It was July 15th, and numerous people were gathered at one of the many hot spring resorts
in Japan which largely owe their existence to the nation's more than 100 volcanoes.
It was early in the morning, and for the past several days an unusual series of weak earthquakes
had occurred.
Then, the adjacent volcano known as Mount Bandai produced an unusual sound, that of
rumbling.
While the year was 1888, something was not right, although this volcano has not erupted
any lava onto the surface for 25,000 years.
Then, a large earthquake struck the region, causing everything to shake violently.
As the faint rumbling all of a sudden became much louder, a powerful explosion originated
from the top of the peak.
Over the next several minutes, dozens of closely times explosions occurred creating a vertical
moving ash column, which threw large boulders of rock over a distance of several kilometers.
Then, an unusually powerful explosion occurred which instead of going upwards moved in a
completely horizontal manner.
As a deadly pyroclastic flow advanced across the landscape to the north, a large section
of the volcano collapsed in a massive landslide.
This volcanically caused landslide would go on to completely bury several towns at distances
of up to 9.6 kilometers or 6 miles away from the initial site of the collapse, covering
a grand total of 40 square kilometers in debris.
What was just described represents a depiction of the Mount Bandai volcano's deadly July
15th 1888 volcanic eruption.
While catastrophic, this was not the largest such collapse to occur at the Bandai volcano,
with it only representing the largest in recorded history.
As, if you look at the Bandai volcano it has two other visible collapse scars, one of which
is far larger than the scar created in 1888.
While the smaller scar on the eastern section of the volcano formed most likely during a
volcanic eruption in 600 BCE, the much larger debris avalanche directed to the south of
the volcano occurred around 50,000 years ago.
All three of these collapses left behind numerous hills and mounds in the region which was the
site of the landslide deposit in what is known as "hummocky terrain".
This is most prominent in sections of Lake Hibara and Lake Onogawa, where numerous landslide
blocks and debris deposits stick out of the bodies of water and form scattered islands.
Since many of the hummocks produced during the 1888 landslide are more than 35 feet in
height, you can picture just how destructive it was.
The reason why all three of these landslides occurred in the short term was a volcanic
eruption, although the majority of volcanic eruptions at this volcano did not cause such
large volume landslides otherwise Bandai's edifice would have been able to grow in the
first place.
But in broader terms, these collapses were caused by long term hydrothermal alteration
of the volcano's edifice, where proximity to magmatic intrusions and/or interaction
with hydrothermal fluids over time caused hard volcanic rock and layered ash to become
brittler rock including sometimes clay.
If the alteration encompasses enough of a volcanic edifice, it will either collapse
without an eruption occurring or be prone to collapsing through a volcanic eruption
with a horizontal component to it whether it be through an explosion or a shallow intrusion
of magma which pushes the unstable slope outwards significantly, starting a landslide.
This volcano has been intermittently active for more than 250,000 years which during its
early history was far more active, producing numerous andesite lava lobes in additional
to ashfall in primarily vulcanian eruptions.
The lobes and layers of ash built on top of one another and with different cones being
dominant at different times created a volcanic edifice with three scenic volcanic cones built
over the timespan of 200,000 years.
After a major collapse associated with a VEI 5 explosive eruption 50,000 years ago, a new
cone would grow out of the collapse scar although it would still not reach the height of its
predecessor.
Magmatic eruptions would then continue for some time before seemingly ceasing 25,000
years ago.
Since then, Bandai has only produced phreatic eruptions where groundwater heated by shallow
magmatic intrusions flashes to steam and explodes, such as what occurred during the 1888 eruption.
However, during the last 10,000 years, the Bandai volcano has erupted only approximately
a dozen times meaning that eruptive events are few and far between, with its last eruption
as of Febuary 22nd of 2023 being its infamous 1888 eruption.
Thanks for watching!
If you would like to request a specific topic, please leave a comment below.
Additionally, I would like to thank this channel's patrons on Patreon and channel members on
YouTube!