Geologists cannot examine subsurface rocks directly, but they can map the strongest layers by tracing earthquakes, which occur only in strong, brittle rock.

Previous studies have suggested that continents are held up by two strong layers, the upper crust and the upper mantle, separated by a weak quake-free layer in the lower crust.

However, Keith Priestley at the University of Cambridge says this may not be so. He analysed the seismic waveforms of earthquakes, which give a more accurate measurement of their depths than the traditional method of measuring the differences in arrival times of seismic waves travelling through the Earth.

He found that many quakes thought to have been triggered in the upper mantle, between 45 and 50 kilometres underground, actually started in the upper crust, only 15 kilometres down.

Priestley found some deeper quakes, but only in zones where the crust was thicker than the usual 35 kilometres. For instance, one quake in Pakistan which was thought to have occurred at a depth of 95 kilometres actually began 40 kilometres down in a zone where the crust was 45 kilometres thick, he says.

These depths paint a new picture of the Earth's structure, Priestley told the Seismological Society of America in San Diego, California, last week. If earthquakes aren't occurring in the upper mantle, then it must be weaker than was previously thought. This means that most continents are being supported entirely by the upper crust.

The findings could also explain why mountains are where they are, Priestley suggests. In the Himalayan region of northern India, earthquakes occur at depths of 40 kilometres, showing that a thick strong crust is supporting mountains 5 kilometres tall.

In contrast, on the Iranian plateau where elevations reach only 2 kilometres, quakes stop between 15 and 20 kilometres underground, suggesting it is being propped up by a more flimsy layer of rock.

Geologists who have spent years developing different models are not convinced, however. "I don't think the final words have been written," says Wang-Ping Chen of the University of Illinois in Urbana-Champaign. He says that Priestley studied only selected areas, and ignored some zones where quakes are thought to have occurred in the uppermost mantle. "It's not a global, exhaustive study."

This article appeared in the April 22 issue of New Scientist New Scientist. Copyright 1999 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

TERRADAILY.COM
Life's Volcanic Start
by Tony Fitzpatrick
Washington - March 29, 2000
Geologists at Washington University in St. Louis have developed new theoretical calculations on how life might have arisen on Earth, Mars and other celestial bodies from volcanic gases.