Yellowstone National Park is known for its bubbling hot springs and steam-spewing geysers. These hydrothermal wonders are powered by a giant cauldron of partially molten rock. It contains enough boiling magma to build several Mount Everests.
Yellowstone’s volcano last erupted 70,000 years ago. If it erupted again, it could bury a vast area under lava.
Most scientists consider this unlikely, at least for the next several thousand years. But another serious threat lurked – even more sudden and treacherous than lava. To understand its destructive power, it’s important to know what happened a decade ago at Mount Ontake in Japan.
Then trouble struck. And it came with very little warning.
The windows of a nearby cottage suddenly shook. They shook with a powerful shock wave, too low for human ears to hear.
Then a huge gray cloud rose up from the mountain’s southwestern slope.
It blew off the summit, blinding people in a cloud of dust. They couldn’t see, as a million tons of rock and dust rained down on them from the mountain. 63 people were killed. Most were killed by falling debris.
But this volcano’s sudden eruption wasn’t caused by lava or fire. It was caused by water.
A pool of water suddenly heated up because of volcanic gases, or magma, rising from beneath the ground. The water turned to steam almost instantly. As it did so, it expanded to hundreds of times its original volume. This caused the mountain slope to collapse and rocks to shoot into the air.
This kind of steam explosion is called a phreatic (free-AAH-tik) eruption. It’s triggered by a sudden pulse of heat within an active volcano. But similar steam explosions, called hydrothermal explosions, can also occur far from active volcanoes.
Yellowstone is riddled with craters created by these explosions. There have probably been thousands of craters over the past 14,000 years. These include a massive explosion last July 23; it threw out rocks and forced tourists to flee.
In the past century, there have been “only small eruptions,” says Paul Bedrosian. He’s a geophysicist with the U.S. Geologic Survey (USGS) in Lakewood, Colo. “But we know that [Yellowstone] is capable of very large eruptions,” he says — ones even larger than Ontake.
To learn what triggers them, researchers have explored the depths of Yellowstone Lake (in the eastern part of the park). There are hundreds of hot-water vents on the lake’s bottom. Also on that bottom are some of the world’s largest hydrothermal-eruption craters. And hard, brittle domes rise from the lake floor that could one day explode.
“Hydrothermal eruptions are very, very dangerous,” says Lisa Morgan. She’s a volcanologist with the USGS in Denver, Colo. For 25 years, she has studied Yellowstone’s biggest eruptions.
And another, she says, “could very well happen today.”
A Swampy Disaster
Scientists have studied Yellowstone’s hot springs and geysers since the late 1800s. But not until 1966 did they realize it was the site of violent steam explosions.
That summer, Patrick Muffler made his first trip to Pocket Basin. It’s near Yellowstone’s western edge. He was a young scientist with the USGS. He traveled with his boss, Donald White, a USGS scientist who studied hot springs and geysers.
Pocket Basin is a wide, bowl-shaped meadow. A rocky ridge surrounds it on three sides. Hydrothermal pools and waterfalls are scattered throughout the meadow. They perfume the air with the sour smell of hydrochloric acid. That acid is constantly seeping from hot water seeping upward from below.
As the two scientists explored the area, White recognized something he had seen before. In 1951, he had visited Lake City, California, to investigate something strange.
Five nights earlier, a cluster of hot springs that once lay quiet in a swampy meadow had erupted. That eruption threw 300,000 tons of mud and rock into nearby fields. The rocks were made of gravel and sand. They were cemented with minerals — white-colored zeolites and opals.
White knew that these substances form when hot water full of dissolved minerals comes close to the surface. As the rising water cools, the dissolved minerals turn into solids, forming these rocks.
White concluded that underground water had suddenly turned to steam. Its instant expansion had thrown those rocks out.
When White and Muffler climbed the ridge surrounding Pocket Basin, their shoes hit something similar. White realized that this grassy field was the crater created by a hydrothermal explosion. It covered an area the size of 10 football fields! And that surrounding ridge? It was made up of rocks from the explosion.