Simple experiments were carried out to study the fracturing of the outer crust of lava domes during emplacement. Analog magma was injected vertically from a reservoir into a feeder conduit and flows on a rigid planar base. A cohesive mixture of sand and flour poured on the dome simulated the lava dome crust generated by cooling. Results showed that two opposite end members have to be considered: symmetrical versus asymmetrical deformation for thin and thick brittle shells, respectively. Thin crusts produce gently dipping slopes with mainly radial fractures. In contrast, thick crusts generate steeply dipping flank slopes on which deformation is restricted. Between the end members, there is a general change from one style to the other. For natural domes, the experiments indicate that a certain thickness of crust is necessary to produce explosive activity: thick crust will cause more violent events.