Pyramid Mountain, once mistaken for a cinder cone, is now known to have formed below several thousand metres of glacial ice. Hemmed in by the surrounding ice, the volcano erupted vigorously, creating layers of glassy, but vesicular, scoria intermixed with smoothed cobbles of granite and metamorphic rock. These rocks were picked up by a glacier many kilometres away and melted out of the ice because of heat from the volcano. Although it had a vigorous start, the eruption that formed Pyramid Mountain was not sufficiently sustained to form a larger edifice that could break through the surrounding ice and water to form a tuya.