A fundamental question regarding the dynamics of mantle convection is whether some intraplate volcanic centers, known as “hotspots,” are the surface manifestations of hot, narrow, thermally driven upwellings, or plumes, rising from the lower mantle. Shown here is a global negative correlation between the thickness of the mantle transition zone (near 410–660 km depth) and petrologically determined potential temperatures of mid-ocean ridge and hotspot magmas. Hotspot potential temperatures are systematically higher than those for mid-ocean ridges, and the transition zone thicknesses beneath these hotspots are thinner. Thus, the majority of oceanic intraplate magmatic centers are associated with deep-seated thermal anomalies, suggesting that such magmatism is probably associated with thermal plumes.