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

CHEYENNE, 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.

The explosion happened in or near Black Diamond Pool, a 120-foot-long (37-meter-long) hot pool that is 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 100 yards (91 meters) away retained its usual blue hues.

A hydrothermal explosion happens when water suddenly flashes to steam underground. Such blasts are relatively common in Yellowstone. Park geologists were investigating what specifically happened in this case.

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