Steam seeping from a fracture atop the lava dome in Mount St. Helens' crater and the mountain's first noteworthy seismic activity since 2004 have caught scientists' attention this week as signs that something is moving inside it.
While the likelihood of a major eruption seemed low, scientists have quit venturing into the volcano's crater and are checking the monitoring equipment along St. Helens' flanks.
"We're just being cautious. It's not that we're anticipating any activity," Cynthia Gardner, scientist in charge of the U.S. Geological Survey's Cascades Volcano Observatory, said Wednesday.
Geologist John S. Pallister was flying over the volcano in southwestern Washington on Sunday when he spotted the steam.
"It was interesting enough to take some pictures," said Pallister, a private pilot who works in the hazards section of the volcano observatory.
After landing, he learned that a magnitude-2.9 earthquake had registered on seismographs at an observatory in Vancouver. That was followed by a small tremor that lasted nearly an hour and a half, an unusually long period, punctuated by a second quake of magnitude 2.7 — all in the same period in which he saw the steam.
Tiltmeters also registered alternate ground swelling and deflation near the lava dome, which has been growing in the crater since fall 2004.
All are typical signs that magma, superheated gases or both are moving through conduits beneath St. Helens, which blew its top with devastating force on May 18, 1980, leveling 230 square miles (596 square kilometers) of forest and killing 57 people.
The last noteworthy tremor at the volcano lasted 55 minutes on Oct. 2, 2004, and was much more powerful, registering on seismometers from Bend, Ore., to Bellingham and causing a hasty evacuation of the Johnston Ridge Observatory 5 miles (8 kilometers) north of the crater.
No evacuations had been ordered by Friday, because the seismic activity had slowed down.
The precise cause of the recent activity was not entirely clear, Gardner said.
"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," he said Tuesday.
The last precise measurements, drawn from images in July, indicated the latest eruptive phase has pumped 123 million cubic yards (94 million cubic meters) of material into the crater. The rate has slowed considerably, but the episode Sunday showed that could change at any time, Pallister said.
"It's still got some surprises," he said.