We currently have a supervolcano problem on our hands.
And no, I am not referring to the increased rate of ground deformation occurring at Italy's
Campi Flegrei volcano, which in recent years has sometimes been called a supervolcano.
Rather, the problem relates to all volcanic complexes which are classified as a supervolcano.
Specifically, us scientists cannot agree with what exactly the term means, despite there
being a literal definition in place.
As, while the term supervolcano has been used in literature since 1949, it was not until
the beginning of the 21st century that the term became popularized.
At first, supervolcanoes only referred to volcanoes which at some point in their history
had generated at least 1 VEI 8 or larger eruption, meaning at least 1 eruption occurred that
ejected 1,000 or more cubic kilometers of tephra, aka ash, pyroclastic flow deposits,
and ignimbrite.
This meant that volcanoes such as Yellowstone in Wyoming, Toba in Indonesia, and Taupo in
New Zealand were classified as supervolcanoes.
Yet, over the years, despite there being a literal definition in place, the word seemingly
changed in meaning not in the scientific sense but rather as the apparent working definition
people used.
Several years later, the word supervolcano was also used to describe volcanoes which
had produced an eruption that ejected at least 500 cubic kilometers of tephra and had at
least 1,000 cubic kilometers of molten rock in its underlying magma chamber.
In other words, the term supervolcano now meant any volcano that had produced a very
large caldera forming eruption and had the potential to produce a supereruption, now
including volcanoes such as the Long Valley Caldera.
Yet, definition creep continued, and soon all volcanoes that had produced at least 1
eruption that ejected 500 cubic kilometers of tephra were considered supervolcanoes,
such as the Tondano volcano in Indonesia or even the Valles Caldera in New Mexico.
Then, a few reports were published that called Campi Flegrei a supervolcano, seemingly indicating
that the term supervolcano now applied to any volcano which had at least once produced
an eruption that ejected more than 300 cubic kilometers of tephra.
If the most recent working definition was correct, then the planet would have 13 active
supervolcanoes.
Yet, if we were to only go with the traditional, original definition, there would only be 7
active supervolcanoes.
So, exactly how many active supervolcanoes are there?
To the people in my audience, you might be confused why I and some other scientists simply
don't go back to the original definition.
While this would be easy, the original meaning of the term had a few problems.
For example, are we to consider a volcano which ejected 999.9 cubic kilometers of tephra
in a single eruption not as a supervolcano just because it is one hundredth of a percent
short of a seemingly arbitrary threshold of 1,000 cubic kilometers?
Additionally, volcanoes such as Campi Flegrei, the Valles Caldera, and Yellowstone really
do seem to behave differently than simple volcanoes with a slightly smaller yet still
large caldera such as the Fisher Caldera, Rabaul, Aniakchak, Crater Lake, or Mount Tambora.
One example includes the ability for purported supervolcanoes to undergo very high levels
of seismic unrest without an eruption occurring.
Take for example the ongoing 78 centimeters a year uplift and thousands of earthquakes
a year at Italy's Campi Flegrei volcano.
If I was to see that same amount of uplift and earthquake activity at the Crater Lake
volcano in Oregon, I'd say "it looks like a volcanic eruption is going to occur".
Yet, Taupo, Campi Flegrei, and Yellowstone have produced multiple such instances of unrest
within an order of magnitude to what was just described in the last 300 years and a volcanic
eruption was never even close to happening.
Another difference between what are considered to be supervolcanoes when compared to normal
volcanoes which have produced large eruptions in the past is the inclusion of a feature
termed as a resurgent dome.
In resurgent domes, without a volcanic eruption necessarily occurring, the ground following
a caldera collapse is pushed upwards by as much as 100s of meters as magma refills the
center of the underlying magma chamber over thousands to hundreds of thousands of years.
Yet, this may not be a perfect criteria for a supervolcano as Japan's Aira caldera does
not to the best of my knowledge contain a large resurgent dome, while some smaller volcanoes
like Iwo Jima do contain a resurgent dome.
Thus, I propose three solutions on what to classify as a supervolcano which are as follows.
They include the original definition, the definition I had previously used, and a third
proposal I have.
For volcanic complexes which are just left out of any specific one of these criteria,
I suggest calling them "large caldera complexes" or "LCCs" if they have a caldera with an area
of greater than 150 square kilometers and have produced multiple explosive eruptions
which ejected at least 50 cubic kilometers of tephra.
So, what do you think of these potential proposals, and which one do you think is best?
Be sure to let me know in the comments section below.
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