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Drilling shortcut penetrates Earth's mantle.

While physicians might dream of curing cancer, geoscientists have longed to drill through the Moho - the boundary between the planer's thin crust and its interior mantle. By taking a shortcut, researchers have come close to realizing that goal.

An international group of oceanographers returned late last month from an expedition near the Galapagos Islands, where they bypassed the crust and drilled directly into the upper mantle. The rocks they collected will help scientists attempting to understand the formation of the ocean floor --which forms twothirds of the Earth's outer skin.

"The most important thing is that we have finally got these rocks. It's amazing to me that after all the years we've been drilling, this is the first time we've ever managed to catch these rocks," says Kathryn Gillis of the Woods Hole (Mass.) Oceanographic Institution. Gillis served as one of the chief scientists on the twomonth-long drilling cruise, part of the Ocean Drilling Program (ODP).

The desire to pierce the Mohorovicic boundary, or Moho, goes back to the late 1950s, when leading oceanographers conceived "Mohole," a megascience project aimed at drilling straight through the 6-kilometer-thick ocean crust. But cost overruns and political problems sank the Mohole project soon after it began.

To drill through the entire ocean crust would take years. Gillis and her colleagues took a quick route into the mantle by drilling in an area called the Hess Deep, where rock from the deep crust and the mantle lies exposed on the seafloor, without kilometers of crustal covering. The interior rocks have reached the surface because plate tectonic forces are ripping the seafloor in the same way that bread crust might split as it bakes, exposing the dough inside.

Gillis and her colleagues did not succeed in drilling through the Moho, which would require passing from the crust into the mantle; that feat must wait for future cruises. Instead, they bored two major holes into rocks from the upper mantle that once lay close to the Moho.

The researchers found remarkable differences in the kinds of rocks present within the upper mantle. While typical mantle rocks appeared in one of the holes, the other yielded mantle rock in combination with a significant amount of gabbro, a rock that makes up the lower layer of the ocean crust. Gabbro forms from molten rock, called magma, that rises out of the mantle and hardens deep within the crust. Because they found this rock in the mantle, the researchers believe they penetrated a fossilized conduit though which mantle magma rose into the crust. The researchers also drilled a major hole into the gabbro rocks of the deep crust at a site nearby,

Geoscientists have long wondered how magma forms in the mantle and migrates upward. The rocks collected from Hess Deep suggest that magma in the mantle funnels into rising conduits that exist in some areas but not in others.

The new discoveries verify some lessons geologists have learned by studying ophiolites - slabs of the ocean floor that have been pushed up onto the continents. Because they are easily accessible, ophiolites have served as a model for understanding how the ocean floor forms. In recent years, some scientists have suggested that ophiolites represent special cases that are not good analogs for typical ocean crust. But the mantle rocks from Hess Deep vindicate those who study ophiolites, Gillis asserts. "They tell us we were on the right track:' she says.

In 1986, an ODP Atlantic voyage collected a small amount of exposed mantle rock, but the Hess Deep rocks are the first mantle known from near the Moho.

The ODP drill crew is currently attempting to extend the deepest hole in the ocean floor, a 2-kilometer-long hole that they have drilled on and off for 13 years. Though far from the Moho, the bottom of the hole lies within reach of the gabbro layer that forms the bottom 4 kilometers of the ocean crust.
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Title Annotation:National Science Foundation's Ocean Drilling Program
Author:Monastersky, Richard
Publication:Science News
Date:Feb 20, 1993
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