by Ker Than
February 28, 2007
Scientists scanning the deep interior of Earth have found evidence
of a vast water reservoir beneath eastern Asia that has at least the
volume of the Arctic Ocean.
The discovery marks the first time such a large body of water has
found in the planet's deep mantle.
The finding, made by Michael Wysession, a seismologist at
Washington University in St. Louis, and his former graduate student
Jesse Lawrence, now at the University of California, San Diego, will
be detailed in a forthcoming monograph to be published by the
American Geophysical Union.
The pair analyzed more than 600,000 seismograms - records of waves
generated by earthquakes traveling through the Earth - collected
from instruments scattered around the planet.
A slice through the
Earth, and the whole thing;
red shows unusually
soft, weak rock saturated with water,
and blue shows unusually stiff
They noticed a region beneath Asia where
seismic waves appeared to dampen themselves, or "attenuate," and
also slow down slightly.
"Water slows the speed of waves a
little," Wysession explained. "Lots of damping and a little
slowing match the predictions for water very well."
Previous predictions calculated that if
a cold slab of the ocean floor were to sink thousands of miles into
the Earth's mantle, the hot temperatures would cause water stored
inside the rock to evaporate out.
"That is exactly what we show here,"
"Water inside the rock goes down with the
sinking slab and it's quite cold, but it heats up the deeper it
goes, and the rock eventually becomes unstable and loses its
The water then rises up into the
overlying region, which becomes saturated with water.
"It would still look like solid rock
to you," Wysession told LiveScience. "You would have to put it
in the lab to find the water in it."
Although they appear solid, the
composition of some ocean-floor rocks is up to 15 percent water.
"The water molecules are actually
stuck in the mineral structure of the rock," Wysession
explained. "As you heat this up, it eventually dehydrates. It's
like taking clay and firing it to get all the water out."
The researchers estimate that up to 0.1
percent of the rock sinking down into the Earth's mantle in that
part of the world is water, which works out to about an Arctic
Ocean's worth of water.
"That's a real back-of-the-envelope
type calculation," Wysession said. "That's the best that we can
do at this point."
Wysession has dubbed the new underground
feature the "Beijing anomaly," because seismic wave attenuation was
found to be highest beneath the Chinese capital city.
Wysession first used the moniker during a presentation of his work
at the University of Beijing.
"They thought it was very, very
interesting," Wysession said. "China is under greater seismic
risk than just about any country in the world, so they are very
interested in seismology."
Water covers 70 percent of Earth's
surface, and one of its many functions is to act like a lubricant
for the movement of continental plates.
"Look at our sister planet, Venus,"
"It is very hot and dry inside Venus, and Venus
has no plate tectonics. All the water probably boiled off, and
without water, there are no plates. The system is locked up,
like a rusty Tin Man with no oil."