The caldera complex started forming 42 million years ago in the Eocene Period with the formation of a laccolith.
The final ash flow sequence resulted in formation of two additional calderas on the crest of the dome.
Further out, from 18 to 23 kilometers (11 to 14 mi), the apron consists mostly of mudflow and hyperconcentrated flow deposits separated by distinct disconformities, erosional surfaces, that show that rapid deposition of sediments during episodes of volcanism was interrupted by periods of erosion.
The most distant part of the apron, from 30 to 35 kilometers (19 to 22 mi) from the caldera complex, is composed of a sequence of mudflow and pumice air fall deposits overlain by sheet flood and stream channel deposits with no distinct disconformities.
Laccocalderas differ from conventional calderas, being relatively small and developing over thin, shallow laccolithic magma chambers rather than deeper bodies under Earth's crust.