Some of the most dangerous volcanoes in the world are known for their dramatically large
eruptions.
A few examples include Vesuvius, Taal, and Santorini.
However, the size of an eruption does not necessarily determine how destructive a volcano
can be.
Within southwest Japan is the nation's most dangerous volcano, Mount Unzen.
This volcano has produced two destructive eruptions in the last 250 years, each of which
were more than 1,000 times smaller than the eruptions from the aforementioned volcanoes.
So, why is Unzen so dangerous?
This video will discuss this volcano, and mention what its future hazards are.
When scientists refer to Mount Unzen, we are not just referring to the peak which bears
this name.
Rather, we are referring to a much wider area, encompassing several different lava domes
which have erupted over the history of this volcano.
As we look around the volcanic complex, you will note a number of U shaped scars in the
landscape.
These scars were the result of dramatic landslides during the course of a volcanic eruption.
These landslides then traveled eastward into the Bay, resulting in the creation of large
tsunamis.
This history of tsunamis, extensive pyroclastic flows during eruptions, and the fact that
2 million people live in potential harms way are why this volcano is so dangerous.
The unzen volcanic complex began forming 500,000 years ago, when magma first reached the surface
in this section of Japan.
When this lava erupted, it had a high viscosity, and as a result formed a large lava dome.
During the course of its formation, gravity would occasionally cause sections of the dome
that had become overly steep to collapse.
These collapses generated pyroclastic flows, which would than travel up to 5 kilometers.
Between 300,000 and 100,000 years ago, 6 other lava domes formed through a similar eruptive
process.
One of these lava domes, called No-dake, formed along the southwestern edge of the modern
volcanic complex between 150,000 and 70,000 years ago.
A subsequent dome, called Myoken-dake, formed on the western edge of the complex between
40,000 and 25,000 years ago.
The lava which erupted from this dome had a high viscosity, emplacing both andesite
and dacite lava.
Eventually, this lava dome became so large, that a large section of it collapsed to the
east 20,000 years ago.
This landslide resulted in the creation of a prominent scarp which remains in the current
landscape.
Since then, the vast majority of volcanic eruptions have occurred at a single lava dome,
Fugen-dake.
This lava dome is largely overlaid by gray dacite rock from the 1990 to 1996 eruption.
However, there is still one other potentially active lava dome in the vicinity.
In the last 5,000 years, a lava dome very close to the Bay called Mayu-dake has erupted
several times, thus forming a large edifice.
The last eruption from this lava dome occurred in 1792.
After a swarm of large earthquakes in the region, and the eruption of a small lava dome
from Fugen dake, this edifice swelled in size.
Then, on May 21st, a 6.4 earthquake hit near the volcano.
This caused the edifice to suddenly collapse into the Bay, resulting in 150 million cubic
meters of rock sliding to the east.
This landslide traveled quickly, and resulted in the creation of a megatsunami as the rock
displaced a large volume of water.
The wave generated by this collapsed was more than 20 meters high, and traveled to dozens
of nearby coastal cities.
By the time the tsunami had subsided, more than 15000 fatalities had occurred in the
deadliest volcanic eruption in Japan's history.
After this eruption the volcano became dormant for approximately 200 years.
It then awakened in 1990, which began a several year long eruptive series.
Over the course of this eruption, a slowly expanding lava dome occasionally collapsed,
producing periodic pyroclastic flows.
The size of these pyroclastic flows widely varied, of which the furthest traveled 5.5
kilometers from the eruptive vent.
This eruptive series ended on May 1st, 1996, since which the volcano has remained dormant.
If Unzen was to erupt again, it would require the evacuation of anyone within a 10 kilometer
radius.
Pyroclastic flows from its eruptions are frequent, and unpredictable.
Additionally, larger explosions could eject boulders of rock in a several kilometer radius.
With this being said, another 1792 sized landslide is unlikely in the near future.
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