What characteristics distinguish mudflows from avalanches? What is a grainflow? A density underflow? What is dispersive presssure? How are avalanches controlled? What is permafrost? What factors result in melting of permafrost? What sorts of surface phenomena are associated with areas of permafrost?

Solifluction lobe

Creep is the very slow downslope motion of the surface soil layers. Wetting and drying, and freeze-thaw cause enough disturbance to allow grains to move down the slope.

Downslope creep in layered beds of sedimentary rocks. Fractures in rock allow slip of smaller blocks, but give appearance of bending of the beds.

Global warming will reduce the extent of permafrost. Consequences: Disruption of ecosystems, roads, foundations, etc. Release of methane hydrates resulting in enhanced greenhouse effects

Mars Earth

A pingo

Rock glacier in central Alaska Range. Rock glaciers are made of rock with a matrix of ice. They flow slowly downslope, like true glaciers, but move only during the freeze-thaw periods of fall and spring.

Solifluction lobes - Alaska. Freeze-thaw of shallow surface materials on a permafrost substrate. Note ‘patterned’ ground.

A cryoturbate soil

Avalanches are initiated by the downslope movement of a mass or slab of snow, as diagrammed above. Oversteepening of cornices and collapse is a common triggering mechanism. As snow moves downslope air is entrained into the snow, and the grains of snow - ice crystals - become separated. As more air is entrained, the avalanche acts like a low-viscosity fluid - a grain flow.

Slide on the Seward Highway, AK about to hit tidewater. Photo by Terry Onslow

Slab avalanche sequence on Mount Superior, Utah, triggered by helicopter bombing. Photos by Dan Judd Avalanche on the Milford Road, New Zealand, Photo by Peter Weir

Identification of avalanche source areas (left) and run-out tracks (right) are key to avalanche control and protection.

Crown fracture of a large dry slab avalanche in the Cariboo Mountains. Research staff perform stability tests done near small and large avalanches such as this one. The shearframe test.The shearframe is used to testthe shearstrength ofweak snowpack layers. The shearframe test.The shearframe is used to testthe shearstrength ofweak snowpack layers. The shearframe test.The shearframe is used to testthe shearstrength ofweak snowpack layers. The shearframe test.The shearframe is used to testthe shearstrength ofweak snowpack layers. The shear frame test. The shear frame is used to test the shear strength of weak snowpack layers. Once buried, surface hoar (frost) forms a weak layer that can sometimes remain weak enough to release slab avalanches weeks after the layer was buried.

Avalanches are common occurrences in mountainous regions. Destruction of property and loss of life may result. Ski areas are particularly prone to avalanches - the combination of steep slopes and abundant snowfall.