The volcanically active axes of mid-ocean ridges are very narrow (<1 km), so existing theoretical models to explain magma supply to active mid-ocean ridges feature either a narrow zone of mantle upwelling beneath the axis or a broader zone of upwelling, with an additional process needed to focus melt migration narrowly at the axis. Seamounts near active mid-ocean ridges also represent zero-age active volcanism in the plate-boundary zone. Their distribution and lava chemistry tend to favor a broad axial zone of upwelling. Furthermore, average near-East Pacific Rise seamount lavas apparently result from shallower mantle melting than the East Pacific Rise axis lavas. This suggests that mantle upwelling beneath the axis is not perfectly adiabatic. Instead, the broad mantle upwelling is cooler at the edges than in the center.