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Earth's mantle
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There are very few places in the world where dynamic activity taking place beneath Earth's surface goes undetected.
Volcanoes, earthquakes, and even the sudden uplifting or sinking of the ground are all visible results of restlessness far below, but according to research by Arizona State University (ASU) seismologists, dynamic activity deep beneath us isn't always expressed on the surface.
The Great Basin in the western United States is a desert region largely devoid of major surface changes. The area consists of small mountain ranges separated by valleys and includes most of Nevada, the western half of Utah and portions of other nearby states.
For tens of millions of years, the Great Basin has been undergoing extension--the stretching of Earth's crust.
While studying the extension of the region, geologist John West of ASU was surprised to find that something unusual existed beneath this area's surface.
West and colleagues found that portions of the lithosphere--the crust and uppermost mantle of the Earth--had sunk into the more fluid upper mantle beneath the Great Basin and formed a large cylindrical blob of cold material far below the surface of central Nevada.
It was an extremely unexpected finding in a location that showed no corresponding changes in surface topography or volcanic activity.

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