NSF PR 96-63 - October 23, 1996
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The Swallowing of Earth's Ocean Floors
Ocean Drilling Program Scientists
Probe 'Recycling' of Oceanic Crust Near Costa Rica
An international team of scientists representing nine
countries will this month board the drill ship JOIDES
Resolution -- currently docked in San Diego, California
-- to study the ocean floor off Costa Rica. The geologists
hope to determine the age and composition of the area's
rocks and sediments, and their physical and chemical
properties. To conduct their research, they will use
a newly developed logging tool called Logging While
Drilling, or LWD. LWD samples physical and chemical
properties within the drill hole while drilling is
occurring, giving "pristine" results that are unaffected
by vertical movements of the drill.
The team is led by geologists Eli Silver of the University
of California at Santa Cruz and Gaku Kimura of the
University of Osaka in Japan. "The Costa Rica margin
is an important location where tectonic plates collide,"
explains Silver, "forcing one plate to slide under
the other, creating a subduction zone."
Subduction zones are the most active features on Earth,
adds Bruce Malfait, director of NSF's ocean drilling
program. "They control the movements of plates, produce
most of the world's volcanic and seismic activity,
and play a key role in recycling surface material
to great depths within the Earth." The recycling of
this material plays a major role in both volcanic
and seismic activity. Through this expedition, scientists
hope to make major inroads into understanding the
recycling process.
The theory of plate tectonics postulates that the
amount of crust destroyed at subduction zones each
year is balanced by the formation of new crust at
spreading ridges. But how much of the sediment being
carried to subduction zones is scraped off the descending
plate and left behind, and how much goes down into
the mantle with the plate, is an unanswered question.
Many subduction zone trenches are currently flooded
with sediments from the last million years of glacial
climates, when large amounts of such material washed
off the land, making it difficult to determine the
proportion of downgoing to offscraped sediment. Costa
Rica may be one of the few places on Earth where this
determination can be made. Preliminary images of Costa
Rica's continental margin show that about 1,300 feet
of sediment now lie on the incoming Cocos plate, with
about 250 feet of the incoming sediment being scraped
off at the base of the slope.
The research team is also interested in understanding
why the seafloor off Costa Rica has some of the lowest
heat flow on Earth. Measurements show that this zone
of low heat flow is widespread and probably indicates
chilling of earth's crust by unusually strong flow
of sea water there. Fluids flowing through from deeper
sources may provide one explanation for this increase.
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