Geology of The Pacific Northwest - Coast Mountains

Coast Mountains

The Coast Mountains are the western range of the North American mainland cordillera, covering the Alaska Panhandle and most of costal British Columbia. The range is approximately 1,600 km (1,000 mi) long and 200 km (120 mi) wide.

Most of the Coast Mountains are composed of granite, which is part of the Coast Plutonic Complex. This is the single largest contiguous granite outcropping in the world, which extends approximately 1,800 km (1,100 mi) in length. It is a large batholith complex. Its formation is related to subduction of the Kula and Farallon tectonic plates along the continental margin during the Jurassic-to-Eocene periods. The plutonic complex is built on unusual island arc fragments, oceanic plateaus and continental margin assemblages accreted between the Triassic and the Cretaceous periods. In addition, the Garibaldi, Meager, Cayley and Silverthrone areas are of recent volcanic origin.

The Coast Mountains consist of a single uplifted mass. During the Pliocene period the Coast Mountains did not exist and a level peneplain extended to the sea. This mass was uplifted during the Miocene period. Rivers such as the Klinaklini River and Homathko River predate this uplift and due to erosion occurring faster than uplift, have continued to flow right up to the present day, directly across the axis of the range. The mountains flanking the Homathko River are the highest in the Coast Mountains, and include Mount Waddington west of the river in the Waddington Range and Mount Queen Bess east of the river, adjacent to the Homathko Icefield.

The Pacific Ranges in southwestern British Columbia are the southernmost subdivision of the Coast Mountains. It has been characterized by rapid rates of uplift over the past 4 million years unlike the North Cascades and has led to relatively high rates of erosion.