3 - 6 Precursory phenomena of extrusion of lava
(Part 1)

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Approaching the lava extrusion, there occurred various types precursory surface activities.
Phreatomagmatic eruption
As magma is approaching the surface of the earth and contacting with the groundwater, thick smoke and debris are emitted. This is called phreatomagmatic eruption, which is a precursor of lava extrusion.

(4 Apr. 1991)

Photomicrograph of vesiculated glass in volcanic ash
A piece of glass found in the volcanic ash erupted on 4 Sept. 1991 contained bubbles (arrows) that might be made by vesiculation with decompression of the gas inside the magma. This suggested that new magma was rising. (Prof. K. Watanabe, Kumamoto Univ.)

Phreatic eruptions became heavier
After March, 1991, three craters, the Kujukushima crater (left), the Byobuiwa crater (center) and the Jigokuato crater (right), simultaneously erupted, emitting an exceptionally large amount of volcanic ash.

(5 Oct. 1991)

Fissures emerged surrounding the crater
As magma rised, there emerged many fissures running from east to west surrounding the Jigokuato crater. Finally, the fissures became about 10m in width (observed by Geological Survey Institute). The following picture was taken two days before the lava extrusion.

(18 May 1991)

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