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 the subject of volcanoes comes up, you’ll often hear the words “active,” “dormant,” and “extinct.”  But what precisely do they mean?  Admittedly, use of these terms by volcanologists has been somewhat inconsistent and evolved with time, which has resulted in understandable confusion among non-volcanologists.  A simple search for the terms online produces a number of contradictory results!  So let’s try to clear this up, and describe the terms in the way that most volcanologists would use them (although we’ll note right off the bat that opinions vary, and not all volcanologists would wholeheartedly agree with the definitions below).

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We’ll start with perhaps the most confusing of these terms: “active.”  Most volcanologists would say that a volcano or volcanic field that has erupted within the Holocene (the current geologic epoch, which began at the end of the most recent ice age about 11,650 years ago), or that has the potential to erupt again in the future, should be considered “active.”  This does not mean that the volcano is erupting right now, although such a volcano would certainly qualify as “active.”  Rather, it means that the volcano has a youthful magmatic system, and if it is not erupting now, it could be in the future.

A case could be made for subdividing the “active” category into “actively erupting” and “potentially active.”  Volcanoes like Kīlauea in Hawaiʻi, Stromboli in Italy, Erebus in Antarctica, and Santiaguito, in Guatemala, would be considered “actively erupting,” since magma is reaching the surface in all those places (and many others!) right now.  Non-erupting volcanoes that have experienced eruptions in the Holocene would then be considered “potentially active”—examples include Mount Hood in Oregon, Cotopaxi in Ecuador, Fuji in Japan, and Katla in Iceland.

This is where the “dormant” term comes into play.  Volcanologists use that term as shorthand for “potentially active,” so a “dormant” volcano is one that is not erupting now, but that is considered “active” because it could erupt in the future.  Mount Hood is an active volcano that is currently dormant.  Kīlauea is an active volcano that is currently erupting.  Some volcanologists add qualifiers to these terms—for example, a “long-dormant system” might refer to a volcano that has not erupted in thousands of years, while a “recently active system” might have erupted a few years or decades (or even centuries!) ago.

Even among “dormant” volcanoes there could be a sub-category for those that are “restless”—these are volcanoes that are not erupting, but that are experiencing some signs that magma is accumulating or moving beneath the surface.  Mauna Loa, in Hawaiʻi, could qualify for this category, since it experiences ground inflation and earthquakes that are a testament to changes within the volcano’s magma chamber.  So Mauna Loa is “active” and “restless” but not erupting—at least, not yet.  It will at some point, which is why monitoring restless volcanoes is so vital for forecasting their behavior on timescales that are helpful to society.

Clear as mud?  Well, at least “extinct” is easy, right?  These are volcanoes that will not erupt again.  They’re dead, Jim.  Except that it’s not unheard of for volcanoes that are considered “extinct” to occasionally erupt!

For some volcanoes, it is easy to say with confidence that they are extinct.  Take Mount Thielsen, in Oregon.  That mountain is all that remains of a volcano that might have once looked like Mount Hood does today, but it last erupted about 300,000 years ago, and since that time erosion has worn away much of the ancient volcano’s cone.  Today, the mountain is cold and quiet.  Volcanism has moved on from Mount Thielson, and that particular mountain will never erupt again.

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There are many volcanic systems, however, that can go quiet for thousands of years and then reawaken.  Mount St. Helens, for example, was dormant from about 11,000 years ago to about 4,000 years ago, after which time it has erupted very frequently.  For some of these volcanoes, it may be that we don’t know enough about their geologic history to say with certainty whether they have erupted in the Holocene—the data don’t exist to say one way or the other.  These status terms depend heavily on knowledge of the geological deposits of volcanoes, which are a window into their eruption histories, so often additional research is needed before any assessments can be made with confidence.  But in other cases, it might just be that a particular volcano experiences long periods of quiet between its eruptions.  The Holocene cutoff is completely arbitrary, after all, and the Smithsonian’s Global Volcanism Program (which has a website at https://volcano.si.edu/ that we highly recommend and that includes information about all currently erupting volcanoes worldwide, as well as those that are active but not erupting) has expanded their catalogs to cover all volcanoes that have erupted as far back as the Pleistocene geological epoch, which started about 2.5 million years ago, to better capture all volcanoes that might still be active.  For this reason, it is important to pay attention to the characteristics of the volcano itself.  If there is evidence for magma beneath the ground—for example, as indicated by magma-related seismicity and/or ground deformation, or by an active hydrothermal system—then the volcano should be considered “active” but currently “dormant,” regardless of the time since its last eruption.

And this is the category into which volcanic systems like Yellowstone fall, as well as the other two caldera systems in the USA: Long Valley (California) and Valles (New Mexico).  Yellowstone last erupted about 70,000 years ago, while Long Valley’s most recent activity was about 16,000–17,000 years ago, and Valles last erupted about 68,000 years ago.  All three might be considered “extinct,” since none have erupted in the Holocene, but we know that magma is present beneath all three caldera systems because of geophysical unrest (including seismicity and ground deformation) and/or hydrothermal activity.  The sheer size of these systems is also important, since big volcanic systems tend to experience long periods of inactivity between eruptions.  They are therefore considered active but dormant.

There you have it!  Hopefully this terminology is now active within your head.  Over time, the knowledge might go dormant in your brain, but it could spring back to life when you need it.  Just don’t let it go extinct!