A hydrothermal explosion sends Yellowstone visitors running

Editors Note: This story features a video of the explosion that contains profanity.

A large explosion sent visitors running down the boardwalk of one of Yellowstone National Park's famed thermal pools on Tuesday as boiling water, mud, rocks and steam spewed high into the air.

A video posted on Facebook showed park visitors running from a massive black cloud as it rose.

The park said there were no reports of injuries, though the boardwalk was damaged — a Park Service photo showed a walkway covered with gray and black debris and with fencing torn apart. The explosion took place at the Biscuit Basin thermal area, near Black Diamond Pool and some 2 miles northwest of the Old Faithful geyser.

The cause was a hydrothermal explosion, the park said.

In hydrothermal systems such as those in Yellowstone, the water often can be very close to the boiling point, says Marianne Karplus, a professor of geological sciences at the University of Texas at El Paso who has studied geysers at Yellowstone. When there's a drop in pressure underground, it "can cause basically the liquid water to flash into steam," she says. The steam takes up more space than the liquid — an expansion. "And when that happens, it can cause an explosion."

Hydrothermal explosions have taken place at Yellowstone's Norris Geyser Basin in 1989 and at Biscuit Basin in 2009, and "a small event" was recorded at Norris Geyser Basin in April of this year as well, the U.S. Geological Survey said.

"In modern times, most of the hydrothermal explosions have led to relatively small craters, sort of a few meters across, or that kind of scale," Karplus says.

Despite the size of the explosion seen on the video, the park and the USGS characterized it as a small one. Hydrothermal explosions can reach more than 1 mile high and leave craters a mile in diameter. Larger explosions happen every 700 years on average, according to the USGS.

The Park Service said Biscuit Basin is now closed for safety reasons.

Karplus said she knows the park has been trying to install more instruments in an attempt to detect similar events before they happen. But because of how dynamic geothermal systems are, "these types of events are very hard to predict or hard to anticipate. And so that's why in this case this morning, as well as in 2009 and other times that this has happened, it's been a total surprise."

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