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.

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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.

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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.

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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]).