Yellowstone: Monitoring the Fire Below
Three of the most catastrophic volcanic eruptions in geologic history occurred at a place now visited by nearly four million people a year: Yellowstone National Park. The magma chamber responsible still lies beneath, and continues to steam, heat, and shift the park landscape. Science Bulletins talks with the geologists regularly monitoring these disquieting signals to understand where this active region lies in its volcanic life span.
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"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.
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.
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.
What Dangers Await?
“If a lava flow were to occur here today, it certainly would have an effect,” says Lowenstern. “But it wouldn’t cause many, if any, deaths.” Lava is what magma is called after it breaches Earth’s surface. About 80 lava flows since Yellowstone’s last big eruption 640,000 years ago have filled in much of the three calderas, so that their entire circumferences are only detectable with careful fieldwork. Lava erupting from existing or new cracks at Yellowstone would likely be thick and viscous and have little gas left in it. Thus, it would ooze, not explode, and be unable to flow long distances easily. “Tourists just wouldn’t be allowed in certain areas,” says Lowenstern.
The remaining molten rock in Yellowstone’s collapsed magma chamber is now cooling. It donates heat to the water table above it, which creates Yellowstone’s more than 10,000 hydrothermal features. The hot groundwater can flash as steam in geysers like Old Faithful or belch through cauldron-like mud pots. The water also collects in pools, some of which are acidic, near boiling, blue-green with minerals and microbes, and reeking of rotten-eggy hydrogen sulfide. (“The smell of life,” Lowenstern calls it.) An unanticipated hydrothermal explosion could scald or severely injure park visitors and staff.
Still, a lava flow or a hydrothermal explosion does not herald a new catastrophic eruption. A surer sign would be a dramatic shift in the ground level at Yellowstone, a hint that the magma chamber was moving upward or significantly refilling. Scientists would also look for serious “swarms” of earthquake activity, which would suggest the malleable magma chamber was rupturing the brittle rock above it. Recent monitoring has detected both ground level rises (8 to 10 cm in the past 19 months) and seismic signals, but they’re not dramatic enough to warrant worry. They simply remind geologists that Yellowstone is naturally a place of change.
“Thermal features can change over one day,” says Heasler. “Yellowstone is an interesting place where we can see geologic processes changing on a day-to-day basis rather than a million-year-by-million-year basis.”Monitoring this change is the essence of the geologic work ongoing at Yellowstone. It is the key to predicting the park’s next big moment.
University of Utah professor Robert B. Smith has spent nearly his entire 40-year scientific career studying the volcanic setting of Yellowstone National Park and geologic evolution of the Teton Range. He directed and installed much of Yellowstone’s 23-station seismographic and 15-station Global Positioning System networks. We were curious how Smith’s ample experience as a geophysicist affects the way he sees majestic Yellowstone National Park.
Can you describe Yellowstone National Park for one who has not yet seen it?
Right now, I’m in Jackson, Wyoming, looking up at the high mountain range of the Tetons. When you drive into Yellowstone, as I’ve done hundreds of times, you drive 50 kilometers up a long, flat plateau with maybe 300 meters of elevation gain to heavily forested terrain. But once you break out of the forest and you look for the mountains you’d expect in a Rocky Mountain setting, they're gone--they were destroyed during Yellowstone’s three gigantic volcanic eruptions 2, 1.3 and 0.6 million years ago. Some mountains were either blown away or subsumed back into the big magma chamber. The plateau is the product of that destruction. Later, many smaller, but still large volcanic eruptions flowed lava across the terrain, smoothing it out.
But Yellowstone has cliffs and hills. How did they get there?
In the last few thousand years frequent earthquakes have broken Yellowstone’s ground unevenly. You can’t see the faces of these faults well, as they are covered with timber, but you’d see one clearly if an earthquake occurred. The largest earthquake recently was the magnitude 7.5 just northwest of the park at Hebgen Lake, Montana, in 1959. It killed 28 people and made a fault 6 meters high and 60 kilometers long. Such earthquakes have helped break up Yellowstone’s topography, added hills and valleys, cracks and cliffs and escarpments. There are also a couple of places where the magma lifted the topography above magma chambers and created a dome: one at Old Faithful and another to the north called Sour Creek dome. These domes remain 500 meters higher than the surrounding plateau.
So without mountains in the way, is it easy to see all of Yellowstone’s geology?
Yellowstone is too big to see at once. It’s about 160 kilometers long from one end to the other. It’s not like you can just park your car and walk around the entire volcano like you can at Kilauea in Hawaii. Also, a lot of Yellowstone is still wilderness, and inaccessible. The things the average visitor sees are the geysers and valleys because that’s where they built the roads.
Have you explored in many of those inaccessible areas?
I’ve done a lot of work in the backcountry. I’ve installed earthquake seismographs, made studies of deformation using GPS, and mapped out some of the big fault escarpments that were not well understood. Some are 30 kilometers from the nearest road and only accessible by horseback.
What are you studying now?
We’re working with geology that is “alive.” Calderas tend to breathethey rise and fall and rise and fall. And earthquakes are the heartbeat of this active volcanic system. With the seismic network in place, we’re keeping track of the earthquakes. We had one just this morning at 4 o’clocka magnitude 4. The newer technology that’s providing information is the GPS system, which we began installing in the mid-80’s. These stations measure the upward or downward movement of the ground, whether driven by earthquakes or by magma or hydrothermal fluids moving underground. Between 1923 and 1975 the entire Yellowstone caldera rose 1 meter. And suddenly in 1985 it reversed and started subsiding. Then in 1990 it started to go back up. Right now we’re studying an uplift surge that began about two years ago.
Can you share a moment when the volcano was particularly “alive”?
One day I was flying in a small plane with a National Geographic photographer--we were taking pictures of the caldera at an altitude. We had just turned back from the Norris Geyser Basin, low on fuel and out of film, when I saw clouds of haze below. I thought it was a forest fire, but it was Steamboat Geyser erupting. It’s the tallest geyser in the world, and it hadn’t erupted in five or ten years. I only had four pictures left in my camera, and used them all. What are the chances? The thing erupted for 10 to 20 minutes. We actually flew through the steam cloud that had risen a thousand feet into the air.
With such a history behind you, how do you see Yellowstone now when you arrive for work?
Whether you’re in the frontcountry or backcountry you’re always seeing something new at Yellowstone. Just over my career it has changed immensely. A huge earthquake occurred in my career. I’ve seen a caldera go up and down almost a meter. At Yellowstone, the changes are short term enough that you can observe them and see how they operate. Every time I go to Yellowstone I’m impressed and educated by new things. It’s always exciting. It’s just such a wonderful place.