in the mantle, the enormously thick layer of rock between Earth’s crust and inner core. One line of evidence for the reservoir was reported this month by scientists who analyzed seismic waves passing through the mantle beneath the United States. Another line of evidence, reported earlier this year, came from a battered diamond found in Brazil.

Scientists were quick to explain that the reservoir does not look like an underground sea. Instead, the water is locked within tiny grains of a kind of mineral called ringwoodite that forms in the searing temperatures and crushing pressures of the mantle. The water is not even a liquid or a vapor. It consists of hydroxide ions, negatively charged atoms of hydrogen and oxygen bound together. One scientists described the reservoir as a layer of “mushy rock.”

Beneath Earth's solid crust are the mantle, the outer core, and the inner core. Scientists can learn about the inside of Earth by studying how waves from earthquakes travel through the planet. (World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago)

The seismic evidence was collected by scientists analyzing data collected by the Transportable Array (TA), an ambitious earth science observatory funded by the National Academy of Sciences. The TA consists of hundreds of portable seismic stations that measure waves of energy passing through Earth’s layers during earthquakes or other seismic events. Arranged in a grid pattern, the stations were rolled eastward across North America over a 10-year period. The scientists found that the speed of the waves changed as they passed through a part of the mantle called the transition zone. This suggested that the ringwoodite in this zone was melting. Finding evidence of melting at such an unusual depth, the scientists concluded, indicated that the ringwoodite was releasing water as it was carried deeper into the mantle.

The other study involved an analysis of a diamond shot from the transition zone to the surface through a volcanic eruption called a kimberlite. The diamond, which scientists said, looked like it had been “to hell and back,” contained a sample of water-bearing ringwoodite. It is the only known piece of ringwoodite ever found naturally on Earth’s surface. All other samples of the mineral, which lies too deep to be obtained by drilling, have come from meteorites or have been produced in scientific laboratories.

Scientists said that in addition for providing evidence of Earth’s hidden ocean, the research may shed new light on the movement of the tectonic plates that make up Earth’s outer surface. The findings also bolster the theory that the water in Earth’s oceans and lakes came from rocks that formed early in Earth’s history, not from icy comets that bombarded the surface.

Additional Woirld Book articles:

• Water cycle