One of Japan's most active volcanoes is a gray colored volcanic cone which rises 1500
meters above the surrounding landscape.
Known as Mount Asama, this complex once produced an eruption so large that it emitted more
than 6 million metric tons of sulfur dioxide into the atmosphere that it created an aerosol
layer around the planet which blocked a double digit percentage of sunlight.
This was sufficient enough to cause a severe famine in much of Europe while in Japan similarly
destructive effects also occurred.
The eruption which was just described occurred in 1108, representing one of Japan's largest
to occur in the last 2,500 years, and covered many what at the time were farmers fields
in more than a meter thick of ash even in some spots 15 kilometers away from the volcano.
Then again in 1783, another plinian eruption occurred, destroying many nearby villages
with heavy ashfall and some pyroclastic flows in addition to creating a tall blocky andesite
lava flow which crept across the landscape to a distance of 6 kilometers north of Asama's
summit.
Luckily, these larger scale events are fairly uncommon at this volcano as during the Holocene
epoch eruptions from Asama of a similar magnitude to what occurred during 1108 and 1783 have
only occurred about once every 1,200 years on average.
However, many far smaller explosive eruptions have occurred since via the Kamayama cone
at the complex's peak which has 2 large craters, the last eruption of which occurred in 2019.
Yet, apart from occasional large explosive eruptions, Mount Asama also has a number of
fascinating qualities such as a horseshoe shaped collapse scar formed when much of the
volcano collapsed 24,000 years ago.
Or, the fact that Mount Asama has occasionally erupted outside of its main cone through flank
lava domes up to 10 kilometers from its summit.
Like many other volcanoes on northern Honshu and Hokkaido, Mount Asama exists due to an
offshore tectonic plate collision where the Pacific Plate is subducting underneath the
Okhotsk Plate.
The melted material from this collision then migrates upwards until it occasionally reaches
the surface and erupts, forming a chain of volcanoes.
Despite its name, Mount Asama does not merely encompass the peak which it is named after.
Rather, it also includes two older volcanic peaks and a scattered assortment of lava domes.
This complex as a whole has been intermittently erupting since approximately 92,200 years
ago, when its first major VEI 4 explosive eruption occurred.
Through subsequent eruptions over a timespan of 68,000 years which were mostly vulcanian
in nature, overlapping masses of andesite lava and ash built a stratocone which reached
a height of 2,700 meters, taller than the modern Asama volcano.
However, during the latter half of this period, repeated intrusions on the eastern half of
the volcano occurred, causing that section of its edifice to turn shades of red and yellow
due to intense hydrothermal alteration.
Eventually, this section of the volcano became too brittle, and the eastern half of the volcano
including its peak collapsed to the east in a massive landslide, which triggered a plinian
eruption.
In the aftermath two vents erupted; one constructed a lava dome while the other constructed a
dacite dome complex which is now buried by younger eruptive products.
Once this became inactive, then the modern Kamayama cone began to grow, primarily being
built out of andesite composition ash through explosive eruptions but also occasional andesite
lava flows.
A complex larger crater on the summit measures 1250 meters across which the inner crater
lies within, with the larger crater likely forming through repeated plinian eruptions.
During historical times, this volcano has erupted more than 100 separate times, with
only 3 of the group being fairly large and the rest being much smaller.
Interestingly, these eruptions largely occur in a pattern.
Much like the Taal volcano in the Philippines, this volcano prodcues a cluster of eruptions
which occur about every other year for a period of time before not erupting for several decades.
It is uncertain which of these two phases we are currently in, although Asama last erupted
in 2019.
These frequent eruptions have for the most part been relatively small, consisting of
phreatic eruptions with minor ashfall and vulcanian eruptions which often eject incadescent
lava while also occasionally producing pyroclastic flows.
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