AbstractCharacterizing the physical properties and mechanical behavior of melt reservoirs is essential for enhancing geophysical models that aim to understand the evolution of subvolcanic systems and support hazard forecasting. Increasing evidence suggests that shallow magmatic reservoirs consist of variably packed crystal frameworks with small volumes of interstitial melt, commonly referred to as “mushes.” Current volcano deformation models often implement static magma sources with a cavity and thus provide little insight into dynamic internal reservoir processes; they also ignore the presence of crystals, melt and other fluids, and therefore the likely poroelastic mechanical response to melt addition or withdrawal. Here we investigate the influence of poroelastic mechanical behavior on reservoir pressure evolution and resultant spatio‐temporal surface deformation. We consider the melt reservoir to be largely crystalline (10%–50% melt fraction) with melt distributed between crystals; we show that the presence of crystals affects the spatial and temporal mechanics of magma reservoir behavior. In contrast to classical models for volcanic surface deformation, our results suggest that a poroelastic surface deformation response continues to develop after withdrawal/upward emplacement of melt has terminated, and importantly that the withdrawal/injection point can affect the evolution of the relative magnitudes of vertical and radial deformation over time. These protracted displacements are caused by melt diffusion, which depends principally on mush hydraulic properties and melt characteristics. Following an intrusion/withdrawal event, a steady state is eventually reached when the fluid pressure is uniform in the mush reservoir.