When a fluid flow is imposed on a porous medium, the infiltration flow may interact with the reaction-induced porosity variations within the medium and may lead to fingering instabilities. A nonlinear model of such interaction is developed and morphological instability of a planar dissolution front is demonstrated using a linear stability analysis of a moving-free-boundary problem. The fully nonlinear model is also examined numerically using finite-difference methods. The numerical simulations confirm the predictions of linear stability theory and, more importantly, reveal the growth of dissolution fingers that emerge as a result of these instabilities.

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Although terra rossa has long been thought to form by residual dissolution of limestone and/or by accumulation of detrital mud, ash, or dust on preexisting karst limestones, we present conclusive new field and petrographic evidence that terra rossa forms by replacement of limestone by authigenic clay at a moving metasomatic front several centimeters wide. The red clay's major chemical elements, Al, Fe, and Si, probably come from dissolved eolian dust. The replacement of calcite by clay exhibits a serrated, microstylolitic texture that helps prove that replacement happens not by dissolution-precipitation, as conventional wisdom has it, but by pressure solution of calcite driven by the crystallization stress generated by the growth of clay crystals. The acid produced by the isovolumetric replacement of limestone by clay quickly dissolves out additional porosity/permeability in an adjacent slice of limestone within the front, triggering a reactive-infiltration instability that should, theoretically, convert the moving reaction front into a set of wormholes, then funnels, then sinks-the very karst morphology that in nature does contain the terra rossa itself. This beautifully explains why terra rossa and karst are associated.

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