Yellowstone has been nicknamed the “American Serengeti” because, like its East African counterpart, it is also a vast ecosystem of open plains, savanna and woodlands. It consists of approximately 12,000 square miles and is even home to its own “Big Five” for wildlife sightings— bears (grizzly and black), wolves, bison, elk and moose. When envisioning the landscape, adventurers often picture jagged peaks, wide-open valleys and free-flowing rivers, but did you know that much of the entirety of Yellowstone National Park is actually an active supervolcano? While this may not be immediately evident, the signs are there. Most notably, the bubbling geysers and hot springs found throughout America’s first national park are an indication of the immense churning volcanic activity right below the surface.
Here, where northwestern Wyoming, southern Montana and eastern Idaho meet, the earth’s crust has been compressed, pulled apart, glaciated, eroded, you name it. All of this geologic activity formed the gorgeous mountains, deep canyons, and dramatic plateaus that visitors can see at Yellowstone National Park. Along with neighboring Grand Teton National Park, federally managed lands such as Shoshone National Forest and Grays Lake National Wildlife Refuge, and additional state-managed and private properties, the Greater Yellowstone Ecosystem is actually one of the largest nearly intact temperate-zone ecosystems and one of the most geologically dynamic areas on Earth. Due to a shallow source of magma and resulting volcanic activity, it boasts the largest concentration of active geysers in the world—approximately 500—which is impressively more than half of the world’s total.
The Yellowstone Supervolcano
So we clarified that Yellowstone is not just a volcano, but a “supervolcano”. But what does that even mean? It implies an eruption of magnitude 8 on the Volcano Explosivity Index, indicating an eruption of more than 250 cubic miles of magma. Yellowstone qualifies because it has had not just one but at least three such eruptions: 2.1 million years ago, 1.2 million years ago and 640,000 years ago. To put things into perspective, these would have been about 6,000, 700 and 2,500 times larger than the May 18, 1980 eruption of Mt. St. Helens in Washington State. The magmatic heat that powered those three eruptions created a sunken giant crater (or caldera) 1,500 square miles in area and still powers the park’s more than 10,000 hydrothermal features, such as its famous geysers (including Old Faithful). Since the supereruptions, 80 smaller eruptions have occurred. Approximately 174,000 years ago, one of these created what’s now the West Thumb of Yellowstone Lake. After these explosive eruptions, 60 to 80 post-caldera lava flows, including viscous rhyolitic lava and less voluminous basalt lava flows, slowly began to fill the caldera floor and surrounding terrain. The youngest of these lava flows is the 70,000-year-old Pitchstone rhyolite flow which you can check out in the southwest corner of Yellowstone National Park.
One does not need to be a history or geology buff to be able to appreciate the consequences of volcanic activity at Yellowstone. On a trip to the park, travelers can enjoy the beauty of five main hydrothermal features: hot springs, geysers, mudpots, fumaroles and travertine terraces.
Hot springs are pools of hydrothermally heated water and are the most common hydrothermal features in Yellowstone. They begin when rain seeps through the bedrock and gets superheated by the Yellowstone magmatic system. An open plumbing system lets the hot water rise back to the surface and convection currents keep the water circulating, preventing it most of the time from getting hot enough to create an eruption. But sometimes boiling waters within a hot spring (like Crested Pool) can explode and shoot water into the air, making it seem like a geyser.
Travertine terraces are hot springs, but ones that rise up through limestone, dissolve the calcium carbonate and deposit the calcite that builds the travertine terraces. At the surface, carbon dioxide is released and calcium carbonate is deposited, forming travertine, the chalky white mineral forming the rock of travertine terraces. These then get almost painted by colorful stripes which are formed by thermophiles—”thermo” for heat, “phile” for lover. Different types of picky thermophiles live at different specific temperatures within a hot spring and cannot tolerate when conditions are the least bit cooler or warmer than their ideal. These extreme organisms can also live in places with acidity that would burn holes into clothes, or an environment as basic as baking soda. Yellowstone’s hot water systems often show obvious gradations of living, vibrant colors where the temperature limit of one group of microbes is reached and another group begins to thrive, creating the striped effect. One of the few places in the world where active travertine terraces are found is Mammoth Hot Springs, one of the many places visited on our Nat Hab adventure to Yellowstone National Park. As one early visitor described the Mammoth Hot Springs, “No human architect ever designed such intricate fountains as these”.
Geysers are hot springs, just ones with constrictions in their plumbing (usually near the surface) which causes them to periodically erupt to be able to release the pressure that builds up. There are more geysers in Yellowstone than anywhere else on Earth. Old Faithful, being by far the most well known, is certainly impressive, but each geyser plays out in its own unique way. Riverside Geyser, in the Upper Geyser Basin, shoots at an angle and usually forms a rainbow in its mist. Grand Geyser explodes in a bunch of powerful bursts that tower above the surrounding trees. Echinus Geyser spouts out to all sides like a splash of fireworks. And Steamboat Geyser, the largest in the world, pulsates like a massive steam engine, reaching heights of 300 to 400 feet.
Hot springs that are so acidic that they dissolve the surrounding rock are called mudpots. A mudpot is basically nature’s double boiler. Surface water collects in a shallow, impermeable depression that is often lined with clay and that has no direct connection to an underground water flow, then thermal water beneath heats the collected surface water. Hydrogen sulfide gas is often present, making mudpots smell like rotten eggs. Microbes that use hydrogen sulfide for energy help convert the gas to sulfuric acid, which breaks down rock into clay and makes a mucky goo through which gasses gurgle and bubble. Minerals, often iron oxides, tint the mudpots with so many pinks, beiges, and grays that mudpots are sometimes called “paint pots.”
Last but not least of the hydrothermal features are the fumaroles, also known as steam vents. They happen when a hydrothermal feature has so little water in its system that the water boils away before reaching the surface, only releasing hissing steam and other gasses that can reach temperatures as high as 280°F. Black Growler in the Norris Geyser Basin is one of Yellowstone’s most visited steam vents, but at Roaring Mountain fumaroles dot an entire mountainside, making it an especially dramatic show.
If you want to go on a Yellowstone safari but are concerned about so much volcanic-related activity, rest assured that the Yellowstone Volcano Observatory (YVO) has everything under control. YVO is a partnership of scientists from the US Geological Survey, National Park Service, University of Utah, University of Wyoming, University NAVSTAR Consortium (UNAVCO) and the state Geological Surveys of Wyoming, Montana and Idaho. YVO scientists keep a close watch on the Yellowstone volcano with a real time and near real-time monitoring network of 26 seismic stations. They collect data on temperature, chemistry, and gas concentrations at selected hydrothermal features and chloride concentrations in major rivers. These experts assure us that Yellowstone’s volcano is likely to erupt again…but only in the next thousands to millions of years. The most likely activity in our lifetime is lava flows, but those ooze slowly over months and years, allowing tons of time for park managers to evaluate the situation and protect visitors. So no worries, you can plan a trip to Yellowstone National Park focusing simply on taking in the excitement of moose and wolf sightings and spectacular geyser shows.