- ANN ARBOR---Scientists from the University of Michigan and Utrecht University
have located a piece of Earth's ancient history buried 1,550 miles below
its surface.
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- Some 200 million years ago, during the
Jurassic period when dinosaurs roamed the world, it was part of a now-extinct
ocean bed. Today it rests at the bottom of a churning layer of superheated
rock that makes up the Earth's mantle. It is the oldest section of subducted
lithosphere ever identified.
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- "Originally this piece of Earth's
crust was located at the bottom of the Mongol-Okhotsk Ocean separating
what is now Siberia and Mongolia," said Rob Van der Voo, U-M professor
of geological sciences. "As the Siberian and Mongolian continental
plates converged between 200 million and 150 million years ago, this material
was forced down or subducted deep into the Earth. It has been sinking ever
since at an average rate of one centimeter (about one-half inch) per year."
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- Van der Voo and Utrecht University scientists
Wim Spakman and Harmen Bijwaard used seismic tomographic imaging to identify
the slab among a "graveyard" of slab remnants in the mantle beneath
Siberia's Lake Baikal. Results of their study are published in the Jan.
21 issue of Nature.
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- The study is significant because it confirms
that subducted slabs do eventually reach the bottom of the mantle, according
to Van der Voo. The study also illustrates how seismic tomography can provide
valuable data to validate theoretical models of what's happening inside
our planet.
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- "Greater understanding of deep Earth
dynamics will help scientists understand the Earth's internal engine which
drives the global convection system and the movement of continental plates,"
Van der Voo said.
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- The Earth's crust is broken up into eight
major segments or plates and about a dozen minor plates---all moving over
the planet's surface at a rate of several inches per year. Continental
plates move because they are pushed and pulled by convection currents in
the Earth's mantle, which is located just beneath the continental and oceanic
plates that make up the Earth's thin crust. When plates collide, one is
forced down beneath the other into the mantle creating what geologists
call a subduction zone. Because subducting slabs are colder and denser
than surrounding mantle material, they tend to sink.
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- Van der Voo and his colleagues selected
the Lake Baikal area for their study because it is the site of an ancient,
well-documented subduction zone and is located in a part of the world with
many earthquakes and an extensive network of seismic monitoring stations.
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- Seismologists study the time and trajectory
of sound waves traveling through the Earth from earthquake sites to seismic
monitoring stations to learn more about the temperature, density and composition
of the mantle layer. Acoustic energy travels through different types of
mantle material at different speeds. The warmer the material, the longer
it takes for sound energy to pass through. Using supercomputers to analyze
the travel times and paths of thousands of seismic waves, seismologists
can create a three-dimensional image, similar to a CAT scan, of the Earth's
interior.
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- Seismic tomographic images in Van der
Voo's study clearly show the slab of subducted material descending to the
bottom of the mantle layer. "We didn't expect to see such a strong
signal at these depths," Van der Voo added. "Whether its visibility
is the result of differences in temperature, composition, pressure or a
combination of these remains unclear."
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- The fact that the slab is located almost
directly below the site of its original subduction indicates that the Siberian
continental plate has moved very little in the last 150 million years.
North America, on the other hand, is steadily moving toward Asia at a rate
of about 1.5 inches annually.
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- "Eurasia will be the next supercontinent,"
Van der Voo said. "If North America continues drifting west, the Pacific
will gradually shrink and eventually the two continents will merge into
one."
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- The research was supported by Utrecht
University, the Netherlands Organization for Scientific Research and the
U.S. National Science Foundation.
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