View to the west across the ice-filled summit caldera of Mt. Wrangell, which is a large shield volcano with a volume of about 900 km³. The 4 by 6 km caldera apparently formed when magma withdrew from reservoirs beneath the summit area and not from a violent explosive eruption. Three small post-caldera craters, all geothermally active, occur along the west and north margin of the caldera. Mt. Drum is in the right background.

Historical activity, which has been limited to fumaroles and minor phreatic (steam) eruptions in the summit craters, apparently waxes and wanes in response to changes in the summit heat flux. Some evidence suggests that increases in the heat flux and concomitant increases in fumarolic and phreatic activity have followed major earthquakes in south-central Alaska. Photographs of the ash-covered summot of Mt. Wrangell that appeared in early 20th century reports may reflect an increase in activity following the September 3, 1899, Yakutat earthquake. Earlier reports of eruptive activity in 1784 and 1884-85 suspect.

Mt. Wrangell lavas range from basalt to dacite (52 to 66 percent SiO₂), but it has erupted predominantly andesite lava.