The main volcanics at Hoodoo Mountain are phonolite and comendic trachyte lavas and hyaloclastites, although some pyroclastic rocks also exist. The trachyte and comendic magmas erupted from Hoodoo Mountain and other Northern Cordilleran volcanoes are thought to have been created by fractionation of primary alkali basaltic magma in crustal reservoirs. The estimated volume of erupted material from Hoodoo Mountain is , and at least six phases of eruptive activity are recognized at the volcano. The first phase of eruptive activity occurred 85,000 years ago in the Late Pleistocene stage of the Pleistocene epoch, producing lava flows, lava domes, and breccia deposits. These volcanics were erupted under more than of glacial ice to produce subglacial eruptions.