Metamorphism Due To Indirect Weather Effects

1. Metamorphism Due To Indirect Weather Effects • Learning Outcomes • Understand temperature gradients. • Understand rounding and its effect on the snowpack. • Know the conditions that are conducive to rounding. • Understand sintering and its effect on the snowpack. • Know the conditions that are conducive to sintering. • Understand faceting and its effect on the snowpack. • Know the conditions that are conducive to faceting

2. Metamorphism Due To Indirect Weather Effects Once snow grains are buried in the snowpack, they become insulated from direct weather effects. Metamorphism still occurs in these deeper layers, however, and the weather still plays a role, but the effects of weather are indirect.

3. Metamorphism Due To Indirect Weather Effects When indirect weather is playing a role, weather factors influence the environment in the snowpack rather than the grains themselves. In differing environments, snow grains metamorphose differently and the overall effect on the grains and the layers varies. Metamorphism due to indirect weather generally occurs more slowly than metamorphism due to direct weather factors.

4. Metamorphism Due To Indirect Weather Effects The primary engine that drives metamorphic processes deep in the snowpack is the temperature gradient. Temperature Gradient Temperature gradient is defined as: “The change in temperature over height.”

5. American Institute for Avalanche Research & Education Level II Avalanche Course Metamorphism Temperature gradients are measured in ºC per 10 cm. Measure the gradient directly at regular 10 cm intervals. If the change in temperature was 3 ºC per 10 cm, The temperature gradient is 3 ºC per 10 cm. In reality, temperature gradients are present in many snowpacks much of the time and they do not always have a detrimental effect.

6. Metamorphism (indirect) To obtain a “calculated” temperature gradient we can use the following formula: T10 – Tgnd = cTG HS/10 Where: T10 is temperature of the snow 10 cm below the surface Tgnd is temperature of the ground HS is the height of snow in centimeters cTG is the calculated temperature gradient

7. Metamorphism: Temperature gradients • Lets try a few examples and calculate the temperature gradient. • T10 = -20ºC, Tgnd = 0, HS = 100 • T10 = -5ºC, Tgnd = 0, HS = 100 • T10 = -10ºC, Tgnd = 0, HS = 50 • T10 = -10ºC, Tgnd = 0, HS = 200 T10 – Tgnd HS/10  = cTG

8. Temperature Gradients Calculate the temperature gradient: T10 = -20, Tgnd = 0, HS = 100, TG = ______ HS = 100 -20º HS, Depth (cm) T10 – Tgnd HS/10  = cTG 0º -10º Tº C

9. Temperature Gradients Calculate the temperature gradient: T10 = -5, Tgnd = 0, HS = 100, TG = ______ HS = 100 -5º HS, Depth (cm) T10 – Tgnd HS/10  = cTG 0º -10º 20º Tº C

10. Temperature Gradients Calculate the temperature gradient: T10 = -10, Tgnd = 0, HS = 50, TG = ______ HS = 50 T10 – Tgnd HS/10  = cTG HS, Depth (cm) -10º 0º -10º -20º Tº C

11. Temperature Gradients Calculate the temperature gradient: T10 = -10, Tgnd = 0, HS = 200, TG = ______ -10º HS = 200 T10 – Tgnd HS/10  = cTG HS, Depth (cm) 0º -10º -20º Tº C

12. Metamorphism Primary factors: Air temperature Ground temperature Snow height TG < 1ºC per 10 cm = weak TG > 1 º C per 10 cm = strong

13. Metamorphism • When the temperature gradient is less that 1ºC per 10 cm (e.g., TG is weak), snow grains are constantly trying to attain a state of equilibrium. • To do this, they reduce their surface area. • Initially, the original grain decomposesintofragments. Decomposition & Fragmentation / The symbol for DF grains is: When the original snow grains no longer exist but their original form can still be inferred from the fragments, the snow grains are defined as “decomposed and fragmented”.

14. Metamorphism Rounding/Sintering • If the temperature gradient remains weak (<1oC/10 cm) the snow grains continue to decrease in size as they attempt to reach and equilibrium state. • The ice that makes up the grain sublimates and becomes water vapor. Stellar crystal begins to round

15. Metamorphism Rounding/Sintering • Vapor pressure or concentration is higher over convex areas of the grain and lower in concave areas. • The vapor tends to move from convex areas of the grain (regions of high vapor pressure) to concave areas (regions of low vapor pressure). Stellar crystal begins to round

16. When the concentration of vapor pressure becomes great enough in the concave areas, the vapor condenses or deposits in the hollow by shifting phase from vapor to ice. Metamorphism Rounding/Sintering Field symbol Rounding  As this process continues, the convexities are reduced and concavities are filled in resulting in a rounded grain of smaller size than the original from which it was created. This process is called rounding. Rounding

17. In latter stages, the entire grain becomes rounded (convex) and the process begins to deposit ice in the joints between the grains. • A neck grows and increases in size over time which greatly strengthens the bonds between grains. • This process is called sintering. • The smaller and more rounded the grains become, the closer they pack together and bond, and the more sintering is present the stronger the snow will be. Metamorphism Rounding/Sintering sintering

18. Rounded grains Rounded grains bond together into simple chains. The polarized light results in different grains generally having a different color. Round snow grains. Red color is pore space between snow grains.

