Geological History
These Insular Mountains are not yet fully emerged above sea level, and Vancouver Island and the Haida Gwaii are just the higher elevations of the range, which was in fact fully exposed during the last glacial period (maximum ice extent about 18,000 years ago) when the continental shelf in this area was a broad coastal plain.
The Insular Mountains formed when a chain of active volcanic islands (the Insular Islands) collided against the North American continent during the mid Cretaceous period. The type of rocks that form the Insular Mountains are turbidites and pillow lavas. Granitic plutons seldom occur in the Insular Mountains, unlike the Coast Mountains. The Insular Mountain range covers some 133,879 km² (51,691 sq mi). It experiences frequent seismic activity, with the Pacific Plate and the Juan de Fuca Plate being subducted into the Earth's mantle. Large earthquakes have led to collapsing mountains, landslides and fissures.
During the last glacial period, ice enclosed nearly all of these mountains. Glaciers that ran down to the Pacific Ocean sharpened the valley faces and eroded their bottoms. These valleys were transformed into fjords when the ice melted and the sea level rose. Ice Age remnants may still be noted, such as the Comox Glacier in the Vancouver Island Ranges.
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