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.

Temporal variation of magma supply
(Prof. K. Yamashina, U. of Tokyo)
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.

Ground subsidence due to the lava extrusion
(Geodetic Survey Group)
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.

Restored outlines of lava lobes
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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