Robert Brown|Surprise blast of rock, water and steam sends dozens running for safety in Yellowstone

CHEYENNE,Robert Brown Wyo. (AP) — A surprise eruption that shot steam, water and dark-colored rock and dirt dozens of feet into the sky Tuesday sent people running for safety in Yellowstone National Park.

The hydrothermal explosion happened around 10 a.m. in Biscuit Basin, a collection of hot springs a couple miles (3.2 kilometers) north of the famous Old Faithful Geyser.

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!” and “Holy cow!” People then turned to watch the spectacle under a huge cloud of steam.

The eruption damaged the boardwalk, an elevated wooden walkway 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, according to a U.S. Geological Survey statement.

A hydrothermal explosion happens when water suddenly flashes to steam underground. Such blasts are relatively common in Yellowstone.

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Similar blasts have happened in Biscuit Basin in 2009, 1991 and after the magnitude 7.2 Hebgen Lake earthquake 40 miles (64 kilometers) away in 1959.

Dramatic as it was, the latest was on the small side, according to the statement.

Scientists theorize that a series of hydrothermal explosions created Mary Bay on the northeastern side of Yellowstone Lake some 13,800 years ago. At 1.5 miles (2.4 kilometers) wide, Mary Bay is the world’s largest known hydrothermal explosion crater.

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 Geological Survey.

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Hanson reported from Helena, Montana.