5 - 4 How is a volcano formed?

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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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