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The pyroclastic flows
in the 1990 - 1995 eruption
of Mt. Fugen
were caused
by the collapses of
the lava domes
containing a lot of
vesicles (bubbles)
of high-pressure gas.
The rate of the volume
of vesicles to the total volume
of the lava is 15-20%.
The flows consisted
of a mixture of
the released gas
and volcanic ash
ran down along
the valley at a very high speed.
The following pictures
show the pyroclastic flow deposits
in its forefront.
These pictures were
taken on 1 June 1991
at the upstream of
the Mizunashi River.
When we took them,
we felt the radiant heat
from the deposits.
Trees stood in
the area of the flow
were completely destroyed,
and the deposits
at the forefront
were accumulated
about 2m high.
(Photo #1, #2)
Behind the forefront,
there accumulated blocks,
volcanic ash,
and burned fallen timber.
(Photo #3, #4, #5)
The size of some blocks
were larger
than 1m in diameter.
It was still too hot
to touch with our hand.
Gas is being emitted from the ash.
(Photo #6)
Sulphur sedimented
(settled) around
the vent of the gas.
(Photo #7)
The pyroclastic flows
were accompanied by
high-temperature glowing cloud
originated with the gas
released from the lava.
It reached higher
and further than the
flow itself, and
burned tall trees around.
(Photo #8, #9)
A large-scale pyroclastic flow
occurred two days
after we had taken
these pictures.
It reached further downstream,
and killed 43 persons.
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