The massive Chief rock of Squamish, British Columbia [Canada]
Intro: There is a giant rock that you can’t ignore along the famous Sea to Sky highway in British Columbia (B.C.), Canada. This is called “the Chief” and is actually a glacially exposed ancient magma chamber! It is well known to the locals in Squamish, B.C., which is my home-town. Thus, as I recently came back to Canada for the Geological Society of America (GSA) conference in Vancouver, I got to spend some time back home. Though I’ve hiked it many times growing up, I don’t think I’ve quite appreciated the awesomeness that is this giant granodiorite intrusion. And with a newly built gondola providing accessible views, I think this is a good time for a quick blog about the how this giant rock came to be, from its magmatic beginnings to its glaciated and carved present.
Science Spiel: A glaciated volcanic heart (Geology, Geomorphology)
The Chief formed ~ 100 million years ago (mya) at depths of 15 to 30 kilometres beneath the earth’s surface. This was during a highly active period on the west coast of Canada that was subjected to scores of magmatic activity (plutonsim) (Rusmore & Woodworth, 1991; Monger & Journeay, 1993). These plutons are expressed as the beautiful mountains of the northwest coast of North America that range from B.C. through Yukon and Alaska (Monger et al., 1982). The Stawamus Chief (known as the Squamish Chief) crystallized slowly from a felsic igneous stock with a granodiorite composition (i.e. mostly consisting of the interlocking minerals of quartz, feldspar and biotite). It was intruded by multiple dark mafic dykes around ~ 40 mya.
How it came to be protruding 700 m above the highway today is owe to the extensive glaciation history from 2.5 mya to 10,000 years ago. As glaciers advanced and retreated over the land they carved and plucked steep slabs off the face of The Chief. This plucking was synchronous with exfoliation weathering due to the expanding and cracking response of minerals and rocks at the surface which initial formed under higher temperatures and pressures. The light patches on the face of the Chief (i.e. what the local’s call the “witch” or “face” in the Chief) is an example of recent slab breakage that exposed fresh (i.e. less-weathered), lighter coloured rocks.
Final Thoughts: The Chief is a giant monolith that casts a shadow upon, and defines the town of, Squamish. Not only it is a impressive landmark with a newly accessible gondola and rock climbing meca, it also is a fantastic exposure of a giant rock with a neat geological history. What was once a slowly cooling magma chamber (possible the heart of an extinct volcano), is now exposed and towering over the town due to uplift, erosion and intense glaciation that took place on the West Coast of Canada. Hopefully its a reminder to all passing Sea to Sky drivers on the way to and from Vancouver/Whistler how awesome rocks are, and what an interesting place Squamish is to live… no bias from me of course…
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