Scientists made ‘tiny earthquakes’ to find where Yellowstone’s magma begins

Viewed from above, the Grand Prismatic Spring in Yellowstone National Park, Wyoming, in 2022. It is the largest hot spring in the United States. Photo / Salwan Georges, The Washington Post

Underneath Yellowstone National Park, magma bubbles and flows. But exactly how close it is to the surface has long been a mystery to scientists - until now.

For the first time, researchers have detected the top of the area’s magma reservoir, roughly two miles below Yellowstone’s surface. There, rock abruptly gives way to bubbling magma, the geoscientists found, creating a magma cap - a sign that the reservoir is dynamic and active.

The data adds to existing evidence that Yellowstone is not facing an imminent volcanic eruption. The magma cap acts as a lid on the pressure and heat beneath, and the system is releasing gas at a steady pace that prevents buildup, according to the findings, which were published last week in the journal Nature.

The discovery offers a new image for Yellowstone visitors: About two miles beneath their feet, the rock starts to get gooey - a reservoir of solid crystals with a mixture of lava melt and bubbles that form as the magma releases gases. New magma comes in from the bottom of the system. And above the cap, going up to the surface of the earth, is brittle rock with cracks that allow gases to escape.

“We didn’t know if it was a very gradual transition or if it was a sharp boundary, and if it was a sharp boundary, we didn’t know exactly where it’s located,” said Rice University geophysics and seismology professor Brandon Schmandt, one of the study’s authors. “Knowing if it’s a few kilometres deeper or shallower really changes the properties we’d expect in the magma.”

To conduct the study, the researchers developed a method for detecting what’s below the surface that could be used for future research on volcanoes, Schmandt said.

“It pushes us to be a little more ambitious about what we could know about volcanoes in Yellowstone,” Schmandt told The Washington Post. “It’s zooming into a different part of the magmatic system.”

Yellowstone, carved by volcanic eruptions, is home to unique hydrothermal features, rocks that are hundreds of millions of years old and about half of the world’s active geysers, according to the National Park Service. A massive volcanic eruption about 631,000 years ago, the last of three major outbursts in the region in the last 2.1 million years, created the Yellowstone caldera, a giant volcanic depression.

How magma is stored there and whether it could erupt again have long been open questions. In January, a different group of researchers found that the magma was segregated enough in different reservoirs across the area that it was unlikely to erupt but would remain active.

The new research, done by scientists from Rice University, the University of New Mexico, the University of Utah and the University of Texas at Dallas, also indicated that the system was lively but not imminently threatening, Schmandt said.

An aerial view of Mammoth Hot Springs in Yellowstone National Park, Wyoming, in 2022. Photo / Salwan Georges / The Washington Post

In volcanic areas, it’s important for scientists to know whether bubbles are accumulating, how easily the gas is escaping and whether the magma reservoir has a sharp boundary to help gauge the eruption risk, Schmandt said. Understanding active magma reservoirs can also help with scientific understanding of inactive ones, which crystallise over time, hold valuable ores and metals, and can become targets for mining.

“What drives most volcanoes to erupt hazardously is the accumulation of bubbles, so we don’t want to be blind to that process. We want to be able to see it, and this is a great step toward seeing it,” Schmandt said.

The researchers found that the gases in the Yellowstone magma reservoir are bubbling up through the porous rock and being vented at a steady pace, releasing the pressure from below.

Whether volcanic systems are stable is strongly influenced by how deep the upper crust capping the magma is. Gathering that type of data below the surface can be difficult, and the research team went through some trial and error.

The scientists worked in the northeast Yellowstone caldera along a road that parallels one of the park’s rivers. Working in the dead of night when the park was empty, they used a 53,000-pound (24,040kg) truck that creates seismic vibrations to cause “tiny earthquakes”, according to Rice University.

The seismic waves bounced off layers below Yellowstone’s surface and were detected by sensors, allowing the scientists to collect data and use seismic imaging and advanced computer models to draw their conclusions. The raw data was difficult to interpret because of Yellowstone’s complex geology, which scatters seismic waves, Rice University researcher Chenglong Duan said in a statement, but he made repeated attempts to find the right method.

“When you see noisy, challenging data,” Duan said, “don’t give up.”