The nation of Japan contains more than 100 active volcanoes which come in a variety of
shapes and sizes.
This includes Mount Fuji, which has produced particularly large eruptions in the past,
Mount Aso which is a supervolcano, and Iwo Jima, where the entire island has been uplifted
by 15 meters since 1945.
However, there are also a number of other dangerous volcanoes in the country which few
are aware of.
One of these lesser known volcanoes produced a deadly tsunami in 1741, resulting in wave
heights as tall as 90 meters.
I am not referring to Mount Unzen, but rather the Oshima Oshima volcano.
The Oshima volcano is considered to be the westernmost point of the prefecture of Hokkaido.
Specifically, it is an island located 62 kilometers west of the city of Matsumae.
The vast majority of Oshima Oshima is located below the ocean surface, meaning the volcano
is much older than it appears.
This volcanic complex began forming approximately 500,000 years ago as a volume of magma intruded
onto the ocean floor beginning at a depth of 2300 meters.
It started as a basaltic volcano, which deposited numerous layers of black lava.
As this volcanic cone continued to climb higher and higher, the lava which it erupted began
to change.
Soon, it began to erupt both basalt and andesite, which is characteristic from volcanoes with
more explosive eruptions.
After a long period of numerous submarine volcanic eruptions, the Oshima volcano breached
the ocean surface 18,000 years ago.
Due to the interaction of rising magma with seawater, a plateau was built out of both
andesite and layers of ash.
From this plateau erupted a volcanic cone known as Mount Higashi, which quickly built
itself into a prominent volcanic cone.
Within only a few thousand years, it had grown to more than 800 meters above sea level.
However, its steep profile had caused the volcano to become unstable to the north.
Eventually, its structural instability became too great, and a large landslide occurred,
causing more than a cubic kilometer of rock to slide off the volcano into the ocean.
After the collapse, this individual volcanic cone became extinct as activity had now shifted
a kilometer to the west.
This new volcanic cone, called Mount Nishi, soon climbed above the ocean's surface.
Unlike its counterpart to the east, it erupted slightly darker colored lavas.
Over several thousand years, this volcanic cone grew in size to a height of 700 meters.
However, just like its counterpart to the east, the volcano once again became oversteepened.
After eruptions in 800 BC and 250 AD, signs of structural instability became very prevalent
on the volcano with a series of large cracks.
Thus, when Oshima erupted again in August of 1741, a landslide became inevitable.
In the first ten days of the eruption, up to 20 centimeters of ash fell in Hokkaido
as large particles of fine ash fell like rain.
Then, on August 29th, a larger explosion occurred.
Suddenly, the northern flank of the mountain collapsed, causing material to be funneled
into the ocean in a northern direction.
This collapse occurred not only on the island but also under the ocean.
As 2.5 cubic kilometers of material fell into the ocean, a 90 meter high tsunami was generated.
This tsunami quickly raced outwards at a speed of several hundred kilometers per hour, inundating
nearby islands.
Soon, a 19 meter high wave crashed into western Hokkaido, sweeping away numerous coastal villages.
To the south, on Sado Island, a 13 meter high tsunami hit.
Several hours later, a 4 meter high wave hit the eastern coast of the Korean Peninsula,
destroying many coastal structures.
What had just occurred was one of Japan's worst volcanic disasters, with a death toll
of approximately 2,000.
In the aftermath of this landslide, a new cinder cone began growing outwards from the
collapse scar.
By 1786, this cinder cone had grown in size and largely filled this scar with new volcanic
rock.
After a small eruption in 1790, the volcano became inactive and has not erupted since.
Today, this island remains one of the most dangerous volcanoes in all of Japan.
Much of its edifice is oversteepened, and could easily collapse with little warning
following a large earthquake or another volcanic eruption.
Thus, the main threat from this volcano are tsunamis, which could once again effect the
western coast of Japan and areas as far away as South Korea.
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