The chemical transformation of a rock by a pervading convecting fluid in disequilibrium with it is modelled by a non-linear partial differential equation of hyperbolic type. Starting from a continuous concentration profile, the solution may become discontinuous within a finite time in some cases. We show that this is due to the instability, in the sense of the second principle of thermodynamics, of a range of the possible concentrations with respect to this dynamical phenomenon. The simple model exposed gives a good account for the characteristic pattern of this phenomenon, which is the self spatial organization of the concentrations in different domains separated by strong gradients.