January 17, 2008
Mount St. Helens Shows Signs of Activity
Until earlier this week, the most recent significant tremor at Mount St. Helens was in October of 2004. On Sunday January 13th, when geologist and private pilot John S. Pallister was flying over the volcano in southwestern Washington he was surprised to spot some steam rising from a fracture in Mount St. Helens' crater. This not only caught Pallister's attention, enough to take some pictures; it also gained the attention of scientists who now think something is moving inside the volcano.
Tiltmeters, tools used to measure very small horizontal changes, registered some swelling and deflation in the ground near the lava dome. This dome has been growing in the crater since the fall of 2004, and is where the steam seemed to come from on Sunday.
Sunday's steam seepage may have been caused by a magnitude 2.9 earthquake registered on seismographs at a Vancouver observatory. That earthquake was followed by a tremor that lasted an hour and a half, 35 minutes longer than the tremor of 2004. That tremor ended when another earthquake of magnitude 2.7 registered. The steam was visible during this same period of time.
The 2004 tremor, despite being shorter was much more powerful and caused an evacuation of a nearby observatory. This particular event simply caused scientists to stop venturing into St. Helens' crater and start taking precautions. As of Wednesday, no evacuations had been ordered because the seismic activity had dwindled.
On Wednesday, Cynthia A. Gardner, the scientist in charge of the U.S. Geological Survey's Cascades Volcano Observatory, stated, "We're just being cautious. It's not that we're anticipating any activity." Gardner also admits that the true cause of Sunday's activity is not completely clear.
Each of the factors of Sunday's event appear as signs that magma or superheated gases might be moving through conduits below St. Helens.
Pallister, who works in the hazards section of the volcano observatory, has an idea about what might have happened. He stated Tuesday, "The settling of the growing lava dome might have caused some fracturing and might have changed the subsurface openings so that water was either being squeezed out of openings or opening new areas."
Images from last July show the last precise measurements from the volcano. These indicated that 123 million cubic yards of material have been pumped into the crater by the latest eruptive phase. That rate has slowed, but could change at any time.
The last eruption of Mount St. Helens, on May 18,1980 flattened 230 square miles of forest and killed 57 people.