Motivated by a geological problem, the continuum conservation equations governing the transport of heat, mass and momentum in a deformable multicomponent mush undergoing solid-liquid phase change are presented, and a one-dimensional (1-D) model of a layer heated from below investigated numerically. The results indicate that phase transport exercises a strong control on heat transport. The form of the spatial distribution of liquid depends upon the relative transport rates of heat and buoyant liquid; a high porosity wave may develop at the top of the mixed phase region, the amplitude of which increases with time until the contiguity of the solid matrix breaks down and a slurry forms. Phase compositions are fixed by the requirement of local thermodynamic equilibrium, and are deduced from published equilibrium phase change experiments.