Underneath The Surface, Yellowstone Still Rumbles

Feb 9, 2021

This geologic map shows the outlines of three large calderas and where Yellowstone Lake is located in relationship to the 631,000-year-old Yellowstone Caldera. The two smaller calderas are older, with the one outlined in purple forming 2.08 million years ago and the smaller one outlined in blue forming 1.3 million years ago.
Credit Lisa Morgan

Yellowstone National Park is famous for its hydrothermal activity. Geysers like Old Faithful and Steamboat have delighted tourists and fascinated scientists for years. But a lot is still unknown about the hydrothermal features that make the park so unique.

Much of the landscape of the park was formed by hydrothermal explosions. These are similar to geysers except that in addition to water and steam, they also spew out pulverized rock and mud from the area, creating large craters that are usually filled with lakes and surrounded by raised rims. This explosion of material leaves an unmistakable fingerprint that researchers can look at to learn more about the events that left it.

"As the boiling water [and] steam front descends, it starts breaking up all of these rocks, and those rocks are ejected in a generally a vertical - but we're finding out not always vertical - eruption column or explosion column," said U.S. Geological Survey scientist emeritus Lisa Morgan. "And it usually creates a crater, but it fills in the crater, and then it puts out an ejecta rim. And this breccia is composed of two components. There are rock fragments, which used to be the subsurface rocks that hydrothermally altered, and then there's the matrix, and the matrix is finely pulverized [rock] or mud fragments that are in between the rock fragments."

The breccia can tell scientists how intense an explosion was and when it happened. How much the steam appears to have altered the rock can also indicate how long hydrothermal activity was happening before the explosion.

"So you can go all around within the Yellowstone Caldera, and you find all these thermal basins like Old Faithful, like Norris, like Lower Geyser Basin, and they're these beautiful features, and they go on and on," said Morgan. "And they do change over time, but we know from studying hydrothermal explosions that these hot springs were in place long before the explosion ever took place."

And Yellowstone is still experiencing hydrothermal explosions.

"I think most people thought all of these hydrothermal explosions in Yellowstone, they happened when the last ice sheet receded from Yellowstone plateau, which was about 14,000 years ago, but our studies have shown that's not the case and that they have continued up until today," said Morgan. "And in fact, we get a small - usually less than 10 meters in diameter, only a couple meters, maybe - we usually get one of those every year or two in Yellowstone. The small ones are pretty common. You really don't want to be there when it happens. But they are more manageable."

This bathymetric map of Yellowstone Lake shows the locations of the 8,000 year-old Elliott's Crater hydrothermal explosion crater and the older and much larger 13,000 year-old Mary Bay hydrothermal explosion crater.
Credit Lisa Morgan

Yellowstone Lake has been formed by hydrothermal explosions in places like Mary Bay, on the north edge of the lake. This is partly because the magma chamber under the Yellowstone Caldera part of the lake is relatively close to the surface and according to Morgan, it's one of the highest heat flow values in the whole park, which helps explosion craters form. Scientists are still mapping the underwater landscape of Yellowstone Lake and other bodies of water in the park.

"I think when most people walk or drive by a lake, they think, 'Oh, isn't that nice and serene' and 'it's calm and quiet.' And in fact, the floor of Yellowstone Lake is anything but quiet," she said. "And what we have found in our bathymetric and seismic surveys, is in fact, there have been multiple hydrothermal explosions. There have been landslides or mudslides. There are over 600 active hydrothermal vents on the floor of Yellowstone Lake. There are major faults on the floor of Yellowstone Lake. Many of those are still active. And the lake has been anything but quiet."

Knowing where active hydrothermal vents are located is a matter of public safety. Hydrothermal explosions have recently been recognized as one of the most serious hazards in Yellowstone. Another important thing that scientists are trying to understand is what causes these explosions in the first place.

"We know that there are major seismic events that triggered some of the hydrothermal explosions. We know that to be a fact. And we also know that some seismic activity doesn't trigger hydrothermal explosions," said Morgan.

Some of the most popular areas in the park are thermally active areas, which Morgan says is a big reason to continue learning more about these events and what can trigger them.

"We know when people go to Yellowstone, where is it they like to go? They like to go to thermal basins," said Morgan. "So while these explosions aren't going to have a regional effect, they will have a profound local effect and could have significant impacts, no pun intended, to the visitors and to the infrastructure in Yellowstone."

Have a question about this story? Contact the reporter, Ivy Engel, at iengel@uwyo.edu.