Metamorphism of Snow

Once new snow crystals are added to the snowpack they begin to metamorphose (change). From this point on, snow crystals are technically referred to as grains (although the word crystal is still used by many practitioners). Through metamorphism, the form and size of snow crystals and grains inside a snowpack change continuously, altering the strength characteristics of the snowpack.

Metamorphism of snow in a seasonal snowpack is the result of sublimation and deposition. (Sublimation is the process of ice becoming vapour without going through a liquid state and vice versa.) During metamorphism in the snowpack, ice from grain surfaces changes into water vapour which is then deposited as ice at other grain surfaces as follows:

1.) Vapour moves from warm surfaces to cold surfaces. Because the snowpack is usually warm (at or near 0 degrees C) at the ground and cold near the surface, ice sublimates from lower, warmer grains and is deposited as ice at other grains sites as shown in the diagram to the right.
2.) Vapour typically moves from convex surfaces (points) to concave surfaces (hollows). The sharp ends of new snow crystals becomes blunt and the space between the branches is filled. In the same manner, large grains with broad curvatures grow at the expense of small grains with sharp curvatures.

The Temperature Gradient

The temperature gradient is the most important factor determining the type of metamorphism, the resulting grain form, and the rate of growth of the grains,

Temperature Gradient is the difference in snow temperature across a given vertical distance in the snowpack. In practice it is expressed in degrees Celsius per 10 centimetres. As a general rule, a temperature gradient less than 1 degree / 10cm is considered weak. A strong temperature gradient is greater than 1degree / 10cm. Strong Temperature gradients promote greater vapour movement than weak gradients.

The nature of the temperature gradient influences the type of metamorphic process that will be dominant in a given portion of the snowpack. The primary processes are faceting and rounding. (For a detailed explanation of faceting and rounding see the sections that follow below.)

Faceting and rounding take place in the snowpack interchangeably. When the temperature gradient is strong and the snow density is low, the faceting process dominates. When the temperature gradient shits from strong to weak (usually the result of warming at the snow surface), faceted grains, depth hoar and surface hoar grains begin rounding. These large angular grains resist rounding much more than branched new snow crystals and may remain weak for long periods.

(For an explanation of the graphical symbols drawn in the following diagrams within each layer to denote snow grain type see Classification of Snow Crystals.)

Strong Temperature Gradient

The cross section of a snowpack at left shows a strong temperature gradient. The height of the snowpack is 100cm and the snow temperature near the top of the snowpack is -15 degrees C. This creates an "average" temperature gradient within this snowpack of 1.5 degrees / 10cm. The actual gradient in any particular layer varies and may be greater or less than this average, but it can be expected that in this sample snowpack the faceting process will be predominating. (This process has gone by other names including temperature gradient metamorphism, TG metamorphism, Constructive metamorphism, recrystallization and kinetic growth. The term faceting is preferred.)

This example is fairly typical of a snowpack that you may find in early winter in many regions or in the Canadian Rockies even during mid-winter or later. If this temperature gradient does not change, the snowpack will continue to lose strength over time and a base of weak depth hoar will continue to develop. Faceted grains and depth hoar formed in this way will persist in the snowpack and can cause cycles of avalanche activity for the rest of the winter and even into the spring or, in some cases, summer.

Weak Temperature Gradient

The cross section of a snowpack at left shows a weak temperature gradient. The height of the snowpack is 200cm and the snow temperature near the top of the snowpack is -15 degrees C. This creates an "average" temperature gradient within this snowpack of 0.75 degrees / 10cm. The actual gradient in any particular layer varies and may be greater or less than this average, but it can be expected that in this sample snowpack the rounding process will be predominating. (The rounding process has gone by other names including equi-temperature metamorphism, ET metamorphism, destructive metamorphism, and equilibrium growth. The term rounding is preferred.)

In this sample snowpack, the temperature gradient is weaker near the base and stronger near the top. There is no place in this sample snowpack that the faceting process will be predominating.

