How Snow Fails and fractures:
Avalanches don't "strike without warning", as we so
often read in the press. They are only the most spectacularly
visible event in a long series of precursors leading up to the
grand finale.
It all begins many hours--or even days--before, usually when
new snow or wind-blown snow begins to pile weight on top of
a buried weak layer. Added weight causes the underlying snow
to deform; rapidly added weight causes snow to rapidly deform.
On an inclined slope, the deformation tends to concentrate within
buried weak layers in the form of shear.
Inside of a weak-layer under stress, we can think of this as
a race between bonds being broken and bonds being re-formed.
Let's look at three different rates of deformation, slow medium
and fast:
Slow deformation rate
If the weak-layer shears slowly, it either deforms the bonds
between the ice grains or more bonds form than break. This means
that the weak-layer adjust to its load and actually gains strength.
Snow can lazily drape over the terrain like a cat draped over
the back of the couch—like a limp rubber band—and
if you’ve every tried to cut a limp rubber band with a
knife, you know what a stable snowpack is like.
Medium deformation rate
With an increasing rate of deformation, we reach a point where
nearly as many crystalline bonds break as form and the strength
of the weak layer remains about the same. With sensitive microphones
we can actually hear the rupture of individual bonds between
the ice grains, like the sound of slowly ripping Velcro.
Rapid deformation rate
If shear occurs too rapidly--past a certain critical threshold--then
more bonds break than form. The weak-layer inexorably looses
strength and begins the slippery slide towards disaster. We
call this "failure"--when the snow begins to progressively
loose strength. We also call this "strain softening."
To understand failure and strain softening, do this experiment:
Take a paper clip and bend it in the same place repeatedly,
and after about ten bends you'll notice that it is getting weaker
(failure) and after about 15 bends, it snaps right off (fracture).
Got that, the difference between failure and fracture?
Having said this, scientists still don't know exactly how avalanches
fail and fracture because snow is such a devilishly difficult
substance to study. First, large variations commonly exist over
both distance and time and second, as you can imagine, catching
a natural avalanche in the act is stupendously difficult and
dangerous, or as Monty Atwater put it, "an occupation something
like trailing a wounded African buffalo." So ten or twenty
years from now, the following paragraphs--like so many of the
"facts" we believe about avalanches--may seem like
yet more quaint, geezer-ramblings. Nevertheless, this is what
scientists generally believe about how avalanches fail and fracture.
Failure is thought, by some, to precede fracture--perhaps over
the course of minutes to hours. But others believe that fracture
doesn’t always require a preceding failure. There’s
a lot of evidence that they can both occur at the same time.
When we add a rapid change to the snowpack, just like silly
putty, it introduces stored elastic energy into the system making
it capable of propagating a fracture. Just like a stretched
rubber band, you can cut it with a knife with very little force.
Failure occurs slowly at perhaps centimeters per hour and fracture
occurs catastrophically and has been measured at around 20 meters
per second. Whamo! The slope shatters like glass.
When a person triggers an avalanche, it means that they have
found a trigger point of the avalanche. Perhaps it’s a
place where the slab it thinner allowing more of the victim’s
weight to tickle the weak layer. Perhaps it’s a place
where the weak layer is more poorly bonded than the rest of
the slope. Perhaps it’s part of the slope were the snowpack
has more stored elastic energy. Perhaps it’s all three.
I don’t think anyone knows for sure. But we do know that
snow is very sensitive to the rate at which it is deformed and
the extremely rapid deformation caused by the weight of a person
is exactly the kind of thump needed to initiate a fracture.
Without this final trigger, unstable slopes can teeter on the
brink of disaster for quite some time, giving us the illusion
that all is well. After a storm, we never know how many slopes
would come down if they just had a proper trigger. But with
enough good snowmobiles and skilled riders, we can certainly
find out. As avalanche researcher Rand Decker likes to say,
"Avalanches are like a bar room brawl. No matter how much
tension you have, you need to give somebody a thump to get things
going."
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