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Continental roots not as deep as some people thought
The
roots of the continents go down between 200 and 250km, forming a distinct
boundary with the underlying mantle like that seen under the oceans, according
to a team of seismologists at the University of California, Berkeley. Their
interpretation of seismic data resolves a debate within the geophysical community
about the depth of the boundary between the rigid lithosphere that floats
on the Earth's surface and the hot convecting mantle that underlies it. "It
was very clear that you had a lithosphere about 80km thick, on average, in
the ocean basins, and under that is the asthenosphere - the very top of the
mantle" says Barbara Romanowicz, Professor of Earth and Planetary Science
at UC Berkeley and Director of the Berkeley Seismological Laboratory. "But
under the continents, it wasn't as clear. What we are now saying is that
we are seeing the same thing under the continents - it's a little more subtle
signal, but it's there."  A
cross-section of the Earth's surface shows that the lithosphere (green) is
thicker under the continents than under the oceans. The asthenosphere, the
very top of the mantle, can be detected because of its anisotropy - horizontally
polarized shear waves generated by earthquakes travel faster through this
rock than vertically polarized seismic waves. (UC Berkeley Seismological
Laboratory)
The
ocean lithosphere is thin when spewed out at the mid-ocean ridges, thickening
to as much as 80km as it is pushed outward to make room for more ocean bottom.
Continents, however, are much older and much thicker, and vary a lot more
around the globe. Romanowicz and UC Berkeley graduate students Yuancheng Gung and Mark Panning claim in a paper in the April 17 issue of Nature
that some estimates of the thickness of the continental lithosphere are based
on a misinterpretation of the earthquake-generated seismic waves recorded
after they pass through the base of the lithosphere and the top of the mantle.
The
continental lithosphere is composed of a thin, 30-50 kilometre-thick layer
of crust atop a hot layer of rock. Seismic waves tend to travel faster through
the lithosphere, then slow down when they pass through the asthenosphere. The
UC Berkeley team argues that only the vertically polarized seismic shear
waves slow down in the asthenosphere, while the horizontally polarized waves
continue to travel faster down to a depth of about 400km. This has confused
geophysicists looking only at horizontally polarized shear waves, leading
them to conclude that the lithosphere descends down to 400km. By
taking into account the different shear wave velocities, the boundary between
the lithosphere and asthenosphere works out to be between 200 and 250km,
in agreement with other methods. "Overall,
(the UC Berkeley seismologists) have provided a satisfying way forward in
tackling a long-standing puzzle in earth science" says Brian Kennett of Australian
National University in a Nature commentary. "One
consequence of this is that the asthenosphere is a prevailing global feature"
Romanowicz added. "I think there is something very special about the asthenosphere
of the Earth that is very important in the character of the plate motion." What
distinguishes Earth from the solar system's other planets, which don't have
plate tectonics, that is, moving surface plates, is that the Earth has a
very well developed asthenosphere which helps move the plates around, she
says. "Somehow
the asthenosphere needs to be sustained over geological times, and I think
that heat is being pumped from below and is actively keeping the asthenosphere
less viscous and more deformable" she says, and thus able to move the lithosphere
around. In a Science
paper a year ago, Romanowicz and Gung proposed that this heat pump consists
of massive plumes of hot rock, or superplumes" rising through the mantle. Previous story
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