MICHELE NORRIS, host:
Mount St. Helens, the volcano in Washington State that so energetically blew its top in 1980, is still erupting. It's not like a big explosive eruption, more like a steady pulsing. Scientists have been keeping an eye on it - and an ear - and they've discovered that the volcano is making a sort of rhythmic drumbeat.
NPR's Christopher Joyce has more on what scientists think is causing these subterranean sounds.
CHRISTOPHER JOYCE: They've ruled out giants or trolls drumming in some subterranean cavern. No, this is all about magma and dome building. Richard Iverson is a geoscientist at the U.S. Geological Survey in Vancouver, Washington. He says what he's been hearing is odd and puzzling.
Dr. RICHARD IVERSON (U.S. Geological Survey): These are small earthquakes that typically have occurred about once a minute and have done so repetitively for more than a year and there have been more than a million of these to date. And it's truly remarkable for earthquakes to occur in such a repetitive fashion like this.
JOYCE: Iverson and a team of earth scientists recorded the sounds of these quakes.
(Soundbite of pounding)
JOYCE: These are actually very low frequency sounds that are difficult to hear, so the scientists have made them 60 times faster so they can analyze them.
(Soundbite of pounding)
Dr. IVERSON: What we're hearing is a record of ground vibrations just a few hundred meters from the vent where rock has been extruded from Mount St. Helens.
JOYCE: Molten rock is rising up the volcano's central conduit trying to get out. As it nears the vent at the top, it solidifies into a solid rock. In fact, a huge plug of rock some 600 feet across and half a mile high. That's twice as tall as the Empire State Building.
Dr. IVERSON: That solid rock in turn slides against and grinds against the adjacent rock forming the walls of the volcanic conduit. And what we believe we're recording with these seismometers is the ground vibration that's produced every time there's a little pulse of this sliding type motion. It's not unlike a squeaky piece of machinery. And in a sense, each one of these earthquakes represents a squeak.
JOYCE: Iverson published this hypothesis in the journal Nature this week. He says the pulsing is not something scientists have encountered before.
Dr. IVERSON: In the first few months, none of us would have predicted that it would still be going now after two years. And I think at this point, none of us are really willing to predict when it might end because it's been so stable and long lived up to the present.
JOYCE: Iverson says there's no reason to believe the volcano is likely to get violent anytime soon.
(Soundbite of banging)
JOYCE: Christopher Joyce, NPR News.
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