Lithospheric thickness can act as an important physical control on the composition of asthenosphere-derived melts by restricting the minimum depth at which decompression melting can occur. A case study of plume-related Tertiary lavas from the Isle of Mull, Scotland, has revealed the presence of three successive magma types, which become progressively more depleted in the incompatible trace elements. Geochemical modeling suggests that melting initially took place within the garnet-Iherzolite stability field but subsequently involved more extensive melting (>10%) of spinel Iherzolite at shallower mantle depths. These results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center. The observed degree of lithospheric thinning cannot be explained by extension alone, and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required. Theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process, and this has important implications for the various starting plume models.