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A
layer of partially molten rock about 22 to 75 miles underground can’t
be the only mechanism that allows continents to gradually shift their
position over millions of years, according to a NASA-sponsored
researcher. The result gives insight into what allows plate
tectonics—the movement of the Earth’s crustal plates—to occur.
“This
melt-rich layer is actually quite spotty under the Pacific Ocean basin
and surrounding areas, as revealed by my analysis of seismometer data,”
says Dr. Nicholas Schmerr, a NASA Postdoctoral Program fellow. “Since it
only exists in certain places, it can’t be the only reason why rigid
crustal plates carrying the continents can slide over softer rock
below.” Schmerr, who is stationed at NASA’s Goddard Space Flight Center
in Greenbelt, Md., is author of a paper on this research appearing in
Science March 23.
The
slow slide of Earth’s continents results from plate tectonics. Our
planet is more than four billion years old, and over this time, the
forces of plate tectonics have carried continents many thousands of
miles, forging mountain ranges when they collided and valleys that
sometimes filled with oceans when they were torn apart. This continental
drift could also have changed the climate by redirecting currents in
the ocean and atmosphere.
The
outermost layer of Earth, the lithosphere, is broken into numerous
tectonic plates. The lithosphere consists of the crust and an underlying
layer of cool and rigid mantle. Beneath the oceans, the lithosphere is
relatively thin (about 65 miles), though beneath continents, it can be
as thick as 200 miles. Lying beneath the lithosphere is the
asthenosphere, a layer of rock that is slowly deforming and gradually
flowing like taffy. Heat in Earth’s core produced by the radioactive
decay of elements escapes and warms mantle rocks above, making them
softer and less viscous, and also causes them to convect. Like the
circulating blobs in a lava lamp, rock in the mantle rises where it is
warmer than its surroundings, and sinks where it’s cooler. This churn
moves the continental plates above, similar to the way a raft of froth
gets pushed around the surface of a simmering pot of soup.
Although
the basic process that drives plate tectonics is understood, many
details remain a mystery. “Something has to decouple the crustal plates
from the asthenosphere so they can slide over it,” says Schmerr.
“Numerous theories have been proposed, and one of those was that a
melt-rich layer lubricates the boundary between the lithosphere and the
asthenosphere, allowing the crustal plates to slide. However, since this
layer is only present in certain regions under the Pacific plate, it
can’t be the only mechanism that allows plate tectonics to happen there.
Something else must be letting the plate slide in areas where the melt
doesn’t exist.”
Other
possible mechanisms that would make the boundary between the
lithosphere and the asthenosphere flow more easily include the addition
of volatile material like water to the rock and differences in
composition, temperature, or the grain size of minerals in this region.
However, current data lacks the resolution to distinguish among them.
Schmerr
made the discovery by analyzing the arrival times of earthquake waves
at seismometers around the globe. Earthquakes generate various kinds of
waves; one type has a back-and-forth motion and is called a shear wave,
or S-wave. S-waves traveling through the Earth will bounce or reflect
off material interfaces inside the Earth, arriving at different times
depending on where they interact with these interfaces.
One
type of S-wave reflects from Earth’s surface halfway between an
earthquake and a seismometer. An S-wave encountering a deeper melt layer
at the lithosphere-asthenosphere boundary at this location will take a
slightly shorter path to the seismometer and therefore arrive several
tens of seconds earlier. By comparing the arrival times, heights, and
shapes of the primary and the melt-layer-reflected waves at various
locations, Schmerr could estimate the depth and seismic properties of
melt layers under the Pacific Ocean basin.
“Most
of the melt layers are where you would expect to find them, like under
volcanic regions like Hawaii and various active undersea volcanoes, or
around subduction zones—areas at the edge of a continental plate where
the oceanic plate is sinking into the deep interior and producing melt,”
said Schmerr. “However, the interesting result is that this layer does
not exist everywhere, suggesting something other than melt is needed to
explain the properties of the asthenosphere.”
Understanding
how plate tectonics works on Earth could help us figure out how other
rocky planets evolved, according to Schmerr. For example, Venus has no
oceans, and no evidence of plate tectonics, either. This might be a clue
that water is needed for plate tectonics to work. One theory proposes
that without water, the asthenosphere of Venus will be more rigid and
unable to sustain plates, suggesting internal heat is released in some
other way, maybe through periodic eruptions of global volcanism.
Schmerr
plans to analyze data from other seismometer networks to see if the
same patchy pattern of melt layers exists under other oceans and the
continents as well. The research was supported by the NASA Postdoctoral
Program and the Carnegie Institution of Washington Department of
Terrestrial Magnetism Postdoctoral Fellowship.
Source: NASA Goddard Space Flight Center
