Core softening in cavernously weathered tonalite

James Conca, George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.


Tonalite exhibiting cavernous weathering at Catavina, Baja California, was investigated to determine the factos 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 effiedt of the coating is insignificant comared to the core-softening of the interior. The hardness, measured by an abrasion resistance hardness testere, is inverserely correlated with kaolinite content in the tonalite. A one-dimensional water flow madel 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 hydroulic conductivities induced by core softening. Therefore, the cavern interior wass should have the greatest weathering rate in the boulder.