NSF PR 96-12 - March 28, 1996
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Discovery Explains How Deep-Earth Rocks Reach Surface
The continental collision between Africa and Europe
more than 40 million years ago brought to Earth's
surface in the Swiss Alps a piece of mantle rock from
perhaps 300 miles deep, a depth inconsistent with
scientists' previous understanding of Earth's tectonics,
National Science Foundation (NSF)-supported research
suggests.
The rock came to the surface in the Alpe Arami region
of the Alps from at least 180 miles and perhaps as
much as 300 miles deep in the Earth's mantle, more
than twice the depth previously established for any
other set of rocks found on the surface, according
to a report published by University of California
at Riverside geophysicists in the March 29 issue of
the journal Science.
The geologically astounding finding suggests that
the lighter crustal rocks that now envelop the deep
rocks of the Alpe Arami were subducted into the Earth
during continental collisions several tens of millions
of years ago, according to Harry Green, one of the
study's authors. Then, as the lighter rocks were lifted
back to the surface by buoyant forces, they picked
up along the way small pieces of the mantle, the largest
of which now forms a part of the Alpe Arami, about
2,500 feet long by 1,500 feet wide. Green likened
the apparent phenomenon to a bar of Ivory soap submerged
in a bathtub which, as it floats, has enough buoyancy
to lift a washcloth to the surface.
"The lighter rocks must have been entrapped within
cold, heavy mantle rocks and carried down to these
great depths by tectonic forces associated with the
collision of Africa and Europe," he says. "When the
heavier rocks that pulled them down heated up and
became soft, the crustal rocks were released and rose
back up toward the surface, apparently picking up
the Alpe Arami rocks along the way."
The discovery implies that the theory of plate tectonics
-which explains the present-day positions of the Earth's
continents as well as the formation of mountains and
volcanoes -- may need to be extended to account for
major gravitational instability in which lighter continental
rocks can be carried to extraordinary depths before
returning to the surface, says Green. In the course
of plate tectonics, one slab of Earth's crust is "subducted"
beneath another. When that leads to continental collisions,
such as is occurring today between India and Asia,
this instability is apparently triggered.
Subduction of continental rocks to depths of more
than 60 miles has been reported previously from four
other locations on Earth -- the Kokchetav area of
Kazaskhstan, the Dabie mountain range of China, in
western Norway and in the Dora Maira Mountains of
the Italian Alps. All are places where continents
collided millions of years ago. The Alpe Arami rocks
have now revealed that during the return to the surface
of continental rocks, pieces of the mantle can come
along for the ride, says Green.
By examining the structure of the crystals within
the rocks, scientists can make inferences about their
history. What they found were tiny crystals of iron-titanium
oxide in the form of tiny "rods" oriented parallel
to one another within individual crystals of the mineral
olivine, the most abundant mineral in the rock. "Despite
the thousands of mantle rocks I have examined over
my career, never had I seen anything like this," says
Green. "This rock had to come from a set of conditions
where these rods could dissolve in olivine -- the
only possible explanation had to be extraordinary
depth." As the rock rose, Green says, lower pressures
and temperatures caused the elements to "unmix" from
the surrounding olivine and form rods, much like sugar
would settle out of heavily sweetened coffee as it
cools.
"The high pressures at work at least 300 kilometers
deep in the Earth had to be what led to formation
of the rods," says Green. "Moreover, it is possible
the Alpe Arami rocks do not contain the deepest subduction
of continental rocks. Now that we know to look for
such things, there is no reason that we might not
find rocks from even greater depths."
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