A surprise eruption in Yellowstone National Park shot steam, water and dark-colored rock and dirt high into the sky Tuesday and sent sightseers running for safety.
The hydrothermal explosion happened around 10 a.m. in Biscuit Basin, a collection of hot springs a couple miles north of the famous Old Faithful Geyser, The AP reported.
Video posted online showed a couple dozen people watching from a boardwalk as the eruption sprayed and grew in front of them. As water and debris began to fall, they ran to keep clear, some yelling “Back up!" People then turned to watch the spectacle under a huge cloud of steam.
No injuries were reported, but the Biscuit Basin area was closed for visitor safety. The eruption damaged a boardwalk that keeps people off Yellowstone’s fragile and often dangerous geothermal areas.
Vlada March was on a tour in the basin when her guide said something unusual was happening. March started taking video.
“We saw more steam coming up and within seconds it became this huge thing,” said March, a California real estate agent who was with her mom, husband and their two kids. “It just exploded and became like a black cloud that covered the sun.”
“I think our tour guide said ‘Run,’ and I started running and I started screaming at the kids, ‘Run, run, run,’ and I continued filming what I could,” she said.
Rocks that fell from the sky smashed the boardwalk they had been walking on. March's mom, who had been sitting on a bench near the explosion, was shaken and dirty but otherwise fine, she said.
Walking back on the broken boardwalk “was a little scary,” she said, “but thankfully it didn’t break under us.”
Photos and video of the aftermath showed damaged guardrails and boards covered in rock and silt near muddy pools.
The explosion could have resulted from a clogged passageway in the extensive natural plumbing network that underlies Yellowstone's world-famous geysers, hot springs and other thermal features, said scientist Mike Poland with the Yellowstone Volcano Observatory.
A clog would have caused a buildup of heat and pressure such as happens inside a pressure cooker, he said, until the water suddenly flashed to steam, causing an instantaneous and huge expansion in volume and triggering the explosion.
After viewing video from the event Poland estimated that the explosion sent material about 100 feet (30 meters) into the air.
He said the explosion was “on the big side” of eruptions that occur periodically — usually when no one is around let alone throngs of tourists.
On occasion they get much bigger: The largest known crater from a hydrothermal explosion on Earth is in Yellowstone and measures 1.5 miles (2.4 kilometers) across, Poland said. Scientists theorize that a series of hydrothermal explosions created that crater some 13,800 years ago in the Mary Bay area on the northeastern side of Yellowstone Lake.