19. Rounded grains Note the sintering (bonding) that occurs where two different grains touch as a result of "neck" growth by vapor deposition to the concavity.

20. Rounded grains Bonding between two different grains touch as a result of "neck" growth by vapor deposition to the concavity.

21. Rounded grains In weak temperature gradients(<1°/ 10cm) sublimation typically moves ice from convex surfaces (points) to concave surfaces in stages:

22. If the temperature gradient is strong (>1oC/10 cm) there is a strong heat fluxin the snowpack. • That simply means that heat is moving through the snowpack from warmer regions to cooler regions as nature tries to equalize the temperature of the ground and the air. • This heat flux changes the metamorphic process. The ice that makes up the grains continues to sublimate. Metamorphism Faceting Strong TG heat flux & sublimation

23. Metamorphism Faceting • The strong movement of heat through the snowpack causes the vapor to move from warmer regions to cooler. • Vapor sublimates from grains in the warmer area (usually nearer the ground) and condenses into ice on grains in the cooler area (generally higher in the snowpack). Early in the faceting process, the grains become angular in shape; flat sides and corners are readily observed. Bonds between grains weaken and the layer becomes softer.

24. Metamorphism Faceting This vapor tends to condense on convexities and larger grains are created at the expense of smaller ones. This process is called faceting. Early in the faceting process, the grains become angular in shape; flat sides and corners are readily observed. Bonds between grains weaken and the layer becomes softer.

25. Metamorphism Faceting & Depth Hoar In latter stages, visible lines (striations) form on the angular grains. The striations represent subsequent deposits of vapor. When grains are fully angular and have more than two striations, they are referred to as depth hoar. It is not uncommon to see 10 mm depth hoar which is visible to the naked eye, especially early in the season.

26. Metamorphism Faceting & Depth Hoar In latter stages, visible lines (striations) form on the angular grains. The striations represent subsequent deposits of vapor. Depth hoar grains can become quite large when the faceting process is strong. It is not uncommon to see 10 mm depth hoar which is visible to the naked eye, especially early in the season.

27. Metamorphism Faceting & Depth Hoar As the grains become larger and more faceted, they become less packed and bond more poorly, resulting in weaker snow. Faceted grains have a dense molecular structure and as a result, once formed, they resist rounding even if temperature gradients become weak. Field symbols  Faceted grains Depth hoar ^ Depth hoar crystal

28. Water vapor is moving upwards, from the bottom of the image towards the top of the image. Depth hoar grains grow downwards and into the source of water vapor. As each wave of water vapor condenses on the depth hoar grain, the grain becomes larger. The result is a very unstable grain that acts like a lever Depth Hoar - facets Image is about 5 cm. Note that each grain is pointed towards the top of the image and widest towards the bottom of the image.

29. Depth Hoar - facets Another example of depth hoar. Again, the depth hoar grain is growing from the top of the screen towards the bottom of the screen. 

30. Depth Hoar - facets Angular grains with poor sintering. Each different color is a different facet within the depth hoar grain. Each facet represents a wave of water vapor that depositied as a single unit onto the existing grain. A depth hoar grain. photograph using polarized light.

31. Water vapor is moving upwards, from the bottom of the image towards the top of the image. Hence the depth hoar grains are growing downwards and into the source of water vapor. As each wave of water vapor condenses on the depth hoar grain, the grain becomes larger. The result is a very unstable grain that acts like a lever Depth Hoar - facets Image is about 5 cm. Note that each grain is pointed towards the top of the image and widest towards the bottom of the image.

32. New Snow DF Faceting & Mixed Forms If new snow or DF grains are exposed to a strong temperature gradient, they may become faceted without becoming a rounded grain. This generally happens at or near the surface of the snowpack. DF = decomposition and fragmentation What is a strong temp. gradient?

33. New Snow DF Faceting & Mixed Forms Since temperatures in the upper layers of the snowpack often fluctuate with day/night temperature swings ---- Gradients in the upper regions of the snowpack are often not sustained for long periods. It is unusual to see depth hoar type grains grow on the surface. DF = decomposition and fragmentation

34. New Snow DF Faceting & Mixed forms Often new snow and DF grains retain some of their original form and faceting takes the form of angular or striated additions to the original grain form. Since a faceted grain is reluctant to round, facets often persist in the upper part of the snowpack even though they may never become well defined or large.

35. New Snow DF Faceting & Mixed forms The process of constant change in the surface temperature gradient. If maintained over time, often results in a mixture of grain types including DFs, poorly developed rounded grains, and early stage facets. These layers are often referred to as mixed forms.

