Rockslide

A rockslide is a type of LANDSLIDE common on high, steep rock slopes that occurs when a mass of rock moves quickly downslope. When the mass moves through the air, the movement is a rockfall. If the upper part of the mass initially rotates outwards, the movement is better described as toppling. Subsidence - the downward movement of rock into closed depressions in the ground - may result from the collapse of natural underground openings called CAVES and is typical of KARST LANDFORMS. Sliding describes the motion of a mass that remains undeformed except along its base. In slides, a rupture surface separates the displaced mass of rock from the rock over which it moves. Numerous small falls from a cliff of hard rock produce a talus or scree, an accumulation of loosely packed rock fragments sloping outward from the cliff. Such a landform is sometimes popularly called a rockslide. This entry, however, concentrates on slope movements in rock.

Slope Movement Characteristics
Rock-slope movements occur in mountainous regions or where erosion or excavation has exposed the rock. They are known throughout the Canadian CORDILLERA and along rocky coastlines and in deeply cut river valleys in eastern Canada. Rockslides also occur in artificial excavations, in cuts for roads or excavations for mines and quarries. Fatalities were caused in the Lower Town of Québec City by rockfalls from the slopes below the citadel in 1841 and 1889.

Movements can vary in size from the fall of a single block less than 1 m³ to slides of whole mountainsides involving many millions of cubic metres. A small rockfall derailed a CP Rail train at Kootenay Lake on 20 January 1995, causing the death of the crew. Larger slides may travel kilometres in a few minutes; the FRANK SLIDE lasted about 100 seconds and transported some boulders 2 km. Such high velocities have caused catastrophic rockslides to be called rock avalanches by analogy with the rapid downslope movements of snow avalanches.

Natural weaknesses often control the shape and location of rockslides. Sedimentary rocks, such as those at Frank and at Brazeau Lake, have surfaces of weakness separating the layers of sediment (or beds) from which the rocks were formed. If the sedimentary rocks are later compressed and folded into mountain chains, the beds are tilted to steep angles. As valleys are eroded through the mountains by rivers or glaciers, surfaces of weakness sloping into the valley may become exposed. At these exposed surfaces of weakness a typical Rocky Mountain rockslide can then take place. COAL MINING at the base of Turtle Mountain may also have contributed to the Frank Slide. A contemporary official report commented that, although destructive landslides were common in the Alps, they were very uncommon in the Canadian Rockies. This comment suggests that there may have been an artificial cause for the slide.

Recent research has mapped the debris of hundreds of rockslides, comparable in size to the Frank Slide, throughout the Cordillera. These prehistoric slides and those at Brazeau Lake, Hope and Rubble Creek in Garibaldi Provincial Park, BC, clearly have natural causes. Processes that destroy cohesion or bonding across potential rupture surfaces can trigger rock-slope movements. For example, water infiltrating the rock mass may freeze and expand, lengthening natural cracks along the growing rupture surface. At Hope, shaking caused by an EARTHQUAKE may have triggered the slide. The 7 million metre³ rockslide at English Chief River, 150 km west of Fort Simpson, NWT, was caused by the magnitude 6.6 Nahanni earthquake on 5 October 1985. Rupture may have occurred at Rubble Creek when debris or freezing obstructed the large springs that at present flow from the scarp of the slide. In limestones, like those at Frank, karst processes are often active in dissolving rock along bedding planes, effectively removing the natural glue holding the rock mass together. A number of different processes may contribute to a single rock-slope movement.

Remediation
Large rockslides rarely occur without such precursors as cracking of the ground at the crown of the slide, or bulging of the ground surface above the toe of the rupture surface. Surveying systems have been designed to monitor slopes so that work in any excavations around the gradually accelerating, displaced rock mass may continue until slope failure approaches. Movements can be reduced by decreasing gravitational forces disturbing the rock mass through off-loading the head of the slide or by draining water from it. Resistance to movement can be increased by loading the foot of the slide or by artificially reinforcing the rupture surface. Modern engineering practice can eliminate loss of life and movable property from rock-slope movements.

See ROCKSLIDES: TABLE.

Author D.M. CRUDEN

Suggested Reading
J.J. Clague, "National Hazards," Geology of the Cordilleran Orogen in Canada, Vol 4 (1991); S.G. Evans and O. Hungr, "The Assessment of Rock Fall Hazard at the Base of Talus Slopes," Canadian Geotechnical Journal, No. 3 (1993).

Links to Other Sites
Turtle Mountain Monitoring Project & Field Laboratory
About the Turtle Mountain Monitoring Project, which involves geological studies related to the development of an avalanche early warning system in the South Peak area. Check out the stunning Turtle Mountain web cams, slide show, and videos. Also provides an illustrated overview of key equipment used in this project. An Alberta Geological Survey website.

The 1903 Frank Slide, Alberta, Canada : A Review Of One Hundred Years Of Investigation
A brief academic article about the geological forces responsible for the Frank Slide and similar phenomena (a pdf file). From the European Geophysical Society.

The Mountain That Moves
An illustrated article about the monitoring of geological activity in Turtle Mountain. From the University of Calgary "OnCampus Weekly.”