The potential threat of a major volcanic eruption at Yellowstone is a gripping story but hydrothermal eruptions are a much more common and bigger risk.

In the boundaries of Yellowstone National Park, there are approximately 500 geysers and about 10,000 hydrothermal features.

The geysers are some of Yellowstone’s most striking features but also some of the potentially most dangerous ones.

According to the United States Geological Survey’s (USGS) Observatory (YVO), these hot water features are a source of frequent eruptive activity.

How often do large Yellowstone hydrothermal eruptions occur?

Yellowstone is estimated to have last erupted about 640,000 years, 1.2 million years and two million years ago.

Hydrothermal eruptions are incredibly more frequent and data collected on the past 16,000 years shows they are something to watch out for.

The USGS said: “Although large hydrothermal explosions are rare events on a human timescale, the potential for additional future events of the sort in Yellowstone National Park is not insignificant.

“Based on the occurrence of large hydrothermal explosion events over the past 16,000 years, an explosion large enough to create a 100-m-wide, 328-ft-wide, crater might be expected every few hundred years.”

On average, these happen every 700 years and at least 25 past eruptions have created craters wider than 328 ft (100 m).

Hydrothermal eruptions at Yellowstone volcano occur when shallow reservoirs of boiling water underlie the volcano’s thermal fields.

These eruptions are “violent and dramatic events” spewing vast amounts of scorching water, steam, mud and rock fragments.

The explosions can spew jets of water as high as 1.2 miles high (two kilometres) and leave behind craters between tens of feet and more than 1.2 miles (two kilometres) in diameter.

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The ejected material spewed during these eruptions can found as far as 1.8 to 2.5 miles (three to four kilometres) from the craters.

The USGS said: “Hydrothermal explosions are a potentially significant local hazard and can damage or even destroy thermal features.

“In Yellowstone, hydrothermal explosions occur within the Yellowstone Caldera and along the active Norris-Mammoth tectonic corridor.”

Some of the past eruptions have been known to be triggered by but none have been connected to Yellowstone volcano’s magmatic activity.

How often does Yellowstone volcano eruptions occur?

The USGS estimates major volcanic eruptions at Yellowstone occur in intervals of about 600,000 to 800,000 years.

Lesser basalt and rhyolitic lava flows occur more frequently, with at least 27 rhyolite flows noted since the last caldera-forming eruption 640,000 years ago.

The USGS said: “The most recent was about 70,000 years ago.

“Many of these eruptions were separated in time by several tens of thousands of years.

“Because the evidence of earlier eruptions may have been either buried or destroyed, we do not really know how often the volcano has actually erupted.”

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