The Black Tusk, a black pinnacle of volcanic rock on the northwestern shore of Garibaldi Lake, is the glacially eroded remnant of a much larger volcano that formed during two periods of volcanic activity. The first between 1.1 and 1.3 million years ago erupted hornblende andesite lava flows and tuffs. These volcanics compose mountain ridges southwest, southeast and northwest of the prime volcanic structure. Subsequent erosion demolished the newly formed volcano. This ultimately exposed the roots of the cone, which currently form the rugged edifice of The Black Tusk. After the cone was eroded, a series of hypersthene andesite lava flows were erupted between 0.17 and 0.21 million years ago. These end at adjacent ice-marginal lava flows that form cliffs. This eruptive phase also produced a lava dome that comprises the current high pinnacle. Consequently, the regional Late Pleistocene ice sheet carved a deep north-trending U-shaped valley into the eastern flank of the second-stage cone. Here, subsequent lava flows from Cinder Cone filled the valley.