Falling rocks are a special category of the large family of gravitationally-driven phenomena called landslides. What are commonly called rockfall events generally fall into four technical definitions: rockfall, rock topple, rock avalanche, and rock slide. Obviously nature doesn’t always follow our pigeon-hole classifications, so rockfalls commonly grade into one another. Generally an individual rockfall includes one to only a few rocks, and sizes from gravel to boulders (~2 inches to 5 feet dia.). When a large mass of rock fails and the resultant fall spreads out into a fan of utter devastation, it is referred to as a “rockslide” or even a “debris avalanche.”
Rockfall is the fastest type of landslide and is common in mountainous areas near cliffs of broken, faulted, or jointed bedrock, on steep slopes of rocky soils, or where cliffy bedrock ledges are undercut by erosion or human activity. The loss of support from underneath, or detachment from a larger rock mass destabilizes the rocks and gravity does the rest. The criteria for rockfall is simply an exposure of broken rock, gravity, and a slope steep enough that when a rock detaches or dislodges from the ground surface, it will move down the slope rapidly. Complex interactions between physical parameters of both the rock and the slope cause the falling rocks to move down the slope in a high-velocity, seemingly random and erratic manner. When people, buildings, vehicles, or highways are in the path, these rockfall events can lead to tragedy — property loss, personal injury, or even loss of life.
It is important to note that rockfall is a natural catastrophic erosional process that has been occurring in steep terrain for as long as the Earth has existed. Young mountain ranges, like Colorado’s, do not weather away grain by grain, but rather they tumble down in a punctuated but perpetual sequence of rockfalls, rockslides, landslides, and debris-laden floods over millions of years. Many regions and towns in Colorado’s mountains and cliff-rimmed mesas of the Western slope are exposed to rockfall hazards. More information about rockfall and other case histories in Colorado can be found in CGS RockTalk, Vol. 11, No. 2
Colorado Geological Survey