Haloclasty

Haloclasty, or salt weathering, is a physical weathering process in which salts crystallizing within rock pores and cracks generate mechanical stress that leads to rock fragmentation. It operates without chemical alteration of the mineral framework and is common in arid, semi-arid, or coastal environments where saline solutions are present.

Two main mechanisms drive haloclasty: crystallization pressure and salt wedging. Crystallization pressure occurs when evaporating saline solutions leave behind salt crystals that grow within pore spaces, exerting tensile and shear stresses on the surrounding rock; as cycles of evaporation and crystallization repeat, microcracks propagate. Salt wedging involves growth of crystals in cracks and fissures that physically pry the rock apart, often aided by wetting–drying cycles. In addition, salts that undergo hydration state changes (for example gypsum/anhydrite systems) alter their volume with humidity, contributing to crack opening. Salts commonly implicated include halite (sodium chloride), gypsum (calcium sulfate dihydrate), anhydrite, thenardite, mirabilite, and other sulfates and carbonates.

Haloclasty is especially important in desert sandstone, limestone and granite in coastal settings, and in cultural heritage stonework exposed to marine spray. Resulting forms include pitting, honeycombing or tafoni, exfoliation, and the gradual loss of surface laminae. Salt weathering can act in concert with other processes such as freeze–thaw where ice formation exacerbates crack propagation.

Understanding haloclasty informs preservation strategies for stone monuments and architectures in saline environments. Mitigation measures include controlling moisture ingress, improving drainage, removing saline crusts, sheltering exposed surfaces, and selecting low-salt materials. Researchers study pore-scale processes and salt cycling to predict deterioration patterns and to design preventive conservation plans.