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Questions About Yellowstone Volcanic History

Answers to questions about volcanism at Yellowstone.

When was the last time there was volcanism at Yellowstone?

The most recent volcanic activity consisted of rhyolitic lava flows that erupted approximately 70,000 years ago. The largest of these flows formed the Pitchstone Plateau in southwestern Yellowstone National Park.

How much volcanic activity has there been at Yellowstone since the most recent giant eruption?

Since the most recent giant caldera-forming eruption, 631,000 years ago, approximately 80 mostly nonexplosive eruptions have occurred. Of these eruptions, at least 27 were rhyolite lava flows in the caldera, 13 were rhyolite lava flows outside the caldera and 40 were basalt vents outside the caldera. Only a few of these eruptions are associated with significant deposits of ash, indicating an explosive component.  The largest of these explosive events, about 173,000 years ago, was similar in size to the one that created Crater Lake in Oregon, and it resulted in the formation of a collapse caldera that is now occupied by the West Thumb of Yellowstone Lake. The most recent volcanic eruption at Yellowstone, a lava flow on the Pitchstone Plateau, occurred 70,000 years ago.

How often do volcanic eruptions occur at Yellowstone?

Three extremely large explosive eruptions have occurred at Yellowstone in the past 2.1 million years with a recurrence interval of about 600,000 to 800,000 years. More frequent eruptions of basalt and rhyolite lava flows have occurred before and after the large caldera-forming events. For example, scientists have identified at least 27 different rhyolite lava flows that erupted after the most recent caldera eruptions, about 631,000 years ago, from vents inside the caldera. The most recent was about 70,000 years ago. Many of these eruptions were separated in time by several tens of thousands of years. Because the evidence of earlier eruptions may have been either buried or destroyed, we do not really know how often the volcano has actually erupted.

What was the extent of ash deposition from the largest Yellowstone eruptions?

Map of the known ash-fall boundaries for several U.S. eruptions
Map of the known ash-fall boundaries for major eruptions from Long Valley Caldera, Mount St. Helens and Yellowstone. (Public domain.)

During the three giant caldera-forming eruptions that occurred between 2.1 million and 631,000 years ago, tiny particles of volcanic debris (volcanic ash) covered much of the western half of North America, likely a third of a meter deep several hundred kilometers from Yellowstone and several centimeters thick farther away. Wind carried sulfur aerosol and the lightest ash particles around the planet and likely caused a notable decrease in temperatures around the globe.

How many caldera-forming eruptions have occurred from the long-lived hotspot that is currently beneath Yellowstone?

Many eruptive units found along the path of the Yellowstone hotspot have been dated, but only a few of them represent large caldera-forming eruptions. At least five volcanic fields centered on large caldera complexes have been identified. Some of these caldera complexes erupted climatically more than once; probably 15 to 20 caldera-forming eruptions have occurred along the hotspot as it left a trail from western Idaho to Yellowstone within the past 16.5 million years.

How many giant eruptions have occurred in the Yellowstone National Park region and how large were they?

Graphic depicting the path left by the Yellowstone hotspot as the c...
Map showing the path of the Yellowstone hotspot. (Public domain.)

Volcanic activity began in the Yellowstone National Park region a little before about 2 million years ago. Molten rock (magma) rising from deep within the Earth produced three cataclysmic eruptions more powerful than any in the world's recorded history. The first caldera-forming eruption occurred about 2.1 million years ago. The eruptive blast removed so much magma from its subsurface storage reservoir that the ground above it collapsed into the magma chamber and left a gigantic depression in the ground- a hole larger than the state of Rhode Island. The huge crater, known as a caldera, measured as much as 80 kilometers long, 65 kilometers wide, and hundreds of meters deep, extending from outside of Yellowstone National Park into the central area of the Park (see map in question above for location information).

Later, activity shifted to a smaller region within the Island Park area of eastern Idaho, just southwest of Yellowstone National Park, and produced another large caldera-forming eruption 1.3 million years ago. Subsequent activity has been focused within the area of the National Park, and another huge eruption 631,000 years ago formed the Yellowstone caldera as we now see it.

The three caldera-forming eruptions, respectively, were about 6,000, 700, and 2,500 times larger than the May 18, 1980 eruption of Mt. St. Helens in Washington State. Together, the three catastrophic eruptions expelled enough ash and lava to fill the Grand Canyon.

In addition to the three climactic eruptions, activity associated with each of the three caldera cycles produced dozens or even hundreds of smaller eruptions that produced both lava and pyroclastic materials.

How do the giant eruptions in the Yellowstone National Park region compare to other large historic eruptions?

Volumes of Yellowstone's giant volcanic eruptions compared with vol...
Volumes of Yellowstone's giant volcanic eruptions compared with volumes of other major eruptions. Graphic used with permission from "Windows into the Earth, The Geologic Story of Yellowstone and Grand Teton National Park", Robert B. Smith and Lee J. Siegel, Oxford University Press, 2000.

This graphic shows that the three largest Yellowstone eruptions emitted much more material than the eruptions of Mount St. Helens (1980), Mount Pinatubo (1991), Krakatau (1883), Mount Mazama (7,600 years ago), and Tambora (1815).