14Equilibrium-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.
19Temperature-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
20Temperature-Gradient Metamorphosis Well rounded crystalsRounded crystals with developing facets
21Melt-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-crystalsAll photographs by S. Colbeck
22MORPHOLOGICAL CLASSIFICATION PROCESS-ORIENTED CLASSIFICATION ADDITIONAL INFORMATION ON PHYSICAL PROCESSES AND STRENGTHBasic ClassificationSubclassShapePlace of FormationClassificationPhysical ProcessesDependence on most important parametersCommon effect on strengthWet GrainsClustered rounded grainsClustered rounded crystals held together by large ice-to-ice bonds; water in internal veins among three crystals or two-grain boundariesWet SnowGrain 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 strengthRounded poly-crystalsIndividual crystals are frozen into solid polycrystaline grain; may be seen either wet or frozenPoorly bonded; rounded single crystalsHigh liquid content; equilibrium form of ice in waterWater drainage blocked by impermeable layer or ground; high energy input to snow cover by solar radiation, high air temperature or water inputLittle strength due to decaying bonds
23Shear Density Relationship, Snoqualmie Pass Casson et al. 2008
24Determining 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. Theslope angle is 8 and the wedge angle is Beta. (La Chappelle and Ferguson 1980)