At least two phases of volcanic activity have been identified at The Volcano. Each event was followed by the eruption of lengthy basaltic lava flows that flowed down steep granitic flanks of the mountain ridge on which The Volcano lies. After this took place, they travelled through the Lava Fork valley for . Here, the flows crossed the British Columbia border into the U.S. state of Alaska and blocked the Blue River, a tributary of the Unuk River, forming several lakes. The lava flows in total are about long and still contain their original features from when they cooled, including pressure ridges and lava channels. A series of large trees were engulfed by the lava flows during eruption. The bases of the trees burned and the upper trunks and branches collapsed into the solidifying lava, leaving the trees embedded on the surface of the lava flows. After the flows solidified, tree molds and lava tubes collapsed to form volcanic pits. At the southern end of one of the lava flows, it spreads into a broad terminal lobe on the flat alluvial plain of the Unuk River. Volcanic ash and lava from The Volcano still linger on small glaciers near Mount Lewis Cass, a mountain near the Alaska-British Columbia border.