Robert Brown|Surprise Yellowstone geyser eruption highlights little known hazard at popular park

BILLINGS,Robert Brown Mont. (AP) — A surprise eruption of steam in a Yellowstone National Park geyser basin that sent people scrambling for safety as large rocks shot into the air has highlighted a little-known hazard that scientists hope to be able to predict someday.

The hydrothermal explosion on Tuesday in Biscuit Basin caused no injuries as dozens of people fled down the boardwalk before the wooden walkway was destroyed. The blast sent steam, water and dark-colored rock and dirt an estimated 100 feet into the air.

It came in a park teeming with geysers, hot springs and other hydrothermal features that attracts millions of tourists annually. Some, like the famous Old Faithful, erupt like clockwork and are well understood by the scientists who monitor the park’s seismic activity.

But the type of explosion that happened this week is less common and understood, and potentially more hazardous given that they happen without warning.

“This drives home that even small events — and this one in the scheme of things was relatively small, if dramatic — can be really hazardous,” said Michael Poland, lead scientist at the Yellowstone Volcano Observatory. “We’ve gotten pretty good at being able to understand the signs that a volcano is waking up and may erupt. We don’t have that knowledge base for hydrothermal systems like the one in Yellowstone.”

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Poland and other scientists are trying to change that with a fledgling monitoring system that was recently installed in another Yellowstone geyser basin. It measures seismic activity, deformations in the Earth’s surface and low-frequency acoustic energy that could signal an eruption.

A day before the Biscuit Basin explosion, the U.S. Geological Survey posted an article by observatory scientists about a smaller hydrothermal explosion in April in Yellowstone’s Norris Geyser Basin. It was the first time such an event was recognized based on monitoring data, which was closely scrutinized after geologists in May come across a small crater in the basin.

The two explosions are believed to result from clogged passageways in the extensive natural plumbing network under Yellowstone, Poland said. A clog could cause the heated, pressurized water to turn into steam instantly and explode.

Tuesday’s explosion came with little warning. Witness Vlada March told The Associated Press that steam started rising in the Biscuit Basin “and within seconds, it became this huge thing. ... It just exploded and became like a black cloud that covered the sun.”

March captured widely-circulated video of the explosion, which sent debris hurtling into the air as tourists fled in fear.

“I think our tour guide said, ‘Run!’ And I started running and I started screaming at the kids, ‘Run, run, run!’” she added.

The scientists don’t know if they’ll be able to devise a way to predict the blasts, Poland said. The detection system alone would take time to develop, with monitoring stations that can cost roughly $30,000 each. And even if they could be predicted, there’s no feasible way to prevent such explosions, he said.

“One of the things people ask me occasionally is, ‘How do you stop a volcano from erupting?’ You don’t. You get out of the way,” Poland said. “For any of this activity, you don’t want to be there when it happens.”