The products of many intermediate arc volcanoes and batholiths manifest compositional complexity with mafic and mixed material occurring early and through-out a period of eruption and pluton assembly. We present a vesiculation model suggesting that the progressive and repeated vesiculation of volatile-rich magmas yields a super-linear buoyancy acceleration. This style of non-stationary entrainment provides a mechanism for compositional reversal, the intimate co-occurrence of mixed and un-mixed magmas, and rapid, chaotic crystal and melt dispersal. Under some circumstances, entrainment can cease entirely, producing eruptions with compositional tunneling where nearly unmixed mafic, or newly intruded silicic magma, exits first. ¿ 2001 American Geophysical Union