The origin of hotspot trails ranges controversially1 from deep mantle plumes rising from the core-mantle boundary2 to shallow plate cracking. But these mechanisms cannot explain uniquely the scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell3, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province, which marks a massive thermo-chemical ‘pile’ at the core-mantle boundary4,5,6. Here we use 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – to show that age progressive hotspot trails formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province. We show also that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We conclude that if plume melts formed synchronous age progressive hotspot trails wherever and whenever they could penetrate the swell lithosphere then hotspot trails in the South Atlantic are controlled by an interplay between deep plumes and the motion and structure of the African plate.