Deep down below us is a tug of war
moving at less than the speed of growing fingernails. Keeping your balance is
not a concern, but how the movement happens has been debated among geologists.
New findings from under the Pacific
Northwest Coast by University of Oregon and University of Washington scientists
now suggest a solution to a mystery that surfaced when the theory of plate
tectonics arose: Do the plates move the mantle, or does the mantle move the
plates.
The separation of tectonic plates, the
researchers proposed in a paper online ahead of print in the journal Nature
Geoscience, is not simply dictating the flow of the gooey, lubricating
molten material of the mantle. The mantle, they argue, is actually fighting
back, flowing in a manner that drives a reorientation of the direction of the
plates.
The new idea is based on seismic
imaging of the Endeavor segment of the Juan de Fuca Plate in the Pacific Ocean
off Washington and on data from previous research on similar ridges in the
mid-Pacific and mid-Atlantic oceans.
"Comparing seismic measurements of
the present mantle flow direction to the recent movements of tectonic
plates, we find that the mantle is flowing in a direction that is
ahead of recent changes in plate motion," said UO doctoral student
Brandon P. VanderBeek, the paper's lead author. "This contradicts the
traditional view that plates move the mantle."
While the new conclusion is based on a
fraction of such sites under the world's oceans, a consistent pattern was
present, VanderBeek said. At the three sites, the mantle's flow is rotated
clockwise or counterclockwise rather than in the directions of the separating
plates. The mantle's flow, the researchers concluded, may be responsible for
past and possibly current changes in plate motion.
The research—funded through National
Science Foundation grants to the two institutions - also explored how the
supply of magma varies under mid-ocean ridge volcanoes. The researchers
conducted a seismic experiment to see how seismic waves moved through the
shallow mantle below the Endeavor segment.
They found that the middle of the volcanic
segment, where the seafloor is shallowest and the inferred volcanic activity
greatest, the underlying mantle magma reservoir is relatively small. The ends,
however, are much deeper with larger volumes of mantle
magma pooling below them because there are no easy routes for it to travel
through the material above it.
Traditional thinking had said there
would be less magma under the deep ends of such segments, known as
discontinuities.
"We found the opposite,"
VanderBeek said. "The biggest volumes of magma that we believe we have
found are located beneath the deepest portions of the ridges, at the segment
ends. Under the shallow centers, there is much less melt, about half as much,
at this particular ridge that we investigated.
"Our idea is that the ultimate
control on where you have magma beneath these mountain ranges is where you can
and cannot take it out," he said. "At the ends, we think, the plate
rips apart much more diffusely, so you are not creating pathways for magma to
move, build mountains and allow for an eruption."
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