36. Metamorphic Combinations Rounds to Facets If rounding has been occurring due to a weak temperature gradient and the gradient changes to strong, the rounding process will be arrested and faceting will begin to occur. Layers that consist of small rounded grains, are hard, and have a high density resist faceting because there are few pore spaces for the heat flux to move through the snowpack. Early stage rounds that are well developed and not yet very hard or dense are highly susceptible to faceting if the temperature gradient shifts from weak to strong.

37. Metamorphic Reversal Facets to Rounds If faceting has been occurring due to a strong temperature gradient and the gradient changes to weak, the faceting process will be arrested and rounding will begin to occur. The process of changing a facet to a round is slower than the process of changing a round to a facet.

38. Metamorphic Reversal Facets to Rounds Faceted grains and depth hoar can and do become rounded and gain hardness and strength but, due to the molecular density of faceted grains, the process of changing a facet to a round is slower than the process of changing a round to a facet. The more well developed a facet or depth hoar grain is, the slower it is to revert to a rounded form.

39. Metamorphic Reversal Facets to Rounds Where well developed depth hoar has formed (e.g. continental climate, shallow snowpack) facets and depth hoar persist for long periods until the snowpack melts. Is this true in Maritime climates?

40. Metamorphic Reversal Facets to Rounds • Facets and depth hoar do gain hardness if not density over time under the influence of weak temperature gradients. • The grains lose their sharp corners, striations become blurred or disappear, and eventually bonds between grains do form. • Bonding and hardening in well-developed facet layers may be due to sintering between the grains (highly angular grains are composed of largely convex features so vapor may be moving to the joints between the grains).

41. Metamorphic Reversal Facets to Rounds • Faceted/depth hoar layers probably also gain strength later in the season due to the pressure of the overlying snowpack and moderating temperatures both of which are conducive to settlement and bonding. • Metamorphic reversals often result in layers that contain mixed forms including rounded, partly rounded, faceted, and/or partly faceted grains. • These mixed forms may eventually become one or the other if the reversal in metamorphic processes occurs before a grain type has become well developed and/or if the reversal remains in effect for a long period or permanently.

42. Metamorphic Combinations It is possible that faceting will occur in one part of the snowpack and rounding in another if the temperature gradients fluctuate from one area to another. Temperature gradients may vary on the vertical scale due to air temperature fluctuations (e.g. strong near the surface, weak near the base). Weak TG HS Strong TG Weak TG 0º Tº C Temperature gradients may vary horizontally due to variation in spx depth (e.g. a shallow area on a windswept rise, a deep area in a windloaded hollow).

43. In a 200 cm snowpack with a temperature of 0º at the bottom and -10º at the top, we have a TG of 0.5o/10cm. A weak temperature gradient. If we shift that so the temperature is -10º at the bottom and -20º at the top, the TG remains 0.5º /10cm. The former is a warm temperature regime and latter is a cold temperature regime. In both cases, rounding will be the dominant metamorphic process. Temperature Regime Review HS = 200 cm -10º -20º HS 0º -10º Tº C

44. Temperature Regime Review Rounding will occur more quickly in the warm regime (0º to -10º). Warm air can carry more vapor than cold and physical processes are faster in warmer temperatures. It is possible that faceting will occur in one part of the snowpack and rounding in another if the temperature gradients fluctuate from one area to another. HS = 200 cm -10º -20º HS 0º -10º Tº C

45. Temperature Regime Review Temperature gradients may vary on the vertical scale due to air temperature fluctuations (e.g. strong near the surface, weak near the base). Temperature gradients may vary horizontally due to variation in snowpack depth (e.g. a shallow area on a windswept rise, a deep area in a windloaded hollow). HS = 200 cm -10º -20º HS 0º -10º Tº C

46. In a 50 cm, snowpack with a temperature of 0º at the bottom and -10º at the top, has a TG of 2º/10cm. A strong temperature gradient. If we shift that so the temperature is -10º at the bottom and -20º at the top, the TG remains 2º/10cm. In both cases, faceting will be the dominant metamorphic process. Faceting will occur more quickly in the warm regime (0º to -10º). Temperature Regime Review HS = 50 cm HS -10º -20º 0º -10º Tº C

47. Temperature Gradient Review Temperature gradient is: A weak temperature gradient is: __________ A strong temperature gradient is: __________

48. Temperature Gradient Review When we have a weak temperature gradient (<1oC/10cm) Ice sublimates into water vapor and vapor pressure moves vapor from: ________________ Vapor condenses as ice in: ________________ As a result, grains break down into: ________________ Eventually, the grains become: ________________ If this goes on long enough: ________________ The result is: ________________

49. Temperature Gradient Review When we have a weak temperature gradient (<1oC/10cm) Ice sublimates into water vapor and vapor pressure moves vapor from: convex regions to concave regions. Vapor condenses as ice in: concave regions. As a result, grains break down into: smaller pieces (decomposed & fragmented forms). Eventually, the grains become: smaller and rounded. If this goes on long enough: sintering occurs. The result is: stronger snow.

50. Review: Rounding and Temperature Gradients • The factors that promote rounding are: • Warm climate • Deep snowpack • Weak temperature gradient • Warm temperature regime • High density snow