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Iceclimb

Points on Ice:

information on how ice-climbs form, deform, and fail

"Praise not…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 there’s the objective danger. Ice-climbs are temporary features of winter, and are in a perpetual state of falling down during their short life-spans. That’s the part of ice-climbing that’s 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-climb Formation

Ice won’t 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.

The suspension 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 climb’s 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.

Ice-climb Deformation

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 it’s 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 crystals.

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 applied pressure.

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 creep rate.

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 brittle fracture.

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.

Ice-climb Destruction

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 won’t 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 doesn’t 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 doesn’t 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 sounds.

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 aren’t around, gently pecking a hole before swinging with more force to stick the pick is an effective technique.

In summary

What is known about ice can be digested into the following common-sense conclusions:

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.

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