Points on Ice:
information on how ice-climbs form, deform, and fail
the ice until it has been crossed."
--From the Viking saga Havamal, circa 1000A.D.--
Ice is weird stuff, though climbing it might just be weirder.
Ice-climbing is also potentially painful: half the equipment has sharp metal points (like
tools, crampons, and ice-screws) that mix well with neither the other half of the
equipment (like clothes, pack, and rope), nor with the soft flesh of a climber. And then
theres the objective danger. Ice-climbs are temporary features of winter, and are in
a perpetual state of falling down during their short life-spans. Thats the part of
ice-climbing thats potentially lethal. The paradox of ice-climbs is that they can
provide the easiest and safest means of ascent of a cliff, or a mountain. The trick is to
determine when an ice-climb is safe, and to do that requires knowing all about ice.
Ice wont form from liquid water at the freezing point
unless there exists a seed for the crystal. A seed works by dissipating the energy of
colliding water molecules so they stay locked in a lattice framework. In the absence of a
seed, the water must be supercooled to a temperature below the freezing point
before crystal nucleation spontaneously begins. The rate of supercooling affects how many
crystals will form. With a faster rate of cooling,
more crystals nucleate. In Nature, there's almost always the presence
of a seed, and it's usually ice crystals themselves.
of small ice crystals in the water is termed "frazil.".
Frazil most often conveniently arrives in the form of
snow falling, so snowfall accentuates ice formation.
In flowing water
especially for a thin film frazil can agglomerate into hanging dams
which inhibit the flow and form a sort of terrace-works on the ice surface. Hanging dams
create washboard texture, and are responsible for the ribbed texture
of many icicles.
The ridges that result from hanging dams can be fractions to many inches in
period and height.
Frazil can adhere to underlying rock, dirt, or plants to form
bulbous anchor ice. Often, for less-than-vertical ice-climbs, anchor ice is what
first begins the climbs formation. The resultant ice is comprised of an aggregate of
randomly oriented crystals. Decreased temperature and increased wind increase the
ice-climb growth rate.
Pillars and curtains of ice are comprised of collections of
icicles. Icicles are polycrystalline, but the crystals are usually oriented such that
their optic axis tends to lie perpendicularly to the icicle length, so they snap more
readily than they split. Icicles grow in length by freezing at the tip of a thin tube
filled with water (the water-filled hollow may extend for a couple of inches back up the
forming icicle). Girth is increased by the freezing of the flowing film of water on the
outer surface. The complete solidity of an icicle occurs with inward freezing of the
central liquid core. Icicles grow in length at rates of 8-32 times that of the diameter
depending on the water-supply rate and temperature. The rate of growth in length drops off
with increased water supply, but the rate of growth in diameter stays about the same since
the thickness of the flowing film of water over the icicle surface varies little.
Decreased temperature and increased wind both lead to increases in diameter and length
growth rates of icicles.
A distinction of "water-ice", or "alpine-ice"
is sometimes made in ice-climb rating; This distinction qualifies the source of the water.
Ice-climbs with a water-ice (WI) rating derive ice from liquid water whereas ice-climbs
with an alpine-ice (AI) rating are formed primarily from snow. Alpine-ice is formed by
pressure sintering, or by recrystallization of snow. For alpine-ice,
temperature and the possible presence of liquid water affect the rate of snow to ice
transformation. Water runoff, or rain, hastens the metamorphosis of snow into alpine ice.
Rainfall on snow will overnight result in accretion of snowflakes into larger crystals
averaging 2mm in diameter. More time allows further metamorphosis of snow into blue ice as
air is excluded and crystal size increases. Late Summer and early Autumn are therefore
usually the best time for alpine ice-routes which rely on snow as the water source.
