Features & Images » Feature Story

A legacy of fire and ice

Volcanoes of the Garibaldi Belt add dramatic scenery to Whistler's mountain landscape

by

comment

Only the tops of the highest mountains project above the ice. The summits of Wedge, Blackcomb and Whistler are mere islands of rock – nunatacks – surrounded by a vast expanse of ice. Moraines of broken rock, stripped by glacial erosion from each nunatack, wind across the ice surface and merge like roads entering a superhighway where the glaciers converge at the head of Howe Sound. South of Whistler, beyond the battered remains of the Black Tusk, the steaming dome of Garibaldi towers above the ice surface. A slab spalls off the spine of hot rock projecting from its summit and sends an avalanche of incandescent blocks rushing down the cone and fanning out onto the ash-covered surface of the ice. Off to the west a plume of white vapour rises perpetually from a circular pit thawed into the ice by the lava of Ring Mountain. And a kilometre below the surface the invisible fury of the Cheakamus Valley sub-glacial eruptions sends tremors through the overlying ice.

Of course no one was around to take a video. But given a smattering of geological insight and a pinch of imagination it's possible to recreate a mental image of the time, not that long ago, when the view from the top of Whistler was very different than it is today. The trick is learning to read the clues that are written in the rocks and Bill Mathews, my U.B.C. geology professor, friend, and colleague, was among the first to interpret what happens when molten lava and glacier ice collide. During the Quaternary Period, the last two million years of geological time, the ebb and flow of regional glaciers through Sea to Sky country was punctuated by the episodic eruption of volcanoes, and it was Bill's pioneering work that broke the geological code and first gave us some insight into that turbulent period of our local history.

He did his Ph.D. thesis on Mount Garibaldi back in 1948 and later went on to study several other features in what is now known as the Garibaldi Volcanic Belt. This scattered family of at least 25 young volcanoes strung out between Howe Sound and the Bridge River plateau is a northern extension of the high Cascade volcanoes of western U.S. and part of the Pacific Ring of Fire. In the context of global plate tectonics, it's the result of convergence between North America and the Juan de Fuca plate, a relatively small chunk of the Pacific Ocean crust that takes a nose-dive west of Vancouver Island and slides, very slowly, under the continent. At a critical depth, which just happens to be directly under the Sea to Sky corridor, things get hot enough to melt and magma, a brew of molten rock infused with dissolved gasses, periodically pops up to the surface as lava. Fortunately it only pops up every few thousand years or so but the volcanoes and lava flows left behind by each eruption have become a fascinating and permanent part of Whistler's mountain landscape.

Next time you go up the Peak Chair or the 7th Heaven Express pause long enough between runs to pick out the volcanoes. Six of them are visible from either mountain: The Black Tusk, Mount Garibaldi, Mount Fee, Mount Cayley, Powder Mountain, and Ring Mountain. And if you happen to arrive at the top on one of those glorious calm days when the valley is filled with fog and only the tops of the mountains rise into sunshine above the brilliant white cloud layer, let your imagination turn the fog to ice and you will have an image of how the world looked when many of the Garibaldi Belt volcanoes were active.

Known as the Fraser Glaciation, the period between 24,000 and 10,000 years ago began with a drop in global temperature that triggered the growth of alpine glaciers. These rapidly expanded into valley and piedmont glaciers that ultimately coalesced into the Cordilleran Ice Sheet which, at its maximum about 14,000 years ago, covered most of British Columbia. At that time huge valley glaciers, flowing south out of the main ice sheet past Whistler/Blackcomb and into Howe Sound, extended as far south as central Washington State. And as Bill Mathews began to unravel the story of Mount Garibaldi he discovered an amazing correlation between the volcanic and glacial history.

From the top of Whistler only the uppermost dome of Mount Garibaldi is visible against the southern horizon. The best place to get an overall view is from the top of Black Tusk. It's a bit of a scramble up a chimney on the back side but you not only get the view, the climb gives you bragging rights that are guaranteed to impress visitors who see only the steep, edge-on profile of the Tusk from Whistler or Blackcomb. In fact, the chimney has been climbed by so many people that the route is worn smooth – not even any loose rocks to worry about – but beware of tourists in running shoes.

