Which phenomenon causes explosive volcanic eruptions at subduction zone stratovolcanoes?

💡 Explanation

When oceanic crust subducts, fractional crystallization concentrates silica in the magma as mafic minerals settle out, which radically increases viscosity because silica tetrahedra bond to form complex chains. Therefore, increasing silica content leads to highly explosive eruptions, rather than exsolution, accumulation, or hydration, because those mechanisms affect magma volume not viscosity.

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