snow pack metamorphosis. m. williams, cu boulder diurnal temperature profile in snowpack

Snow Pack Metamorphosis

M. Williams, CU Boulder

Diurnal Temperature Profile in Snowpack

M. Williams, CU Boulder

Snowcrystals.com; Libbrecht 2005

Snowcrystals.com

Types of Snow Metamorphism in Snow pack

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 (hollow)in 2 stages:

"Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Equilibrium-Temperature-Metamorphism

Stage 1: In the initial stage of rounding

the sharp ends of new crystals and the points of faceted grains sublimate and the resulting water vapor 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.



Stage Two:

Under weak temperature gradients, water vapor moves from warm areas to cold, but the rate of movement is much slower than in strong temperature gradient environments.

Slow moving vapor 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 vapor 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)



During the rounding process the particles bond together by forming necks. Sintering is the process by which two particles weld together without a liquid present. Notice that there are bonds or "necks" between the rounded crystals in the right hand image.


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.


Temperature-Gradient Metamorphism

Also called Constructive metamorphism

Synonyms: constructive metamorphism temperature-gradient metamorphism kinetic growth depth hoar sugar snow

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 vapor from warm areas to cold)

Loose, low density snow (which facilitates the free movement of water vapor between grains)Presence of crusts (which concentrate water vapor, promoting vapor transfer in the concentrated area)

Moderate snow temperature (which maximizes the amount of vapor the snowpack can hold but does not reduce the overall temperature gradient significantly)



TG metamorphosis occurs in response to a strong temperature gradient (> 10°C /m).

As water vapor is deposited on the grains, they grow larger, and can eventually form the large crystals known as depth hoar.

Depth hoar normally forms near the base of the snow pack, where the vapour pressure gradient is strongest and most persistent.

Sublimation: The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point

Temperature-Gradient Metamorphosis

Well rounded crystals Rounded crystals with developing facets

Temperature-Gradient Metamorphosis

Melt-Freeze Metamorphism

Melt-freeze metamorphism occurs when the sun melts the upper layers of the snowpack during the day, but freezing still takes place at night. With MF Metamorphism, larger grains grow at the expense of smaller ones, helping to strengthen the snowpack in most cases. With MF metamorphosis, the density of the snow pack can increase to 0.6 g cm-3.

Melt cluster (clustered rounded grains) Rounded poly-crystals Slush (snow grains

completely surrounded by liquid water)

All photographs by S. Colbeck

MORPHOLOGICAL CLASSIFICATIONPROCESS-ORIENTED

CLASSIFICATIONADDITIONAL INFORMATION ON

PHYSICAL PROCESSES AND STRENGTH

Basic Classificatio

n Subclass Shape Place of

Formation Classification Physical

Processes

Dependence on most

important parameters

Common effect on strength

Wet Grains Clustered rounded grains

Clustered rounded crystals held together by large ice-to-ice bonds; water in internal veins among three crystals or two-grain boundaries

Wet Snow Grain clusters without melt-freeze cycle

Wet snow at low water content, pendular regime; clusters form to minimize surface free energy

Meltwater can drain; too much water leads to slush; freezing leads to melt-freeze particles

Ice-to-ice bonds give strength

Rounded poly-crystals

Individual crystals are frozen into solid polycrystaline grain; may be seen either wet or frozen

Wet Snow Poorly bonded; rounded single crystals

High liquid content; equilibrium form of ice in water

Water drainage blocked by impermeable layer or ground; high energy input to snow cover by solar radiation, high air temperature or water input

Little strength due to decaying bonds

Casson et al. 2008

Shear Density Relationship, Snoqualmie Pass

Fig. 4. A snow wedge vertically driven into the backside of a snow column to find failure planes within a snow-pack. Theslope angle is 8 and the wedge angle is Beta. (La Chappelle and Ferguson 1980)

Determining Shear in Snow Pack

snow pack metamorphosis. m. williams, cu boulder diurnal temperature profile in snowpack

Documents

metamorphism slide

snowpack slide

snow pack slide

high snow temperature

colder grains

weak temperature gradients

cu boulder slide

snow pack metamorphosis