Where is the volcano?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Morgan Nasholds, Geologist/Geophysicist Scientist-in-the-Park at Yellowstone National Park.

There are numerous questions posed by curious visitors to Yellowstone National Park. Some are questions of orientation: “Where is Artemisia Geyser?”, “How do I get to the Grand Canyon of Yellowstone?”. Some questions relate to timing: “When will this geyser erupt?”, “How often does Old Faithful go off?”, “When will Yellowstone explode?”. Other questions are those borne of scientific curiosity: “Does this spring vary in temperature?”, “What causes the colors in the hydrothermal springs?”, “What is this type of rock?”. Among these questions is one of the more unintentionally thought-provoking: “Where is the volcano?”. 

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This question regarding the location of the volcano in Yellowstone comes from many visitors who expect to see a large, singular mountain that is prominent above the landscape, such as Mount Rainier above Seattle. However, the actual location of the volcano at Yellowstone is more complex. The short answer is that the volcano in Yellowstone is right beneath where one stands in many of the popular basins of the park, like the Old Faithful area, Midway Geyser Basin, Lower Geyser Basin, and West Thumb. However, this may stimulate a follow-up question: “What do you mean it’s beneath me? Why or how is it beneath me?”

Yellowstone is a caldera, or a volcanic depression caused by the collapse of an underground magma chamber after a massive eruption emptied that chamber. The actual outline of Yellowstone caldera is immense—it is an irregular oval shape covering approximately 67 x 42 kilometers (42 x 26 miles), oriented in a northeast-southwest fashion. The caldera is so large that a popular misconception has arisen that it was only recognized when satellites beamed back pictures of the area from space, but this is not correct—geologists were able to map out the gigantic structure starting in the 1950s.  The entire caldera is considered “the volcano,” and instead of having a single edifice or centralized series of vents, it includes multiple, complex vent structures spread throughout its area. The most recent caldera-forming eruption 631,000 years ago occurred through these vents, which emptied the underlying magma chamber and caused the collapse of Yellowstone Caldera.

Over the last 17 million years, the North American tectonic plate has traveled over a mantle plume that expresses itself at the surface as hotspot volcanism. This hotspot volcanism has left behind a series of calderas that created the Snake River Plain. At one end of the plain is the McDermitt Caldera (about 17 million years old) on the border of southeastern Oregon and northwestern Nevada. At the other end is an area we call the Yellowstone Plateau Volcanic Field that contains three caldera systems: Huckleberry Ridge (2.1 million years old), Henry’s Fork (1.3 million years old), and Yellowstone (631,000 years old). In essence, there are three huge volcanic structures on the Yellowstone Plateau Volcanic Field, and two (Yellowstone and Huckleberry Ridge) are predominantly within the boundaries of Yellowstone National Park. Only within Yellowstone Caldera, however, do we see activity in the form of ground deformation, gas emissions, earthquakes and geothermal processes. The Henry’s Fork and Huckleberry Ridge calderas are “extinct.”

The formation of a caldera produces a sequence of large, curved faults, commonly known as ring faults, around crustal blocks that have collapsed into underlying void spaces. In addition, resurgent domes form where post-collapse magma intrusion causes uplift, and this results in further faulting of the crater-like landscape. After a caldera-forming eruption, subsequent intrusions of magma may exploit these faults to produce additional, smaller eruptions. For example, the formation of Yellowstone Caldera was followed by voluminous rhyolite lava flows sourced from vents along many of these faults. The most recent of these post-caldera eruptions, the Pitchstone Plateau flow, occurred about 70,000 years ago. Although the volumes of some individual rhyolite flows at Yellowstone are similar to those of smaller stratovolcanoes (up to ~70 km³), each is considered to be an individual vent of the greater volcanic system.

The Yellowstone volcano is a caldera with multiple eruptive vents distributed across its area. This same phenomenon is seen at nearly every caldera volcano around the world. In the United States alone, calderas with multiple vents and post-caldera features include Valles Caldera in New Mexico and Long Valley Caldera in California. The systems are so immense that it can be difficult to appreciate their scale.  When someone at Old Faithful asks, “Where is the volcano?” the answer is “Right beneath you, spanning across the park roughly from the Madison Junction until around halfway through Yellowstone Lake.  It encompasses many of our favorite geyser basins, from Lower, Midway, and Upper, through West Thumb and Mud Volcano!”