Yellowstone explosion, explained: A dive into national park's recent thermal activity

Yellowstone National Park visitors were startled last month by an unexpectedly fierce hydrothermal explosion – an eruption of underground hot water – that spewed steam, boiling water, mud, and grapefruit-sized rocks hundreds of feet into the air.

No one was injured, but the July 23 explosion in the Biscuit Basin area, just 2 miles north of the famous Old Faithful geyser, damaged a boardwalk and nearby thermal pools. The park service has closed Biscuit Basin as it assesses the area.

While tourists flock to Yellowstone for its natural beauty, the park is also known for its geysers, earthquakes and hydrothermal explosions that show the power below the its surface – a supervolcano.

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What caused Yellowstone’s hydrothermal explosion?

Part of the active volcanic area that spans northern Nevada, Idaho, and Wyoming is the Yellowstone Plateau Volcanic Field , which includes the caldera. This Yellowstone caldera is what is causing geysers to erupt.

The U.S. Geological Survey says that the geological activity in the park can appear to be a sign of an impending disaster, the explosion at the Biscuit Basin is not a brewing volcanic eruption. Most hydrothermal explosions are small and go unobserved, according to Michael Poland , the scientist-in-charge at the Yellowstone Volcano Observatory.

Exploring Yellowstone's hydrothermal basins

According to the National Park Service , Yellowstone has more than 10,000 thermal areas which include hot springs, mudpots and steam vents. The park is also a hotspot for seismic activity, with an average of 1,500 to 2,500 earthquakes per year. In 2023, there were 1,623 earthquakes, which is considered average. The largest earthquake reported last year was 3.7-magnitude, according to the Yellowstone Volcano Observatory .

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

Last month, there were 64 earthquakes reported in Yellowstone, according to the University of Utah Seismograph stations. Clusters of earthquakes can be frequent and contribute to about 50% of the overall seismic activity in the Yellowstone area.

How big is the magma chamber under Yellowstone?

The size of Yellowstone Caldera is about 55 miles long and close to 25 miles wide. The word "caldera" basically refers to an enormous crater. It's the largest volcanic vent in North America.

After effects of the Biscuit Basin eruption

What causes hydrothermal eruptions

According to the USGS, when water in the underground chambers is heated by bodies of magma, the water can quickly flash to steam. This causes a hydrothermal explosion. A geyser may form to relieve the pressure if the cavity is connected to the surface.

A geyser's water is typically around 200 degrees Fahrenheit and the hot springs can reach temperatures from 200 to 459 degrees Fahrenheit.

In the Upper Geyser Basin of Yellowstone, Old Faithful Geyser erupts on average every 90 minutes. 3,700 to 8,400 gallons of boiling water are released during 1.5- to 5-minute eruptions that reach a height of 106 to 184 feet.

Past Yellowstone volcanic eruptions

It's been about 640,000 years since Yellowstone's last volcanic eruption.

The largest of the park's three major eruptions would have left more than 20 feet of lava, rock, and ash covering an area the size of California. Even the smallest eruption had a volume 280 times larger than the 1980 explosion of Mount St. Helens in Washington.

Massive volumes of volcanic ash, gas, and magma were released into the atmosphere during each of these eruptions. Most of the continental United States was blanketed with debris.

Is Yellowstone's supervolcano overdue for an eruption?

The short answer – No. Not anytime soon.

Hydrothermal explosions are not an indicator that a volcanic eruption is brewing, according to USGS . Scientists are researching how to predict these explosions.

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

Although before an eruption, the subterranean system will probably exhibit decades' worth of warning signs, such as several, significant earthquakes, a shift in the hot springs' chemical composition, and a rise in temperature. According to Poland, the ground would also gradually move tens of feet and release gases.

Contributing: N'dea Yancey-Bragg /USA TODAY

SOURCES: U.S. Geological Survey, National Park Service, The University of Utah, National Geographic, Smithsonian Magazine, Volcanodiscovery.com and USA TODAY research

This article originally appeared on USA TODAY: Yellowstone explosion, explained: A dive into national park's recent thermal activity