Yougan (English)

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Observation of the lava from Mt. Fugen
with a microscope

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What type was

the 1990-1995 eruption of Mt. Fugen ?

To see this,

we have investigated

the lava extruded in this eruption

with a microscope.

It has been clarified

that the lava was a mixture of

two types of magma

mixed under the ground.

How to watch a rock with a microscope

When a sample of rock is cut

into a plate so thin to be 0.01

- 0.03 mm thick,

light can penetrate the glass

and the silicate minerals in it.

Accordingly, we can observe

and study

the minerals composing

the rock and its structure

with a microscope.

(Light cannot penetrate the

minerals with metallic luster

like magnetite or iron sulphide,

even if they

are very thin.

However these types of

minerals are not contained very much

in a rock (less than 1%),

so that they give

no trouble for observation.)

Crystals in lava

In a magma chamber

in the upper crust,

the crystallization

of dacite magma

proceeds at low temperatures.

Minerals so formed are, therefore,

stable at such low temperatures.

When a hot basaltic magma

is newly injected into the chamber,

these minerals are heated

and partly melted or,

sometimes, completely resolved.

The andecite lava

explained on the previous page is usually

in this state.

As seen in the photomicrographs

below (Photo 1,

Photo 2 and Photo 3),

the lava extruded

in the present eruption of Mt. Fugen

was just of this type.

Photo 1 : Quartz in the lava of Mt. Fugen

This is a photomicrograph of

quartz in the lava.

The diamond-shaped

white (transparent) part

at the center-left of the photo

is quartz.

Since it was promptly cooled down

after the extrusion to the surface,

the surrounding which represents the melt

before the cooling contains a lot of

tiny crystals.

In the quartz, there are three colored spots.

They are melts

produced by melting from quartz,

which contain opaque minerals.

Quartz appears at the final

(low-temperature) stage

in the course of

the crystallization of magma.

It is quite stable at a temperature lower than 900C.

It has a very short,

prismatic, hexagonal form

terminated on both side

by hexagonal piramids and

hence its section is

diamond-shaped.

If a quartz is heated

and resolved,

the diamond form becomes rounded,

deformed and sometimes

makes a hollow inside.

The quartz in Photo 1 is

just in this state;

it shows to start melting

by being heated up.

Photo 2 : Plagioclase in the lava of Mt. Fugen

A plagioclase in the lava

is shown at the

center of Photo 2.

Its central part

(blue arrow)

and the outer layer

(red arrows)

are transparent,

but the second layer

in between is quite dirty,

because it contains a lot

of micron-size tiny particles

of glass.

Plagioclase phenocrysts

in the dacite magma are

heated by the newly injected hot magma,

and their surface

were melted incongruently.

The second layer

is just the melt

solidified directly.

The surrounding outer layer

is a rim grown

from the mixed magma immediately

before its extrusion.

Photo 3 : Amphibole in the lava of Mt. Fugen

This is a photomicrograph

of amphibole melted

incongruently in the lava

(red arrows).

Amphibole is originally

greenish brown in color,

but in Photo 3

it has now been converted

to a dense aggregate

of very fine grains of opaque minerals.

Ferrous iron

contained in the amphibole is

oxidized to form these

magnetite grains by

oxidation and heating

associated with the magma injection.

This amphibole originally

crystallized from the low-temperature

dacite magma was later

disintegrated and melted

incongruently, being

heated by newly injected hot magma.

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