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SANTA CRUZ DE TENERIFE, Canary Islands (AP) - British scientists have embarked on a mission to study a huge area on the Atlantic seabed where the Earth's crust is mysteriously missing and instead is covered with dark green rock from deep inside the planet.

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It is part of a globe-spanning ridge of undersea volcanos, the kind of structure that forms when Atlantic tectonic plates separate and lava surges upward to fill the gap in the Earth's crust.


But that apparently did not happen this time. Where there should be a four-mile-thick layer of crust, there is instead that much mantle _ the very dense, dark green rock that makes up the deep inner layer of the Earth.


Scientists have seen chunks of mantle that have been spewed up with lava, but never such a large, exposed stretch.


"It is like a window into the interior of the Earth," Bramley Murton, a geophysicist who is taking part in the six-week mission, said Tuesday from the research ship RRS James Cook as it headed to the site, still five days away.

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As the Iapetus Ocean closed, new oceanic crust developed when magma from the Earth's mantle welled upwards to form peridotite and dunite followed by gabbro. The transitional contact zone between them represents the base of the ancient ocean crust. Western Newfoundland is one of the few places on Earth where this boundary, the Mohorovicic Discontinuity, and the upper mantle can be studied directly.

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