A landslide is the rapid sliding of large masses of bed rocks or regoliths. Whenever mountain slopes are steep there is a possibility of large disastrous landslide. Landslides are triggered by earthquakes or sudden rock failures. They can also result when the base of a slope is over steepened by excavation or river erosion.
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Severe earthquakes in mountainous regions are a major cause of landslides. In the Himalayas, the Western Ghats, and along the river valleys landslides are a common feature. Natural removal of soil and rock from slopes is known as mass wasting. Landslide as a hazard has long been recognised by the people living in the mountains. It becomes specially dangerous when there is heavy rainfall or snowfall on the slip and break.
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The extent of landslides depends on the steepness of the slope, the bedding plane of rocks, the amount of vegetation cover and the extent of folding and faulting of the rocks. It is the rocks that break and carry with it the soil and debris.
A major cause which triggers off the landslide is the weight of the overlying material and the presence of a lubricating material like water, this is known as solifluction. Freezing and thawing of the rocks on mountain slopes cause them to break and roll down the slopes. The overbearing weight of snow or ice or water which has seeped into the soft permeable rocks also lead to slipping and breakage of hill slopes.
Other causes of the landslides are the volcanoes and earthquakes. In areas which have sedimentary rock and steep slopes tremors dislodge the rock structures and cause falls. Often near sea coasts as for example the Kanara coast; cliffs are eroded at the base by sea waves and the rocks jutting out on top break off and fall. Landslides occur frequently during the rains.
Deforestation as a result of felling of trees for timber and removal of vegetation cover for developmental activities are also responsible for soil erosion and destabilization of slopes.
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It is estimated that the construction of just one kilometre long road requires removal of 40,000 to 80,000 cubic metres of debris, which slide down the slopes, killing vegetation and choaking mountain streams. Humans often make changes in the natural slope for construction of roads and buildings. Such changes make hill sides more vulnerable to mass wasting and landslides.