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

National Post

6 hours ago

Science
National Post

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

A large rockslide in Banff National Park left up to 15 hikers injured Thursday, leading to one potential death and raising a series of questions about how and why the disaster occurred. Article content With officials still working on rescue and treatment of survivors, it will be some time until an investigation concludes why the rockslide happened. But a look at published research and archive news articles on rockslides provides some general information about the dangerous occurrences. Article content Article content Article content Article content 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. Article content 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. 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.' Article content What's the difference between a landslide and a rockslide? Article content 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. Article content 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. Article content 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. Article content 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.'