Observations related to flood basalts suggest the existence of mantle plumes with large heads and thin trailing tails (cavity plumes). Seismic data suggest the existence of mantle plumes with thick tails (diapir plumes). The conditions required for diapir versus cavity plume generation are different, and in a chemically homogeneous mantle both types are not predicted to coexist. We show, however, that if a variable thickness chemical layer exists at the base of the mantle, consistent with seismic observations, then the coexistence of morphologically distinct plumes types is expected. The chemical layer governs temperature, and thus viscosity variations, in the thermal boundary layer from which mantle plumes rise. A locally thick layer leads to small viscosity variation instabilities and hence to diapir plumes. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. A chemical layer that can move in response to changing flow patterns allows for the potential that plumes can morphologically transition over their lifetimes. An expectation that the morphology and thermal structure of mantle plumes should vary according to the thickness of a chemical layer is consistent with correlations between seismic observations of chemically distinct material at the core-mantle boundary, the varied morphology of mantle thermal anomalies, and the inferred diversity in hotspot buoyancy fluxes and excess temperatures.