Supraglacial & Englacial Environments, Processes

1. Supraglacial & Englacial Environments, Processes chapter 6

2. Supra- and Englacial Processes [Andrews, 1975]

3. Supra- and Englacial Processes • Topics • Ice flow • Ice structure • Sources of glacial debris • Glacial debris transport • Character of glacial debris • The glacier terminus

4. Glacier (summary) • Cirque glacier-Heap Steep, WY • Snowfield/glacier (bergshrund) • Firn/ice • Debris around, on, in, below, beyond • Flow/structures

5. Tributary Flow • Blue Glacier (WA) • Multiple cirques • Icefall

6. Tributary Flow

7. Crevasse types • Chevron • Longitudinal • Transverse • Splaying • Bergschrund • Randkluft

8. Mechanics of crevassing • Results from rapidly-applied stress • Form many distinctive patterns • Observed patterns relate the strain directly to the mechanics of stress couples

9. Basic Crevasse Formation (Sharp, 1960)

10. Crevasse examples • Depth <40 m ? • Tensional and marginal • Terminal splays • Complex systems

11. Crevasse examples

12. Crevasses • Crevasses are principal points of input of water & debris into glaciers • moulin (glacier mill) = a crevasses open across a glacial stream • randkluft • bergschrund

13. Crevasses • Input of water & debris into glaciers • moulin • randkluft = break between ice and rock at valley wall • bergschrund = deep crevasses in ice, near valley wall

14. Subsurface Crevasse Formation Nath and Vaughn (2003) wanted to investigate the formation of crevasses at depths of ~10–30 meters Used ground penetrating radar (GPR) to show that crevasses occur several meters below the surface even where there are none at the surface Used linear elastic fracture mechanics (LEFM) to investigate feasibility of fracture at depth

15. LEFM Assumes all materials have small cracks and defects, near which stresses are concentrated LEFM describes the initiation and propagation of fractures in brittle materials If initial cracks are more than a few centimeters long then they can propagate into a crevasse

16. GPR Data (Nath and Vaugn, 2003)

17. Initiation • Starter cracks are generally initiated in brittle layers • Re-frozen meltwater • Sun crusts • These cracks propagate during plastic flow • Varying dynamic tensile strength with depth • folding

18. Results They found very significant evidence for the feasibility of crevasse initiation at depth More work is currently in progress to determine if these cracks must propagate upward to eventually form surface crevasses

19. Icefalls

20. Ogives • “Ogives are one of the most enigmatic indicators of glacier flow and are of two main types: wave ogives and band ogives” (Goodsellet al.) Ogives on Juneau icefield

21. Ogive Basics • (aka ~ Forbes or Alaskan bands) • Two major types : wave and band • Occur down-ice from icefalls • Useful in velocity calculations and to identify basal features

22. Wave (swell-and-swale) Ogives • Alternating crests, convex down ice • Velocity is a function of wavelength and amplitude

23. Wave (swell-and-swale) Ogives Ogives are formed annually, alternating crest = 1 year advancement Icefall travel time < 6 mo. James Forbes (mid 19th century) indicator of velocity

24. Band Ogive • Alternating convex bands of dark and light • Color can come from debris or ice density

25. Conclusion Ogives are alternating colors or ridges on glaciers Can form on surging glaciers Used to determine velocities or surge intervals Can be used to predict crevasse formation by identifying crevasse scars

26. Deformation Fabrics Common fabrics found in ice and metamorphic rock • Layering (stratification) • Foliation surfaces • Lineations • Folds

27. Foliation • Defined in rocks (Yardley 1989) = preferred orientation, caused by recrystallization of minerals into a planar fabric • Oriented perpendicular to maximum compressive stress • Defined in ice by alternating fine-grained, granulated ice and coarse-grained bubbly ice (Rigsby, 1960) • Developed parallel to edges and bottom of glacier – induced shear couple

28. Foliation orientation

29. Lineations • Defined in rocks (Yardley 1989) = elongation of recrystallized minerals • Induced under tensional stress environments – long axes parallel to stretching direction • Elongation of polycrystals • Elongation axes perpendicular to c-axis (optic and crystallographic) • Rapid growth encourages elongation (Owston, 1951)

30. Stereographic projection

31. Folds As observed in rocks • Classically have been interpreted as having formed during contractional and extensional tectonism As observed in ice • Folding is expressed by alternating dirty bands and clean, hummocky ice (Malaspina Glacier) • Results from differential shearing along foliation planes and not compression of ice itself (Rigsby, 1960)

32. Glacier ice folding • Recumbent folding (Tien Shan) • Thrusting (no photo)

33. Sources of Glacial Debris • Supraglacial • (dust, tephra, meteorites, bugs) • rockfall • Englacial • crevasse fill • thrusting • Subglacial • plucking

34. Rockfall • Penny Ice Cap (Canada) – outlet glacier • Rock walls • Marginaldebris • Lateral/medial moraines

35. Rockfall II • Mer de Glace (France) • Holocene trimline

36. Trimlines • Big Timber Creek • Moraines and trimline

37. 1964 M 8.9 “Good Friday EQ” • Sherman Glacier rock avalanche • Glacier outcomes?

38. 2002 M 7.9 Denali EQ • Black Rapids Glacier panorama • Rock avalanches – effects? By USGS; from AK DNR - http://wwwdggs.dnr.state.ak.us/earthquake.html

39. Debris in / on Ice • Tulsequah Glacier (BC) • Surface area • Debris introduction to ice

40. Glacial Transport • Mooneshine Gl. (Canada) • Note 5’9” Bill Locke for scale • Estimate shear strength? • Rock wall source – angular • Note fines in foreground and meltwater

41. Supra- and Englacial Processes II:the glacier terminus

42. Sources of Glacial Debris • Supraglacial • rockfall • Englacial • crevasse fill • thrusting • Subglacial • plucking

43. Medial Moraines • Mooneshine Glacier (Canada) • Ridge ~3 m tall – how much is debris?

44. Multiple Medial Moraines • Muldrow Glacier (Alaska Range)

45. Tributary Flow

46. Medial Moraine Evolution • Penny Ice Cap • Outlet glacier • Concentration of debris • Supraglacial drainage • Debris-covered terminus

47. The Glacier Terminus • Black Rapids Glacier • active ice • stagnant ice • (surges) • local reworking

48. Ablation Zone • Chugach Mountains • Debris accumulation • Surplus of water and debris • Dynamics of flow of ice and debris • Evolution of local topography

49. Sources of Terminal Debris

50. Ice-cored Moraines • Melt-out of ice over time • f (climate) • Last for decades to centuries (+?)