RESUMEN

This paper describes a model representing the process of partial melting of deforming mantle rock, and the associated melt migration due to differential buoyancy. The model is a double free-boundary problem of degenerate type, and is typical of such slow, reactive two-phase flows. Prescription of boundaryconditions is problematical, but in some sense the model picks its own; a realistic asymptotic solution involving rather novel boundary layer behaviour is presented, for the case of two fixed boundaries, and it is then shown how the free boundaries can be determined a posteriors. The implications for geophysical behaviour are discussed.