About 170,000 years ago the Black Tusk was an active volcano. It once had the conical shape of a miniature Fuji but centuries of erosion, including the ravages of the Fraser Glaciation, have stripped away the outer pyroclastic cone of loose bombs, blocks, and ash, leaving only the solid plug-dome of lava that once filled the central conduit and now forms the narrow summit spire. It was already deeply eroded when Garibaldi burst onto the scene.

If you climb the Tusk bring a good pair of binoculars and a copy of Mathews guide to "Garibaldi Geology." The scenery spread out around you is a chronicle of fire and ice, an entire landscape of lakes, mountains and valleys that were born and reshaped during the brief final chapter of our local geological history.

South of your perch atop the Tusk, across Garibaldi Lake and beyond the rounded summit of Mount Price, the pyramid-shaped cone of Mount Garibaldi rises above the surrounding granitic mountains. Twenty thousand years ago the older mountains didn't look much different than they do today but Mount Garibaldi, which now fills the southern horizon, didn't even exist. It was built during the waning stages of the Fraser Glaciation when the ice surface stood at about 1,300 metres above sea level. Composed of dacite, a viscous lava prone to explosive behavior, the summit cone of Garibaldi was built in several stages. The final eruption produced a spine of glowing semi-rigid lava that repeatedly collapsed onto the growing cone. And Mathews presents compelling evidence that the cone actually extended onto the ice surface.

When the regional ice sheet stood at more than 2,000 metres above sea level, the eruption of a satellitic vent on the north flank of Garibaldi, built the steep-sided, flat topped lava pile known as The Table. Mathews called it a "Tuya" after similar features he had mapped on the Tuya Plateau of northern B.C. The name stuck and became part the lexicon of technical geological terms. The Table, like all Tuyas, was formed by lavas that erupted into and were contained by a vertical pipe thawed into the ice. Having nowhere to go, successive flows piled up like a stack of giant hotcakes.

The final gasp of Garibaldi's long and varied life came sometime after the last ice had disappeared from the valleys. A massive effusion of lava issued from Opal Cone on its southern flank producing the Ring Creek lava flow, 17 km long and two km wide, which rumbled down Mamquam valley almost to Squamish.

Garibaldi Lake, which fills the foreground of your view from the Tusk, is an even more recent addition to the landscape than Mount Garibaldi. After the Cordilleran Ice Sheet had disappeared from the highlands, leaving only isolated alpine and valley glaciers, an eruption from Clinker Peak on the shoulder of Mount Price, sent massive lava flows into upper Culliton and Rubble Creeks. Garibaldi Lake is now ponded behind a lava dam that was formed by an arm of the Clinker Peak flow.

When Clinker Peak erupted the lower part of Rubble Creek valley was still filled with ice and where the ice and lava met in what must have been a spectacular outburst of steam explosions the lava front was quenched and deflected. In 1855, long after the ice had disappeared, its oversteepened, unstable snout collapsed, triggering a massive rockslide. The headwall of that slide, now known as The Barrier, is still considered by many to be a potential hazard. Mathews, the guy who identified the origin of the Barrier, was among the expert witnesses in a court case that assessed the risk and ruled against any development on or adjacent to the Rubble Creek slide.

Bill Mathews passed away in 2003 but his pioneering work on Mount Garibaldi paved the way for many others who continue adding to our knowledge of the Volcanic Belt as a whole. Modern isotopic dating has given us the ages of individual volcanoes. The nature of past eruptions has been interpreted from the morphology and chemistry of the lavas, and detailed regional mapping has provided a link to global tectonics. But there is still much more to learn.

Not all the Garibaldi Belt volcanoes share Mount Garibaldi's brief, eventful history. Many of them, like the Black Tusk, were already dormant by the time the Fraser Glaciation began. Others were born and died beneath the ice and, despite their common Juan de Fucan heritage, the life of each volcano is a unique blend of nature and nurture – the chemical "DNA" of its parent magma, and its encounter with the surface world.

My favorite volcanoes: Mount Fee, Mount Cayley, and Ring Mountain are all visible from the ski slopes but, because they are hard to reach, their pristine alpine meadows nestled against a backdrop of dramatic cliffs and spires must be among the most awesome campsites in the world. From the top of Whistler it's 25 km to Mount Fee and nearby Mount Cayley, but even at that distance their incredibly steep serrated profiles are an imposing sight against the western skyline. Too precipitous to hold snow, the bare rock of their jagged upper slopes contrasts with the rounded, snow-covered summits of surrounding granitic peaks.

