for National Geographic News
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."
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."
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