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[Photo: Scenic Yellowstone]
Yellowstone simmers and seethes in an
ever-changing spectacle of water, steam, gas, and mist. Beneath the ground, a huge mass of magma and hot rock powers a complicated plumbing system. Water seeps down, is heated, and circulates back to the surface, producing thousands of hot springs and geysers.


The North American Plate has been moving slowly over this hot spot, generating volcanic eruptions. When you visit Yellowstone Park, you drive through calderas formed by three titanic eruptions.

  [Photo: North American Plate]

Above and Below Yellowstone

Basaltic magma rising from a hot spot beneath Yellowstone accumulates deep in the crust. It heats up local rocks, which melt to form viscous, water-rich magmas. Three times in the past 2.1 million years, large batches of these magmas have erupted explosively, forming huge calderas.

  [Photo: Cross section]

Before Yellowstone

Large eruptions above the Yellowstone hot spot began about 16 million years ago, when present-day Idaho was above it. Since then, steady motion of the North American Plate has produced a trail of volcanoes that extends from southeastern Oregon to Yellowstone National Park nearly 800 km (500 mi).

  [Photo: Hot Spot Trail]

Yellowstone’s Explosive Past

In the last 2.1 million years, three major eruptions have occurred in and near Yellowstone National Park - one about every 600,000 years. These eruptions were of awesome magnitude, dwarfing any in historical times, and produced three overlapping calderas.

  [Photo: Satellite image]

Truly Colossal Eruptions

[Graphic: Volumes of Three Major Eruptions]

These cubes represent the volumes emitted by Yellowstone's three major eruptions. As you can see, they dwarfed the 1980 eruption of Mount St. Helens in Washington State.

Showers of Ash

Airborne ash from each of Yellowstone's three major eruptions blanketed much of the western and central United States.

  [Graphic: Extent of Ash Falls from Yellowstone Eruptions]

Steamy Present, Uncertain Future

The magma accumulating beneath Yellowstone today may erupt sometime in the future. Such an eruption could cover Denver with 20-30 cm (8-12 in) of ash and devastate much of the central and western United States. Should you keep your car engine running while you visit the park? Not really. Large eruptions are rare. Besides, scientists closely monitor earthquakes and ground deformation in Yellowstone for warning signs that usually precede even small eruptions.

  [Photo: Old Faithful]

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits