The rock "might provide a very nice, excellent analog to the deep crust beneath the Andes," Tang said. This rock, called a xenolith, originally formed deep at the base of the Sierra Nevada when they were an active continental arc - the researchers found the rock in Arizona.
Tang, along with Rice University petrologist Cin-Ty Lee, and their colleagues analyzed a sample of rock that formed some 66 million years ago and was pushed to the surface in a volcanic eruption about 25 million years ago. Enter the xenolithįortunately, the Sierra Nevada mountains of the western United States used to be an active mountain-building region, like the Andes today. At nearly 50 miles (80 kilometers) below the surface, these rocks are inaccessible.
Unfortunately, the chemistry of the rocks at the bottom of this crust are a mystery. Regions like the Andes - which form atop a subduction zone - are known as continental arcs, and they're special because the crust there is about twice as thick as regular continental crust, Tang said. As the oceanic crust off the coast of South America crunches below the continental crust, the restless Andes rise, and magma spews from some of the highest-elevation volcanoes on Earth, Tang said. The Andes are a massive mountain-building region, powered by the nearby tectonics of a subduction zone.
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