A mass-wasting event can occur any time a slope becomes unstable. Sometimes, as in the case of creep or solifluction, the slope is unstable all of the time and the process is continuous. In other instances, triggering events can occur causing sudden instability.
Modification of a slope, either by humans or by natural causes, can result in a slope angle that is no longer tenable. A sudden shock, such as an earthquake, may trigger slope instability. Minor shocks like heavy trucks rambling down the road, trees blowing in the wind, or human-made explosions, can also serve this purpose over time. A mass-wasting event can then restore the slope to its natural angle of repose.
Example: In the 1870s there was a large demand for slate to make blackboards throughout Europe. To meet this demand, miners near Elm, Switzerland, began digging a slate quarry at the base of a steep cliff. Slate is a metamorphic rock with an excellent planar foliation that breaks smoothly along the foliation planes. By 1876 a "v"-shaped fissure formed above the cliff, about 360 m above the quarry. By 1881, the quarry had been excavated to where it was 180 m long and 60 m into the hill below the cliff. The "v"-shaped fissure had opened to 30 m wide.
Falling rocks were frequent in the quarry and there were almost continuous loud noises heard coming from within the overhang above the quarry. Realizing that the slope had become unstable, the miners abandoned the quarry, thinking that the rock mass would probably collapse and fall.
On September 11, 1881, the mass of rock above the quarry suddenly did fall, sliding rapidly down the slope. But it did not stop when it hit the quarry floor. Instead, the 10 million m3 of rock broke into pieces and rebounded into the air. Residents in Untertal, on the opposite side of the valley, saw the rebounded wall of shattered rock coming uphill at the them. The rock continued up the walls of the valley and buried the residents of Untertal alive. The avalanche then turned and ran an additional 2,230 m as a dry avalanche traveling at 180 km/hr and burying the entire village of Elm. The event killed 148 people.