Causes of landslides

What ever the scale, landslides occur when the forces for movement (gravity) overcomes the strength of the hill slope itself. In progressive slope failures such as soil creep and even large scale rock slope failures, movement may be intermittent over very long time periods, but can be enhanced by changes in environmental conditions, especially changes in water flow within soil and rock.

In Scotland single event slope failures, such as debris flows and rock falls, are closely associated with weather conditions, such as freeze thaw cycles or prolonged or intense rainfall. Water or ice crystals reduce the internal strength of the rock or soil by prizing rough surfaces apart, and allowing movement along gaps or smoother surfaces.

The time before a landslide occurs is just as important as the trigger event.

• Weathering along fissures in the rock face will reduce the strength of the rock, so that it is susceptible to failure, requiring only a trigger mechanism to overload the slope and make it fail.
• Availability of susceptible rock or sediment.
• Changes in vegetation cover also affects the ability of the slopes to remain intact.  

Trotternish Ridge landslides like the Quirang have been active for thousands of years, and have surviving being over run by one or more glaciations (photo SNH) 

St Cyrus National Nature Reserve cliff landslide (photo SNH) 

Large scale rock slope failure along the glacially oversteepened slopes of Sgurr a' Bhealiach in Glen Shiel, Kintail (Photo David Jarman)