Geologists witness rare hydrothermal explosion in Yellowstone National Park

Yellowstone National Park geologist Hank Heasler was lecturing a group of colleagues in Biscuit Basin on the rarity of hydrothermal explosions recently when Boom!

A hot pool behind him exploded, spewing mud, rocks and hot water 50 feet in the air. Geologists know of only a handful of such unpredictable explosions in Yellowstone's recorded history.

Heasler on May 17 was addressing a group of geologists, geophysicists, graduate students, U.S. Geological Survey employees and university faculty members on a tour of the basin led by University of Utah Professor Bob Smith. The group was just out of reach of the scalding water and debris.

"I couldn't decide; should I run or grab my camera," Smith said in a telephone interview from his home in Moose. "By the time I got people down, to stand back, it was over."

The event lasted five or 10 seconds, Smith said. "I wasn't afraid, but I was very prudent, very anxious."

Smith had guided his group of about 25 people from the annual meeting of EarthScope, a national consortium of earth-science professionals who gathered this year in Boise. The post-conference tour followed the track of the Yellowstone hotspot across Idaho to the world's first national park.