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.
"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.
Arlene Ducao for AMNH
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.
Park geologist Henry Heasler sets out to measure the temperature and pH of a shallow thermal pool in Norris Geyser Basin.
David Rasmussen
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.
Mud pot, Lower Geyser Basin
David Rasmussen
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.
The magma chamber responsible for Yellowstone’s volcanic activity is invisible and inaccessible, buried about 8 km beneath the surface. Therefore, its geologic moves must be analyzed indirectly. A hundred or so researchers from universities, the USGS, and other institutions check satellite data, seismometers, Global Positioning System (GPS) stations, temperature gauges, infrared cameras, gas sampling apparatus, and other equipment to monitor Yellowstone’s every geologic move. A spike in activity at any one of the monitored aspects may mean nothing. But if a number of features collectively and rapidlyover days or weeksact strangely, the Yellowstone Volcano Observatory (YVO) would issue an alert of volcanic unrest. This year, the YVO adopted the new USGS standard alert-level system, which assesses the volcano at one of four degrees: from Normal (no eruptions), through Warning (highly hazardous eruption underway or imminent).
Yellowstone Volcano Observatory scientist-in-charge Jake Lowenstern captures gases rising from a thermal feature at Norris Geyser Basin.
David Rasmussen
Ups and Downs
The Monitoring
Walking the ground at Yellowstone is like treading on the back of a giant, lumbering beast. The caldera floor rises and falls over the years, indicating that fluidsmaybe magma, maybe gas, maybe water and steamare moving in and out of the rock beneath Yellowstone. “We watch the GPS stations to see how they’re moving north, south, east, west, up, or down over time,” says Jake Lowenstern, a USGS geologist who heads the Yellowstone Volcano Observatory. A newer system called Interferometric Synthetic Aperture Radar (InSAR) uses satellites to measure uplift and subsidence. Once or twice a year, the European Space Agency’s ERS-2 satellite passes over Yellowstone and beams out radio waves. By measuring the time of the return signal, precise changes in the ground level relative to the satellite can be detected and then checked against the GPS data.
Recent Unrest
Currently, the floor of the Yellowstone caldera is in an uplift phase. From October 2004 through May 2006, it rose a maximum of 10 cm at both the Yellowstone Lake and the White Lake GPS stations. The Yellowstone Volcano Observatory considers the shift business as usual for such an active volcano, warranting a “Normal” alert level.
Earthquakes
The Monitoring
Each year 1,000 to 3,000 earthquakes shake Yellowstone. These vibrations are caused by rock breaking in the volcanic system and by movement along faults. The molten magma’s pressure on the brittle rock above the chamber can trigger strain and breakage, as can high-pressure hydrothermal fluids winding through fractures.
“When you see a series of earthquakes get shallower and shallower, that means fluids are rising,” says Bob Smith, a geophysics professor at the University of Utah who runs Yellowstone’s seismographic network. Major shifts, like when the floor of the Yellowstone caldera switched from rising to falling in 1985, can be accompanied by intense swarms of earthquakes. Indeed, unusual earthquake activity is the first place scientists look for clues of upcoming eruption.
Recent Unrest
The last destructive earthquake in the area, a magnitude 7.5, was at Montana’s Hebgen Lake in 1959. More than three swarms have occurred since 1985; the last was in 2004 when 400 earthquakes rumbled the northwest section of the park over a three-day period. Many small earthquakes (magnitudes 1, 2, 3, or 4) occur at Yellowstone every day, some strong enough for people near the epicenters to feel. Geologists consider Yellowstone’s recent seismic activity to be at “background levels”: i.e., typical for such a geologically active area. To learn about Yellowstone’s latest earthquakes, visit the University of Utah’s Recent Earthquake Activity site.
A typical roadside sign in Yellowstone National Park.
David Rasmussen
Heat
The Monitoring
The daily air temperature at Yellowstone averages a near-freezing 2.2 degrees C. But the magma chamber can heat the soil to upwards of 100 degrees C, and the groundwater flowing into springs, geysers, and mud pools can be beyond boiling. Last summer, on separate occasions, three Yellowstone visitors scalded themselves by walking off-trail and unwittingly into three of Yellowstone’s 10,000 thermal features.
