"The real trick for an ash plume is to incorporate enough air to become buoyant before it loses its upward momentum and collapses," says James E. Gardner, a geologist at the University of Texas at Austin.
Now, for the first time, recent analyses of images captured that day provide vital clues about what happens inside a volcano's thick ash plume, he
and colleague Benjamin J. Andrews report in the October 2009 Geology.
Although the May 18 eruption is one of the best documented eruptions in history, scientists don't have images showing the ash plume's transition, says Gardner
notes, it's not possible to discern how quickly that transition took place or whether the plume displayed advance warning of its impending collapse.
It's tantalizing to think that scientists could spot early indications that a plume was on the verge of falling back to Earth, says Gardner
And even though data from Mount St. Helens can't answer that question, he
team's findings nearly 30 years after the explosion open up new avenues for studies of future eruptions.