The stability of the flow of interstitial Liquid in a mushy layer driv
en by expansion or contraction upon solidification is analysed. The fu
ll perturbation equations are reduced in a particular aymptotic limit
that allows the principal mechanisms controlling instability to be ide
ntified. Comparisons are made with the acid-etching instabilities in p
orous rocks. The full equations are then solved to determine the param
etric dependences of the instability. It is found that, though the pot
ential for instability exists, it is unlikely to occur in practice.