Only the hexagonal crystalline form of ice --referred to with the
symbol Ih-- is found naturally on Earth (there are a total of eleven
known crystalline forms of ice). The pattern in which the molecules of water are packed in
ice Ih controls its inherent mechanical behavior. In ice Ih, the
oxygen atoms of the water molecules can be imagined as being arranged in layers of
hexagonal rings and the layers are stacked on one another. The layers form what is
referred to as the basal plane of a water crystal. The direction perpendicular to
the basal plane is referred to as the optic axis. When a single ice crystal is
subjected to stress, the region between adjacent rings will slip more readily than a layer
will tear, and there is no apparent preferred direction for this slippage much like
a stack of playing cards will readily slip, but its tough to tear the deck.
Ice deforms under stress by a combination of one to five
recognized mechanisms. Which of the deformation mechanisms predominates depends largely on
the rate of application of the stress and the temperature of the ice. A low rate of
applied stress (such as gravity acting on an ice mass) affects plastic deformation of ice.
That is, ice will "creep." The mechanisms by which creep occurs are:
Elastic deformation of inter- and intra-molecular bonds. This
mechanism causes the reassuring pinch on tool picks and crampon points, and also makes
tool removal difficult. Colder temperatures limit the elasticity of ice.
Individual ice crystal deformation --usually by slippage along
the basal plane of the crystal, though some slippage along the optic axis will occur.
Rearrangement of crystals within the polycrystalline aggregate.
Crystal growth via the migration of crystal boundaries.
Dynamic re-crystallization via the nucleation and growth of new
Ice crystals creep even at small stress levels, and the creep
rate accelerates as time proceeds! Elastic deformation initially occurs with applied
stress. Thereafter, all of the deformation mechanisms are thought to be engaged with their
respective contributions depending on several factors. The following factors affect creep
rate: temperature, crystal size and orientation, liquid water, impurities, density, and
Temperature: For temperatures from near and above freezing to
about -10 C° (14 F°), grain-boundary sliding contributes greatly to creep, and is
assisted by the presence of liquid water at grain boundaries. Somewhere between -10 C°
and -14 C° (14 F° to 7 F°), there appears to be a change in the mechanism that
predominates the deformation process. Below -15 C°, the
rate of creep slows by a factor of 10.
Crystal size and orientation: Crystal size does not affect
the magnitude of the creep rate, but does affect how quickly the maximum acceleration of
rate is reached. Large crystals in an aggregate cause a faster acceleration in the creep
rate than do small crystals. Crystal orientation profoundly affects creep rate since
slippage along the optic axis requires about 100 times the stress than does slippage in
the basal plane. Ice that is small-grained with randomly oriented crystals deforms more
slowly than does large-grained ice with favorably aligned crystals. Thus, alpine ice
formed from snow often exhibits less "flow" deformations than does water ice.
All ice undergoes recrystallization in which the crystals become oriented to favor flow
and the creep rate accelerates.
Liquid water: Liquid water between crystals can act as a
lubricant as well as a media for melting and refreezing. Its presence accelerates the
Impurities: Dissolved and occluded impurities lower the
melting point of ice and increase the creep rate. Typically, a discolored icefall
indicating the presence of impurities will deform more easily and will be more
plastic in texture than "pure" ice in the same vicinity. An example of this is
the mushy Avocado Falls in the Crystal River Valley of central Colorado.
Density: High density ice creeps more slowly than low density
ice. In other words, blue ice will deform more slowly than white ice, all other variables
being the same.
Pressure: Because of the disparate densities of liquid and
solid water, increased pressure produces liquefaction of ice. The consequence of this
phenomena for ice pillars and curtains is that they will deform and eventually collapse
with the fracture typically occurring up high where pressure is often negative (i.e.
where the ice is actually under tension rather than being compressed). Frozen waterfalls
often initially melt where they contact the underlying rock and where pressure is most
intense (though, there can be, and often are, other extenuating circumstances contributing
to this such as warmer underlying ground and the presence of flowing water).
