Magma Surge Moves Nevada Mountain, Study Says

Stefan Lovgren
for National Geographic News

August 6, 2004

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 depths—and about the role that deep magma plays in the process.

The research is described in this week's issue of the journal Science.

Deep Earthquakes

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 2.2—struck 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.

"We have not seen earthquakes in the Tahoe area deeper than 15 to 18 kilometers [9 to 11 miles]," said study coauthor Kenneth Smith, a research seismology professor at the University of Nevada Seismological Laboratory in Reno.

At the same time, Blewitt's GPS (global positioning system) instruments on top of Slide Mountain showed that the mountain had moved six millimeters [a little less than three-tenths of in inch] horizontally outward from the quakes' epicenter and eight millimeters [a little more than three-tenths of an inch] upward.

The movement was too large to have been caused solely by the earthquakes.

"In that range of magnitude, the earthquakes are so small that even with 1,600 events, you wouldn't expect to see anything more than a millimeter [four-hundredths of an inch] of movement on the surface," Blewitt said.

"It had to be something else that was creating the earthquakes and also causing the mountain to move."

Enter the Magma

Magma is formed when a part of the Earth's lower crust or upper mantle melts. The movement of magma causes volcanic activity. As rock inside the Earth melts, it can rise toward the surface and in some cases erupt in a volcano.

In the Lake Tahoe region, there is two-million-year-old volcanic material on the surface of the Earth. However, scientists don't believe the magma could reach the surface today.

"It is too far down, about 20 miles [32 kilometers] below the surface, and there is no evidence to suggest magma from this event is still moving," Smith said.

Instead, the scientists believe that the magma in this case was injected up into an area of 15 square miles (40 square kilometers). The injection created a magma pocket about one yard (about one meter) tall deep inside the Earth. This caused the Earth's surface to rise.

"It's like taking your fist and putting it under the pillow," study co-author David von Seggern. Von Seggern is the seismic network manager at the University of Nevada Seismological Laboratory in Reno.

"Your fist is several inches wide, but your pillow only seems to rise a little bit. When you inject a volume of material at depth, the surface has to go up a little bit."

The magma injection pushed the crust apart, producing high strains in the rock, which in turn caused earthquakes.

"We're confident that we're dealing with a magma-related type of process," Smith said.

Boundary in Action

Scientists know that the Basin and Range area is encroaching west into the Sierra region. They hope that the magmatic phenomenon will help them better understand how changes in the Earth's lower crust affect the lay of the land.

"There is a lot of debate over how extension really takes place in the Basin and Range—and how the interface between the Sierra and the Basin and Range evolves," Smith said. "We may actually see this boundary in action in the lower crust, and that's an important finding."

The discovery could also help explain geological phenomena elsewhere in the world.

"For all we know, this type of activity could be quite common," Blewitt said. "Because of recent technological developments, we're able to combine the GPS analysis and the seismological analysis and, for the first time, really start looking at the lower crust, which has always been a bit of a mystery."

For more seismology news, scroll down.