Equations of state of the Earth

The major results of research in the United States for 1971–1974 in the areas of static, high‐pressure X ray, ultrasonic and shock wave measurements, and progress in theoretical equations of state are summarized. This 4‐year period has seen the massive accumulation of new data pertinent to describing the properties, and to some extent geodynamic processes, of the upper mantle as well as exposing the gauntlet of scientific challenge with regard to our lack of detailed understanding of the major changes in crustal seismic velocities, which seem to offer so much promise for earthquake prediction. In the area of very high pressure geophysics, another challenging problem uncovered is the question of explaining the low adiabatic gradient in the outer core, which seems to be required to retain liquid iron alloy material at the melting point. Important advances in what until recently has been quite empirical elastic moduli ‘systematics’ have demonstrated how ionic radii and ionic packing densities may be quantitatively employed to predict unmeasured equations of state as well as tie together an ever increasing body of data for the elasticity of compounds which are crystal‐chemical analogs of mantle minerals.