Analysis of seismic waves passing through the deep earth appears to confirm that an ocean of water is tied up in the mantle, 400 miles below the surface. Any voyager to the centre of the earth could leave the wet suit behind, however. The water is not liquid, but rather bound in minerals that exist at the extreme pressures found at such depths.
The finding suggests that processes that occur in the shallower mantle and that cause volcanoes and related activity at the surface are also occurring farther down.
“It’s a new view of the structure of this part of the earth,” said Brandon Schmandt, a geophysicist at the University of New Mexico and an author of a recent paper in the journal Science describing the research.
The work also adds credence to the idea that the earth’s water accumulated in the interior during the planet’s formation, rather than arriving later through the bombardment of icy comets. In this view, water bound up in minerals in the mantle, the 1,800-mile-thick layer between the thin crust and the hot metallic core, degassed over time and reached the surface.
The scientists studied a part of the mantle called the transition zone, from 300 to 440 miles deep. The ability of this zone to contain water — and apparently to retain a lot of it — “may have something to do with stabilising or buffering the size of the oceans,” said Steven D Jacobsen, a mineralogist at Northwestern University and another author of the paper. “It may be fortunate that the earth’s interior can act like a sponge.”
Jacobsen synthesises deep-earth minerals by replicating the extreme pressures that exist hundreds of miles below the surface. Over the years when he has made a blue mineral called ringwoodite, which is ubiquitous in the transition zone, it has formed with water. But that was just lab work, Jacobsen said, adding, “We haven’t known whether it could happen down there.”
Schmandt analysed seismic data from the US Array project, in which 400 mobile seismometers have been deployed across the US to create high-resolution images of the mantle. The analysis showed signs of melting in the transition zone, in areas where convection was causing the mantle to flow downward.
Melting of the mantle occurs close to the surface, creating the magma that is responsible for volcanic hot spots around the world. The process is called dehydration melting, because as parts of the mantle slide deeper at places where the earth’s tectonic plates meet, the increasing pressure causes minerals in the mantle to release their water, lowering the melting temperature.
But finding evidence of melting much farther down, as Schmandt did, was difficult to explain “unless you invoke water,” Jacobsen said. The work is all part of trying to understand how the dynamics of the deep earth affect what happens at the surface. “We’re trying to connect the rock cycles — i.e., plate tectonics — with water cycles,” Jacobsen said. “The more we look, the deeper it goes.”
“It’s no longer liquid water that we’re talking about at these great depths. The weight of hundreds of kilometres of rock and very high temperatures above 1,000 degrees Celsius (1,832 Fahrenheit) break down water into its components. And it’s not accessible. It’s not a resource in any way,” Jacobsen added.
“Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles (80 km),” Schmandt said in a statement. “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.”
The research built on another study in March showing that a commercially worthless diamond found in Brazil contained ringwoodite that entrapped water amounting to more than 1 percent of its weight. Ringwoodite has been found in meteorites, but this was the first terrestrial sample because it normally is so deeply buried.