A lava dome was built in the crater of Mount St. Helens by intermittent intrusion and extrusion of dacite lava between 1980 and 1986. Spectacular ground deformation was associated with the dome building events and included the development of a system of radial cracks and tangential thrust faults in the surrounding crater floor. These cracks and thrusts, best developed and studied in 1981-1982, formed because the crater fill was displaced upward and radially outward from the feeder conduit, owing to rising magma. Radial cracks formed first and, as some evolved into strike-slip tear faults, influenced the subsequent geometry of thrusting. Once faulting began, deformation was localized near the thrust scarps and their bounding tear faults. The magnitude of displacements systematically increased before extrusions, whereas the azimuth and inclination of displacements remained relatively constant. The thrust-fault scarps were bulbous in profile, lobate in plan, and steepened during continued fault movement. The hanging walls of each thrust were increasingly disrupted as cumulative fault slip increased. The crater fill was unconsolidated to weakly consolidated near the surface and influenced the morphology of the thrust-fault scarps. Similar structures have been produced by active thrust faults in other areas with unconsolidated alluvium at the surface, and in laboratory sandbox experiments.