Mount St Helens, a volcano long believed to be dormant in Washington State in the U.S., freaked people out back in 1980 when it suddenly erupted and spewed tons of lava and boiling mud into the air (as you can see in this picture). It calmed down for a few years, but in 2004 it started slowly erupting, and has been oozing sticky clumps of lava continuously since then. As it erupts, it also unleashes constant small earthquakes in the areas nearby. Now a Michigan Tech researcher has braved the lava-slicked slopes of Mount St Helens do do some of the most detailed seismic research on the volcano ever. And he thinks he knows what's causing all the shakes.
Geophysics professor Gregory P. Waite published his work recently in the Journal of Geophysical Research. A summary of it explains:
Volcanoes don't always erupt suddenly and violently. The most recent eruption of Mount St Helens, for example, began in October 2004 and is still going on. It's what Waite and other volcanologists call a passive eruption, with thick and sticky lava squeezing slowly out of the ground like toothpaste from a tube . . . When a volcano such as Mount St Helens erupts, it can cause a series of shallow, repetitive earthquakes at intervals so regular that they've been called "drumbeat earthquakes."
The fluid in the crack most likely is steam, derived from the magma and combined with water vaporized by the heat of the molten rock. A continuous supply of heat and fluid keeps the crack pressurized and the "drumbeats" beating.
A Fresh Look Inside Mount St Helens [Michigan Tech]