The arrival of a new hot batch of magma changes the thermal and the chemical state of a volcanic plumbing system. Formation of hybrid magma by chemical dilution (liquid miscibility model) of different magma batches is generally accepted. Observations of drop-like domains of less evolved (newer) magma in more evolved (host) magma point to the contribution of a mechanical mixing process (liquid immiscibility). The interplay of both mechanisms seems to control the time scale from the dormant to the active state of a volcano. We carried out laboratory experiments using melts from natural volcanic rock compositions with the aim to validate model calculations for shear rate dependent domain sizes. The results confirmed the model and also the importance of hydro-mechanically mixing for chemical dilution. The key process, however, that is able to change the state of a magma reservoir on a critical time scale is hydrodynamic magma mixing.