The processes before volcanic eruption can be divided into three stages: magma formation and initial ascent stage, magma chamber stage and leaving the magma chamber to the surface stage.
(1) Magma formation and initial ascent stage. There must be two processes for magma formation:partial melting and separation of molten body from parent rock. In fact, the two processes are unlikely to be independent of each other, separation of melt from parent rock may occur at the beginning of melting. Partial melting is the coexistence of liquid (i.e., magma) and solid (crystallization), partial melting occurs when temperature increases, pressure decreases and solid line decreases. When the partially melted material rises with mantle flow, the separation of liquid and solid will also occur in the flow, thus resulting in the movement and aggregation of liquid, which is called melting.
(2)Magma chamber stage. Magma chamber is the area filled with magma under the volcano, where magma is relatively enriched in crustal or upper mantle rock media. Temperature fluid is generally regarded as a pore (or fracture) fluid in rocks similar to oil reservoirs, it is generally believed that magma accounts for only 5%-30% of the total volume in the mantle plume. From the local point of view, it can be regarded as the liquid collection of internal relative flow. Magma is a mixture of magma melts, volatiles, and crystals.
(3)From the magma chamber to the surface stage. The rise of magma pathway from magma source area to near surface is related to the excess pressure of magma chamber, the formation and connection of magma passage, and the crystallization and degassing process during magmatic ascent. When the tensile or tensile shear stress in the crust is greater than the fracture strength of local rocks, tensile or tension-shear fractures may be formed, if these fractures are interconnected , they can be used as channels for magma eruption.
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