This example is fairly typical of a snowpack that you may find in early winter in a deep snowpack region with moderate climate (such as the Coast Ranges of British Columbia or the US). Similarly this type of snowpack may exist in the Columbia Mountains in the Interior of British Columbia in early winter during a heavy snowfall winter and certainly by mid winter in an average winter. The Canadian Rockies would typically only have this type of condition later in winter or spring or in a good snow year.

If this temperature gradient does not change, the snowpack will continue to gain strength over time and any base of weaker facets as shown in this example will continue to strengthen. Even with a weak temperature gradient which promotes rounding and strengthening of the snowpack, hidden weak layers may exist. In this sample snowpack, a layer of surface hoar is buried just above 130cm. Buried surface hoar may persist in the snowpack and can cause cycles of avalanche activity for the next several weeks or more. The weak temperature gradient will eventually round out the surface hoar and promote bonding with the layers above and below but this gain in strength of this insidious layer can take a very long time in some cases.

Rounding

The rounding process builds rounded grains (rounds) which bond well to one another creating a snowpack (or layer) that is generally increasingly strong.

In weak temperature gradients(<1 degree / 10cm) sublimation typically moves ice from convex surfaces (points) to concave surfaces (hollows) in 2 stages:

In the initial stage of rounding, the sharp ends of new crystals and the points of faceted grains sublimate and the resulting water vapour is deposited in concave areas. At high temperatures, molecules also glide along the grain surface from convexities to concavities. As well, large grains with broad curvatures grow at the expense of small grains with sharp curvatures. The result is a concentration of mass with a minimum surface area.
Under weak temperature gradients, water vapour moves from warm areas to cold, but the rate of movement is much slower than in strong temperature gradient environments. Slow moving vapour is deposited on the colder surfaces in a more homogenous manner and the faceted, stepped pattern associated with a strong temperature gradient does not occur.

The following conditions promote rounding:

A weak temperature gradient generally less than 1 degree C per 10 cm (which moves water vapour slowly from warm areas to cold)
Dense, tightly packed snow
Small grains (which produce denser snow)
A high snow temperature, typically above -10 degrees C (which promotes weaker temperature gradients)

Faceting

The faceting process builds angular grains (facets) which bond relatively poorly to one another and other grains creating a snowpack (or layer) that is generally increasingly weak.

When the temperature gradient is strong (> 1 degree / 10cm) water vapour moves rapidly from warm grain surfaces to colder surfaces. Because the snowpack usually is warm (at or near 0 degrees C) at the ground and colder at the surface, ice sublimates from lower, warmer grains and is deposited onto colder grains higher up in the snowpack. These colder grains first develop sharp corners, then stepped facets.

If the faceting process continues, large, six - sided hollow or filled cup shaped grains called depth hoar are formed. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin. The following conditions promote faceting:

A strong temperature gradient, generally greater than 1 degree / 10cm (which quickly drives water vapour from warm areas to cold)
Loose, low density snow (which facilitates the free movement of water vapour between grains)
Presence of crusts (which concentrate water vapour, promoting vapour transfer in the concentrated area)
Moderate snow temperature (which maximizes the amount of vapour the snowpack can hold but does not reduce the overall temperature gradient significantly)

Sintering

Sintering is usually associated with the rounding process. Water Vapour is deposited at the contact points between snow grains forming necks. These necks create strong bonds between grains, increasing snow strength.

Mechanical Hardening

Compaction from any mechanical disturbance such as boots, skis, snowmobiles, groomers, wind and avalanches breaks up large grains and brings grains into close contact, producing rapid sintering.

Melt-Freeze Metamorphism

The change in snow grains as the snowpack becomes wet (snow temperature reaches 0 degrees C) and subsequently refreezes is known as melt-freeze metamorphism. This process usually occurs during late winter and spring when air temperatures are high, solar radiation is high, and cycles of melting and refreezing are common.

Melt-freeze crust layers that exist in the freeze part of the cycle can be very strong.

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