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NatureSpeak Articles

How old is a mountain? Or are they ageless?

The question sounds simple enough.

I am standing outside the Roundhouse with a group of summer visitors. The tag on my Mountain Host jacket puts me on a first name basis with the man in the Budweiser T-shirt (I will call him Bud). He points across Fitzsimmons Creek toward Wedge and repeats his question. "Say Jack, how old is that big mountain over there?" I tell him that Blackcomb and Wedge are part of a large Jurassic pluton about 150 million years old. "I do declare", says Bud, "that is some old mountain. Come on over here Martha," he calls to his wife. "We got to get a picture of us with that mountain."

As I framed the couple in the viewfinder against the backdrop of our local mountains, I wondered what image of creation was in their minds – mountains popping up full-blown from a Jurassic swamp or dropping out of a celestial twister like Dorothy's house into the land of Oz? But of this I was sure: the question was not simple and then was the wrong time and place for the long answer.

To begin with the question assumes some sort of beginning – a birth, the sprouting of a seed, or the completion of a structure – some point in time that defines the beginning of "age." But when does the life of a mountain begin and when is it completed? As geologists we commonly side-step that one, as I just did, and talk instead about the age of the rocks. The stuff of which mountains are made.

Blackcomb is indeed made of granitic rock (granodiorite) which was intruded into the earth's upper crust as a hot, semi-molten "pluton" during the Jurassic Period. Its "age," about 150 million years, is the time elapsed since the pluton cooled enough to allow its atomic clock to start ticking. In contrast, the rocks forming Whistler Mountain consist of shale, a sedimentary rock, and andesite, a volcanic rock, both deposited in a marine basin during the Cretaceous Period, about 80 million years ago.

Though carved out of very different material both Blackcomb and Whistler Mountains were shaped during the same relatively recent interval of time. Like all erosional mountains, they are slowly but constantly changing as the opposing forces of tectonic uplift and erosion maintain them in a state of dynamic equilibrium. Here in the Coast Mountains of B.C., the rate of tectonic uplift is about 100 metres per million years – just enough to compensate for the wear and tear of streams, glaciers and rockfalls eroding them from the top. Our mountains are, in fact, "works in progress" without any defined beginning and no foreseeable end.

Bud and Martha, heading down after a short walk, smile and thank me for taking their picture. They have had a great mountain experience and are happy with their bit of new knowledge. It’s only us geologists who get picky about the details.

Written by: Jack Souther

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