for National Geographic News
In late 2003 a swarm of small earthquakes struck 19 miles (30 kilometers) below Lake Tahoe, which straddles the border of California and Nevada. At the same time, Slide Mountain, 11 miles (18 kilometers) away in Nevada, moved dramatically, according to satellite readings.
Scientists were puzzled. They suspected that the two events were related. But the earthquakes were too small and too deep to cause the mountain to move.
But something else was at play: magma. The scientists were astonished to discover that an injection of magma, or molten rock, into the lower crust of the Earth had caused both the earthquakes and the movement of the mountain.
"This is the first time we have seen anything like this," said study co-author Geoffrey Blewitt, a research professor at the Nevada Bureau of Mines and Geology at the University of Nevada in Reno.
This magma "intrusion" could provide vital information about the evolution of the boundary between two tectonic provinces at lower crust depthsand about the role that deep magma plays in the process.
The research is described in this week's issue of the journal Science.
Lake Tahoe, a popular ski resort, sits atop the boundary between the Basin and Range region (which includes most of Nevada, southern Oregon and Idaho, and western Utah) and the Sierra Nevada area(which includes California).
In the Basin and Range, the crust of the Earth is 19 to 22 miles (30 to 35 kilometers) deep. (The layer below the crust is called the upper mantle.)
The boundary under Lake Tahoe is volatile, with the Sierra region pulling away from the Basin and Range. Scientists are not sure what happens to the crust of the Basin and Range area as it reaches into the Sierra block.
During five months in late 2003, a swarm of more than 1,600 earthquakes ranging up to a relatively light magnitude of 2.2struck the area. Earthquakes are common in Lake Tahoe, but these were unusual, because they occurred at such great depth, more than 19 miles (30 kilometers) into the Earth.
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