Using global correlations of effusive eruption style with convergence rates and surface heat flux at volcanic arcs, this study identifies several arcs with excess surface heat flux, related to the presence of unusually hot and H2O-poor magmas. Geophysical evidence suggests that these melts are linked to discontinuities in the subducting slab, which would facilitate upwelling of hot sub-slab asthenospheric mantle. It is demonstrated that excess heat flux arcs are sites where Sr-rich adakitic compositions are generated, and that the arc proportion erupting Sr-rich adakitic magmas is directly proportional to the degree of observed heat flux excess. A model is presented in which the formation of Sr-rich adakitic compositions, which require garnet as a residual phase, is facilitated at excess heat flux arcs as a result of a decrease in H2O content through contributions from sub-slab asthenospheric mantle, leading to an expansion of the garnet stability field. Future geochemical, geophysical and experimental work required to test this model is addressed.