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Up the Down Escalator: Minerals from Deep in the Earth

Up the Down Escalator: Minerals from Deep in the Earth

(April 1, 1999)

Scientists at the University of California, Riverside have found more evidence that rocks found in the Swiss Alps near the Italian border came to the surface from more than 150 miles deep, a discovery that could rekindle the scientific debate over the origin of the rocks.

Reporting in the April 2 issue of the journal Science, three UCR geoscientists write that they have found new evidence that suggests the rocks of the Alpe Arami region of the Alps came to the surface from the Earth's mantle by hitching a ride on lighter crustal rocks that were subducted - or driven down into the Earth - during the continental collision between Africa and Europe more than 40 million years ago.

"This breaks a paradigm of belief that rocks so far along on the downward escalator of 'subduction' cannot actually come up that escalator. We have found a completely independent set of observations that, by themselves, make the case again for the Alpe Arami rocks having come to the surface from great depth, in this case from more than 250 kilometers," said Harry W. Green II, UCR professor of geophysics and vice chancellor for research. Green co-authored the study with Larissa Dobrzhinetskaya, adjunct professor of earth sciences and formerly of Moscow's Institute of the Lithosphere, and Krassimir Bozhilov, mineralogist, crystallographer and manager of UCR's Analytical Electron Microscopy Facility.

The rocks in question contain red garnets with bright apple-green crystals of the mineral diopside clustered within and around them. Ironically, they had languished unstudied in Green's collections for more than two decades before his laboratory began examining them about four years ago. Green collected the rocks during a 1973 trip to the Alpe Arami region, but it wasn't until 1994 that he became intrigued that they might hold evidence of very great depth, following a presentation he heard at a meeting of the American Geophysical Union.

"We have had the extraordinarily good fortune to discover a particularly well-preserved volume of rocks which retains the 'memory' of its travels from great depth," said Green.

In 1996, the UCR scientists first published in Science that they had found 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.

The UCR team reasoned that as the rock rose, 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 finding caused a stir in the geological community because the implied depth, more than 180 miles, was more than twice the depth previously established for any other set of rocks found on the Earth's surface. Some scientists have argued that the concentration of iron-titanium oxide found in the minerals was not as high as the UCR group claimed and argued for a shallower origin. However, with funding from the National Science Foundation, the UCR researchers performed experiments verifying that under conditions existing at 180 miles depth, the solubility of iron-titanium oxide in olivine is greatly enhanced, in agreement with their hypothesis.

Now, Green and Dobrzhinetskaya and a new colleague, Bozhilov, have found independent evidence in the same rocks that also argues for great depth. By sophisticated examination in UCR's state-of-the-art, high-voltage transmission electron microscope, they have shown that the apple-green mineral (diopside) in and around the garnets contains crystalline lamellae of a different mineral, clinoenstatite, indicating the rocks originated at least 150 miles deep in the Earth where the pressures are great enough to dissolve clinoenstatite in diopside. As the rocks rose, the clinoenstatite was "spit out" in the form of lath-shaped crystals, Green said.

It is the characteristic shape of the clinoenstatite minerals and detailed crystallographic information contained within them that indicate the rocks came to the surface from deep in the Earth's mantle, rather than being formed at shallower depths. "The finding of two independent sets of evidence in different minerals of the same rocks greatly enhances the probability that we are correct", Green said.

Exactly how the deep rocks rose to the surface is still a scientific mystery. Because the deep rocks found at Alpe Arami are enveloped with lighter crustal rocks, the UCR team reasons that the surface rocks were subducted deep into the Earth during collision of Africa and Europe several tens of millions of years ago. When these lighter rocks rose back to the surface driven by their buoyancy, like ice cubes rising to the surface of a glass of water, they picked up along the way a small piece of the mantle, which now forms a part of the Alpe Arami about 2,500 feet long by 1,500 feet wide.

Alternatively, the very old memory retained by the rocks could have been imprinted during an earlier collision between these two continents that occurred several hundred million years ago and they have had a more complicated voyage to their current home in the Swiss Alps, Green said.

If the scenario of very deep origin is ultimately found to be correct, 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 surface rocks making a round trip to the Earth's mantle.

The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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