Initial volcanic activity of the Mount Cayley massif 4,000,000 years ago corresponded with changes to the regional plate tectonics. This involved the separation of the Explorer and Juan de Fuca plates off the British Columbia Coast, which had some significant ramifications for regional geologic evolution. After this reorganization ceased, volcanism shifted westward from the Pemberton Volcanic Belt to establish the younger and currently active Garibaldi Volcanic Belt. The westward shift in volcanism may have been related to steepening of the Juan de Fuca slab after the formation of the Explorer Plate.