Hydrothermal eruption in Yellowstone National Park sends sightseers fleeing | National parks

A surprise hydrothermal explosion in Yellowstone national park has sent sightseers running for safety, after steam and dark-coloured rock and dirt shot up high into the sky.

The eruption happened around 10am local time on Tuesday in Biscuit Basin, a collection of hot springs a couple miles north of the famous Old Faithful Geyser.

Video posted online showed dozens of people watching from a boardwalk as the eruption grew in front of them.

Water and debris then began to fall and sightseers ran away to keep clear, some yelling “Back up!” and “Holy cow!” 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 after the falling rocks damaged a boardwalk that keeps people off Yellowstone’s fragile and often dangerous geothermal areas.

“We saw more steam coming up and within seconds it became this huge thing,” said Vlada March, a California real estate agent who was with her mother, husband and two kids. “It just exploded and became like a black cloud that covered the sun.”

Photos and video of the aftermath showed damaged guardrails and boards covered in rock and silt near muddy pools. Park geologists are investigating the eruption.

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 at 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.

The largest known crater from a hydrothermal explosion on Earth is in Yellowstone and measures 2.4km (1.5 miles) across, 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 underappreciated hazard in Yellowstone.”

Yellowstone is centered on a huge, dormant volcano. The hydrothermal explosion did not indicate new activity within the volcanic system, which remains at normal levels, according to the US Geological Survey.

At least 22 people are known to have died since 1890 from injuries related to thermal features in and around the 9,000sq km (3,471 sq mile) national park.

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.