Fire and ice have created a doubly dangerous and disruptive volcanic disaster in Iceland that is being felt around the world as locals are evacuated and thousands of flights to and from Northern and Western Europe have been grounded.

At least 800 people have been evacuated from the area around the Eyjafjallajokull glacier (about 120 kilometers outside of the capital Reykjavik and from under which the volcano is erupting), to protect them from rushing floodwaters. River levels have surged some three meters above average as the volcanic heat melts the 200-meter thick glacier, creating mountainside runoff, the Associated Press reported.

Air traffic in Belgium, Denmark, Finland, France, the Netherlands, Sweden and the U.K. has been at least partially suspended, as volcanic ash from the eruption (which began in the early hours of Thursday morning) had spread 6,000 to 11,000 meters up into the stratosphere as of midday, drifting eastward over parts of northern Europe. This height is also the frequent cruising altitude of commercial airliners. The intersection of volcanic ash (which contains particles of silicates that can melt and shut down jet engines) and congested flight paths has resulted in canceled flights, grounding hundreds of thousands of airline passengers who had planned to travel to or from parts of Europe, likely until at least Friday morning.

So how long will the volcano continue to keep flights grounded? We spoke with volcanologist Sally Kuhn Sennert of the Smithsonian Institution's National Museum of Natural History to find out.

[An edited transcript of the interview follows.]

Was the eruption of the Eyjafjallajokull volcano a surprise?
The initial eruption happened on March 20, and as of just a few days ago that had sort of stopped. There was a fly-over on the 13th, and scientists had not seen any lava. People were wondering if it was finished. The activity migrated northward to the central part of the volcano. It was visually confirmed yesterday.

Anytime there's an eruption under a glacier there's a lot of potential for what Icelanders call a jökulhlaups. It's basically just a giant flood of water from a rapidly melting glacier. There are just standing waves in these waters. There's already a lot of structural damage.

Are volcanoes erupting under glaciers rare?
I don't think it's unusual for Iceland. It's certainly dangerous because it can cause such massive flooding that can come on very suddenly. It has already caused damage. I was reading that local farmers said their pastures had just turned green and now they're flooded with mud.

Does this glacier volcano make a difference in what is in the air?

When I first saw the eruption plume, it was a very white, steam-rich plume. Now, this morning, it's very dark with a lot of ash.

It's really the ash that causes the problem. It can get into the airplane engines and melt down. It can basically cause the engine to seize. Even if the airplane does not lose engines, it still causes very expensive damage.

So can other aircraft, such as helicopters or prop-planes that don't have jet engines, fly through these conditions?
No, because the ash—the ash is pulverized rock—it can scratch up the windshield, too. It just does body damage. I wouldn't want to fly in a helicopter through the ash.

How long is the ash from this eruption going to keep planes on the ground?
It really just depends on the weather patterns and how long the eruption is going to last—and if there are going to continue to be explosive components to the eruption.

I think people could eventually get to where they're going. Planes can be diverted. It wouldn't be fun for travelers who go three hours away and have their flights diverted 10 hours. I'm glad I'm not snagged in that area now.

How long is the eruption likely to last?
It's so difficult to tell. The last eruption is very similar to this one. It erupted from the central crater from December 1821 and went to January 1823. I'm not sure the magnitude of the eruption was the same all along. But it's really, really tough to predict.

The local scientists are working on that now. They've collected samples of the ash to analyze to get a sense of whether the chemistry is similar to the last eruption. But that takes time. I think it's too soon to say too much about what's really happening there.

Are there volcanoes that might erupt soon that would pose a threat to U.S. airspace?
In Alaska in the Aleutian Islands—it's a volcanically active area—and there's a lot of flight pathways that would cross where that ash would potentially drift. The airspace and volcanoes in Alaska are monitored very closely in that area. The potential is always there, and sometimes these things can come on pretty quickly.