Iliamna volcano is a large stratovolcano on the western side of the Cook Inlet, in Lake Clark National Park 220 km SW of Anchorage or 100 km NW of Homer. There are several uncertain reports about activity in the past 250 years, but the only confirmed known historical eruptions were small to moderate explosive eruptions in 1876, 1867, and between 778 and 1879. A larger eruption with pyroclastic flows was dated by radiocarbon method and occurred between 300 and 140 years ago, but was not observed. Most of the sparse eruption reports from Iliamna are probably referring to strong activity of fumaroles located east and SE of the summit.

Iliamna volcano has no distinct summit crater, but forms a flat top joined on the south by a 5 km long ridge with the prominent flank lava domes of North and South Twin Peaks. Another important flank vent is the Johnson Glacier dome on the NE flank.

Steep headwalls on the southern and eastern flanks expose an inaccessible cross-section of the volcano.

Several major glaciers radiate from its summit; the 4 largest together contain an estimated volume of 15 cubic km of ice.

Eruptions producing pyroclastic flows have been dated at as recent as about 300 and 140 years ago (into the historical period), and elevated seismicity accompanying dike emplacement beneath the volcano was recorded in 1996.