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.

When discussing the size of an earthquake, it is common to refer to its magnitude—a single number that describes an earthquake’s strength.  Earthquake magnitude is determined instrumentally from data recorded by seismometers and provides a quantitative way to compare earthquakes regardless of their depth, geological conditions, and other factors.

It would be nice if there were a similar scale for volcanic eruptions—a quantitative way of describing their size.  But what should such a scale be based upon?  The volume of erupted products?  The height of the eruption plume?  The area over which ash falls?  The eruption duration?

Unfortunately, there are no consistent instrumental means of determining an eruption size in the same way that earthquake magnitudes are calculated.  But there is a semi-quantitative eruption magnitude scale—the Volcanic Explosivity Index, or VEI.

The VEI scale was proposed in 1982 by volcanologists Chris Newhall and Steve Self.  They recognized the need for a way to quickly and easily describe explosive volcanic eruptions and their impacts, but also saw the challenge in determining the measurements that best represent the size of a volcanic eruption and that data for historical eruptions can vary in quantity and quality.  Some eruptions might have excellent records of ash volume and plume height, whereas others might not have left a recognizable deposit and be known only from the ambiguous written accounts of a distant observer.  To address this variability, Newhall and Self decided that it would be best to utilize numerous different datasets to describe the size of explosive eruptions.

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A primary quantitative criterion for assigning a VEI is the volume of erupted ash.  Like the earthquake magnitude scale, each VEI scale is 10 times different in terms of ash volume (except for VEI 1, which spans two orders of magnitude, since those eruptions are very small in volume).  For example, a VEI 5 eruption might have an ash volume of about 1 cubic kilometer (0.24 cubic miles)—the 1980 eruption of Mount St. Helens is an example.  A VEI 4 eruption would be closer to 0.1 cubic kilometers (0.024 cubic miles), like the 2021 eruption of La Soufrière on the island of St. Vincent in the Caribbean, while a VEI 6 eruption would have a volume near 10 cubic kilometers (2.4 cubic miles), like the 1991 eruption of Pinatubo in the Philippines. 

The eruption volume is not a rule—other factors are also important, like the duration of the eruption.  As an example, take Paracutin, Mexico—the volcano that was famously “born in a Mexican cornfield” in 1943.  The eruption explosively ejected 1.3 cubic kilometers (0.31 cubic miles) of ash and rock fragments, which would seem to qualify it as a VEI 5 eruption (there was a similar amount of lava erupted, but lava is not considered in the determination of VEI). However, since that volume came out over 9 years and the activity was never violently explosive, the eruption is classified as VEI 4.  Of course, if the duration were unknown, it might be classified as VEI 5, given the deposit volume.  This highlights a challenge with employing the VEI scale—modern eruptions are assigned VEI values based on deposit volumes plus observations, while prehistoric eruptions are known only from the volumes and distributions of their deposits.

There are also no hard and fast rules when it comes to the relation of plume height to VEI, since the altitude of an ash column depends on many factors, including plume water content and volcano latitude.  Occasionally the plume height and ash volume are inconsistent in terms of the implied VEI—the spectacular January 2022 eruption of Hunga Tonga-Hunga Haʻapai is a great example.  The extraordinary height of that eruption plume wasn’t a good indication of the VEI because the volume of ash erupted into the atmosphere was more similar to that of a smaller eruption. This is where volcanological expertise and consensus are needed to establish a VEI that is useful for comparative purposes.  For Hunga Tonga-Hunga Haʻapai, the 2022 eruption was assigned as VEI 5, although even this value is still considered preliminary because it includes insights from ocean-floor surveys that have yet to be comprehensively analyzed.  This eruption is unique so far in modern experience in terms of the column height, and application of VEI to this type of explosion is clearly problematic.

Eruptions that are strictly lava flows or lava dome activity with no explosions are classified as VEI 0 or, if there is a small explosive component—like lava fountaining or minor ash emissions—could be VEI 1.  These describe most eruptions in Hawaiʻi, except in the event of massive lava fountains or ash-rich explosive eruptions with tall plumes.

The biggest explosive eruptions—so-called “super eruptions”—are VEI 8 or, in the case of the Toba (Indonesia) eruption 74,000 years ago, possibly VEI 9.  These are epically big eruptions, with ash-deposit volumes in excess of 1000 cubic kilometers (240 cubic miles) and ash plumes that reach the stratosphere, usually occurring over many hours to a few days.  Although, again, since no VEI 8 or 9 eruption has ever been observed, the duration of such eruptions is unknown.

These rare and very large eruptions are what has made Yellowstone famous.  The biggest Yellowstone eruptions, like the one that formed Yellowstone Caldera 631,000 years ago, were VEI 8 eruptions because they emitted at least 1000 cubic kilometers (240 cubic miles) of explosive products and had massive stratospheric ash plumes, as indicated by the widespread ash deposits.  There is some evidence, though, that at least some of the Yellowstone VEI 8 eruptions might have been composed of multiple smaller eruptions, with considerable time gaps—up to decades!—in between.  Perhaps, rather than a single VEI 8 eruption, some of these eruptions were actually a combination of multiple VEI 7 events?  Additional geologic field work is needed to address this question.

The VEI scale incorporates many factors and can be challenging to apply to some eruptions—especially those that were not observed—but it remains a useful tool to compare the relative sizes and impacts of volcanic events.