Fig. 1. Fracture propagation through a rock mass, such as a volcanic edifice or a sedimentary basins, requires less energy (is ‘easier’) when the mass is composed of mechanically similar layers than when it is composed of mechanically dissimilar layers. (a) A basaltic edifice is composed primarily of mechanically similar layers so that, once initiated, landslide faults, ring faults, and dikes have a comparatively high probability of reaching the surface, as indicated here by the many feeder dikes. (b) A stratovolcano is composed of mechanically dissimilar layers so that faults and dikes have comparatively low probability of reaching the surface, as indicated here by the many arrested and deflected (into sills) dikes.