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The Yellowstone volcanic system has hosted some very large eruptions. But there have been much larger volcanic explosions in geologic history—including several in the western USA!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

La Garita Mountain (elevation 4179 m [13711 ft]), Colorado
La Garita Mountain (elevation 4179 m [13711 ft]), Colorado.  The mountain is a resurgent block of Fish Canyon Tuff that is more than 1 km (0.6 mi) thick—the top is eroded and the base is not exposed.  The tuff formed during the eruption of La Garita caldera about 27.8 million years ago and has a volume of more than 5000 km3 (1200 mi3)—one of the largest eruptions known on Earth.  USGS photo by Peter Lipman, 1986.

The Yellowstone volcanic system is well known for being the source for some of the largest volcanic eruptions on Earth.  About 631,000 years ago, an eruption about 1,000 times bigger than the 1980 Mount St. Helens eruption resulted in the formation of Yellowstone caldera.  Before that, 2.1 million years ago, the Yellowstone system was the site of an even larger eruption, about 2,500 times larger than that of Mount St. Helens in 1980!  Caldera-forming eruptions from the Yellowstone hotspot system have occurred repeatedly over the past 16 million years or so, with several of these eruptions being larger than that which formed Yellowstone caldera.  The Snake River Plain, in southern Idaho, contains evidence for buried calderas that were the sites of these massive explosions.

Yellowstone is not the only place on Earth where large eruptions have occurred.  Caldera-forming eruptions have taken place across the globe from volcanic systems that are similar to Yellowstone.  So how does Yellowstone rank in the geologic history of explosive volcanic eruptions?

It may surprise you to know that there have been several eruptions that were much bigger—including in geologically recent times when humans were on the planet!

But first, a definition.  The general threshold for defining the largest volcanic eruptions—so-called “super eruptions”—is 1000 cubic kilometers (km3), or 250 cubic miles (mi3), also defined as “8” on the Volcano Explosivity Index (VEI) scale.  This was the size of the eruption that formed Yellowstone caldera, and it is equivalent to 400 million Olympic swimming pools, 400,000 Great Pyramids of Giza, and 1/4 the volume of the Grand Canyon.

View looking northeastward across Long Valley Caldera to Glass Mountain
View looking northeastward across Long Valley Caldera to Glass Mountain, about 30 km away. Image from USGS Professional Paper 1692.

And second, a caveat.  Assessing the size of a past volcanic eruption is not easy.  These calculations use the distribution of volcanic material to determine the volume of the source eruption.  Sometimes that material is not well preserved, may have fallen in places that are difficult to access (like the ocean), or may also be buried beneath more recent deposits.  In addition, the farther back in time you look, the more likely it is that volcanic material will have been reduced or reshaped by erosion.  So there can be a high degree of uncertainty when it comes to deposit volumes, and therefore eruption sizes.

With that out of the way, it is impossible to select one single eruption as definitively the largest in terms of volume.  But we can point out three that vie for the title and that were at least 5 times bigger than the eruption that formed Yellowstone Caldera.

The most recent of these massive eruptions occurred from Toba, Indonesia, only 74,000 years ago and had a volume of about 5300 km3 (1272 mi3).  For many years, this eruption was suspected as causing the near-extinction of humanity, but subsequent research has cast doubt on this hypothesis.

Aerial view of Lake Taupo, New Zealand
Aerial view shows Lake Taupō, New Zealand, looking to the southwest. This lake fills the caldera of a volcano that continues to alter the surrounding seismic and geothermal landscape. The Ruapeho and Tongariro volcanoes are visible in the background.

Another massive explosive eruption occurred in Colorado 27.8 million years ago.  The formation of La Garita caldera deposited the Fish Canyon Tuff, which has an estimated volume of 5000 km3 (1200 mi3).

Also in the western USA, the Wah Wah Springs caldera, in southern Utah, erupted roughly 30 million years ago with a volume of about 5900 km3 (1415 mi3).  This was the largest of several massive eruptions that occurred in that area at about that time.

The most recent “super eruption” on Earth was from the Taupō volcanic system on the North Island of New Zealand.  About 26,500 years ago, an eruption with a volume of approximately 1200 km3 (288 mi3) occurred there.

As you can see, the formation of Yellowstone caldera 631,000 years ago isn’t the record holder in terms of global eruptions.  It’s not even the largest from the Yellowstone hotspot system!  But the eruption was still bigger than many, including those that formed other well-known calderas, like Campi Flegrei in Italy about 40,000 years ago (200–300 km3 [48–72 mi3]), Long Valley caldera in California about 760,000 years ago (650 km3 [156 mi3]), and Valles Caldera in New Mexico about 1.1 million years ago (300 km3 [72 mi3]).

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