The Swallowing of Earth's Ocean Floors

October 23, 1996

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.

National Science Foundation

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