5 - 5 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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