Geophysical evidence has accumulated rapidly in the last few years for hypotheses of plate tectonics, which include the ideas of continental drift, sea-floor spreading, transform faulting, underthrusting of the lithosphere at island arcs and the coherent movements of large blocks of the crust and uppermost mantle. The success of these concepts in explaining and synthesizing a great amount of geophysical data strongly suggests that these ideas should have broad application in geology. Plate tectonics, which is largely a kinematic description of relative movements of the earth's surface, also has stimulated interest in the processes driving plates and in the mechanical properties of the upper mantle below the lithosphere.
To explore some of these new directions a Symposium on Mechanical Properties and Processes in the Mantle was held in Flagstaff, Arizona, June 29–July 3, 1970, under the auspices of the International Upper Mantle Project and the U.S. National Academy of Sciences. This was the first major upper mantle symposium held in the United States. The scope of the conference included physical properties of materials likely to be present in the mantle, rocks of possible mantle origin and their mode of emplacement, convection currents, and other possible driving forces of global tectonics. The conference was not intended to be a review of geophysical evidence for plate movements, but rather to seek out new directions influenced by these ideas.h such special areas as satellite altimetry.