Life is a risk, but Yellowstone National Park's tumultuous geology offers a bit more for worrywarts.

Lately, the U.S. Geological Survey has been trying to gauge the park's geological hazards and figure the odds that something dangerous - such an earth-shattering volcanic eruption or an exploding geyser - will happen.

The agency's first stab is a recently released report on volcanic and hydrothermal hazards in and around Yellowstone. There are plenty of potential hazards, the report said, but the chances of something catastrophic happening on any day are extraordinarily slim.

Comment: But what about over a month? A year? A decade? Our lifetime?

"There's a lot more risk crossing the street or driving to Yellowstone than having one of these things happen while you're there," said Robert Christiansen, a USGS geologist and lead author of the report.
The most immediate danger is a large earthquake, like the magnitude 7.6 quake near Hebgen Lake in 1959 that killed 28 people and caused $13 million in damage.

"We certainly could have others like that," Christiansen said.

The USGS is planning an assessment of seismic hazards but most recently has focused on volcanic and geothermal threats. The report will be used as park managers and others put together emergency plans.

One of the most likely events is what's called a hydrothermal explosion, when hot water just below the surface flashes into steam, breaks rocks on the surface and sends debris flying.

That happened in 1989 at Porkchop geyser at Norris Geyser Basin, flinging rocks and debris more than 200 feet.

At least one "rock-hurling event" happens in Yellowstone about every two years but rarely when people are around or in danger, the report said. Most occur in the Firehole River basin, beneath or around Yellowstone Lake or north of Norris, the report said.

There's increased danger with rarer but larger hydrothermal explosions.

The explosion that created Mary Bay on the northern edge of Yellowstone Lake left a crater more than 1,600 yards across.

"If you're near the site of one of those explosions it could be very dangerous, even within a mile or so," said Jake Lowenstern, a USGS scientist and head of the Yellowstone Volcano Observatory.

Scientists estimate that blasts leaving a crater larger than a football field happen about once every 200 years.

By far the most publicized hazard has been the potential for a massive eruption of what some have dubbed Yellowstone's "supervolcano."

The largest of those eruptions, about 2 million years ago, was 280 times larger than Mount St. Helens' eruption in 1980. It destroyed mountains, left a hole larger than Rhode Island and spread a layer of rock hundreds of yard thick for about 60 miles.

A similar eruption would have disastrous consequences in much of the Western United States and affect the Earth's climate and create long-running problems for those who survived.

The probability of that happening is "extremely low," the report said. Simply calculating the number of years between the last three caldera-forming eruptions isn't enough to predict that another will happen, Christiansen said.

It also doesn't take into account apparent changes in the volcanic system.

"We consider the likelihood of there being enough interconnected magma that could erupt in one event - big enough for a caldera-former eruption - is pretty darn small," he said.

In recent years, Yellowstone's busy volcanic system has become increasingly wired with gauges that monitor its huffing, puffing and gurgling. In most cases, excluding earthquakes, there would be enough signs to tip off scientists before a major event.

Still, there's a lot to learn about the dangers of Yellowstone.

"It's not an easy thing to do, using the record of past events as a guide to the future," Christiansen said. "But we're getting more confident."