Though ice creeps with slow application of stress, it fractures
with rapid application. Ice fractures start with the formation of voids between crystals
which link to form micro-cracks. The micro-cracks can develop to macro-cracks which may
result in spalling of large pieces. Rates of applied stress of 1mm/s, or greater, will
affect brittle fracture. In other words, any ice-tool strike, or crampon kick, causes
The same factors that influence creep play a role in brittle
fracture. Higher temperatures and the presence of intercrystalline water inhibit brittle
fracture. Unfortunately, these same influences which make ice plastic and easier to climb
can also make the ice-fall itself unstable and prone to collapse.
In a sense, all of the deformation processes of ice are
destructive. For example, many pillar and curtain climbs succumb to their own creep by
eventually fracturing near their top where tension overcomes inter-molecular bonding.
However, the most destructive entity at work on ice-climbs is water itself. Liquid water
has an enormously high heat capacity. That is, liquid water can absorb, or emit, a lot of
energy without much change in its temperature. Many high-flow waterfalls wont freeze
simply because not enough energy can be carried away by the surrounding cold air and
ground. Liquid water can also melt a lot of ice. Once water starts flowing over, under, or
through ice, it deteriorates and weakens quickly.
The melting of ice will also destroy an ice-climb, and ice melts
in a curious way; It doesnt solely melt from the surface inward, but melts
throughout at all of the crystal boundaries. Melting initiates the percolation of liquid
water which in turn creates a vacuum and pulls in air. A white color reveals the presence
of air, and is indicative of poor cohesion of ice crystals, low density, and weak ice.
Eventually a melting ice-fall will reach a consistency of spring snow if it doesnt
collapse first. Also, the percolating water often winds up at the underlying rock-ice
interface where it may accumulate into significant flow to melt even more ice. Melting
will therefore be greatest at the areas where the ice adheres, or once adhered, to the
underlying substrate, and will be hidden from view. If running water is audible, the ice
will likely be poorly affixed to the underlying rock, or dirt. Be wary of any gurgling
Ice-tool and Crampon Penetration
The ice-pick acts as a wedge as it pushes aside ice. The
ice-pick penetrates through a combination of three deformation mechanisms: 1) elastic
deformation; 2) intercrytalline glide (the sliding of crystals past one another); and 3)
individual crystal deformation. These mechanisms are enhanced by near-melting-point
temperatures when much liquid water is present. With low temperatures, the ice surface
topography can be utilized to place the pick where the directed forces affect ice
displacement and minimize micro-crack formation. Where the ice is concave, the force of the
tool-strike is directed into the ice, and micro-crack formation is thwarted. Where
concavities arent around, gently pecking a hole before swinging with more force to
stick the pick is an effective technique.
What is known about ice can be digested into the following
Near the melting temperature (which is 0 C°/32 F°, or lower, if
impurities are present, and they probably are), ice gets creepy especially pillars
and curtains where ice is supported by itself. The creep rate of ice slows significantly
with lower temperatures (by about a factor of ten at -15 C°/5 F°).
Water flow will melt ice quickly and can do so both internally
and at the rock-ice interface. An ice-climb being eroded by water will be structurally
weaker than an ice-climb of similar thickness with no water-flow. Whitened ice and/or
gurgling sounds indicate that melting has occurred, or is underway. A white colored
ice-fall that is spalling chunks under the force of gravity, or is making cracking noises,
is dangerously unconsolidated and will soon fail catastrophically.
Impure water-ice melts more readily and behaves more plastically
than pure water-ice. Green, or brown, ice will melt before blue ice.
Aged ice will have undergone re-crystallization to an extent that
the ice-crystals will be oriented to favor creep deformation. Therefore, late season
climbs will be creeping at a faster rate than in early season, and will be more subject to
micro-crack formation and subsequent fracture failure.
While ice is not actually alive, it is certainly animate. It
exists in a dynamic state of perpetual vibration, flux, and movement. Enjoy its existence,
and its safe passage in the mountains.