ABSTRACT: Techniques are presented to quantify the rock strength of a volcano edifice (summit), enabling some understanding of failure processes to be obtained from modeling studies. Rock strength and structural information has been obtained from field studies on volcano edifices in the Cascade Range Volcanoes, Washington state and from the eastern portion of the Mexican Volcanic Belt. Field information consists of geologic mapping, point load and in-situ vane testing, and discontinuity mapping. Laboratory rock testing and RMR determination for various rock types have augmented the field data. The rock mass strength for fresh and hydrothermally altered rock was calculated for lava flow, dome and pyroclastic litholigies. Volcano modeling utilizing the distinct element method (UDEC) and the limit equilibrium technique, was used to assess stability and deformation of the edifice. Modeling results give indications of probable failure volumes, velocities and direction. The models also highlight the crucial role played by weak lithology in reducing the strength of the volcano edifice

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