Yellowstone: Monitoring the Fire Below
Yellowstone National Park is a Volcano
Yellowstone, we feel, is a very, very safe place to visit,” says Hank Heasler, one of two park geologists at Yellowstone. It’s true that acrid, piping-hot groundwater flows just under the park’s rocky plateau, forming a landscape bubbling, steaming, and spraying with hydrothermal activity. It’s also true that three of the most astonishing volcanic eruptions in the geologic record—each hundreds to thousands of times the volume of 1980’s Saint Helens eruption—occurred around what is now Yellowstone National Park, which includes parts of Wyoming, Montana, and Idaho. Over three million visitors step onto this charged volcanic landscape every year. Yet the geologists that monitor it are unconcerned about a large, imminent eruption. Far more unnerving is an encounter with one of the park’s wolves or bears.
Each of the three largest eruptions of Yellowstone's magma chamber resulted in the collapse of the chamber's roof rock, resulting in a circular caldera tens of kilometers wide.
A Restless History
“To the public, an active volcano is one that’s erupting now,” says United States Geological Survey geologist Jake Lowenstern, who heads the Yellowstone Volcano Observatory. Yellowstone is not erupting, but it is active. About 400 km below it, in the Earth’s upper mantle, lies a hot spot: a fixed region of partly molten rock far from any tectonic plate boundary. “You can think of the hot spot as a blowtorch,” explains Lowenstern. “It's creating melt in the mantle, and that melt is rising and melting the continental crust above it.” At the moment, a 50 km wide chamber of molten rock—magma—sits about 8 km beneath Yellowstone. When the crust above the chamber no longer can withstand the upward pressure of the swelling magma chamber, it fractures and the magma erupts.
The first of Yellowstone’s three big eruptions was 2.1 million years ago, the next was 1.3 million years ago, and the last was 640,000 years ago. During each event, gas-laden magma erupted explosively like an uncorked champagne bottle. The explosions shattered magma and overlying rocks into fragments and ash particles. Fluid magma exploded through the fractures and paved the Yellowstone soil.
Only about 10 percent of the magma chamber exploded in each “supervolcano” event; still, that amounted to more than a thousand cubic kilometers of material per eruption. “Two of the three eruptions put out enough volcanic ash to spread a cloud all the way to the Mississippi River and the Gulf of Mexico,” says Heasler. This blocked the Sun’s rays and cooled the Earth’s atmosphere, which took years to recover. After each eruption, the roof of the partially emptied magma chamber collapsed, forming an enormous surface depression called a caldera. When the magma chamber filled again to a pressure point, it erupted in a slightly different location. Remnants of the clifflike walls of Yellowstone’s three calderas are still visible. (See part of one caldera in this interactive.)
Park geologist Henry Heasler sets out to measure the temperature and pH of a shallow thermal pool in Norris Geyser Basin.
Basic math on Yellowstone's eruption cycle (one event every 600,000 to 800,000 years) seems to suggest a fourth event, well, about now. Heasler demurs. ”Three data points do not make a compelling argument for almost anything in science,” he says. Geologists are uncertain whether Yellowstone is winding down from the third eruption or ramping up to a fourth.
Technically, the next eruption could happen anytime. However, catastrophic eruptions occur so infrequently in the geologic record that it is statistically not likely anytime soon. More importantly, if Yellowstone were preparing to blow another big one, its heavily monitored signs of unrest would also clearly indicate imminent eruption. (They don’t.) More likely hazards are localized lava flows and hydrothermal explosions, which are just symptoms of the park’s volcanic underbelly.
