THE CORE-MANTLE BOUNDARY REGION

The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of th e transition zone between the Earth's core and mantle, The core-mantle boundary (CMB) is the most significant internal boundary within our p lanet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding o f the dynamic Earth system. The thermal and chemical processes operati ng near the CMB have intimate relationships to fundamental events in E arth history, such as core formation, and continue to play a major rol e in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gra vitation of the planet, and the mode of thermal convection of the Eart h. The D '' region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and s mall scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge tha t we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious in creases in data, computational power, and experimental methodologies u sed to investigate deep earth structure and processes. In the past dec ade in particular there have been unprecedented multidisciplinary adva nces in understanding the CMB region, and there are excellent prospect s for developing a comprehensive understanding of this region in the n ext few decades. Fundamental issues yet to be resolved include the cau ses of layering extensively observed in D '', whether or not downwelli ng slabs accumulate at the base of the mantle, whether plumes arise fr om the CMB region to feed hotspots at Earth's surface, the extent of c ore-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to whic h mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophy sical investigations of the CMB region.