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University Of North Carolina At Chapel Hill
CHAPEL HILL
-- Working on volcanoes in the remote province of Kamchatka in
easternmost Russia, U.S., Russian and German geologists believe
they have solved a long-standing mystery about volcanoes ringing
the Pacific Ocean.
The question
has been why analysis of hardened lava there, also on Adak Island
in Alaska's Aleutian chain and elsewhere around the globe reveals
a chemical makeup different from what researchers predict it should
be. Pieces of the lava are named adakites because of where the
primitive rocks first were discovered two decades ago.
According
to the standard model of how vast pieces of the earth's surface
move in relation to one another, the Pacific Plate is continuously
thrusting underneath the Eurasian Plate so that North America
and Asia are drawing closer. The process takes water-laced sediments
on the sea floor lying atop the Pacific Plate to depths of about
60 miles deep, changing their nature through intense heat and
pressure.
"That
material then leaks up into the earth's upper mantle, which causes
the mantle to melt, and dark basalts to come out in volcanoes
as lava, the model suggests," said Dr. Jonathan M. Lees,
associate professor of geology at the University of North Carolina
at Chapel Hill. "What we found makes us think something different
also is happening in some places."
Dr. Gene Yogodzinki
of Dickinson College, a geochemist, and colleagues conducted chemical
analyses indicating that pieces of the plate, or slab, also have
melted, an observation contradicting the earlier belief that the
slab is too cold to melt and flow to the surface as lava. Lees,
a volcanologist who has been recording seismic activity on Kamchatka
for the past three years, said his data suggest the current model
should not be abandoned but rather changed.
A report on
the work appears in the Jan. 25 issue of the journal Nature.
"There
appears to be a very large tear in the slab where the Aleutians
and Kamchatka intersect so that the edge of the slab is exposed
to the mantle," he said. "When that happens, as it also
does in some other places like California, it allows the mantle
to erode the slab so that we see this very interesting and unexpected
geochemical signature -- or combination of chemicals -- near some
volcanoes. It's a kind of contamination, or mixing, of mantle
and slab."
Scientists
do not find the unusual lava in Hawaii and the Azores, which are
not near the edge of any slab's subduction zone. These exemplify
a different kind of volcano, one arising from what researchers
call hot spots deep in the earth beneath the oceanic crust. Volcanoes
formed by one slab sliding under another -- rather than just burning
holes through them -- include Mt. St. Helens, Mt. Rainer, Mt.
Pinatubo, Mt. Vesuvius and Mt. Fuji.
"What
we've done is to explain a mysterious chemical signature in rocks
with a very simple new model that has important implications for
our understanding of the mantle," Lees said. "Before,
the origin of adakites was the subject of much controversy because
we did not have a satisfactory model for how they got ocean crust
in them. Now we believe the cold slab melts because it's torn
and exposed to the hot mantle."
The mantle
lies about 18 miles below the continental crust and about three
to five miles beneath oceanic crust. Lava comes from molten mantle
rock about 60 miles beneath the surface.
Co-authors
of the paper work at the Institute for Volcanic Geology and Geochemistry
in Petropavlovsk-Kamchatsky in Russia and the Geochemical Institute
in Goettingen, Germany.
Lees said
that many geochemists and geophysicists he has discussed it with
like the research because it appears to solve the mystery. Others
disagree but have not been able to refute the new model.
"The
more seismic and geophysical evidence we come up with in our experiments,
the more we seem to find that our model is correct," he said.
"For scientists like us, that's fun and pretty exciting.
It's showing us how the earth works, explaining its plumbing system."
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