KasairyuuTaisekibutu_1 (English)

3 - 17

Pyroclastic flow deposits (Part 1)

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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