How_Volcano (English)

5 - 4

How is a volcano formed?

It is quite rare

that the magma generated

in the mantle directly

comes out on

the surface of the earth.

Generally, it would

appear on the surface

through volcanic eruption,

after the modification of its own

property (composition)

in the magma chamber.

	Types of lava
	Lava is roughly classified into the following 3 types.

Basaltic lava	Solidified magma extruded on the earth surface or on the bottom of the sea almost directly from the chamber which is filled with magma made by partial melting from the mantle. Lava at a oceanic ridge or at volcano on the hot spot
Dacite lava	Solidified magma with high silica contents. Dacite magma, by slow cooling, becomes granitic plutons which are the major constituent of continental crust and matured island arc crust.
Andecite lava	Solidified hybrid magma generated by mixing of basaltic and dacitic magmas. Therefore its silica (SiO₂) content lies in between the values of basalt and dacite magmas. The major constituent of continental crust and matured island arc crust.

Longstanding controversy

The lava extruded

on an island arc

is dacite lava

or andecite lava

which has much content of silica

(= 55-60%).

How is such lava made?

This has been a longstanding

"hot" controversy

for these 70 years,

but is still open problem.

Let's explain a theory

that has been proposed by

a group of Kyushu University.

This theory accords rather well

with the data

obtained by observation.

Structure of magma chamber and its mechanism

Let's explain the mechanism of magma chamber

by taking the case of Mt. Sakurajima for

example.

The underground structure

of a volcano consists of

upper magma chamber,

lower magma chamber

and cumulate plug in between.

The upper magma chamber

is located in the crust

and the lower one around the boundary

of the crust and the mantle.

In the upper magma chamber,

crystallization

of magma advances

and the heavy crystals are

deposited (cumulated)

on the floor.

The deposits constitute the plug.

The remaining is light

dacite magma.

When the basaltic magma

is newly supplied into

the lower chamber

by the rising of

the mantle from the depths

(Fig. 1),

the cumulate plug starts

to sink down because it

is heavy (Fig. 2).

Hence, the primary

basaltic magma

in the lower chamber

moves up through openings

(cracks) to the upper chamber,

and mixes with the dacite magma

remained in the upper chamber to make

a mixture, andecite magma.

This migration of magma

from the lower to upper chambers

continues

until the plug touches

the bottom of the lower chamber.

During this period,

there occur several times of

large-scale volcanic eruptions, in which

the extruded lava is accordingly

dacite or andecite.

After the plug touches

the floor of the lower magma chamber,

a part of the plug is

eroded and removed away

by the mantle flow (Fig. 3).

On the other hand,

crystallization proceeds

in the upper chamber, and

heavy crystalline matter

is deposited on the plug.

And then new magma is

supplied into the lower magma chamber.

Thus the above-mentioned process is

repeated.

In the case of Mt. Sakurajima,

it seems probable that this process

would be repeated every 600 years.

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