The Influence of Magma Mixing on the Composition of Andesite Magmas and Silicic Eruption Style

Although inhibited on established fluid mechanical grounds, extensive magma mixing can play a critical role in creating andesite magmas at volcanic arcs and triggering effusive or explosive volcanism. We use analog experiments, scaling theory, and thermodynamic modeling of natural volcanic systems to show that mechanical mixing is enhanced if an intruding magma crystallizes to acquire a yield strength. Mafic magmas intruding highly silicic reservoirs will fragment as crystal‐sized blobs/enclaves to produce textural homogeneity at the outcrop scale and heterogeneity at the scale of individual crystals. Rapid degassing from these enclaves can favor the production of permeable magmatic foams that facilitate gas loss from the reservoir and effusive volcanism. Crystallizing magmas intruding similar composition reservoirs of comparable or smaller effective viscosity will fragment at scales approaching the dike width. Sluggish degassing and consequent volatile retention in the reservoir can enhance volatile exsolution in an erupting conduit and tendency for explosive volcanism.