How Thermochemical Piles Can (Periodically) Generate Plumes at Their Edges

Deep‐rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core‐mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma. Plain Language Summary Deep‐rooted upwellings in the Earth's mantle, so‐called “plumes”, are responsible for volcanism such as Hawaii and seem to cluster around two continent‐sized regions at the core‐mantle boundary that show anomalously low seismic velocities. These regions have been suggested to have a different composition than the surrounding mantle, causing them to be denser and stiffer which allows them to survive billions of years at the base of the mantle without being completely eroded. In this study, we use numerical simulations to show that mantle plumes and dense piles at the core‐mantle boundary interact with each other, potentially resulting in a periodic plume initiation. A starting upwelling increases the pile thickness locally by pulling dense pile material upward. It then cools down, causing a density increase that results in gravitational collapse of the dense material toward the core‐mantle boundary. As a result, this material spreads along the b