How does a rockslide happen? 'The mountain that moves' was Canada's deadliest

Yahoo

22-06-2025

A large rockslide in Banff National Park at Bow Glacier Falls left two hikers dead and up to 13 others injured Thursday, raising questions about how and why the disaster occurred. But a look at published research and archive news articles on rockslides provides some general information about the dangerous occurrences.
A rockslide happens when a large chunk of rock detaches itself from the mountain where it sits and begins sliding down the slope. Why does this occur? Well, natural erosion or seismic activity can cause a rockslide, as can heavy rainfalls. Human activity such as excavation, construction or mining can also lead to a rockslide.
As one chunk of rock begins its downward slide, it can quickly gain momentum and trigger massive amounts of other rock to also begin sliding, leading to devastating effects.
Britannica.com notes a landslide or rockslide can occur 'when gravitational and other types of shear stresses within a slope exceed the shear strength (resistance to shearing) of the materials that form the slope.'
Dr. Dan Shugar, a University of Calgary geoscience professor, said rockslides are a fairly common geological phenomenon, particularly in the Rocky Mountains, due to how steep the slopes are. The composition of rock is largely limestone, which is susceptible to water saturation, making the rocks heavier.
'Ultimately, the cause is gravity,' he said. 'Mountains get built up over geological time and then they get torn down. That's an entirely natural process.
'We see rockfalls, rock avalanches, rockslides — we see a variety of mass wasting in mountain environments all the time. They range from a small boulder that would hurt you if it hit you but wouldn't be that damaging to entire mountain sides collapsing, and everything in between.'
A landslide occurs when sediment or loose dirt disengages from a hill or mountain and begins moving downwards. A rockslide, however, means solid rocks are also being swept down a slope during a similar type of event. Rockslides are also incredibly fast-moving, as they tend to move down a flat surface of a mountain.
The Canadian Encyclopedia notes a rockslide can move up to 100 km/hr.
The most horrific rockslide in Canadian history occurred in 1903 when a huge slab of Turtle Mountain crashed down onto the town of Frank and Crowsnest Pass (about 250 kilometres southwest of Calgary). At least 72 known residents were killed in the natural disaster, as were an undetermined number of others visiting or passing through the area. Some historians thus put the death toll closer to 90.
An estimated 80 to 110 million tonnes of rock were involved in the deadly event that came to be known as Frank Slide. The rockslide only lasted about a minute and a half.
Newspaper clippings and archive stories from the rockslide describe the horrific results that led to the deaths of men, women and children. As those clippings note, information about the state of some of the victims was disturbing, but shed light on how powerful the rockslide was: 'The leg and hip of a man was found lying fifty yards from the Imperial Hotel.'
First Nations people in the area had noticed instability in the mountain decades earlier and even had a name for it that translated to 'the mountain that moves.' The geological structure of Turtle Mountain was said to be the primary cause of Frank Slide, but weather impacts and coal mining were also noted as factors in the deadly rockslide.
An interpretive centre in Frank now tells the story of the slide and history of the area.
Other Canadian rockslides of note include the 1841 rockfall in the Lower Town of Quebec City, killing 32 people and crushing eight homes, and the 1889 rockslide in the same area that killed more than 40, says the Canadian Encyclopedia.
The worst rockslide worldwide was the Haiyuan Landslides of 1920 in China, when more than 200,000 people were killed. An earthquake caused those landslides.
Apart from the Frank Slide, Shugar said Alberta has surprisingly not had that many significant rockfall events. He noted B.C. tends to get more, citing the Hope Slide of 1965 as an example.
'It certainly was a very big, impressive landslide right by the highway,' he said.
The 680-tonne Big Rock, a type of quartzite, is an intriguing tourist attraction at Rocky Mountain House in Alberta, but how did this boulder measuring 9.7 metres by 9.4 metres by 5.5 metres get there? Well, the Rocky Mountain House Mountaineer reported the following 11 years back: 'Right around 20,000 years ago the Late Wisconsinan Glaciation was at its height; it was a glacier that could have been one kilometre thick. We know that all of the rocks in the Foothills Erratic Train come from the upper Athabasca drainage area south of Jasper,' said author and geologist Ben Gadd.
'A rockslide, almost certainly, dropped the rocks on the glacier. The glacier then eventually began to flow eastward until running into the Laurentide ice sheet (a glacier much larger than the one carrying the boulders) right around the Edson area. The larger glacier forced the smaller one to begin to move southeastward, right towards Rocky Mountain House.'
Along with this Big Rock, another famous boulder that is part of the Foothills Erratic Train is the big rock in Okotoks, south of Calgary. The Okotoks Erratic is 16,500 tonnes in size, but was discovered in large pieces rather than a single stone.
As the glacier, now on a new path, moved in the southeastward direction, it slowly began to melt. And as this process continued, the boulders that fell and became embedded in the glacier from the upper Athabasca drainage area began to drop from the flowing glacier.
According to Shugar, the U of C geoscientist, the short answer is probably yes.
The reason for that is due to how climate change is accelerating glacial retreat, which causes rock to become less stable.
Temperature and precipitation changes are other components, as warmer temperatures can melt more ice and increased rainfall can change glacial mass or erode cliffs, making them steeper.
'These landscapes, as they become newly created or newly exposed by glacier retreat, they often are unstable,' Shugar said. 'There's a sort of relaxation time over which they adjust to this new paradigm, new reality for them. Quite often they're very steep because of glacier erosion and so they need time to relax back to a geographical equilibrium.'
In glaciated mountains like in the Rockies, Shugar said that as glaciers retreat, we can expect to see more landslides.
In the case of the Bow Glacier Falls rockslide, he suspects there have been side-effects due to the recent creation of a new proglacial lake, which formed just 70 years ago at the toe of the Bow Glacier.
He suspects that over those seven decades, water from that new lake has been seeping into the rock, saturating it over the years and making it heavier.
'We see this all over the place,' he said. 'This isn't unique to this particular location, but I suspect part of the ultimate cause of this event yesterday (Thursday) was that saturated rock.'