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Underneath an early continent.

Underneath an early continent

The wind-scoured plains of northwest Canada above the Arctic circle present an image of primeval times. So it seems fitting that geologists have traveled there to probe eons back into Earth's history. After studying the ancient rocks preserved in this barren landscape, two scientists conclude that the lithosphere -- Earth's hard, outer skin -- underlying this region was far thinner 1.9 billion years ago than it is today.

Like a cracked eggshell, the lithosphere is broken into more than a dozen rigid plates that slowly rearrange their positions over the planet. The brittle lithosphere covers a hotter, more ductile mantle that can flow over geologic time. In the modern world, the thickness of the lithosphere measures about 100 kilometers under continents and up to 40 km under oceans.

To estimate the thickness of the ancient Canadian lithosphere, John Grotzinger and Leigh Royden of the Massachusetts Institute of Technology in Cambridge studied a basin created when two large continental blocks collided 1.9 billion years ago. In the crash, one block rode over the other. The thickness of the basin sediments revealed how much the lower block warped under the weight of the top one, allowing the researchers to calculate that the continental lithosphere in this region once measured less than half its present thickness. They report their findings in the Sept. 6 NATURE.

The lithosphere's shallow depth is important, say the researchers, because this indicates the ancient continent was sitting over hot mantle rock. Geophysicists believe that in the modern world, continents have thick "roots" of relatively cold mantle material extending down to a depth of 300 to 400 km (SN: 12/13/86, p.380). But grotzinger and Royden believe no such mantle root existed 1.9 billion years ago under the ancient continental block they studied.

These findings bear on a hot debate among geophysicists concerning how and when mantle roots formed. Because there are rocks in South Africa indicating that region had a root as far back as 3.2 billion years ago, some scientits have suggested all roots formed during the earliest era of Earth's history. But Grotzinger says the new work suggets instead that roots continued to form during more recent periods.
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Title Annotation:research of ancient Canadian lithosphere
Publication:Science News
Date:Sep 22, 1990
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