THE Earth鈥檚 crust may be all that is supporting the world鈥檚 tallest
mountains. A study of earthquakes by a geophysicist in Britain has revealed that
the upper layer of the Earth鈥檚 mantle, which underlies the continents, may be
too weak to do the job.
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.
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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鈥檚 structure, Priestley told the
Seismological Society of America in San Diego, California, last week. If
earthquakes aren鈥檛 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. 鈥淚 don鈥檛 think the final words have been written,鈥� 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. 鈥淚t鈥檚 not a global, exhaustive
蝉迟耻诲测.鈥