Mercury appears to have a tectonic framework and diastrophic history not found on other terrestrial planets explored to date (earth, Mars, and the moon). On the part of the planet viewed by Mariner 10, only two localized areas show evidence of tensional stresses, both of which are apparently associated with the Caloris basin. Lobate scarps occur in the remainder of the explored region and appear to be primarily reverse or thrust faults which have resulted from compressive stresses acting on a global scale. The period of compression represented by these scarps occurred during the final phase of heavy bombardment on Mercury and was probably caused by crustal shortening due to a small decrease in the planet's radius. Stratigraphic, volumetric, and albedo considerations together with distribution indicate that the majority of smooth plains on Mercury were produced by volcanism which occurred at the close of the period of late heavy bombardment similiar to that on the moon and Mars. Several generations of plains are evident; the oldest may have resulted in part from an early differentiation of the planet.