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Forecasters can predict where that cloud of ash will spread, but scientists can't say how long the eruption will last. It could actually continue to spew forth ash on and off for months, and it could trigger an even bigger eruption of an even bigger volcano nearby.

NPR's Richard Harris reports.

RICHARD HARRIS: Because volcanic ash can cause extensive damage to jets, even bring them out of the sky, weather bureaus around the world have scientists who specialize in following volcanic clouds.

Greg Gallina works at the Washington Volcanic Ash Advisory Center in Camp Springs, Maryland, and he has been watching the Icelandic fallout carefully.

Mr. GREG GALLINA (Satellite Meteorologist, Volcanic Ash Advisory Center): The nature of the ash is pretty typical.

HARRIS: The particles spewing forth from the Icelandic volcano are mostly microscopic bits of glass. They're hazardous because they can easily melt onto jet engine parts, block intake lines, ding windshields, and scour aircraft bodies.

Mr. GALLINA: What makes this one really special is the location of which it's erupting.

HARRIS: Of course, that's because Europe's busy air corridors are downwind. The volcano itself is no big deal. Gallina says there have actually been much bigger eruptions in the past few years - off in the Aleutian Islands and in Russia.

Satellites pick up information about these ash clouds. Some instruments can even see what they're made of, and forecasters can even get a good idea about where they are heading, just as they forecast the weather.

Mr. GALLINA: It is actually quite like the weather. Because of the fact that these particles are very small, you can actually trace them along through the winds.

HARRIS: Once a cloud is in the air, forecasters can keep planes away. The trick comes in figuring out what's going to happen next with the source of these clouds - the volcanoes themselves. That job is left to geologists on the ground.

In this case, Sigr�n Hreinsd�ttir and her colleagues at the University of Iceland, have been following the activity of the volcano for three months. At first it started with a gentle tourist-style eruption.

Dr. SIGR�N HREINSD�TTIR (Geophysics, University of Iceland): Sometimes magma is just beautiful, quite bubbly thing that comes out of the ground, and sometimes it's more violent.

HARRIS: In this case, the eruption turned violent when the red-hot magma underground changed course and started coming up directly below a glacier. The mix of magma and ice is explosive, and how long that could last is anyone's guess.

Dr. HREINSD�TTIR: We wouldn't be surprised if it went on for months, but on and off, like we have seen in the last few weeks, where it comes up in one location and then it comes up on in another location. That didn't surprise us.

HARRIS: Hreinsd�ttir also wouldn't be surprised if the magma from the Eyjafjallajokull volcano crept underground just a bit to the east and flowed into the chambers of its much bigger volcanic neighbor, Katla. That volcano has been unexpectedly quiet in recent decades, and she says an eruption there could trigger much more disruption than what's happening now.

Dr. HREINSD�TTIR: We were just looking at each other this morning when we realized Katla is probably able to do way much more than that. Yeah, I would be worried when Katla goes up.

HARRIS: And she'll also be among the first to know.

Richard Harris, NPR News.

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