A model is proposed to explain temporal patterns of activity in a class of periodically exploding Strombolian-type andesite volcanoes. These patterns include major events (explosions) which occur every 3–30 min and subsequent tremor with a typical period of 1 s. This two-periodic activity is thought to be caused by two distinct mechanisms of accumulation of the elastic energy in the moving magma column: compressibility of the magma in the conduit and viscoelastic response of the almost solid magma plug on the top. A release of the elastic energy occurs during a stick–slip dynamic phase transition in a boundary layer along the walls of the conduit; this phase transition is driven by the shear stress accumulated in the boundary layer. The intrinsic hysteresis of this first-order phase transition explains the long periods of inactivity in the explosion cycle. Temporal characteristics of the model are found to be qualitatively similar to the acoustic and seismic signals recorded at Karymsky volcano in Kamchatka.