Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Abstract Continually improving resolution of lower mantle structure and understanding of mantle dynamics suggest that, as in the plate‐tectonics era, the upper and lower mantle may undergo little exchange. Although the phase transition at ~660 km forms the sharpest seismologically mapped boundary within the mantle, another at 1,000 (±100) km might be more important for geodynamics. Contrasts in density and viscosity may limit penetration of upper and lower mantle through this boundary. Strong evidence, if still not definitive, calls for chemical heterogeneity of the lower mantle that manifests itself in a few energetic plumes rising from near the core mantle boundary and slowly evolving, larger piles or blobs of chemically distinct material. Those blobs or piles may deform little on billion‐year timescales. Convection in the lower mantle might dictate some aspects of flow in the upper mantle, such as the positions of plumes and hot spots and the breakup of supercontinents, but its contribution to the speeds and directions that plates move might be negligible. The improved understanding of mantle convection has not overturned the simple, 50‐year‐old image that plates “drive themselves,” with forces per unit length pushing them apart at spreading centers, excess mass in downgoing slabs pulling them down, and viscous drag on their bases (and tops at subduction zones) retarding movement. Remaining challenges include determining how convection in the lower mantle affects geologic history and using that history to constrain lower mantle dynamics.