Last September, a swarm of small earthquakes started shaking the volcano. The first eruption of ash and steam rose upward a couple of weeks later, followed by a flood of reporters who crammed news conferences, asking if another blast like the one of 1980 was imminent.
Mount St. Helens tossed up a few more small clouds of steam and ash. The reporters went on there way, elsewhere.
`Pipsqueak'
The work at Mount St. Helens in the 1980s had been under a pall of death and devastation; this time, scientists have a rare, close-up view with relatively little danger. "On the scale of volcanic eruptions, this is a pipsqueak," said John Pallister, a geologist at the volcano observatory.
Gases like carbon dioxide and sulfur dioxide -- the ingredients that make volcanoes explosively deadly -- are largely missing this time. "It's become an incredible scientific experiment," Pallister said. "It's just a great place to be working right now."
Past eruptions of Mount St. Helens have varied widely. Sometimes it has gone up violently, like in 1980. Other times, the lava flowed out as hot liquid, similar to eruptions at Kilauea in Hawaii.
New magma from deep inside the Earth tends to be full of volcanic gases, so the lack of volcanic gases suggests that the volcano is just emptying out molten rock leftover from the 1980s, like soda that has lost its fizz. The chemical composition of the rock is highly similar to rock from the 1980s, offering more support for that hypothesis.
The question is why magma that has been quietly sitting underground for nearly two decades would start erupting again now.
The scientists say that new magma may have risen from the mantle, pushing the old magma upward. Perhaps heavy rains in August last year percolated downward, hit the hot rocks under the volcano and changed into steam that weakened and broke apart the rocks, allowing lava to rise again.
Beneath the mountain, the magma rises through a conduit somewhat like a drinking straw from a fairly small magma chamber about [7km] below. Beneath that chamber there is probably another strawlike pipe that taps the deeper mantle.
As the current eruption empties the conduit, scientists have detected a slight deflation of the flanks of the volcano, though not quite as much as predicted, which suggests that the chamber has partially been refilled by new magma. The composition of the new magma could help tell what might happen next.
As the eruption continues, so do the earthquakes that announced the reawakening of Mount St. Helens, about one a minute, more than a million in the past 14 months. The earthquakes are shallow, occurring within a few hundred yards of the surface, and small, magnitude 0.5 to 1.5. "They're so regular, we've been calling them drumbeats," Dzurisin said.
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