Rising surface temperatures could mean that magma is inching closer to the surface, or that underground heat and fluid flow has shifted in some manner. It also can be a tip-off to groundwater pressure changes, which can alter the boiling point of underground fluids. Over large areas, surface temperature is detectable by satellite. But in 2002 a research team flew an infrared camera aboard a twin-engine aircraft over Yellowstone Park to get a initial sense of its surface heat patterns in higher resolution. “It’s like sensing the temperature of skinin this case, the “skin” of the hot ground or hot water coming off of thermal features,” says park geologist Cheryl Jawoworski.
The black cylinder in the belly of this twin-engine plane is an infrared camera that senses emitted subsurface heat. Utah State University graduate student Deepak Lal oversaw the most recent thermal data flights over Yellowstone’s Upper, Midway, and Lower Geyser Basins.
David Rasmussen
In fall 2005, Jawoworski and National Park Service colleagues commissioned flights over the park’s geyser basins, which include Old Faithful, to establish a solid set of baseline measurements for Yellowstone’s heat flux. The team will be able to compare future heat maps to this baseline to detect temperature changes and alert visitors to danger zones before they learn the hard way. Other scientists are testing another novel technique to image Yellowstone’s heat aerially: infrared cameras strapped to 2 m wide helium balloons.
Recent Unrest
In summer 2003, a portion of Norris Geyser Basin turned up the heat. Ground thermometer measurements revealed that cool soil along the Back Basin Trail shot up to 94 degrees C. A new mud pot also formed, spurting hot, acidic ooze onto the trail. These events indicated changes not in magmatic activity, but in underground water pressure. By looking back at the heat patterns on the 2002 aircraft map, park staff decided to reroute the Back Basin Trail and add wooden boardwalks to prevent accidents on hot ground. Jawoworski and her colleagues are still analyzing the results from the fall 2005 baseline flight.
Gas
The Monitoring
“We know that magmas are down there, they're putting off a lot of heat,” says Lowenstern. “They also put out a lot of gas. And one way that geologists and volcanologists can understand what magmas are doing is to look at that gas.” Lowenstern and his colleagues manually collect these gases in glass bottles, going face to face with acrid emissions of carbon dioxide, sulfur, helium, and hydrogen sulfide. They find the gases dissolved in the hot waters around Yellowstone and percolating from soils and hissing steam vents called fumaroles.
Bison at Yellowstone.
AMNH
Analyses can reveal the chemical compositions and sources of the gas, whether from magma or shallow groundwater. By sampling gases 30 times a year in different locations, Lowenstern and his colleagues can understand what areas of Yellowstone are more affected by the magma chamber. Knowing if any unhealthy gases are building in particular areas also helps inform visitor safety.
Recent Unrest
A few months before the temperatures increased at Norris, a line of new fumaroles opened in a forested portion of the basin. Beyond this, no major changes in gas activity have occurred recently at Yellowstone. But even normal gas emissions can prove deadly in concert with certain weather and geographical conditions. In March 2004, biologists discovered five dead bison near the Gibbon River in the Norris Geyser Basin. The bison were lying on their sides with their legs straight out. Gas testing and blood and lung samples revealed that gas vents uphill from the bison emitted denser-than-air carbon dioxide and hydrogen sulfide, which stagnated in the low river valley on a cold, windless night. The group of bison roaming into the valley was asphyxiated en masse.
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.
Yellowstone National Park is a plateau in the Rocky Mountains. The mountains once at the park site were removed via volcanic eruption.
David Rasmussen
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.
University of Utah professor Robert B. Smith at the Lake Butte seismograph.
Jim Peaco
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.
A volcanic system with current, measurable activity.
ash
Fragments of volcanic rock less than 2 mm in diameter.
assimilate
To take in.
atmosphere
The envelope of gases surrounding a planet, e.g., Earth's atmosphere consists of approximately 80 percent nitrogen and 20 percent oxygen.
background levels
At typical levels.
caldera
A giant crater-like depression in Earth's surface caused when a magma chamber collapses after a rapid, voluminous volcanic eruption.
carbon dioxide
A molecule consisting of one carbon atom and two oxygen atoms (CO2). Its gas is denser than air.
continental crust
The part of Earth's crust that includes and underlies the continents and continental shelves. The continental crust averages about 40 km thick.
continental plate
A tectonic plate that includes continental crust, which is less dense than oceanic crust.
crust
The outermost and thinnest of the solid Earth's layers, which consists of rocky material that is less dense than the rocks of the mantle below.
