Mantle plumes are buoyant upwellings of hot rock that transport heat from Earth's core to its surface, generating anomalous regions of volcanism that are not directly associated with plate tectonic processes. The best-studied example is the Hawaiian-Emperor chain, but the emergence of two sub-parallel volcanic tracks along this chain(1), Loa and Kea, and the systematic geochemical differences between them(2,3) have remained unexplained. Here we argue that the emergence of these tracks coincides with the appearance of other double volcanic tracks on the Pacific plate and a recent azimuthal change in the motion of the plate. We propose a three-part model that explains the evolution of Hawaiian double-track volcanism: first, mantle flow beneath the rapidly moving Pacific plate strongly tilts the Hawaiian plume and leads to lateral separation between high- and low-pressure melt source regions; second, the recent azimuthal change in Pacific plate motion exposes high- and low-pressure melt products as geographically distinct volcanoes, explaining the simultaneous emergence of double-track volcanism across the Pacific; and finally, secondary pyroxenite, which is formed as eclogite melt reacts with peridotite(4), dominates the low-pressure melt region beneath Loa-track volcanism, yielding the systematic geochemical differences observed between Loa-and Kea-type lavas(3,5-9). Our results imply that the formation of double-track volcanism is transitory and can be used to identify and place temporal bounds on plate-motion changes.