To the northwest, the Mount Cayley massif constitutes the largest and most persistent volcano in the central Garibaldi Belt. It is a highly eroded stratovolcano composed of dacite and rhyodacite lava that was deposited during three phases of volcanic activity. The first eruptive phase started about four million years ago with the eruption of dacite lava flows and pyroclastic rock. This resulted in the creation of Mount Cayley itself. Subsequent volcanism during this volcanic phase constructed a significant lava dome. This acts like a volcanic plug and composes the lava spines that currently form pinnacles on Cayley's rugged summit. After Mount Cayley was constructed, lava flows, tephra and welded dacite rubble were erupted. This second phase of activity million years ago resulted in the creation of the Vulcan's Thumb, a craggy volcanic ridge on the southern flank of Mount Cayley. Lengthy dissection from an extended period of erosion demolished much of the original stratovolcano. Volcanic activity after this prolonged period of erosion produced thick dacite lava flows from parasitic vents 300,000 years ago that extended into the Turbid and Shovelnose Creek valleys near the Squamish River. This subsequently created two minor parasitic lava domes 200,000 years ago. These three volcanic events are in contrast to several others around Cayley in that they do not show signs of interaction with glacial ice.