The role of surface active aqueous environments in chemomechanical weakening of geologic materials is examined in light of the results of hydraulic fracture tests in sandstone, calorimetric studies, and crack propagation tests in synthetic quartz. In hydraulic fracture tests employing Crab Orchard Sandstone it was found that the effective hydraulic fracture pressure was reduced, over that attained with distilled water, when 5¿10-4 M aqueous solutions of dodecyl trimethyl ammonium bromide (DTAB) were used as the hydraulic fracture medium. The degree of branching of the fractures was also increased in the presence of the DTAB solution. Previously reported crack propagation stress values in quartz exposed to distilled water and various DTAB solutions displayed the same trend. When examined in this study, the cracks propagated in the presence of DTAB solutions also displayed a greater degree of branching than those propagated in the presence of distilled water or the ambient atmosphere. These results and results from calorimetric measurements of the heats of adsorption and desorption from quartz of distilled water and DTAB are synthesized in a model relating the weakening and morphological effects to a reduction in the surface free energy of quartz due to adsorption of species from the chemical environment onto the surfaces of the quartz and sandstone.