The Bennett Lake Volcanic Complex was formed when the ancient Kula Plate was subducting under North America during the early Eocene period. Cataclysmic eruptions from the Bennett Lake Volcanic Complex were from vents along arcuate fracture systems that ejected out about of glowing [[avalanche]]s of [[pyroclastic rock]] called [[pyroclastic flow]]s. Evacuation of the underlying [[magma chamber]] was followed by several stages of collapse to form two calderas, one nested inside the other, that produced an elliptical depression by across. The calderas were from to deep. Volcanism continued for some time after the caldera collapse. High level [[andesite]] and [[rhyolite]] [[dike (geology)|dike]]s and intrusive bodies crosscut volcanic flows and [[tuff]]s at all levels. [[Dike swarm]]s are emplaced along ring fractures and [[fault zone]]s at the southwest edge of the caldera. Near the dying stages of the [[volcano]], [[magma]] surged upward and arched the roof of the magma chamber into a broad dome with relief of about .