MECHANISM OF AVALANCHE RELEASE
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Mechanism of Avalanche Release...
avalanches of dangerous size originate on slope angles between 30 degrees
and 45 degrees. They seldom occur
below 30 degrees and
hardly ever below 25 degrees. Above 45 degrees to 50 degrees sluffs and
small avalanches are
common, but snow seldom accumulates
to sufficient depths to generate large slides.
metamorphism or stress development may sometimes initiate snow rupture,
avalanches are often
dislodged by external triggers. An
overload of new snow may dislodge an existing slab. Falling cornices or
snow from trees are common natural triggers,
similar in action to the sunballs or snow wheels which frequently initiate
slides. In the absence of external triggers; unstable
snow may revert to stability with passage of time and no avalanche
Artificial triggers in the form of mechanical disturbance may be intentionally
introduced for control purposes.
are a major cause of accidents; most skiers who fall victim to an avalanche
trigger the slide which
avalanches fall when a well defined
snow later breaks free and slides away. The sliding surface is usually
the interface between distinguishable
layers of snow which has been
formed by variations in weather or snow deposition. In some cases the sliding
surface may be the ground (entire snow cover avalanches).
often exists a lubricating layer of low shear strength which allows the
slab to break free from the
sliding surface. This lubricating
layer may be generated
by deposition of fragile crystal forms (e.g., surface hoar), internal metamorphism,
or the intrusion
of melt water. In some cases the lubricating
layer is absent, instability being provided simply by a poor bond between
slab and the sliding surface.
primary instability develops when the component of force parallel to the
slope due to the weight of the slab exceeds
the shear strength
of the bond to the underlayer (sliding surface). The situation is mechanically
complex due to irregular
attachment of the slab to stable
snow or other anchorages at the head, toe,, and sides. In general., slab
occurs when one of these attachments
is broken by a trigger; redistributed stresses then exceed the strength
of the other
bonds and an avalanche falls. Only part of
the slab attachments may be weak, while others are strong. In this case
trigger will initiate fracturing in the snow but the
slab remains in place and no avalanche falls.
as destructive metamorphism proceeds, and on an inclined surface it also
creeps downhill under the
influence of gravity by internal
plastic deformation and slip on the ground. Creep velocity varies with
type, snow depth, slope inclination
and profile., and ground cover. These variations from one zone of the snow
another develop creep stresses. The zones of creep
tension are favorable locations for slab avalanche fracture lines;
commonly occur on convex profiles or at the head of open slopes where the
snow cover first finds anchorage (trees,
ridge top, etc.).
A snow slab under tension may not only break free when triggered but shatter
into blocks as well when the
stress is relieved. In hard
snow of high tensile strength this release may achieve almost explosive
are in large measure responsible for the dangerous and unpredictable character
of slab avalanches.
In this section, you will
get to see pictures of avalanches happening worldwide...dated from the
Maps of avalanche-affected
Click on the globe to see
frequently hit places...