Supraglacial & Englacial Environments, Processes chapter 6

Supra- and Englacial Processes [Andrews, 1975]

Supra- and Englacial Processes  Topics –Ice flow –Ice structure –Sources of glacial debris –Glacial debris transport –Character of glacial debris –The glacier terminus

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

Tributary Flow  Blue Glacier (WA)  Multiple cirques  Icefall

Tributary Flow

Crevasse types  Chevron  Longitudinal  Transverse  Splaying  Bergschrund  Randkluft

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

Basic Crevasse Formation (Sharp, 1960)

Crevasse examples  Depth <40 m ?  Tensional and marginal  Terminal splays  Complex systems

Crevasse examples

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

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

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

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

GPR Data (Nath and Vaugn, 2003)

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

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

Icefalls

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

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

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

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

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

 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 Conclusion

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

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

Foliation orientation

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)

Stereographic projection

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)

Glacier ice folding  Recumbent folding (Tien Shan)  Thrusting (no photo)

Sources of Glacial Debris  Supraglacial –(dust, tephra, meteorites, bugs) –rockfall  Englacial –crevasse fill –thrusting  Subglacial –plucking

Rockfall  Penny Ice Cap (Canada) – outlet glacier  Rock walls  Marginal debris  Lateral/ medial moraines

Rockfall II  Mer de Glace (France)  Holocene trimline

Trimlines  Big Timber Creek  Moraines and trimline

1964 M 8.9 “Good Friday EQ”  Sherman Glacier rock avalanche  Glacier outcomes?

2002 M 7.9 Denali EQ  Black Rapids Glacier panorama  Rock avalanches – effects? By USGS; from AK DNR -

Debris in / on Ice  Tulsequah Glacier (BC)  Surface area  Debris introduction to ice

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

Supra- and Englacial Processes II: the glacier terminus

Sources of Glacial Debris  Supraglacial –rockfall  Englacial –crevasse fill –thrusting  Subglacial –plucking

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

Multiple Medial Moraines  Muldrow Glacier (Alaska Range)

Tributary Flow

Medial Moraine Evolution  Penny Ice Cap –Outlet glacier –Concentration of debris –Supraglacial drainage –Debris-covered terminus

The Glacier Terminus  Black Rapids Glacier –active ice –stagnant ice –(surges) –local reworking

Ablation Zone  Chugach Mountains –Debris accumulation –Surplus of water and debris –Dynamics of flow of ice and debris –Evolution of local topography

Sources of Terminal Debris

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

Melt-out Tills  Surface melt –supraglacial –character of till?  Basal melt –subglacial –character of till?

Flowtills  Redistribution of supraglacial debris –Character?

Character of Glacial Debris  Pangnirtung Pass (Canada)  Note figure (6’3” Pete Birkeland) for scale  No real limit to debris caliber

Till we meet again…