3 - 17 Pyroclastic flow deposits (Part 1)

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The pyroclastic flows in the 1990 - 1995 eruption of Mt. Fugen were caused by the collapses of the lava domes.
The lava contained a lot of vesicles (vapour bubbles) and was quite fragile. Hence the mixture of high-temperature lava fragments, volcanic ash and vapour flowed down at a very high speed along the Mizunashi River.

Plume due to the pyroclastic flow = emission of gas (vapour)
The reason of high speed = small frictional resistance with the surface of the earth because of the gas (vapour)

Mechanism of vesiculation (making bubbles) in the magma
The magma deep in the earth crust contains water in general.
As is shown on the left-hand side of Fig. 1, the solubility of the water in the magma (= How much water can be dissolved in the magma) depends on the pressure applied on the magma, i.e. the depth of the position of the magma. When the magma comes up near the surface of the earth in the case of volcanic eruption and the pressure goes down, the solubility of the water decreases sharply. Then the water dissolved in the magma separates to be vapour and vesiculation occurred.

Relation between the water content of the lava and the feature of eruption
In the case of the eruption of Mt. Fugen, the water content of the lava is not so much (less than 1%). Accordingly, the extruded lava made a dome on the top of the mountain.
If the water content had been much more, the magma would have vesiculated to explode in a large scale like Mt. Pinatubo, the Philippines.

The following figures, Fig. 2 and Fig. 3, show the change of the rate of vesicularity of lava extruded from the Jigokuato crater with the elapsed time since 20 May 1991. As seen in Fig. 3, the rate of content of vesicles in the lava was almost constant on the average (about 20% in volume) during the whole period of the eruption.
However, only an exception was observed; namely, there occurred a small scale of explosion on the 23rd day after the start of the lava extrusion, 11 Jun 1991. The cinder (bomb) erupted at that time was pumice whose specific gravity was about 1000kg/m3 and rate of vesicles reached 60% in volume (Fig. 2). This was only one case of explosion.
Chemical composition of the lava
Fig. 4 shows the temporal variation of the chemical composition of the lava. It was almost constant during the period of the eruption; namely, it was a composition called "dacite". Rigorously speaking, it contained more crystal at the beginning.
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