Salt Weathering, a Neglected Geological Erosive Agent in Coastal and Arid Environments

Abstract

THE fragmentation of rocks by the crystallization of salts, for convenience termed ‘salt weathering’, is important in a restricted range of environments and produces distinctive topographic forms. Relatively hard rocks can be completely broken down into their component particles by soaking them in a salt solution and allowing the salt to crystallize in the interstices¹. The chemical free energy of a given mass of solid increases with its surface area. Therefore any system tends to reduce the area of its interfaces to a minimum², and in a system containing crystals in equilibrium with a saturated solution larger crystals will grow at the expense of the smaller. It is less obvious why, when the larger crystals entirely fill a pore space, they continue to grow against the constraint imposed by the walls of the pore, expand the pore and fragment the rock. The work required to be done during crystal growth on one face of a crystal (ref. 3) is equal to (P_l–P_s) dV, where P_l is the pressure in the liquid, P_s the pressure in the solid, and dV the increase in volume. This must equal the work done in extending the surface³, which is equal to σdA, where σ is the interfacial tension between the crystal face and its saturated solution and dA is the increment of volume. Then since σ is independent of V: Consider crystallization in a porous solid with large pores and small pores both filled with the saturated salt solution; let water evaporate and escape from the system, or let crystallization be induced by a temperature change. First, the larger crystals in the large pores will grow at the expense of small crystals in the small pores. Let the process continue until salt crystals completely fill the large pores. Now since P_s–P_l = σ dA/dV for the crystal to grow down the capillary pores would greatly increase the area of the crystal, but only slightly increase its volume The crystal will therefore grow in the large pore until the pressure builds up to such an extent that either mechanical fracture occurs or (P_s–P_l)/σ becomes greater than the necessary dA/dV to make the crystal grow down the capillary pore. Thus (for a given crystal and therefore a given σ) whether or not fracture occurs depends on smallness of the small pores and the value of σ compared with the mechanical strength of the porous material. Hence a large rock pore will be enlarged provided that the surface tension of the salt times the dA/dV of the micro-pores is greater than the mechanical strength of the rock. Thus for rocks of equal mechanical strength those with large pores separated from each other by micro-porous regions will be the most liable to salt weathering.