In short:
A surprise eruption in Yellowstone National Park on Tuesday has sent sightseers running for safety.
A collection of Biscuit Basin hot springs north of the famous Old Faithful Geyser erupted at around 10am local time, shooting steam, water, dark-coloured rock and dirt high into the sky.
Although no injuries were reported, the Biscuit Basin area has been closed for visitor safety.
A surprise eruption in Yellowstone National Park on Tuesday has sent sightseers running for safety.
A collection of Biscuit Basin hot springs north of the famous Old Faithful Geyser erupted at around 10am local time, shooting steam, water, dark-coloured rock and dirt high into the sky.
Video posted online showed a couple of dozen people watching from a boardwalk as the eruption sprayed and grew in front of them.
Vlada March was on a tour in the basin when her guide said something unusual was happening. She started taking video.
"We saw more steam coming up and within seconds it became this huge thing," said Ms March, a California real estate agent who was with her mother, husband and her 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.
As water and debris began to fall, tourists ran to keep clear.
Some yelled, "Back up!" and "Holy cow!"
People then turned to watch the spectacle under a huge cloud of steam.
Visitors warned to stay on the trail
Rocks that fell from the sky smashed the boardwalk tourists had been walking on.
Ms March's mother, who had been sitting on a bench near the explosion, was shaken and dirty but otherwise fine.
Walking back on the broken boardwalk "was a little scary", she said, "but thankfully it didn't break under us".
The eruption damaged a boardwalk that keeps people off Yellowstone's fragile and often dangerous geothermal areas.
Photos and video of the aftermath showed damaged guardrails and boards covered in rock and silt near muddy pools.
No injuries were reported, but the Biscuit Basin area was closed for visitor safety.
Visitors are warned to stay on the boardwalks and trails in thermal areas, where some of the pools and springs have a thin, breakable crust covering the scalding and sometimes acidic water.
The hot spring erupted in or near Black Diamond Pool, a 37-metre-long hot pool which marks the basin's widest thermal feature.
A National Park Service aerial photo taken afterward showed the pool somewhat enlarged near the boardwalk compared to recent satellite images, and its water turned a muddy beige.
The deep Sapphire Pool about 91 metres away retained its usual blue hues.
Biscuit Basin explosion 'on the big side' of normal Yellowstone activity
Park geologists are investigating what caused the eruption.
The hydrothermal explosion did not indicate new activity within the volcanic system, which remains at normal levels, according to the US Geological Survey.
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, Yellowstone Volcano Observatory scientist Mike Poland said.
A clog would have caused a build-up of heat and pressure similar to the inside of a pressure cooker until the water suddenly flashed to steam, causing an instantaneous and huge expansion in volume which triggered the explosion.
After viewing video from the event, Mr Poland estimated the explosion sent material about 30 metres 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, a national park centred on a huge volcano.
The crater measures 12.4 kilometres across, Mr Poland said.
Scientists theorise that a series of hydrothermal explosions created that crater some 13,800 years ago in the Mary Bay area on the north-eastern side of Yellowstone Lake.
The crater from Tuesday's explosion will likely be much smaller, Mr Poland said.
"What we saw today was spectacular and definitely hazardous. But on the scale of what the Yellowstone system has done in the past, it was relatively small," he said.
"It's a very good reminder of an under-appreciated hazard in Yellowstone."
Similar blasts have happened in Biscuit Basin in 2009, 1991 and after the magnitude 7.2 Hebgen Lake earthquake 64 kilometres away in 1959.
At least 22 people are known to have died from injuries related to thermal features in and around the 9,000-square-kilometre national park since 1890.
AP
