Leg 170 Pre-cruise
THE SWALLOWING OF EARTH'S OCEAN FLOORS
Ocean Drilling Program scientists probe recycling of oceanic crust beneath Costa Rica
22 October 1996
The team is led by Dr. Eli Silver (Director of the Institute of Tectonics, University of California, Santa Cruz) and Dr. Gaku Kimura (Department of Earth Sciences, University of Osaka Prefecture, College of Integrated Arts and Sciences, Sakai-shi 593, Japan), along with scientists from the U.S., Japan, France, Germany, Great Britain, Canada, Italy, Norway, and Costa Rica. Silver noted that the Costa Rica margin is an important locator where one tectonic plate collides, forcing one plate to slide under the other plate creating subduction zones.
Subduction zones are the most dynamically active features on Earth. 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
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. This expedition seeks to understand how much of the sediment being carried to subduction zones is scraped off the descending plate and how much goes down into the
Using three dimensional seismic imaging, a method of probing the subsurface with high energy sound waves in a very tightly spaced pattern, this part of the offshore margin of Costa Rica can be viewed in exquisitely fine detail.
There are two gaping holes of knowledge scientists would like to fill. Scientists don't know the age of the material and therefore can't determine the rates at which these scraping and subducting processes occur. The second problem is determining how long this process has been occurring. Recent seismic studies using larger energy sources and receivers on the seafloor indicate that a large part of the margin may be underlain by old rocks which are not part of the present cycle of sediment accretion. If this is the case, it limits the amount of material that could have been scraped off the subducting plate.
In addition to determining the balance between accreted and subducted sediment, this team is interested in understanding why the seafloor off Costa Rica has some of the lowest heat flow measured on Earth. Measurements made in 1994 from the R/V Atlantis II and from the submersible ALVIN showed that this zone of low heat flow is widespread and probably indicates chilling of the crust by unusually strong flow of sea water through the oceanic crust. The heat flow through the accreted sediment is twice as great as that through the
The Ocean Drilling Program is funded by the U.S. National Science Foundation, Canada, Australia, South Korea, the European Science Foundation Consortium, Germany, France, Japan, and the United Kingdom to investigate
Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES), an international group of scientists, provides scientific planning and program advice. Joint Oceanographic Institutions, Inc., a nonprofit consortium of 10 major U.S. oceanographic institutions, manages the program.
Texas A&M University, science operator, operates and staffs the drill ship that retrieves core samples from strategic sites in the world's oceans. Lamont-Doherty Earth Observatory of Columbia University is responsible for downhole logging.
U.S. members of JOIDES are: University of California at San Diego, Columbia University, University of Hawaii, University of Miami, Oregon State University, University of Rhode Island, Texas A&M University,
Scientists on the ship:
Dr. Eli A. Silver
Dr. Gaku Kimura
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