Many of the planet's largest recent explosive eruptions are well known.
For example, on May 18th of 1980, after a large bulge pushed the northern section of
Mount Saint Helens upwards and outwards by 450 feet due to the influx of magma, the volcano
exploded in one of the most powerful volcanic eruptions of the 20th century.
As a large section of the volcano collapsed to the north in a massive landslide, it gave
way to an intense lateral blast which traveled at a speed of beyond 300 miles per hour or
480 kilometers per hour.
This blast burned everything in its path, reaching up to 19 miles or 31 kilometers away
from the volcano.
As the eruption plume rose to a height of 80,000 feet or 24.4 kilometers, it moved towards
the east, causing measurable amounts of material to fall as far away as Oklahoma.
By the time this powerful eruption had ended, the volcano had lost around 1,300 feet in
height, 266 square miles or 692 square kilometers of forest had been destroyed just by the lateral
blast, and slightly more than 1.3 cubic kilometers of material had been explosively ejected.
On the 0 to 8 volcanic explosivity index, where with one minor exception, each increase
in a single number equals an eruption 10 times larger and more explosive, the 1980 eruption
of Mount Saint Helens only was scored as a 5.
As a whole, eruptions that are this unusually large are widely known, such as the January
2022 eruption of Hunga Tonga Hunga Ha'apai in Tonga which was attributed as a VEI 5 by
the Smithsonian Institute's Global Volcanism Program, perhaps slightly larger.
However, the 1980 eruption of Mount Saint Helens was not the only VEI 5 or larger eruption
to occur in the 20th century.
During that century, 8 VEI 5 and 3 VEI 6 eruptions occurred.
Two other well known eruptions in the VEI 5 category from that century are El Chichon
in 1982 and Bezymianny in 1956.
Yet, there is another volcano which produced a similarly large VEI 5 eruption in the 1900s
which few have ever heard of.
The volcano in question is not located in the remote Antarctic, but is rather a submarine
volcano off one of Japan's many islands, being known as the Iriomotejima submarine volcano.
The Iriomotejima submarine volcano is located 22 kilometers north-northeast of the island
of Iriomotejima.
Like other volcanoes in the region, it exists due to a tectonic plate collision to the southeast
of the complex.
There, the Philippine Sea Plate is being subducted underneath the Okinawa Plate.
The melted material from this collision then migrates upwards, until it erupts onto the
seafloor, forming a chain of volcanoes.
Not much is known about this volcano other than it rises several hundred meters from
the seafloor and its summit is a mere 200 meters or 656 feet below sea level.
Only one eruption is known to have occurred from this volcano which erupted highly explosive
rhyolite composition material.
This is quite unusual as not many strictly rhyolite composition submarine volcanoes are
known.
However, I question whether or not this represents the true range of composition at this volcano
or is a symptom of a small sample size.
On October 31st of 1924, this volcano unexpectedly erupted in a highly explosive manner, sending
discolored water and a plume of volcanic gasses to the surface.
However, while other large explosive eruptions ejected significant amounts of ash, this eruption
instead erupted approximately a cubic kilometers of rhyolite composition pumice.
In a manner of speaking it was quite similar to the 2021 eruption of Fukutoku Okanoba,
except it was far briefer and was about 3 times as explosive.
The pumice that was produced covered numerous beaches in material as far north as Hokkaido,
with individual fragments of pumice being up to 2 meters in diameter.
Rating in as just barely a VEI 5 event, this 1924 eruption was the 10th largest explosive
eruption of the 20th century.
Although this volcano has not erupted since, its implied rhyolite composition suggests
that this volcano could produce similarly powerful eruptions that could even form tsunamis,
meaning it needs further study.
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