Seismology of the lower mantle and core‐mantle boundary

The thermal and chemical processes operating near the core‐mantle boundary (CMB) play a major role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the configuration of mantle convection. The past decade has witnessed extensive investigation of the core‐mantle transition zone using seismological, geodynamical, geomagnetic, and mineral physics experimental techniques. The many multidisciplinary contributions on this region have been surveyed in several recent review articles (e.g. Loper and Lay, 1994; Jeanloz and Lay, 1993; Bloxham and Jackson, 1991) which provide a comprehensive survey of the recent literature. I confine my attention here to recent seismological evidence for laterally heterogeneous structure in the D″ region, the lowermost 200 km of the mantle. While seismic models for D″ are still low resolution, we have attained general constraints on the extent of heterogeneity which raise provocative questions as to the origins of the complex structures. The idea that chemical and thermal heterogeneity in D″ is a manifestation of boundary layer processes is a long‐standing one (Bullen, 1949), but we are far short of quantifying this notion. In particular, it is not known whether heterogeneities in D″ are relics of subducted slabs, in‐situ accumulations of core‐mantle chemical reaction products, primordial residuum from the core‐formation process, or thermal heterogeneities within a hot, unstable thermal boundary layer.