Re: 3-D model shows big body of water in Earth's mantle
- From: "Carsten Troelsgaard" <carsten.troelsgaard@xxxxxxx>
- Date: Sat, 10 Feb 2007 09:15:50 +0100
"George" <george@xxxxxxxxxxxxxxx> skrev i en meddelelse
news:wqczh.2713$B8.876@xxxxxxxxxxxxxxxxxxxxxxxxx
Go to the link to view the graphics:
http://www.physorg.com/news90171847.html
A seismologist at Washington University in St. Louis has made the first
3-D model of seismic wave damping - diminishing - deep in the Earth's
mantle and has revealed the existence of an underground water reservoir at
least the volume of the Arctic Ocean. It is the first evidence for water
existing in the Earth's deep mantle.
Michael E. Wysession, Ph.D., Washington University professor of earth and
planetary sciences in Arts & Sciences, working with former graduate
student Jesse Lawrence (now at the University of California, San Diego),
analyzed 80,000 shear waves from more than 600,000 seismograms and found a
large area in Earth's lower mantle beneath eastern Asia where water is
damping out, or attenuating, seismic waves from earthquakes.
The traditional method seismologists use to image the Earth below us is to
measure the speed of seismic waves. This will provide a sort of CAT scan
of the Earth's core and mantle. Using wave speeds alone is a problem,
however, because they cannot distinguish between temperature and
composition variations.
The research is described in a forthcoming monograph, Earth's Deep Water
Cycle, which is in press to be published by the American Geophysical
Union.
Analyzing damped-out waves
An increasingly popular method, which Wysession used, is to analyze the
way waves damp out from their source. If you take a hammer and pound it
hard on a desk, waves will go from the source to the end of the table with
the mass of the table lessening, or attenuating, the power of waves. A
picture near the striking point might topple, but a stapler two feet away
might not even budge. Attenuation data tell seismologists how stiff a
region is, which is a function of how hot it is and how much water it
contains. Looking at the seismic wave speeds and attenuation at the same
time can tell whether an anomaly is due to temperature or water.
In analyzing the data, Wysession first saw large patterns associated with
known areas where the ocean floor is sinking down into the earth. Beneath
Asia, the fallen Pacific sea floor piles up at the base of the mantle.
Right above that he observed an "incredibly highly attenuating region,
that is both very damping and slightly slow," he said. "Water slows the
speed of waves a little. Lots of damping and a little slowing match the
predictions for water very well."
Previous predictions calculated that a cold ocean slab sinking into the
earth at 1,200 to 1,4000 kilometers beneath the surface would release
water in the rock that would escape the rock and rise up to a region above
it, but this was never previously observed.
Beijing anomaly
"That is exactly what we show here, the exact depth and high attenuation
amounts right above it," Wysession said. "I call it the Beijing anomaly.
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 water. The water then rises up into the overlying
region, which becomes saturated with water.
"If you combine the volume of this anomaly with the fact that the rock can
hold up to about 0.1 percent of water, that works out to be about an
Arctic Ocean's worth of water."
In recent years, seismologists have become excited at the possibility of a
feature like the Beijing anomaly. The availability of vast amounts of
digital seismograms made possible the discovery by Wysession and Lawrence,
who wrote many thousands of lines of computer codes to do the analyses.
Seventy percent of the earth is covered by water, which is very important
for the earth's geology, serving as a lubricant that allows efficient
convection and plate tectonics and the continental collisions that form
mountains.
"Water is like a lubricant, constantly oiling the machine of mantle
convection
Hi George
I wonder why it's not oily enough to create a path for it's own way out ..
Anyway, are we in for a new session with the creationists ;o/
Carsten
which then drives plate tectonics and causes the continents to move about
Earth's surface," Wysession said. "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."
.
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