Whistler Mountain - Geology

Geology

Whistler Mountain contains shale that originated as mud on the seafloor of a former ocean. This same shale formation forms rocks in other locations throughout southwestern British Columbia. The most common rocks that comprise Whistler Mountain are andesite and dacite lava flows. These lava flows and the associated shale form part of a rock assemblage called the Gambier Group. This geologic group was created within a shallow underwater basin about 100 million years ago during the Early Cretaceous period. Granular material, such as clay, sand and slit, was carried into the ancient ocean by rivers that existed during the Cretaceous period. As Cretaceous rivers continuously sent granular material into the former ocean, it was deposited yearly to eventually form layers of sedimentary material. Once the sedimentary material was compressed, it created the shale that now forms portions of Whistler Mountain. The andesite and dacite lava flows were deposited when volcanic eruptions created a series of volcanic islands and produced lava flows in the ancient ocean.

Once the volcanic and sedimentary rocks of the Gambier Group were formed, they began to deform, crumple and uplift due to the extreme pressures created by movement of the North American Plate and the tectonic plates in the Pacific Ocean. The large masses of solidified lava that formerly created the volcanic island chain and underwater lava flows yielded by demolishing into massive, mountain-sized blocks while the less dense, thinly layered shale was compressed, folded and crushed between the associated lavas. In contrast, nearby volcanic landforms in the Garibaldi Lake area, such as Black Tusk, are of relatively recent volcanic origin and form part of a chain of volcanoes called the Garibaldi Volcanic Belt.