Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Snow Pack MetamorphosisM. Williams, CU BoulderDiurnal Temperature Profile in SnowpackM. Williams, CU BoulderSnowcrystals.com; Libbrecht 2005Snowcrystals.comTypes of Snow Metamorphism in Snow packThe 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 AssociationEquilibrium-Temperature-MetamorphismStage 1: In the initial stage of roundingthe 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 AssociationEquilibrium-Temperature-MetamorphismStage 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 AssociationEquilibrium-Temperature-MetamorphismThe 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 AssociationEquilibrium-Temperature-MetamorphismEquilibrium-Temperature-MetamorphismDuring 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.Equilibrium-Temperature-MetamorphismThe 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. "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche AssociationTemperature-Gradient MetamorphismAlso called Constructive metamorphismSynonyms: constructive metamorphism temperature-gradient metamorphism kinetic growth depth hoar sugar snowWhen 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 AssociationTemperature-Gradient MetamorphismThe 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 AssociationTemperature-Gradient MetamorphismTG 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 MetamorphosisWell rounded crystalsRounded crystals with developing facetsTemperature-Gradient MetamorphosisMelt-Freeze MetamorphismMelt-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. ColbeckMORPHOLOGICAL CLASSIFICATIONPROCESS-ORIENTED CLASSIFICATIONADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTHBasic 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 cycleWet snow at low water content, pendular regime; clusters form to minimize surface free energyMeltwater can drain; too much water leads to slush; freezing leads to melt-freeze particlesIce-to-ice bonds give strength Rounded