Where a Volcano Eruption Like Japan's Mount Ontake Is Mostly Likely in the US

— -- Could a deadly volcano eruption like Mount Ontake's in Japan happen here in the United States?

It's possible, U.S. Geological Survey scientist John Ewert told ABC News today. Luckily, most of the active volcanoes in America aren't near homes, businesses or schools, he said.

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"When you look at the Cascade Volcanoes, which are most like Ontake, you find that these are all on federally managed lands, national parks or national forests and wilderness areas. We don't have much in the way of built environment nearby. An exception to that might be a place like Mt. Hood, which has ski lodges on the side, but these are not up by the crater."

Ewert added that while eruptions are dangerous, "they don't tend to affect large areas."

"The area of lethal effect for a phreatic explosion is as little as a radius of half a mile or less," he said, noting that the damage area varies.

Americans are allowed to climb some active volcanoes, but many require a climbing permit, like Mount St. Helens in Washington, and don't allow climbers to enter the crater.

"Mount St. Helens is a very risky place. It's not open to the general public just to take a hike and walk into," Ewert said, adding that officials also have the authority to shut down access altogether if volcanoes are at a higher level of alert.

The USGS tracks volcanoes to warn people of when they're exhibiting unusual activity. Right now, there's a warning for Kilauea in Hawaii and a watch for Shishaldin in Alaska.

"In Kilauea, lava flows are moving toward an inhabited area," Ewert said. "Shishaldin is producing some low level eruptive activity."

But he suspects the most dangerous volcano in the country is Mount Rainier in Washington, based on the number of people in the surrounding area, "areas we know could be hazardous when that volcano is coming active," Ewert said.

While scientists can tell when a volcano is more active than usual, it's difficult to pinpoint exactly when an explosion will occur. And sometimes, there's no warning at all -- like at Mount Ontake.

"The alert level on that volcano had not been raised ahead of time so there was no reason to think there was an increase of explosive potential," Ewert said. "These things are very difficult to foresee."