Stawamus Chief - Geology

Geology

The Chief is part of a medium-sized pluton of a granitic rock (granodiorite) that was initially formed in the early Cretaceous (approximately 100 million years ago) by the slow cooling and solidification of molten magma deep below the surface of the Earth. Exhumation of the granite body probably occurred mostly by erosion of overlying rocks over tens of millions of years, with glacial erosion processes dominating exhumation over the last 2.5 million years. Once exposed at the surface, the original granite body was shaped by glacial erosion, which is responsible for the tall steep walls that define the Chief, as well as the excavation of Howe Sound, a fjord. Classic hallmarks of glacial erosion are ubiquitous, especially polished, striated surfaces. Polish and striations observable at the very summit of the formation require that, at the peak of glaciation, the entire formation was buried under a substantial thickness of ice.

The striking gullies that separate and define the three summits of the Chief are the result of fracturing and mass-wasting of large blocks along a series of vertical, and roughly north-south oriented deep seated fracture sets (joints).

The Chief may be the root of an extinct volcano because no volcanic activity has occurred in the Squamish area from about 86 million years ago to the beginning of Garibaldi Volcanic Belt volcanism about 2-3 million years ago. In the Squamish area, Garibaldi Volcanic Belt volcanism ceased during or shorty after the end of the last ice age.