Structure of the Core- Mantle Transition Zone: A Chemical and Thermal Boundary Layer

A remarkable diversity of geophysical techniques is being used to probe the tran sition zone between the outer core and the lower mantle. This inaccessible re gion, 2900 km below the Earth's surface, is now recognized to profoundly influ ence the style of mantle convection, thermal and chemical plume formation, sec ular variation, and possibly reversals of the magnetic field, core-mantle exchanges of angular momentum, long-wavelength gravitational variations, and chemical evolution of the Earth. Establishing the thermal and chemical structure of the lowermost mantle and outermost core is critical to our understanding of the dynamic processes near this transition zone, and progress is now accelerating as a result of an interdisciplinary approach. Recent contributions from seismolo gy, mineral physics, geomagnetism, and geodynamics are synthesized in this arti cle, and a model with a combined thermal and chemical boundary layer at the core-mantle boundary is proposed. The recent formation of the IUGG and AGU committees for SEDI (Studies of the Earth's Deep Interior) is facilitating commu nication in this and related interdisciplinary efforts to understand how the interi or of the planet works