Anatomy of an ancient volcano
By: Jack Souther
Date: August 12, 2004
My present status as Mountain Host/Elder Ski-bum does not require me to carry a hammer and go snooping around the mountains in search of geological problems. But old habits die hard. So it was, a few years ago, that I started out for Russet Lake and ended up spending the day on Flute, trying to figure out why it was red, unlike the sombre greyish-green hues of its musical neighbours.
The day’s prowl, plus a look at my rock collection in the lab, convinced me that Flute is the exhumed bowel of an ancient volcano – or more correctly a sub-volcanic pluton. The rock that forms the nearby summits of Whistler and Piccolo is lava that poured out of a volcanic vent and consolidated at the surface in the late Cretaceous, about 100 million years ago. The rock that forms Flute is the stuff that didn’t quite make it to the surface.
The anatomy of any volcano extends deep into the earth’s crust or even deeper into the underlying mantle where molten magma is produced. As magma moves up through cracks into zones of lower pressure some of it may break through to the surface and erupt as lava from a volcano, while some may shoulder aside the confining rocks and swell out into subsurface chambers called sub-volcanic plutons. Unlike magma that issues onto the surface and cools quickly as lava, the magma trapped below the surface is insulated by the enclosing rock and may remain semi-molten and hot for hundreds or even thousands of years. Because it cools and crystallizes slowly the mineral grains grow larger, producing a coarse grained rock. And, like the heating element in your coffee-maker, this subterranean hot-spot heats and sets up convection in the surrounding groundwater. The resulting hydrothermal (hot water) system may deposit quartz veins and, combined with sulphurous gasses released from the magma, cause chemical alteration of both the crystallizing pluton and the surrounding rock. Which brings me back to why Flute is red.
A close look at a freshly broken rock from Flute reveals a relatively coarse texture and the presence of tiny crystals of pyrite (iron sulfide) which weathers to red iron oxide or rust. The coarse texture is evidence of slow cooling, the pyrite is evidence of alteration in a sulphurous hydrothermal system. Next time you hike the Singing Pass trail notice the large quartz veins that cut the rock surrounding Flute. The evidence seems pretty good to me. The texture, the hydrothermal alteration, the numerous quartz veins all suggest that Flute had its origin as a sub-volcanic pluton – a reservoir of molten magma that cooled slowly in the bowels of one of the many Cretaceous volcanoes that produced the lava flows on Whistler and Piccolo.