Historic
eruptions of Mount Fuji
From Wikipedia, the free encyclopedia
Mount Fuji is the highest volcano in Japan. The mountain as it appears
now is the "New Fuji volcano", which began to erupt about 10,000
years ago. Under the "New Fuji volcano" lies the "Old Fuji
volcano", which was active between 100,000 years ago and 10,000 years ago,
and the "Komitake volcano", which became active 700,000 years ago.
Komitake
There has been much volcanic
activity in the vicinity of Mount Fuji for several million years. In the
location occupied by the current Mount Fuji, a volcano known as Mount
Komitake, literally meaning "small
mountain volcano", became active 700,000 years ago. Around this time,
another volcano, Mount
Ashitaka, in the area to the south-east of
Mount Fuji, was also highly active. The peak of the ancient volcano, Komitake,
can be seen from the north face of Mount Fuji at the fifth station, about 2,300
meters (7,500 ft) above sea-level.
Old
Fuji
Around 100,000 years after becoming
inactive, Komitake entered another period of activity. The volcano in this
period is known as Old
Fuji. Older Fuji was known as to have
explosive eruptions, throwing out large quantities of scoria, volcanic ash and lava, forming a large mountain which reached a height of 3,100
meters (10,200 ft).
New
Fuji
Following the Old Fuji period, there
were about 4,000 years of inactivity, ending at around 5,000 years ago, when
Mount Fuji became active again; this period is known as New Fuji, and continues
to the present day. Eruptions of New Fuji exhibit phenomena such as lava flows, magma, scoria, volcanic ash, collapses and side eruptions, leading it to be called
"a department store of eruptions". Ash from New Fuji is often black,
and eruptions are new in terms of geological layers. Valuable data on the
activity of Mount Fuji is recorded in Japanese historical documents dating from
the 8th century onwards. It exhibits a range of representative eruptions.
The
Gotemba mud flow
About 2,300 years ago the east face
of the volcano collapsed and liquid mud flowed down to the Gotemba area as
far as the Ashigara plain in the east and the Suruga bay across Mishima city in
the south. This incident is now called the Gotemba mud flow. Liquid mud piled
up over an area as wide as the city area of Mishima.
Jōgan
eruption
In 864 (the 6th year of the Jōgan
era) there was an eruption on the north-east side of Mount Fuji, which produced
a great amount of lava.
- 864 (Jōgan
6, 5th month): Mount Fuji erupted for 10 days, and it ejected from its
summit an immense quantity of cinders and ash which fell back to earth as
far away as the ocean at Edo bay. Many people perished and a great numbers
of homes were destroyed. The volcanic eruption began on the side of
Fuji-san closest to Mount Asama, throwing cinders and ash as far away as Kai province.[1]
Some of the lava filled up a large
lake Senoumi which existed at the time, dividing it into two lakes, Saiko and Shōjiko This is
known as the Aokigahara lava and at present is covered by the dense Aokigahara
forest.
The latest eruption, in 1707 (the
4th year of the Hōei era), was known as the great Hōei
eruption. It followed several weeks after
the Great Hōei earthquake, an 8.7 in the Richter scale. It severely damaged the city
of Osaka, but more than that, it created enough seismic activity to compress
the magma chamber 20 km deep in the inactive Mt. Fuji. Due to the compression
of the magma chamber, basaltic lava rose from the bottom to the higher dacitic
magma chamber at 8 km deep. The mixing of the two different types of magma
caused a plinian eruption to occur. Previous to the Hoei, another earthquake
named Genroku had struck Japan in 1703. The earthquake affected both Kanagawa
and Shizuoka prefectures; Genroku was measured as an 8.2 in the Richter Scale,
it caused the same effect as Hoei but with less severity. It clamped the dike
of the mountain at 8 km to the surface (where the dacitic magma resides), as
well as the basaltic chamber at 20 km deep. Many articles find a correlation
between the two earthquakes, getting to the conclusion that without one another
the Hoei eruption would’ve not happened.
- November 11, 1707
(Hōei 4, 14th day of the 10th month): The city of Osaka suffers
tremendously because of a very violent earthquake.[2]
- December 16, 1707
(Hōei 4, 23rd day of the 11th month): An eruption of Mt. Fuji;
the cinders and ash fell like rain in Izu, Kai, Sagami,
and Musashi.[3]
This eruption was remarkable in that it spread a vast amount of volcanic ash
and scoria
over a region as far away as Edo.
Records
of eruption
Sixteen eruptions of New Fuji have
been recorded since 781. Many of the eruptions occurred in the Heian era,
with twelve eruptions between 800 and 1083. Sometimes inactive periods between
eruptions lasted for hundreds of years, as in the period between 1083 and 1511,
when no eruptions were recorded for over 400 years. At present, there have been
no eruptions since the Hoei eruption in 1707-1708, around 300 years ago.
Current
eruptive danger
Scientists study the activity of the
magma rising by measuring CO2 emissions in the deeper parts of the volcano.
Studies from before the Tohoku earthquake show the CO2 emissions below 5
gCO2/m2/day, which is the detection limit. If the emissions rise above 5
gCO2/m2/day then seismic activity is occurring and an eruption could possibly
take place. According to a five-stage evolutionary model for the release of
volcanic gas, Mt. Fuji would be considered stage I. Magma is at a considerable
10 km depth and no emissions of gasses can be observed regularly.
Following the 2011
Tōhoku earthquake and tsunami
much attention was given to the volcanic reaction of Mt. Fuji. Experts have
found that the internal pressure of the Mt. Fuji Lava Chamber has increased to
1.6 megapascals. Eruptive pressure for this chamber is 0.1 megapascals, placing
Mt. Fuji in an imminent eruptive state. Additional indicators include steam and
gas being emitted from the crater, hot natural gas and water being in the area,
and a recently discovered 21 mile long fault beneath Fuji. Immediate volcanic
dangers include massive mudslides, lahars, ash, and ejecta. Estimated damage to Japan from a Fuji eruption is at ¥2.5
trillion ($25 billion).
Predictions have also been made by
scientists about Mt. Fuji’s imminent eruption. Professor Masaki Kimura has
predicted on the basis of his many studies an eruption of Mt. Fuji in 2015,
four years after the major Tohoku earthquake in 2011, and similar to the Hoei
eruption four years after the Genroku earthquake. Professor Kimura mentions two
important details that back up his prediction. One, the rise in the water level
in Lake Sai, just after the Tohoku earthquake, which he believes was caused by
the melting of a permafrost layer in the summit of the mountain by rising
magma. And two, the continuous occurrence of phreatic eruptions, explosions of
steam, around the mountain. It should also be noted that Professor Kimura is
the author of two books that refer to his predictions: Fujisan no Funka wa
Hajimatteiru! (The Eruption of Mt. Fuji has Begun), released in last year in
June, and Fujisan Dai Funka! Bukimina Itsutsu no Choko (Major Eruption of Mt.
Fuji! Five Eerie Signs), released in 2011. [4]
[5]
The entire wiki article with images
and references can be found at: http://en.wikipedia.org/wiki/Historic_eruptions_of_Mount_Fuji
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