Very
interesting. We now have an explanation for the effective fluidity of olivine
which is an important part of mantle rocks.
Drawing the conclusion that this is sufficient for plate tectonics may
be premature.
However it is
good to know that actual continuous deformation is possible and that conforms
to the apparent plasticity we observe.
All good work
and tidies up this part of the issue.
There are other reasons to expect plasticity in these rocks.
Tiny Crystal
Defects Help Drive Plate Tectonics
By By Becky Oskin, Staff Writer
http://news.yahoo.com/tiny-crystal-defects-help-drive-plate-tectonics-173611921.html
Inside most of the Earth, olivine is a hot mineral
whose creepy behavior drives plate tectonics.
In the upper mantle — the top of the planetary layer
between the crust and core — olivine's unusual behavior presents a paradox.
These solid crystals must change shape for plate tectonics to work, oozing like
toothpaste over long time scales. (The mantle's flow helps
push and pull Earth's crustal plates.)
By mimicking the extreme pressures and temperatures
of the mantle in a laboratory, scientists have found that olivine crystals move
by contorting along internal defects. The defects allow one part of a crystal
to slip and slide (or shear) over another part. That's how a single crystal
morphs without breaking. The paradox? There's a missing defect. According to
models, Earth's mantle flows in such a way that there should be three
independent directions of movement for olivine crystals. But until now,
researchers had found only two, said Patrick Cordier, a geophysicist at the
University of Lille in France.
"Olivine shows only defects along two
directions, not three," Cordier said. "This is not enough for
achieving a general deformation. However, olivine-rich natural rocks show
pervasive evidence suggesting that olivine deforms very easily in the
mantle."
Cordier and his colleagues recently discovered a new
kind of olivine crystal defect, one that could explain the paradox. The
findings were published Feb. 26 in the journal Nature.
With an advanced electron microscopy technique, the
researchers saw linear defects, called dislocations, at the boundaries between
olivine crystals. The dislocations let individual crystals slide past one
another. Malleable metals also have similar dislocations, which is why jewelry
makers can bend and twist gold and silver into beautiful shapes.
"Dislocations allow crystals to be sheared
along some specific directions and on some specific planes," Cordier said.
"If a crystalline solid has enough different kinds of those defects, it
can exhibit an ability to deform which seems to ignore its crystalline structure.
This is the case for metals. This is also the case for minerals and
rocks," he told Live Science's Our Amazing Planet.
More work is needed before geoscientists will know
whether the new discovery resolves the olivine paradox, said Greg Hirth, a geophysicist
at Brown University who was not involved in the study. However, the study
marks the first time dislocations have been discovered in geological materials,
and will further understanding of the processes that control the mantle, Hirth said in a commentary on the findings that was
also published in Nature.
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