| 3 - 8 | Formation of lava dome |
 
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The lava extruded
from the Jigokuato crater
was dacite magma
in which the content
of SiO2 was 64 wt%.
It had a slight
fluidity and successively
formed lava lobes;
some were grown to be
crown-like and others were
elongated
to be tongue-like hanging
on the eastern slope.
Newly coming lava
pushed up older lobes
to generate a huge
lava dome finally.
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Temporal variation of magma supply
(Prof. K. Yamashina, U. of Tokyo)
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It was clarified
during the eruption of
this time that
the amplitude of
the long-period oscillation
in the data from the
tiltmeters set
around the crater is proportional
to the amount of extruded lava.
We could thereby
predict everyday's magma supply
by using an empirical
formula established
on the basis of the aerial survey
by Geographical Survey Institute.
Prof. Kenichiro Yamashina
(U. of Tokyo)
proposed this formula,
which was a big achievement
in the field of volcanic
observation in the World.
Because the data
from tiltmeter was
in advance of the actual
lava extrusion,
it played an important role
in preventing volcanic
disasters in the local area.
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Ground subsidence
due to the lava extrusion
(Geodetic Survey Group)
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Because of the lava extrusion
to the amount of 200,000,000 m3
in this eruption,
Route 251 faced to
Tachibana Bay
(the Chijiwa caldera)
sank at maximum by 8 cm
at Kitano,
the Obama Town
(the leveling point T119).
It has been clarified
that the origin of ground shrinking
(the second magma chamber)
hides itself 10km deep under
the point
D
(the red mark below),
by observing
the ground deformation
over a broad area
in the western part
of Unzen Volcano.
The position of
main magma chamber
assumed to exist beneath
Tachibana Bay
is very deep
and its influence
to ground subsidence
seems to be concealed
within the above data.
Moreover no sufficient
observation around the bay
has been carried out
because it is in a sea area.
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Restored outlines of lava lobes |
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This figure shows
a superposition of the restored
outlines of the fourteen
lava lobes
in each largest growth.
No original shape
are kept at present
except No. 11B.
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