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NSF PR 98-29 - May 21, 1998
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New, Surprising Picture Emerges of Sub-Seafloor Magma
Formation
For the first time, seismologists have captured detailed
images of the deep underground processes that give
birth to most of the planet's new surface, along mid-ocean
ridges where the seafloor pulls apart. Some of the
underground patterns confirm predictions. Others are
a surprise. The new data represent a major step forward
in understanding the formation of the crust, convection
in the planet's interior, and the source of the most
abundant volcanic activity on Earth.
Funded by the National Science Foundation, the $7
million project is called the Mantle Electromagnetic
and Tomography Experiment, also known as MELT. One
of the largest marine geophysical experiments ever
conducted, its main goal is to find where melted rock,
also known as magma or "melt," is formed and how it
moves to the ridge crest to form new oceanic crust.
"The experiment has set a new standard for the type
and scale of experiments to be done in the oceans,"
says Dave Epp, program director in NSF's marine geology
and geophysics program, which funded the research.
"We expect this line of research to continue for the
next decade or more."
In this week's issue of Science, geophysicists
from Brown University and six other institutions report
that melting rock flows up in a broad zone in the
Earth's upper mantle, rather than in the narrower
plume that some researchers had predicted. Under the
separating seafloor, magma starts forming nearly twice
as deep as scientists had expected and, surprisingly,
wells up slightly-off center, not directly beneath
the ridge where most of the magma eventually erupts
and cools to form new oceanic crust.
"The seafloor spreading process is like a conveyor
belt carrying away crust from mid-ocean ridges," says
Donald Forsyth, a geologist at Brown University and
coordinator of the imaging project. "We're seeing
a suggestion that the upper mantle beneath the oceans
is stirred up by small-scale convection, such that
melting would be pretty much the same no matter where
a ridge opened up."
The early findings are reported in a special section
in this week's Science. An overview
article and seven research papers present direct observations
of the mantle from seafloor seismic recorders and
from other geophysical observations. More details
will be added Friday, May 29, at the American Geophysical
Union meeting in Boston, when results from the second
part of the experiment, using electromagnetic imaging
techniques, will be reported for the first time.
The results show that melt is generated over a much
larger region than many scientists had expected. Some
of the surprising observations will likely lead to
the development of a new generation of models of mantle
flow and magma generation beneath mid-ocean ridges.
Scientists had debated about mantle activity, and
it turns out that their predictions were at least
partly wrong. One model described a broad, shallow
region of passive upwelling and a second model predicted
a narrow, shallow and active upwelling zone. Neither
model predicted the asymmetry of the upwelling magma
or the depth at which melting occurs.
Other institutions involved in MELT are: the Carnegie
Institution of Washington, the Scripps Institution
of Oceanography in San Diego, the University of Colorado,
the University of Oregon, the University of Washington
in Seattle, and the Woods Hole Oceanographic Institution
in Massachusetts.
Editors: visuals are available.
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