The ash cloud that rose from a volcano in Iceland last week to halt air traffic in the U.K. and much of the rest of Europe appears to be easing its stranglehold on transportation. EUROCONTROL, an intergovernmental air traffic control organization based in Brussels, announced Monday that its member states were designating a limited "no-fly zone" beyond which airlines would be permitted to operate by Tuesday morning.

So how soon is such an air travel-impacting event to recur? Major volcanic eruptions are not uncommon, notes John Eichelberger, Volcano Hazards Program coordinator for the U.S. Geological Survey. But many of them occur in remote regions far from population centers, affecting only flyover airspace or the occasional airport in parts of the world such as Alaska. "What's unusual about this is that the cloud has kind of sat on one of the most populated parts of the planet, occupying not just air routes but airports," Eichelberger says. But given the frequency of volcanic eruptions, it was only a matter of time before winds carried a plume over a population center and interrupted air travel.

"It's kind of surprising this hasn't happened before to Europe, because Icelandic volcanoes are very active," Eichelberger says. "I imagine there is, on average, an explosive eruption per decade or something of that order." In the past, he adds, weather patterns have been more forgiving to mainland Europe.

And Iceland is only one hot spot. In the North Pacific, Eichelberger says, there are roughly 100 potentially active volcanoes, many of them under transoceanic flight routes. The past few years saw huge eruptions in that region, including one in 2008 in the Aleutian Islands of Alaska and one in 2009 in Russia's Kuril Islands. "These kinds of problems happen multiple times per year in the North Pacific air routes," Eichelberger says. "You don't hear about them very much because they only occasionally close airports."

But whereas the Aleutians and the Kurils are largely unpopulated, not all volcanically active regions are so remote. "Japan has a tremendous number of active volcanoes," Eichelberger says. "They have had huge explosive eruptions in the past—not so much recently, but there is a great potential for disruption there."

Tom Murray, a U.S. Geological Survey volcano scientist at the Alaska Volcano Observatory in Anchorage, says the Pacific Northwest of the U.S. could also witness a volcanic disturbance of airspace or infrastructure. If one of the Cascade Range volcanoes were to erupt, Murray says, it could cause major problems—possibly even more so than the explosive eruption of Mount St. Helens in 1980. "That was a long time ago. If that were to occur today, it would create even more havoc," Murray adds. The Cascades, he says, act up not infrequently. "Sort of a rule of thumb there is probably a couple times a century," Murray says. "Sometimes it may be rather benign, and sometimes it will be much less so." reached Eichelberger by phone on Monday* in Paris, where he himself was stranded by the volcanic cloud following an advisory committee meeting about French volcano monitoring. He pointed out a historical precedent of an American in Europe during an Icelandic eruption. "In 1783, Benjamin Franklin was sitting here during a much larger eruption in Iceland, and he wrote about the endless haze and identified it as being from an eruption in Iceland," Eichelberger says. "Of course, air travel was not a concern at that time."

* Correction (4/19/2010): The article originally stated incorrectly that Eichelberger had been reached on Thursday.