In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution1, 2, 3. V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume4, 5, 6, 7. Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle8. Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years—providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5–10 °C. From 35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25–30 °C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core–mantle boundary.