マグマの発生と上昇

抄録

The following model is proposed to describe magmatic processes in relation to volcanic eruptions. An ascending mantle flow occurs for some reasons beneath the mid-ocean ridge, hot-spot and arc volcanoes. Magma is generated in the ascending mantle flow in which solidus temperature is lowered due to the pressure release. In the mantle flow, the magma is distributed along the edges and corners of mafic mineral grains and basaltic in composition. When melt channels are widely connected at higher degree of partial melting, the magma migrates through the mantle minerals with permeable flow. The ascending mantle flow finally passes the top of the mantle, and moves out aside and downward. This outgoing flow is gradually cooled so that the residual magma in it is partly solidified and has more abundant incompatible elements. Magma is segregated at the top of the mantle, forming separate magma bodies. The magma body, which may be identified with a moving magma chamber, migrates upward along a preexisting conduit in the crust. Fractional crystallization and chemical reactions with surrounding crustal rocks make magmas more felsic, especially when a small magma chamber ascends through a thick continental crust. Since the chemical differentiation is accompanied by significant increase in the magma viscosity, a more felsic magma becomes less mobile. Therefore an extremely felsic magma may be standing and crystallized in the interior of the crust as plutonic rocks.