2. What types of avalanches are there?

Avalanches are classified by their morphological characteristics, and are rated by either their destructive potential, or the mass of the downward flowing snow.
Here we will present avalanches with their general properties:

  • Slab Avalanches
  • Loose avalanches
  • Slush Avalanches
  • Dry avalanches and powder avalanches

SLAB AVALANCHES: have different names according to, moisture content of snow layers, slope cover (rock, soil or tree), and fracture depth.

Dry Slab Avalanche: in this type of avalanches, a low moisture, cohesive snow layer (snow slab) has a thinner, weaker failure layer beneath it. Once the snow slab is cut out around all boundaries by fractures, the slab snow avalanche occurs. Densities of slab layers are between 100 and 300 kg/m3.

Wet Slab Snow Avalanche: this type occurs by means of three principal mechanisms: 1- loading by new precipitation (rain), 2- changes in strength of a buried weak layer due to water percolation or 3- by water lubrication of a sliding surface, which may be partially or totally impermeable to water. Sometimes, failure affects the whole snow pack and the ground surface is uncovered after the avalanche has occurred. This type of avalanche is called “ground avalanche”.

Figure 2 - Dry slab avalanche: the "crown" of the slab is the fracture cutting the snow pack on the left side of the picture. A small slab is still present just below the crown, while wider slabs are visible in front of the tree. Source: www.thekyrgyzstanplan.com

LOOSE AVALANCHE: loose snow has little or no cohesion. Loose snow avalanches form near the surface and they involve near surface snow on initiation. Loose snow avalanches are easily recognizable because they start from a point and the displaced mass of snow forms a triangular pattern on descent. Natural loose snow avalanches are triggered by a local loss of cohesion due to snow metamorphism or the effects of sun or rain. Often the initiation is near rock outcrops, which cause locally high snow temperatures.

Dry loose avalanches: they commonly form under cold, relatively windless conditions which favour a low density superficial snow pack.

Wet loose avalanches: they take place after heavy melting of the snow pack caused by warming by the sun, rain or heat transfer from warmer surfaces.

Figure 3 - Wet loose snow avalanche, with its classic pear-shape, developing just below trees on a rock outcrop. The localized snow melting is probably occurring at dark surfaces, such as trees and rocks, favoring snow pack instability. The phenomenon is quite widespread in the area, as many other loose avalanches can be seen over the entire slope. Source: www.juneauempire.com

SLUSH AVALANCHES: in slush avalanches the flowing of snow is similar to the flowing of mud with high water content. Measured densities of saturated snow in slush snow avalanche deposits may exceed 1000 kg/m3 due to the mixture of water, ice, snow and entrained earth or rock material. They are usually partially or totally saturated with water. The temperature of the snow is round 0 °C. The highest frequency of occurrence at high latitudes is partially due to the rapid onset of snowmelt in spring as the sun returns to provide direct, intense radiation input to snow packs that have been previously subject to strong temperature gradients and a lack of solar radiation input.

Many examples of this avalanche type can be found in Northern Norway and Alaska.

Figure 4 -Slush avalanche resting below a gully, on an alluvial fan covered by snow. Of note are the many "streams" created by the snow flowing downslope, resembling a debris-flow-like deposit pattern. The high water content of the snow is responsible of this particular deposition pattern. Source: www.unis.no

DRY AVALANCHES AND POWDER AVALANCHES: if moisture content is low, density is near 100 kg/m3 or less, snow temperatures are less than 0 °C and snow depth is more than 1 m, those types of snow avalanches may occur. After the snow avalanche starts, a dust cloud which contains snow and air forms.

Dry Avalanche: in this type of snow avalanche there is a dense core of snow at the bottom of the moving mass. It is estimated that about one-third of the space in the core is filled with snow particles and about two-thirds is air. In the dust cloud that may develop above, only about 1% of the space is filled with snow particles and 99% is air. Therefore the density is about a factor of 10 higher in the core than in the dust cloud. Usually the depth of flow of the core is less than 5 m, but the depth of dust cloud can be tens of meters. Deceleration is rapid in run out zone.

Powder Avalanche: are those in which a dense core of snow at the bottom is absent. Any high-speed dry snow avalanche will have a powder or dust cloud associated with it and tends to make powder avalanches and dry flowing avalanches look similar. To distinguish between powder and dry snow avalanches, one must make an observation of the deposit, of the destructive effects, or of flowing snow properties to determine the presence of a core. In a true powder avalanche, almost all of the material is suspended by turbulent eddies. They often form by falling ice (or sometimes by ice avalanches) from steep icefalls. Their speed may sometimes be more than that of dry avalanches, but since their density is much less, their destructive power is less.