Discovery Explains How Deep-Earth Rocks Reach Surface
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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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