Volcanic Eruptions: The Volcanic Explosivity Index

(April 20) -- For all the travel trouble it has caused, you might think the eruption of Iceland's Eyjafjallajokull volcano would rate as one of the bigger volcanic calamities in modern history. But it isn't so.

For nearly 30 years, scientists have rated volcanic eruptions on a scale known as the Volcanic Explosivity Index, a Richter scale for volcanic activity. The VEI rates eruptions on a variety of factors, including duration, height, and how much lava and ash they spew.

The highest number on the VEI is 8, a rating achieved by just one known eruption -- a cataclysmic blast more than 600,000 years ago in what is now Yellowstone National Park in Wyoming. It pumped out 1,000 cubic kilometers of debris and was the last in a series of three eruptions that produced enough ash and lava to fill the Grand Canyon, according to the U.S. Geological Survey.

So where does that leave Eyjafjallajokull? Geologically speaking, it's in the minor leagues, as the chart shows. The eruption has earned a VEI of just 2 or 3 so far.

Source: usgs.gov, volcano.si.edu, uwgb.edu

Although the volcano's ash plume forced the cancellation of tens of thousands of airline flights in Europe -- the biggest disruption in air travel since the Sept. 11, 2001, terrorist attacks -- it hasn't produced that much ash when compared with other notable eruptions from the past three centuries. As of Tuesday, it had put out just a fraction of a cubic kilometer of ash. The 1815 explosion of Tambora in Indonesia, the large bubble near the top of the chart, produced 150 cubic kilometers.

The chart can't tell all the tales behind the eruptions. In 1783, for example, Iceland's Grimsvotn volcano let loose 15 cubic kilometers of lava, which remains a record. The toll on the nation's inhabitants was devastating -- damage from the eruption triggered a famine that wiped out one-fifth of its population.

Two other eruptions on the chart -- Tambora and Krakatoa, also in Indonesia -- are the deadliest in history. Tambora killed 92,000, while Krakatoa killed 36,000.

It's not clear when Eyjafjallajokull will settle down, but when it does, Iceland and its neighbors may not be able to breathe easy for too long. Scientists are keeping an anxious watch over other Icelandic volcanoes. One of them, Katla, produces more magma than Eyjafjallajokull and could erupt more explosively, scientists say.