Drilling team seeking to tap secrets of Cocos plate

Arrow shows the offshore location where drilling will take place. Graphic: Costa Rica Seismogenic Zone Project/A.M. Costa Rica

The team of scientists who are leaving here Sunday on a special ship hope to drill as deep as possible, perhaps as deep as 2 kilometers (6,560 feet), into the Coco tectonic plate that is beneath the sea.

The French Centre National de la Recherche Scientifique outlined the goals of the mission this week. Four of the 31 international scientists taking part in the mission are French. The drilling boat now is in Puntarenas to change crews.

The holy grail for geological scientists would be to drill 4 to 5 kilometers (2.5 to 3.1 miles) to reach the earth’s mantle. The research center explained that the scientists who will be aboard the JOIDES Resolution are planning to drill as deeply as they can.

The site of the drilling is expected to be 900 kilometers, about 560 miles, off Costa Rica’s Pacific coast. The 143-meter (469-foot) vessel is specially designed to remain stationary in the ocean while its drill penetrates the sea floor.

Costa Rican residents know the Cocos plate well. Interaction with the Caribbean plate is the principal cause of earthquakes here, and the Cocos pushing under the Caribbean plate also is credited with the creating of Costa Rica’s mountains and volcanoes. The Cocos plate also tussles with the Panamá micro plate to the south.

The French research center noted that the departure of the mission coincides with the 50th anniversary of the first drilling of the ocean crust in April 1961 in the Atlantic.

The center explained in a summary of the mission:

Oceanic crust is formed along mid-ocean ridges by the cooling of magma produced by the partial melting of mantle rocks. If the magma reaches the surface of the ocean floor, it cools rapidly, forming the basalts that make up the vast majority of the upper oceanic crust. However, if the magma crystallizes more slowly at depth, the rock that is formed is called gabbro. The oceanic crust is therefore made up of a surface layer of basalt underlain by deeper gabbro.
The mission is part of the Integrated Ocean Drilling Program, which has operations all over the world. The interest in the area off Costa Rica can be explained by the title of the mission: Superfast Spreading Rate Crust 4. This is the fourth cruise to the location identified as Hole 1256D where the drilling crew will attempt to locate and then deepen an existing penetration into the crust. The hole reached 1,500 meters (about 4,900 feet) during a 2005 expedition. The scientists hope to go deep enough to reach the gabbro rock.

The term “super fast spreading” describes the mid-ocean ridge at which the drilling will take place. Fast-spreading ridges are more uniform and homogeneous than the crust formed at slow- spreading mid-ocean ridges, such as those in the Atlantic, the French center said. Around 20 percent of existing mid-ocean ridges have fast spreading rates greater than 8 centimeters (3.1 inches) a year, it added. They have generated nearly half of today’s oceanic crust, making up 30 percent of the earth’s surface, the center added..

The crust of the Cocos plate, formed 15 million years ago, is expanding at 20 centimeters (nearly 8 inches) a year, which is faster that any existing active ridge, the center said.

The samples collected will enable geologists to test models for the formation of these rocks and to characterize their cooling process, as well as the role of hydrothermal circulation at such depths, said the French summary.

The formation of oceanic crust, which covers around 70 percent of the Earth’s surface, is a key process in the dynamics of the planet, which this drilling project should help scientists understand better, it added.

The effort is more than pure science. The Observatorio Vulcanológico y Sismológico de Costa Rica at Universidad Nacional said that scientists are trying to learn more about the process that creates large earthquakes in these subduction zones where one plate is forced under another. Some 80 percent of the world’s earthquakes of magnitude 8.0 or larger take place in the Pacific Rim of which Costa Rica is a part, the observatory noted.

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