Geologists Drill Into Fossil Magma
Chamber Deep Under the Ocean
National Science Foundation
International collaboration brings up first samples of hard
rock called gabbro in intact ocean crust.
Scientists aboard the research drilling ship JOIDES Resolution
have, for the first time, drilled into a fossil magma chamber
under intact ocean crust. There, 1.4 kilometers beneath the
sea floor, they have recovered samples of gabbro: a hard,
black rock that forms when molten magma is trapped beneath
Earth's surface and cools slowly.
The scientists, affiliated with the Integrated Ocean Drilling
Program (IODP), published their findings on April 20 in Science
Express, the online edition of the journal Science.
Although gabbro has been sampled elsewhere in the oceans
where faulting and tectonic movements have brought it closer
to the seafloor, this is the first time gabbro has been recovered
from intact ocean crust.
The borehole into the magma chamber took nearly five months
to drill, and required the use of twenty-five hardened steel
and tungsten carbide drill bits. Getting there "is
a rare opportunity to calibrate geophysical measurements with
direct observations of real rocks," said geophysicist
Doug Wilson of the University of California at Santa Barbara,
lead author on the Science Express paper. "Finding
the right place to drill was probably the key to this success."
Wilson and his IODP colleagues found that place by identifying
a region of the Pacific Ocean that formed some 15 million
years ago when the East Pacific Rise was spreading at a "superfast"
rate of more than 200 millimeters per year, faster than any
mid-ocean ridge on Earth today.
"We planned to test the idea that magma chambers
should be closest to the Earth's surface in crust formed at
the fastest spreading rate," said Wilson.
"These results confirm ideas about the way in which
fast-spreading oceanic crust is built," said Jamie
Allan, IODP program director at the U.S. National Science
Foundation, which co-funds the program. "This new
understanding opens the way to understanding the origin of
oceanic crust, which we can best do by deep drilling."
"We've accomplished a major goal scientists have
pursued for more than 40 years," agreed geologist
Damon Teagle of the National Oceanography Centre at the University
of Southampton, a co-chief scientist of the drilling expedition.
"Our research will ultimately help answer an important
question: how is new ocean crust formed?"
The formation of ocean crust is a key process in the cycle
of plate tectonics, which constantly repaves the surface of
the planet, builds mountains, and leads to earthquakes and
volcanoes.
"Sampling a deep fossil magma chamber will allow
us to compare its composition to overlying lavas,"
said expedition co-chief scientist Jeff Alt of the University
of Michigan. "It will help explain whether ocean crust,
which is about six- to seven- kilometers thick, is formed
from one magma chamber or from a series of stacked magma lenses.
The size and geometry of these lenses affect the composition
and structure of the ocean crust, and circulation of seawater
through the crust."
Such circulation leads to the formation of spectacular hydrothermal
"black-smoker" vents--oases that support exotic
life forms in the deep ocean.
IODP is an international marine research drilling program
dedicated to advancing scientific understanding of the Earth,
the deep biosphere, climate change, and Earth processes by
monitoring and sampling sub-seafloor environments.
IODP is supported by two lead agencies, the U.S. National
Science Foundation and Japan's Ministry of Education, Culture,
Sports, Science, and Technology. U.S.-sponsored drilling operations
are conducted by the JOI Alliance, comprised of the Joint
Oceanographic Institutions, Texas A & M University Research
Foundation, and Lamont-Doherty Earth Observatory of Columbia
University.
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