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Yellowstone is a geological wonderland, home to geysers, hot springs, and stunning landscapes, and subject to dynamic forces that result in near constant earthquake activity and ground deformation. But what is driving this activity? What is the source of Yellowstone's heat? New research suggests that Yellowstone's heat engine originates deep within the Earth.

Yellowstone is a hotspot—an area of anomalously high temperatures and some melting within Earth's upper mantle (the layer between the crust and the core that makes up the bulk of Earth's volume). The intense heat generated by the hotspot causes melting of the crust, forming basaltic and rhyolitic magma. Rhyolite is extremely high in silica content, which makes it very viscous and prevents gas from escaping the magma. This means that when large amounts of rhyolite accumulate within the crust, large explosive eruptions can result.

Shaded relief map showing the path of the Yellowstone hotspot. Yellow and orange ovals outline past caldera eruptions during the time periods indicated (orange calderas are the most recent). The calderas progress from oldest in northern Nevada to youngest in Yellowstone National Park as the North American plate passed over the relatively stationary hotspot. Black lines indicate faults. Figure is from the book "Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks," by Robert Smith and Lee Siegel. Used with permission.

The Yellowstone hotspot has a history of such large eruptions. A track of calderas stretches from northern Nevada and eastern Oregon to Yellowstone National Park. The oldest caldera erupted 16 million years ago, and the calderas get younger to the northeast, with Yellowstone caldera being the most recent at 631,000 years old.

The hotspot is fixed within Earth's mantle, and the tectonic plates that make up Earth's surface move over its top. The North American plate is moving to the southwest at about 2.5 cm/yr (1 inch per year) relative to the stationary hotspot, which is why the eruptive centers get younger to the northeast.

The Yellowstone hotspot has long been suspected to be part of a mantle plume—a region of the mantle that is hot but still solid and that is buoyantly upwelling. Mantle plumes may originate from the boundary between Earth's mantle and core, nearly 3000 km (about 1850 mi) beneath the surface. They are suspected in several places around the planet, like Hawaii, the Galapagos, and Iceland.

For many years, evidence for a plume beneath Yellowstone has been difficult to identify. The chemistry of helium gas released at Mud Volcano, near Hayden Valley in Yellowstone National Park, is indicative of a deep mantle origin, but that evidence is not conclusive.

The speeds of earthquake waves within the Earth could offer evidence for the presence of a plume because seismic velocities are expected to decrease in warm areas. Unfortunately, studies of seismic wave speeds, called seismic tomography, have not had the resolution needed to identify a plume deep beneath Yellowstone. Until recently, the most detailed tomography, produced by scientists from the University of Utah and their collaborators, was able to "see" only the upper few hundred km of the mantle.

A new tomography study by scientists from the University of Texas used earthquake waves that passed through Earth's outer core to improve the resolution of seismic velocity deep within the Earth. They found evidence for a plume that is 350 km (about 220 mi) in diameter extending from the core-mantle boundary all the way to the base of the crust at Yellowstone. The suspected plume is tilted to the northeast, probably due to circulation patterns within the mantle. While these data are subject to some interpretation, they provide a good starting point for further investigations of the source of Yellowstone's heat.

Depth cross section through the Earth, form the surface to the core-mantle boundary. The location of the section is shown by the purple line on the map. Warm colors indicate slower seismic velocities, which are areas of higher temperature. A plume of hot material originates at the core-mantle boundary and extends upward to the Yellowstone hotspot (marked by the volcano symbol at the surface). Figure is from the manuscript "Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves," by Peter Nelson and Stephen Grand and published in Nature Geoscience. Used with permission.

This new finding does not mean that Yellowstone is any more or less likely to erupt. Instead, it provides a clearer picture of the heat engine that drives the activity that we see at the surface, from geysers to earthquakes. But this result is far from the final word on the topic. Much more work is needed to confirm the existence of the plume and to understand the subtleties of how the plume relates to volcanism at the surface. Ultimately, the new research will enable even more detailed studies into the structure at all levels beneath Yellowstone.

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