Rockfall is a form of mass movement or mass wasting in which pieces of rock travel downward through some combination of falling, bouncing, and rolling after they are initially separated from the slope. The sizes of rockfall blocks can range from cubic centimeters to tens of thousand of cubic meters. Although some sliding may occur as the rock is becoming detached, sliding is a minor component of the process. Free fall typically occurs on slopes steeper than 76 degrees, bouncing on slopes between 45 and 76 degrees, and rolling on slopes below 45 degrees. Because slopes are commonly irregular, a rock may alternate between the three modes during its downslope movement. Talus slopes along the bases of cliffs are the products of uncounted rockfalls over thousands of years.
The size of rockfall blocks is controlled by bedding planes, joints, and fractures that form mechanical discontinuities and allow the blocks to become detached from the slope. Fracture lengths, and therefore rockfall volumes, tend to follow power law or fractal distributions, meaning that their numbers decrease exponentially as fracture length or rockfall volume increases. Field studies have also shown that freezing and melting of ice within cracks may control rockfall timing in some areas, although the falls generally seem to be more closely related to melting rather than ice wedging.
Extremely large rockfalls can constitute catastrophic mass movements because their weight and vertical fall distance (which combine to define the potential energy of the rock before it falls) produce potentially destructive kinetic energy. A large rockfall in Yosemite National Park during the summer of 1996, for example, involved two large rock slabs with a total volume of between 30,000 and 49,700 yd3 (23,000 and 38,000 m3) that became detached from a cliff face and became airborne 2,181 ft (665 m) above the valley floor. The estimated velocity of the slabs at impact was approximately 246–268 mph (396–431 kph) and the event was recorded on seismographs as far as 124 mi (200 km) away. The impact produced an air blast with estimated velocities as high as 246 mph, which snapped and uprooted trees, generated a dense cloud of dust that plunged the area into darkness, and killed a hiker. The catastrophic rock avalanches, such as those that destroyed the towns of Elm, Austria and Frank, Alberta probably also involved significant amounts of rockfall that evolved into rock avalanches as they moved downslope.
See also Landslide; Talus pile or Talus slope