The biggest rock avalanches may ride a cushion of their own steam
Some giant landslides travel astonishingly far across flat or even uphill ground. One explanation is that friction heats the base of the moving mass so fast that trapped water flashes to vapour, lifting the rock like a hovercraft; sometimes minerals in that zone even melt.
Every landslide comes down to a balance of forces. Gravity drives the movement, and a slope fails when the shear strength of its material drops, the stress on it rises, or both. Water is the usual culprit: heavy rain, snowmelt or glacial melt saturates soil, raises groundwater and increases pressure inside pores and cracks. Rivers or waves eating at the base of a slope, repeated freezing and thawing, wildfire that strips away vegetation, and earthquake shaking that can liquefy soil all push slopes toward failure. Often a single trigger, such as a storm, a quake or a road cut, sets things off, though sometimes none can be identified.
People make matters worse. Deforestation, mining, construction and urban sprawl alter how much water soaks into the ground or remove the deep roots that anchor loose soil to bedrock. Even abandoning farmland, as happened widely in Europe after the Second World War, changes slope behaviour. Climate change adds more extreme weather that can trigger failures.
Geologists long used the word landslide loosely for almost any movement of rock and soil. In 1978 David Varnes proposed a stricter classification, refined repeatedly until the version by Hungr, Leroueil and Picarelli in 2014, which recognises six movement types. A fall is free-falling blocks; a topple rotates away from a vertical face; a slide moves as a largely intact body over one or more shear surfaces, either flat or spoon-shaped; spreads crack open and expand sideways.
Flows are the most varied. Earthflows of clay, silt or fine volcanic material can creep at a millimetre per year or race at many kilometres per hour. Saturated debris flows in steep alpine gorges can carry off trees, cars and houses, and may dam a river temporarily; when such a blockage bursts, the surge can grow dramatically in volume and power.
Source: Landslide