density
The average mass per unit volume of a substance.
dome (lava dome)
A mound of viscous, gas-poor lava erupted from a volcanic vent. Because its viscosity prevents the lava from flowing far from the vent, it cools in a domed shape.
earthquake
A sudden motion or trembling of Earth's crust caused by the passage of seismic waves radiated from a fault along which sudden movement has occurred.
earthquake swarm
A series of earthquakes occurring around the same time, typically near a volcano.
epicenter
The point on the surface of Earth directly above the subsurface source of an earthquake.
escarpment (scarp)
A steep cliff, either above or below sea level.
fault
A rock fracture or fracture zone along which there has been movement.
fumarole
A vent on Earth's surface that emits volcanic gas.
gas (volcanic)
Steam (water gas), carbon dioxide, sulfur dioxide, and other gases that are dissolved in magma. When magma erupts onto Earth's surface, it releases these gases. Volcanic gas also can issue directly through fumaroles and soil.
geology
The study of Earth, its history, its composition, its structure, and the dynamic processes that shape it.
geophysics
The study of Earth's physical properties.
geothermal
Heat sources under Earth's surface.
geyser
A hot spring that can erupt water and steam. Geysers typically occur when geothermal processes keep fluids in a confined area of a fracture or vent at high temperature and under high pressure.
global positioning system (GPS)
A system of satellites, computers, and receivers that can determine the location (latitude and longitude) of a receiver on Earth.
groundwater
All the water contained in the spaces within the subsurface.
hot spot
A fixed point on the Earth's surface with long-lived volcanism.
hydrogen sulfide
H2S gas. It is toxic, flammable, and smells like rotten eggs.
hydrothermal
Of or caused by the circulation of hot water due to thermal processes in Earth's crust.
hydrothermal fluids
Hot water, steam, and other gases trapped in fractured or porous rocks underneath Earth's surface.kilometer (km)A unit of length equal to 1,000 meters, or 0.62 miles.
lava
Magma when it erupts on Earth's surface.
lava flow
Rivers of magma that travel over Earth's surface during a volcanic eruption.
lithosphere
The outer layer of solid rock that includes the crust and uppermost mantle. This layer, up to 100 km thick, forms Earth's tectonic plates.
magma
Molten or partially molten underground rock.
magma chamber
A reservoir of magma.
magnitude (earthquake)
A measure of the total amount of energy released by an earthquake.
mantle
The layer within Earth that lies between the crust and the core.
meter
A unit of length in the metric system equal to 3.28 feet.
mud pot
A pool of hot water and fine sediment through which steam, water, or volcanic gas escape.
plate
One of several large, interlocking, mobile pieces of Earth's lithosphere.
plateau
Relatively level high ground.
radio
Low energy electromagnetic radiation, with long wavelengths and low frequencies.
rock
Any naturally formed aggregate of one or more minerals, such as granite, shale, or marble.
seismic
Relating to or caused by an earthquake or earth vibration.
seismograph
An instrument that detects and records the vibrations of Earth.
seismology
The study of earthquakes and other seismic waves.
silica
The compound silicon dioxide (SiO2). Silica is an important component of many rocks and minerals. It can be found in several forms, including quartz and opals.
subduction zone
The zone of convergence of two tectonic plates, one of which usually overrides the other.
subsidence
A sudden sinking or gradual downward settling of Earth's surface with little or no horizontal motion.
tectonic plate
One of several large, mobile pieces of Earth's lithosphere adjoining other plates along zones of seismic activity.
uplift
The process or result of raising a portion of Earthàs crust through tectonic mechanisms.
vent
An opening such as a fissure, fracture, crack, or hole in Earth's crust through which magma and gas escape.
viscous
Having a thick consistency somewhere between a solid and a liquid. The more viscous a material, the more resistant it is to flow.
volcanic center
A region of related volcanoes or volcanic activity.
volcano
A vent or fissure in the Earth's surface through which molten lava, ash, and gases are ejected. It is also the name for the structure, usually conical, formed by the materials ejected from the vent or fissure.
In Yellowstone: Monitoring the Fire Below, scientists examine the geologic past -- and future -- of Yellowstone.
This feature can be used to illustrate the process of science. Scientists in the video collect data to test their hypothesis about the likelihood of a volcanic eruption at Yellowstone. Read more about The Scientific Process.
