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Depth doesn't
jibe
The studies beneath Iceland revealed
no deep plume, Dr. Foulger says. Whatever heat source
fuels Iceland's volcanoes must be coming from a broad
reservoir no more than 250 miles down. Plumes were traditionally
thought to come from 1,800 miles deep, at the boundary
between the Earth's core and mantle.
“This took me completely by surprise,”
Dr. Foulger says. “I started telling people that
the plume seems to be shallow. ... And it suddenly occurred
to me that it's not a plume if it's shallow.”
Iceland isn't the only place where
plume evidence has come up lacking. Similar work in
and around Yellowstone National Park found no traces
of a plume, even though the biggest hotspot in North
America lies under the region.
But other seismologists say their research
confirms the presence of plumes elsewhere. Raffaella
Montelli of Princeton will present such pro-plume
findings in December, at a meeting of the American Geophysical
Union.
Dr. Montelli used a new technique to
further clarify how seismic waves bounce around narrow
objects such as plumes. The new analysis found evidence
for about 30 plumes, including a dozen reaching deep
to the core-mantle boundary, says team member Guust
Nolet.
In his view, the work supports deep
plumes. “We scientists are skeptical by nature,
and it is not surprising that the plume hypothesis came
under fire,” he wrote in an e-mail interview.
“However, much of the skepticism is now obsolete.”
Obsolete or not, the skepticism remains.
Dr. Foulger, for instance, says that the seismic data
is too patchy to be able to confirm the Princeton observations
as true plumes.
In the meantime, anti-plumers come
up with a few alternative ideas to explain hotspot volcanism.
One such notion is the “crack” theory, which
originated in the 1840s as a way to explain chains of
volcanic islands in the Pacific, says James
Natland, a marine geologist at the University of
Miami.
In this scenario, plates fracture,
allowing a blob of material to rise upward, melt and
fuel a volcano. The crack theory doesn't require a deep
source of hot rock, says Dr. Natland - just shallow
mantle rock, primed to melt if pressure is relieved
from above.
Chemistry helps determine at what depth
and pressure a piece of rock will melt. In order for
crack theory to work, the mantle would have to be near
its melting point at relatively low pressure.
New experiments support this idea,
says Dean
Presnall, a petrologist at the Carnegie Institution
of Washington who recently retired from the University
of Texas at Dallas.
His work, which simulates the high
temperatures and pressures inside the Earth's mantle,
suggests that rocks don't have to be that hot –
and thus deep – in order to melt. In other words,
given just a crack, the mantle could rise upward and
melt due to decompression without relying on plumes
from deep down.
“ I think plumes have been vastly overplayed,”
says Dr. Presnall.
Perhaps the biggest unknown in the
plumes debate is Hawaii, where the seismology data can
be interpreted as either pro-plume or anti-plume. To
settle the question, next year researchers will begin
the Plume-Lithosphere
Undersea Mantle Experiment, or PLUME.
It will use 64 seismic instruments on the seafloor,
and 10 on the surface, to take pictures of the upper
mantle beneath the Hawaiian Islands.
The 15-month project may finally be
able to determine whether Hawaii is in fact a plume
or not. But no matter what its results are, Dr. Foulger
says the debate has succeeded in getting scientists
to think of plumes in a new way.
“If we want to hang on to our
dear old friend plumes,” she says, “then
we need to acknowledge that they're rather different
than what we thought.”
E-mail: awitze@dallasnews.com |
