Schematic section through a hemispherical dome of external radius b overlying a gas‐pressurized vertical conduit of radius a. The conduit cap is harmonically pressurized as p_a=p_i+cos 2πωt, generating a gas uplift force, P, and rear‐scarp gas, F^g_m, or magma, F^m_m, disturbing forces acting on the wings of a failing block of weight W and included sector angle ξ resting on a detachment surface inclined at an angle α to the horizontal. Failure initiates on release of the block toe, with failure retrogressing to unroof the pressurized core, resulting in the potential for spontaneous disintegration or a directed explosion.