Japan contains more than 100 active volcanoes.
Of these, none are as famous as Mount Fuji.
Since its formation, Mount Fuji has been viewed by hundreds of millions of people, being a
notable often snow covered peak visible from Tokyo and appearing in abundant artwork.
In fact, Mount Fuji is still considered to be the most perfectly symmetrical volcano
by many people as it has a near perfect circular cone shaped profile.
However, on this volcano there is one notable imperfection.
What I am referring to are a series of 3 vast and deep explosion craters that formed on
Mount Fuji's southeastern slope in 1707 in a powerful and destructive eruption which
was 38% larger than the 1980 eruption of Mount Saint Helens.
Although Mount Fuji has not erupted since that 1707 to 1708 event, its 4 dozen eruptions
in the last 10,000 years marks a fairly high rate of activity which means it will one day
erupt again.
Due to the proximity of how many people live and work nearby, and Fuji's highly explosive
nature when it does erupt, I personally consider Mount Fuji to be one of the world's 10 most
potentially dangerous volcanoes.
But first, why is Mount Fuji colored such a beautiful shade of red unlike most other
towering volcanoes?
The answer is that unlike most other volcanoes which are colored gray due to an abundance
of andesite lava, Mount Fuji is largely a less viscous basaltic stratovolcano.
Just like the Ulawun volcano in Papua New Guinea, Fuji's summit region is covered by
an oxidized basaltic rock type known as scoria.
Scoria largely forms during strombolian eruptions, when dissolved gas in volcanic rock is ejected
during an explosion.
The story of Mount Fuji which is located 100 kilometers west-southwest of the city of Tokyo
began 700,000 years ago.
At the time, basaltic composition magma initially reached the surface, erupting in an explosive
manner due to a relatively high gas content.
Over the next 500,000 years, this volcanic cone known as Komitake erupted hundreds of
times, depositing layer after layer of basaltic lava and ash.
By the time eruptive activity at this older volcanic cone ceased, it had a peak 7,500
feet above sea level.
The remnants of this cone can now be found on the north slope of the modern Mount Fuji.
After a period of dormancy, a new cone known as "Old Fuji" began to form 80,000 years ago.
Over the next 64,000 years, numerous eruptions of basaltic lava and ash caused this mountain
to climb to 10,000 feet in height.
4 eruptions during the phase of Old Fuji involved highly explosive plinian eruptions.
What we consider the final cone of Mount Fuji that consists of the top 2,389 feet of the
volcano completely formed during the last 14,000 years.
A heightened rate of eruptive activity has occurred in much of the time since, depositing
layer after layer of beautiful scoria, lava, and ash.
As Mount Fuji continued to grow in height, new eruptive vents opened around this volcano's
flanks, resulting in the eventual placement of 56 flank cinder cones and vents during
this period.
One of these vents is known as Mount Omuro, which erupted and emplaced a basaltic lava
flow in 1030 BCE.
These flank vents largely occurred along a north-northwest trend, which would represent
the same trend that would appear during Fuji's largest historical eruption.
This eventual eruption lasted for 71 days between 1707 and 1708, and was linked to the
occurrence of a major magnitude 8.7 megathrust earthquake occurring 49 days earlier.
This quake merely sped up the timetable of Fuji, which was already primed to erupt in
a fairly short amount of time.
Known as the Hōei eruption, this sequence repeatedly created a 20 kilometer or 65,600
foot high mushroom shaped eruption column, as three powerful blasts carved out 500 to
1400 meter wide overlapping craters on Fuji's southeastern slope.
Continuous ash emissions caused an area of 112,000 square kilometers to receive a layer
of ash, including 40 centimeters thick in Gotemba, 19 centimeters thick in Kamakura,
2 centimeters thick in Tokyo, and 3 centimeters thick of ash in Asashi.
In total, 1.8 cubic kilometers of tephra was ejected, meaning this eruption was assigned
a volcanic explosivity index of a 5.
Today, the mountain is quiet, although on occasion volcanic earthquakes occur in its
vicinity.
Statistically speaking, Mount Fuji during the last 10,000 years has averaged an eruption
about once every 210 years.
If an eruption was to occur, it would present a variety of hazards.
And no, despite what tabloids ridiculously claim, an eruption from Fuji will NOT destroy
Tokyo.
However, it is true that future eruptions could destroy populated areas with pyroclastic
flows, heavy ashfall, potentially long reaching lahars, and lava flows, with lava flows being
capable of striking areas up to 40 kilometers away from Fuji's summit.
As a final note, I would like to thank my new patron Jane Lacher for supporting this
channel!