We have many reasons to believe that the history of the development of the earth's crust is fundamentally dependent on processes in the upper mantle to a depth not exceeding 1000 km. Because of this relation, the Upper Mantle Project was organized as an international program of geophysical, geochemical, and geological studies concerning the 'upper mantle and its influence on the development of the earth's crust.'

Many important results have emerged during the course of the Upper Mantle Project. Probably the most significant is the delineation in some detail of the 'fine structure'—the lateral and vertical variations of properties—of the upper mantle. These results have fully convinced us that there is a fundamental relation between this fine structure, the motions of materials in the upper mantle, and activities at or near the surface of the earth such as earthquakes (and the types of earthquake motions), volcanism, the building and deformation of mountains, and the differentiation of various rock types, including concentrations that are of economic interest.