A baffling, dangerous explosion in Yellowstone: What is a hydrothermal explosion?

An eruption of steam, rock and mud sent visitors scattering at Yellowstone National Park on Tuesday — a terrifying and baffling scene that even scientists struggle to understand or predict.

Dramatic video shows the moment a hydrothermal explosion sent a dark cloud into the air as guests ran for safety. Such explosions are "one of the most important and least understood geologic hazards," Lisa Morgan, an emeritus U.S. Geological Survey research geologist, wrote for the Yellowstone Caldera Chronicles, a Yellowstone Volcano Observatory publication.

First the good news: While unexpected geological activity at the park can seem like a harbinger of doom, the explosion at the Biscuit Basin thermal area is not a sign of an impending volcanic eruption, according to the U.S. Geological Survey. (That's especially good because Yellowstone is home to a supervolcano.)

Here's what to know about the phenomenon:

What is a hydrothermal explosion?

Hydrothermal explosions occur when hot water in a volcano system turns into steam in a confined area, Morgan wrote. A sudden drop in pressure causes rapid expansion of the high-temperature fluids and a crater-forming eruption.

The explosions are "violent and dramatic events" that can reach heights of over a 1 mile and spew debris as far as 2 and-a-half miles away, according to the observatory.

Are they dangerous? Has anyone ever been killed?

Most hydrothermal explosions are small and go unobserved, according to Michael Poland, the scientist-in-charge at the observatory. No one has been killed or injured by a hydrothermal explosion, although between "blowing out rock, mud and boiling water, it's not something you want to be close to," he said.

Tuesday's eruption at Yellowstone damaged a boardwalk, but no one was injured.

"It was small compared to what Yellowstone is capable of," USGS said on X, formerly known as Twitter. "That's not to say it was not dramatic or very hazardous — obviously it was."

Where can hydrothermal explosions happen?

The explosions can happen anywhere there is hydrothermal activity, according to Poland. ("Hydrothermal" refers to heated water in Earth's crust).

Hydrothermal explosions occur as often as a couple times per year at Yellowstone, and other hotbeds include New Zealand, Iceland and Chile, Poland said.

The area northeast of Yellowstone Lake is home to the three largest-known hydrothermal explosion craters on earth: Mary Bay, a mile-and-a-half wide crater formed 13,000 years ago; Turbid Lake, a mile wide crater formed 9,400 years ago; and Elliott’s Crater, which is nearly half-a-mile wide and was formed 8,000 years ago.

Can scientists predict hydrothermal explosions?

Scientists are researching how to predict these explosions, but some are skeptical it can even be done, according to Poland.

"One of the things we don't fully know right now is whether these things can be forecast," he said. "It's still an open question."

An explosion large enough to leave a football field-sized crater can be expected every few hundred years, according to the observatory. Several smaller explosions have been triggered by seismic events like an earthquake.

Is a hydrothermal explosion different than a volcano? Is magma involved?

Hydrothermal explosions are far more common than an eruption of lava or volcanic ash and unlike a volcanic eruption, hydrothermal explosions are not caused by magma rising towards the earth's surface, according to USGS.

Magma is extremely hot molten or semi-molten rock found under the Earth's surface, primarily in the mantle between the core and the crust, according to National Geographic. When magma erupts from a volcano or flows from the Earth through a crust fracture, lava forms.

Does this mean Yellowstone's supervolcano is going to erupt?

Hydrothermal explosions are not an indicator a volcanic eruption is brewing, according to USGS. There is a supervolcano roughly the size of Rhode Island buried deep beneath Yellowstone that has produced some of the largest eruptions in the world and Poland previously told USA TODAY it will erupt again.

But the underground system will likely show decades of warning signs before it blows including multiple, large earthquakes, a change in the chemicals in the hot springs, and an increase in heat. The ground also would slowly shift by tens of feet and release gasses, Poland said.

And an eruption isn't likely to happen for thousands of years. When it does, it will probably resemble Hawaii's Kilauea volcano eruption in 2018, rather than a mass extinction event. But if a supereruption were to occur, millions would die, ash would blanket much of the country, water would be contaminated, the country’s heartland would be unfarmable for years and the climate would change for years or even decades.

“Humanity would survive, but it would not be a fun time,” Poland said.

Contributing: Olivia Munson, Anthony Robledo, Michael Loria, and Katharine Lackey, USA TODAY