The effects of applying a 0.2-T transverse magnetic field on a solidifying Ga-25 wt%In alloy have been investigated through a joint experimental and numerical study. The magnetic field introduced significant changes to both the microstructure and the dynamics of escaping high-concentration Ga plumes. Plume migration across the interface was quantified and correlated to simulations to demonstrate that thermoelectric magnetohydrodynamics (TEMHD) is the underlying mechanism. TEMHD introduced macrosegregation within the dendritic structure, leading to the formation of a stable “chimney” channel by increasing the solutal buoyancy in the flow direction. The resulting pressure difference across the solidification front introduced a secondary hydrodynamic phenomenon that subsequently caused solute plume migration.