Three Fingered Jack shows a similar eruptive history to many High Cascade volcanoes. Activity began with the formation of a pyroclastic cone over shield volcanoes. Later eruptions formed a main volcanic cone from lava flows and more pyroclastic rock. There are dikes that radiate from a micronorite plug, which deformed tephra in the surrounding strata. The northern and southern flanks of Three Fingered Jack feature lava flows made of olivine and augite basalt.