Since March 25th, 2017, the Sakurajima volcano in southern Kyushu within Japan has been continuously
erupting.
Its eruptions have largely consisted of a series of so called "vulcanian" explosions,
where a small plug of lava or volcanic material is cleared once every few hours, generating
a large explosion and on occasion pyroclastic flows.
Generally speaking, 100 to 1000 of these explosions occur each year, so Sakurajima is mostly kept
at an alert level of 3 on a 1 to 5 scale.
This carries with it a 2 kilometer exclusion zone as this radius well represents the typical
maximum distant bombs of lava and boulders of rock thrown by explosions can travel along
with occasional pyroclastic flows.
However, on July 24th something changed.
At 8:05 pm local time, Sakurajima produced a powerful explosion, sending a plume of ash
to a height of 3,000 meters or 9,842 feet.
More importantly, this explosion was unusually powerful, causing large chunks of rock which
were up to the size of boulders to fall up to 2,500 meters distant.
Since a number of people live very close to or within this radius, and the threat of a
far more powerful explosion exists, Sakurajima's alert level was raised from 3 to 5.
This was the first time Sakurajima's alert level had been raised to the maximum level
since the current Japan volcano rating system was implemented.
Scientists immediately ordered evacuations for residents within 3 kilometers of the currently
erupting Minamidake and secondary Showa summit craters.
This meant that several areas of dense structures were evacuated, including parts of Arimura
and Furusato towns.
In addition, residents in the densely populated city of Kagoshima City and Kagoshima prefecture
were advised to remain cautious as both areas could be affected by falling volcanic ejecta
if an even larger explosion does occur.
For context, the Sakurajima volcano is located in southern Japan in south central Kyushu.
It completely formed in the last 13,000 years, constructing two separate peaks.
While the northern Kitadake cone last erupted 4850 years ago, the southern Minamidake and
adjacent Showa Craters have since been the site of recent activity.
Zooming out, you might note that the Sakurajima volcano is located within a circular ridge
in the northern part of Kagoshima Bay.
This is not a coincidence, at the northern part of the bay represents the outline of
2 large calderas.
The larger of these calderas is 19 kilometers wide, and formed in a supereruption which
occurred 29,000 years ago.
That eruption covered much of the island of Kyushu in a thick layer of ash and pyroclastic
flow deposits, causing its underlying magma chamber to be largely drained.
This caused overlying rock to collapse downwards like a sinkhole, forming a massive caldera.
In total, that ancient eruption ejected 400 cubic kilometers of volcanic rock, which was
around 40 to 60 times larger than the January 2022 eruption of Hunga Tonga Hunga Ha'apai.
Thus, since Sakurajima is merely the youngest cone of the broader Aira caldera, it can accurately
be described as a supervolcano which happens to be erupting.
It just so happens that its eruptions are generally quite small, and is unlikely to
collapse into another caldera anytime soon.
However, Sakurajima does not need to produce a supereruption to be potentially deadly.
As one of the most dangerous volcanoes on the planet, all that is needed is an unusually
energetic explosion that throws car sized rocks several kilometers away into a major
city.
Today's earlier explosion represented the first time that large blocks of material were
thrown beyond 2500 meters from the active vent in the 21st century.
On prior occasions, such as on November 29th of 2013 or on June 4th of 2020, smaller cinders
were thrown to even greater distances, but these were unlikely to destroy any buildings.
In the next several hours to days, two things could happen.
It is quite possible that this explosion was a one-off event, fed by an unusually gas rich
batch of magma, which means that the volcano will return to its typical lesser level of
intensity.
Or, more similarly energetic explosions will occur, potentially placing thousands of people
at risk.
We do not know which of these will come to fruition, as only time will tell.
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