Formation and Eruptive History
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 spewed out about 850 km3 (200 cu mi) of glowing avalanches of pyroclastic rock called pyroclastic flows. 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 19 km (12 mi) by 30 km (19 mi) across. The calderas were from 200 m (656 ft) to 2,700 m (8,858 ft) deep. Volcanism continued for some time after the caldera collapse. High level andesite and rhyolite dikes and intrusive bodies crosscut volcanic flows and tuffs at all levels. Dike swarms are emplaced along ring fractures and fault zones 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 1,500 m (4,921 ft).
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