The volcano is a typical composite stratovolcano composed of interbedded lava flows and pyroclastic rocks. Condensate plumes have been observed extending to an estimated 1000 m above the mountain. Except for the summit fumarolic activity, it is uncertain and perhaps unlikely that Iliamna Volcano has been historically active. Most of the flows are light gray and range in thickness from 25 to 120 m. The lava flows contain phenocrysts of plagioclase and subordinate hypersthene and augite in a dense pilotaxitic groundmass composed of randomly oriented microlites with relatively little glass.
Sparse olivine occurs in some of the flows but is generally less than one percent of the mode.