Core Softening in Cavernously Weathered Tonalite

Abstract

Tonalite exhibiting cavernous weathering at Catavina, Baja California, was investigated to determine the factors which contribute to differential hardness within the rock. Soft cores have a high degree of chemical weathering as indicated by kaolinite content. Hematite formed from the leaching of biotite occurs in coatings on rock surfaces, but the hardening effect of the coating is insignificant compared to the core-softening of the interior. The hardness, measured by an abrasion resistance hardness tester, is inversely correlated with kaolinite content in the tonalite. A one-dimensional water flow model was developed for core-softened. cavernously weathered boulders. It indicates that during infiltration and dessication the moisture flux is greatest through the cavern interior wall because of changes in the hydraulic conductivities induced by core softening. Therefore, the cavern interior wall should have the greatest weathering rate in the boulder.

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