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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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