The Mull Tertiary volcano lies on the eastern edge of the North Atlantic Igneous Province, and was active soon after the initiation of the Iceland plume. Detailed sampling of the Mull lava pile has allowed temporal changes in the mantle source to be determined. A significant change at 700 m above sea level from N‐MORB‐like to ‘Icelandic’ marks the first arrival beneath the region of mantle from the core of the ancestral plume. The earlier basalts represent an outer plume sheath of heated upper mantle. From the initiation of magmatism in Mull, it took 1.9±0.4 Ma for this transition to occur.