NSF PR 96-79 - December 5, 1996
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International Research Team Discovers Unsuspected
Molten Layer in Himalayan Crust
At the premier place in the world where two continents
are colliding head on, researchers have traced the
Indian continent's downward plunge under Tibet and
discovered a molten layer within the crust, changing
the way scientists view the evolution of earth's continents.
The scientists conducted their research as part of
project INDEPTH (International Deep Profiling of Tibet
and the Himalaya), sponsored by the National Science
Foundation.
"This discovery may go a long way toward explaining
how large volumes of the earth's crust are redistributed
via magmatism," says Leonard Johnson, director of
NSF's continental dynamics program, which funded the
research. Magmatism is the development and movement
of magma within the earth.
"We tend to think of earth's plates as moving along
discrete faults," says Doug Nelson, an earth scientist
at Syracuse University and author of a paper on the
subject in this week's issue of the journal Science.
"But we should be thinking about flow, about how molasses
looks when it's pushed by a squeegee."
Project INDEPTH's research revealed a molten layer
within earth's crust under the Tibetan Plateau, but
Nelson says this is not magma that has been injected
into the crust from beneath. Rather, this soft area
is the result of an extremely thick crust. The thick
crust's lower regions, since they run so deep, are
exposed to higher than usual temperatures, melting
the crustal materials. "If you thicken a crust enough
and give it tens of millions of years," says Nelson,
"it will melt."
This molten crust may explain the relative flatness
of the Tibetan Plateau, researchers surmise. "It may
be that collisions like this act like big moonshine
stills," Nelson adds. "Continental crust moves around
and, in major collisions, this heating occurs, and
the crust is chemically refined. That's not a new
idea, but now we have the suggestion that collision-induced
magmatism may be the big elephant in the process."
Nelson's research used seismic reflection, a procedure
that involves detonating explosions on the earth's
surface and recording the waves reflected back by
sub-surface structures. "No one method alone would
have presented compelling evidence for what we found,"
says Nelson, so multiple techniques were employed
in searching the earth's hidden layers.
Other methods of exploration included seismic refraction
(measuring return waves that have refracted rather
than reflected), broadband earthquake recording (in
which stations measure waves generated by worldwide
earthquake activity), and magnetotelluric (determining
how conductive geological structures are by measuring
induced electrical and magnetic fields).
Plate tectonics, a view of the earth's crust as an
array of shifting pieces, is only a few decades old.
The current generation of geologists, says Nelson,
is working to clarify this picture. "Those of us involved
with seismic reflection have long thought that if
we could pick one place in the world to do this work,
it would be the Himalayan-Tibetan Plateau, because
this is the one place where a classic continental
collision is happening on a large scale." For 50 million
years, India has been driving north into Asia like
a bulldozer pushing a pile of debris, moving at about
five centimeters a year. Says Nelson, "It's pushed
up the Himalayas and the Tibetan Plateau, and pieces
of central Asia are pushing out like melon seeds into
the Pacific."
Editors: For more information about Project INDEPTH
see: http://www-hl.syr.edu/departments/gol/doug.html
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