The present form of Mount Cayley is probably fairly close to its original shape. The eruption of thick, viscous lava flows was accompanied by the extrusion of near-solid domes that shouldered aside the surrounding granite and expanded like spring mushrooms forcing their way through the forest litter. The first hot, semi-rigid lava broke through the surface about 3 million years ago and grew into the crumbling spine of rock that now forms the summit of Mount Cayley. Other eruptions followed, forming the precipitous spine of Vulcan's Thumb and two smaller domes, the youngest being about 300,000 years old.

If you know exactly where to look from the top of Whistler, the circular mound of Ring Mountain can be seen tucked in among the older peaks and at its base is a beautiful clear lake surrounded by flower-filled meadows. It’s a long day hike in from the Callaghan road but the campsite is worth every step of the way. With its pronounced southerly list Ring Mountain resembles a giant baking failure, a fallen souffle. Its circular form, flat top, and steep sides are characteristic of tuyas but the thick, southerly sloping cap that gives the mountain its skewed profile is a typical dry-land lava flow whose upper surface is strewn with bombs and coarse ash. It seems that Ring Mountain began its life as a tuya surrounded by a moat of glacial meltwater. At some point the enclosing ice-dam gave way, draining the lake and leaving the newly-formed tuya high and dry. The final massive flows of viscous lava issued along its northern rim and spread out across its dry surface while bombs and ash rained down from a fire fountain at the vent.

Not all the lava that issued from ice-covered vents made it to the surface. According to Bill Mathews the sinuous ridges of columnar basalt at the south end of Daisy Lake and around the B.C. Rail quarry were formed where lava was confined to meltwater channels beneath the ice. And in the Cheakamus River valley, near the demonstration forest, Logger’s Lake now occupies the crater of a small andesitic volcano that was born and died beneath a kilometre of glacier ice. Next time you're biking along the Cheakamus Lake road take the time to pause at Crater Lookout or, better still, hike around the crater of this relic of the ice age.

The Cheakamus Valley volcano is a typical "sub-glacial mound," a blister of molten lava that was large enough to thaw a large cavity beneath the ice but not massive enough to melt an open pit – a sort of failed tuya that never developed discrete flows or the classic flat top. Its extremely long columns, formed by shrinkage during cooling, suggest that the Cheakamus lava cooled from a single large mass. Any loose pyroclastic bombs or blocks that may once have mantled it were swept away by the very ice that encased it. The remaining core of resistant andesite is deeply scoured by ice and the surface of the dome is strewn with glacial eratics, abandoned during the final withdrawal of the glacier.

Meager Mountain, not quite visible from the top of Whistler, is the youngest and also among the oldest of the Garibaldi Belt volcanoes. Best known for its hot springs and geothermal potential, Meager Mountain first erupted about two million years ago. It periodically surged back to life and with each eruption added new bulk to its growing mass. Its most recent eruption, less than 2,500 years ago, began with a violent discharge of gas and fine ash from a vent high on its north face. Within minutes the Lillooet River along the base of the slope was choked with rafts of pumice. The forest was set ablaze and ultimately buried under several metres of volcanic ash. Midway through the eruption a massive glowing avalanche of blocks and ash swept down the slope and smothered the valley under several metres of welded pyroclastic deposits and the plume of airborne ash extended east beyond the Rocky Mountains.

In the years following the eruption the Lillooet River has cut through the thick layer of welded pumice and ash exposing the charred remains of the pre-eruption forest still rooted in the underlying soil. Radiocarbon dates of this material pegged the time of eruption at 2,490 + 50 years before present. The ash deposits, known as the Bridge River Ash, have also been dated and traced across central B.C. into Alberta.

At present the volcanoes of the Garibaldi Belt are quiet, presumed dead but still not completely cold. But the flare-up of Meager Mountain 2,500 years ago raises the question, "Could it happen again?" Was the explosive eruption of Meager Mountain the last gasp of the Garibaldi Volcanic Belt or only the most recent event in its on-going life? The short answer is nobody really knows for sure. There is no seismic activity presently associated with any of the Garibaldi Belt volcanoes. But the Juan de Fuca plate continues to slip under us and the chemistry of hot-spring water and geothermal drilling at both Mount Cayley and Meager Mountain indicates that their deep roots are still very, very hot. So just in case I sometimes do a quick check of the old hot-spots when I get off the Peak Chair.