Mount Garibaldi, one of the larger volcanoes in the southern Garibaldi Belt with a volume of , is composed of dacite lavas that were erupted in the past 300,000 years. It was constructed when volcanic material erupted onto a portion of the Cordilleran Ice Sheet during the Pleistocene period. This created the unique asymmetrical shape of the mountain. Successive landslides on Garibaldi's flanks occurred after glacial ice of the Cordilleran Ice Sheet retreated. Subsequent volcanism about 9,300 years ago produced a long dacite lava flow from Opal Cone on Garibaldi's southeastern flank. This is unusually long for a dacite flow, which commonly travel only short distances from a volcanic vent due to their high viscosity. The Opal Cone lava flow represents the most recent volcanic feature at Mount Garibaldi.