Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure). ... ... Lay et al (3 authors at 2 installations, US) review recent ideas concerning the core- mantle boundary layer and deep Earth dynamics. The Earth acquired very early in its history its primary layered structure, which consists of a molten metallic alloy core overlain by a thick shell of silicates and oxides, and the region near the core- mantle boundary surface has undoubtedly played a significant role in both the core and mantle dynamic systems through their subsequent 4.5 billion years of evolution. Recent seismological work has revealed new structures in the boundary layer between the Earth's core and mantle, and the observation are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. The authors suggest that clear challenges for future research in seismological, experimental, theoretical, and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological phenomena observed at the Earth's surface. It seems likely that the core- mantle boundary is about to replace the transition zone between Earth's upper and lower mantle as the region most likely to hold the key to a large number of geophysical problems.

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