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Mount St. Helens's "Drumbeat" Quakes Caused by Stuck Plug?

Richard A. Lovett
for National Geographic News
November 22, 2006
 
The current eruption of Washington State's Mount St. Helens, which began
about two years ago, has been marked by a series of weak, shallow
earthquakes, or "drumbeats," that occur every couple of minutes, a new
study says.

The "slip/stick" motion of the rocky "plug" being pushed out of the volcano is causing those rhythmic quakes, according to scientists from the Cascades Volcano Observatory in Vancouver, Washington (Washington State map).

(Related: "Giant Rock Growing in Mount St. Helens Crater" [May 9, 2006].)

In nature, repetitive events like this are rare, says the U.S. Geological Survey's Richard Iverson, who led the study team.

"So it seemed to me that whatever was causing this had to be something where the physics was quite simple."

Boing, Boing

Iverson likens the process to trying to push a block of wood across a table with a spring.

"If conditions are right, the spring can cause the block to move in a jerky fashion as tension is built up and released."

Specifically, magma, or molten rock, is rising at a steady rate, pushing against the base of the plug until it lurches upward. The lurches are so small—only about a quarter of an inch (0.5 centimeter)—that they can't be seen by video cameras that monitor the crater.

For the process to have continued for so long, new magma must be solidifying against the bottom of the plug at the same average rate at which the plug is rising.

Otherwise, the mountain should pop its cork and spew lava—or the plug should seal the lava strongly enough that the quakes would temporarily halt.

No one knows how long the current eruption will continue, but nobody expects it to stop soon. As long as magma continues to rise from below, Iverson said, "the chances are that it'll keep coming out at the top."

The new research on the tiny quakes, which have magnitudes of less than 2.0, will be published in tomorrow's issue of the journal Nature.

Steam Engine

Not all geologists are convinced. One of them is Bernard Chouet of the U.S. Geological Survey office in Menlo Park, California.

Chouet doubts that the plug's tiny motions can cause quakes within Mount St. Helens.

In an effort to pinpoint the location of the quakes, Chouet and colleagues examined data from 19 seismometers installed on the mountain in 2005. Seismometers measure ground movement.

Chouet's team concluded that the drumbeats were not being created by rock sliding against rock in the magma channel.

Rather, they say, vibrations in a steam-filled horizontal fracture about 330 feet (100 meters) beneath the crater floor are causing the drumbeats.

"This mechanism has nothing to do with stick/slip," he said.

In his scenario, the pressure probably comes from steam, as water inside the volcano is heated by the rising magma.

"There's plenty of water in the ground, and there's plenty of heat," Chouet said.

"It looks like Mount St. Helens is huffing and puffing and basically behaving like a steam engine," Chouet said.

Steam engine or not, none of the scientists are predicting that Mount St. Helens is about to explode like it did in 1980.

Study leader Iverson said in an email, "Large explosions at Mount St. Helens are certainly possible, as they are at any volcano.

"But with the current style and level of volcanic activity at Mount St. Helens, we believe the probability of large explosions is very low."

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