President Obama's visit to Indonesia this week will reportedly be cut a few hours short due to an ash plume from a volcanic eruption in the country. The eruption of Mount Merapi, more than 400 kilometers from Jakarta, where Obama is visiting, has killed more than 150 people since it began erupting in late October, according to The Washington Post.

The 3,000-meter Merapi is a highly active volcano that has erupted as recently as 2007, but the present eruption is Merapi's largest in a century, says John Eichelberger, Volcano Hazards Program Coordinator for the U.S. Geological Survey. Indonesia, an island chain that lies along the Pacific Ring of Fire, is no stranger to powerful seismic and volcanic activity. The 2004 Indian Ocean tsunami, for instance, hit Indonesia especially hard, leaving more than 160,000 dead or unaccounted for in the country.

"Indonesia has incredible volcanoes," Eichelberger says, and the current eruption is just one in a long line of historic events. The eruption of Merapi is much smaller than Krakatau in 1883, which he calls "a one-in-100-year kind of eruption for the Earth." And Krakatau, in turn, was a smaller eruption than Tambora in 1815, which might be called a one-in-1,000-year event. "Subduction zones vary in terms of whether they produce big eruptions or not, and whether they produce earthquakes or not," Eichelberger says. "Indonesia is toward the far end for both."

But unlike sparsely inhabited chains on the Ring of Fire such as the Aleutian Islands in Alaska or Russia's Kuril Islands, Indonesia has both significant geologic activity and a huge population—a recipe for tragedy. With nearly 250 million inhabitants, Indonesia is the fourth most populous country in the world. "Indonesia probably has the greatest problem because the volcanoes are so active and so many people are living on them," Eichelberger says. "It's not a rich country, and people live where they can make a living, so there are farmers on the slopes of these volcanoes."

Photo of Merapi in 1982: USGS