On February 24th, 2022, the alert level at the Mount Aso supervolcano in Japan was raised
from yellow to orange by government scientists, corresponding to a change from level 2 to
3 in the local 5 level alert system.
While the threat of a supervolcano erupting might sound cataclysmic, in this case there
really isn't too much to worry about outside of the immediate vicinity of its summit.
The vast majority of volcanic eruptions from supervolcanoes contrary to popular belief
are quite small.
For example, in Japan a different supervolcano to the south has been erupting near continuously
since 1955.
There, the stratovolcano known as Sakurajima has produced frequent vulcanian explosive
eruptions, ejecting bombs of lava several kilometers into the air and producing short
distance pyroclastic flows.
Sakurajima represents the youngest vent of a massive 23 kilometer wide caldera that formed
in a supermassive eruption 29,000 years ago.
Looking at Mount Aso from satellite, you can see an oval shaped depression in the ground
which is now filled with houses and farms.
This is in fact a large caldera, formed from several prehistoric supermassive eruptions.
4 ancient supermassive eruptions created this 25 kilometer long and 18 kilometer wide caldera,
the largest of which was more than 80 times the size of the 1991 eruption of Mount Pinatubo.
The last of these large eruptions occurred 87,000 years ago, which covered areas as far
away as Tokyo in ash.
Luckily, in the last 10,000 years the Mount Aso complex has only produced small to moderately
sized eruptions.
These recent eruptions have all originated from a series of a half dozen vents in the
center of the complex, forming a group of cinder cones and explosion craters.
These vents exist on an uplifted area which is caused by the gradual refilling of its
underlying massive magma chamber in what is termed a "resurgent dome".
Since 1826, this resurgent dome has produced 95 distinct eruptions which all originated
from the same vent.
This vent is called Naka-dake, which could be best classified as a group of overlapping
explosion craters with an acidic crater lake in its center.
In recent days the ground underlying Naka-dake has produced an exponentially increasing number
of earthquakes and volcanic tremor events caused by the movement of magma.
These earthquakes are almost identical to those witnessed before an eruption in October
of 2021, which produced pyroclastic flows that travelled up to 1500 meters distant.
Also, the crater lake temperature has greatly increased, leading to the emission of thick
white plumes of steam even visible from satellite.
Based on the available evidence, an eruption from Aso's Naka-dake in the next month is
a near certainty, which will most likely occur via a series of phreatomagmatic explosions,
the first of which is most likely to be the largest.
Phreatomagmatic explosions involve a body of magma interacting with groundwater or in
this case water in a crater lake, which caused a series of powerful explosions to occur,
ejecting primarily steam but also small fragments of lava and ash.
These explosions can eject boulders of rock as large as cars while simultaneously releasing
pyroclastic flows that travel several kilometers distant.
Thus, you would not want to be anywhere near Naka-dake's crater when it erupts.
Technically speaking, the stated alert level of 3 carries with it an order to stay outside
a 2 kilometer radius of the crater which is expected to erupt.
However, I personally recommend doubling this distance to 4 kilometers, as Aso's last eruption
in 2021 came a bit too close for comfort to a group of several hundred tourists in a parking
lot marked on screen.
I do not want this area to be placed in danger again, as it should be closed off whenever
an eruption is expected.
If a new eruptive sequence does begin, it could last as short as a few hours or continue
with a series of intermittent explosions for as long as 2 years.
Only time will tell, and we simply do not know what to expect at this time.
Thanks for watching.
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