Webster’s Third New International Dictionary (1971) defines these phenomena as follows:
Landslide: Rapid downward movement, under the influence of gravity, of a mass of rock, earth or artificial fill on a slope. Also, the mass that moves or has moved downwards.
Snow Avalanche: large mass of snow, ice, earth, rock, or other material in swift motion down a mountainside or over a precipice.
The encyclopedia Britannica – Micropeadia (1985) gives fairly detailed descriptions of the phenomena as follows:
Also called landslip; Downward mass movement of earth or rock on unstable slope including many forms resulting from differences in rock structure, coherence of material involved, degree of slope, amount of included water, extent of natural or artificial undercutting at the base of slope, relative rate of movement and relative quantity of material involved. Many terms cover these variations: creep, earth flow, mudflow, solifluction and debris avalanche are related forms in which mass movement is by flowage.
If shearing movement occurs on a surface on consolidated rock, the dislocated mass is a debris slide. Cliffs may become so steep through undercutting by rivers, glaciers or waves that masses of rocks will fall freely and constitute a rock-all type of landslide.
Large mass of snow or rock debris that moves rapidly down a mountain slope sweeping and grinding everything in its path. An avalanche begins when a mass of material overcomes frictional resistance of the sloping surface, often after its foundation is loosened by rains or is melted by a warm and dry wind. Variations caused by loud noises such as artillery fire, thunder or blasting can start the mass in motion.
Some snow avalanches develop during heavy snowstorms and slide while snow is still falling more often they occur after the snow has accumulated at the given site.
The wet avalanche is perhaps the most dangerous of its large weight, heavy texture and the tendency to solidify as soon as it stops moving. The dry type is also very dangerous because its entraining of great amounts of air makes it act like a fluid; this kind of avalanche may flow up the opposite side of a narrow valley. Avalanches carry a considerable amount of rock debris along with snow and therefore are significant geological agents; in addition to transporting unsorted materials to the bottoms of slopes, they may, if repeated, cause an important amount of erosion.
From the above definitions and descriptions, it will be seen that landslides and snow avalanches are phenomena of mountain regions and both involve the swift and sudden movement of large masses of material falling or slipping down a hilly slope. While landslide involves rock, soil and mud; snow avalanche primarily involves snow. While landslide may occur even in smaller hills or rocky terrains, snow. Landslides involve loosened or weakened rocks and mud whereas snow avalanche brings down accumulated or overhanging snow mass although it may collect rock and other debris on its way. Both the phenomena can be triggered by their own weights or by vibrations and also due to loud noise. Earthquakes or even minor tremors are known to have triggered landslides and snow avalanches.
Incidences of landslides are common in the various hilly regions of India but these are more in the Himalayas, in the Western Ghats (including Kerala), Nilagiris. There are occasional reports of landslides in the Vindhyachals and the Eastern Ghats as well. Landslides are more frequent during or after heavy rains.
In India, snow avalanches occur in the Himalayan ranges and more so in the mountain regions of Kashmir, Himanchal Pradesh and the hills of West U.P. this is because the dense forest and vegetation cover in the eastern and northeastern Himalayas act as binding force and inhibit the slippage of snow mass.