Sun, Dec 04, 2005 page19

• Still puffing after all these years

  Mount St. Helen's has been erupting for the past 15 monthes, albeit in a less destructive way than in 1980

  • By Kenneth Chang / NY TIMES NEWS SERVICE
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  The satellite trucks and news reporters have long gone. The crowds of tourists have thinned. No plumes of steam and ash have risen above Mount St. Helens for nine months.

  Daniel Dzurisin, a volcanologist at the US Geological Survey's Cascades Volcano Observatory in Vancouver, Washington, said that people often asked him when St. Helens will erupt again.

  "When I tell them it's erupting today, they're surprised," Dzurisin said.

  

  Mount St. Helens just after the second dome appeared eariler this year. Twenty five years after its cataclysmic explosion, the volcano is steadily pushing solid rock up from its depths.

  PHOTO: NY TIMES

  The mountain has a split personality. The cataclysmic eruption on May 18, 1980, blew off the top 396m of the mountain, flattened 86,000 acres of forest and killed 57 people. The current eruption, now in its 15th month, is quiet, as volcanic eruptions go. It shows no signs of turning violent -- no explosions, no ash thrown into the sky.

  There is not even lava. Instead, what is coming out of the ground is a tube of rock that, while still hot, solidified perhaps over a kilometer underground and then was pushed upward. The process is somewhat like holding a toothpaste tube vertically and squeezing the toothpaste out.

  Each second, about a cubic meter of new mountain -- roughly a pickup truck's worth -- is pushed to the surface, adding to a dome growing inside the crater. In earlier months, the cylinder of new rock, which is about 200m in diameter, toppled to the side as it rose. Now, the new rock is buried beneath earlier material and just pushes up the entire hill. "It's looking pretty impressive," said Jon Major, a hydrologist at the observatory. "There's quite a pile of rock and rubble."

  For the scientists at the volcano observatory, the past year has been an unexpected bonanza, one that is giving them new insight into Mount St. Helens, the youngest and most active of the volcanoes in the Cascades Mountains, and perhaps into the 60 other volcanoes on the US mainland that have erupted in the past 10,000 years and are thus presumed to have the potential to erupt again.

  How many times?

  Among all volcanoes in the US, the long-term average is two eruptions a century.

  "That doesn't sound like a lot," John Ewert, a Geological Survey scientist who was involved in assessing the risk posed by volcanic eruptions. "But when you consider the size of the volcanoes and consider most of them are covered with snow and ice, it becomes a much more significant number."

  An eruption can melt the snow and ice, setting off avalanches and gargantuan flows of debris rolling down the side of the mountain. But for now, the Geological Survey has few instruments keeping watch over them.

  At Mount Shasta in Northern California, there are two seismometers within 10km of the volcano and another four within 27km. "And that's about it," Ewert said.

  Satellite monitoring might be able to give earlier warning of volcanic activity. For example, satellites were able to detect a swelling of the South Sister volcano in central Oregon in 2001.

  Swarms of small earthquakes -- usually the first sign of a reawakening volcano -- did not start until three years later.

  However, the technique does not work flawlessly; it did not work at Mount St. Helens, for example.

  Many of the scientists now observing Mount St. Helens were there when it erupted in 1980 and continued to observe the mountain as a series of about 20 smaller eruptions, some lasting only a few days, continued through 1986. Then the mountain fell quiet, and the scientists did not expect another eruption in their lifetimes.

  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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