By Will Dunham
WASHINGTON (Reuters) - If you want to find
Earth's vast reservoirs of water, you may have to look beyond the
obvious places like the oceans and polar ice caps.
Scientists on Friday said massive amounts of water appear to exist
deep beneath the planet's surface, trapped in a rocky layer of the
mantle at depths between 250 miles and 410 miles (410 km to 660 km).
But do not expect to quench your thirst down there. The water is not
liquid - or any other familiar form like ice or vapor. It is locked
inside the molecular structure of minerals called ringwoodite and
wadsleyite in mantle rock that possesses the remarkable ability to
absorb water like a sponge.
"It may equal or perhaps be larger than the amount of water in the
oceans," Northwestern University geophysicist Steve Jacobsen said in
a telephone interview. "It alters our thoughts about the composition
of the Earth."
"It's no longer liquid water that we're talking about at these great
depths. The weight of hundreds of kilometers 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.
Jacobsen said water is taken down into the mantle with minerals
during the process known as plate tectonics - the slow, inexorable
movement of the colossal rock slabs that make up the Earth's
When the minerals containing this water reach certain depths, they
break down in a process called dehydration and release the water to
form magmas. Such "dehydration melting" is common in the shallow
mantle and forms the source for magmas in many volcanoes.
In a study published in the journal Science, the researchers present
evidence that this is also occurring much deeper in the mantle in a
region called the "transition zone" between Earth's upper and lower
[to top of second column]
The study combined lab experiments involving synthetic ringwoodite
being exposed to conditions simulating the heat and pressure of the
"transition zone" and observations of events in this zone based on
seismic data from a network of more than 2,000 seismometers across
the United States.
A team led by Jacobsen and University of New Mexico seismologist
Brandon Schmandt identified deep pockets of magma, a likely
signature of the presence of water at those depths.
"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.