YELLOWSTONE NATIONAL PARK – Yellowstone National Park geologist Hank Heasler was lecturing a group of colleagues in Biscuit Basin on the rarity of hydrothermal explosions last week 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.
Smith took the group around parts of Yellowstone, ending up at Biscuit Basin, just downstream from Old Faithful. He chose the site not for its geysers there are none but for its various hot springs.
“We had commented that this place had exploded before, one time a year for three years,” Smith said.
Evidence of other hydrothermal explosions was found in Bechler River drainage last year, at Porkchop Geyser in Norris Geyser Basin in 1989 and at Excelsior Geyser in Midway Geyser basin where a tourist made a photograph of the event in 1878, Smith said. Larger prehistoric hydrothermal explosions, including one that created Mary Bay on Yellowstone Lake, produced craters as wide as a kilometer across.
A hydrothermal explosion is similar to a geyser’s eruption except that “it blows the lid out,” not just water, Smith said. “It is so violent it breaks the surrounding rocks out with it.”
Last week, nature’s forces created a display for the most dedicated of students.
“Hank Heasler was making the point these are rare events,” Smith said. “I had my back to the hot springs system.
“It started off with an explosion,” he said. “I heard this roar. I really thought it was a truck coming down the boardwalk.”
After the water, mud, rocks and chunks of broken sinter fell harmlessly to Earth, the group approached the spring. None had instruments thermometers or water gauges to take any immediate readings.
“The remaining water was murky,” Smith said. “It was totally full of debris.”
It was impossible to tell how large the new opening for the spring was because of the murky water, Smith said.
A geophysicist, Smith said he would consider a magnitude 8 earthquake on the San Andreas Fault, a magma eruption in Yellowstone or a big earthquake on the Wasatch or Teton Fault career-defining events. Yet he was able to take an aerial photograph of a rare eruption of Steamboat Geyser at Norris Geyser Basin and now has witnessed a hydrothermal explosion.
“This is once in a lifetime,” he said. “It was a learning experience.”
And for the field trip, “it was a fitting ending,” he said. “They all said I had planned it.”