Snow Pack Metamorphosis

Diurnal Temperature Profile in Snowpack M. Williams, CU Boulder

M. Williams, CU Boulder

Snowcrystals.com; Libbrecht 2005

Snowcrystals.com

Types of Snow Metamorphism in Snow pack

Equilibrium-Temperature-Metamorphism 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. "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Equilibrium-Temperature-Metamorphism 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. "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

The following conditions promote rounding: Equilibrium-Temperature-Metamorphism 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) "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Equilibrium-Temperature-Metamorphism

Equilibrium-Temperature-Metamorphism 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.

Temperature-Gradient Metamorphism 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. Also called Constructive metamorphism Synonyms: constructive metamorphism temperature-gradient metamorphism kinetic growth depth hoar sugar snow "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Temperature-Gradient Metamorphism 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. "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Temperature-Gradient Metamorphism 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) "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

Temperature-Gradient Metamorphosis 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

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) Slush (snow grains completely surrounded by liquid water) Rounded poly-crystals All photographs by S. Colbeck

MORPHOLOGICAL CLASSIFICATION PROCESS-ORIENTED CLASSIFICATION ADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTH Basic Classification 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 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

Shear Density Relationship, Snoqualmie Pass Casson et al. 2008

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