supraglacial & englacial environments, processes chapter 6
TRANSCRIPT
Supraglacial & Englacial Supraglacial & Englacial Environments, ProcessesEnvironments, Processes
chapter 6chapter 6
Supra- and Englacial ProcessesSupra- and Englacial Processes
[Andrews, 1975]
Supra- and Englacial ProcessesSupra- and Englacial Processes
TopicsTopics– Ice flow Ice flow – Ice structure Ice structure – Sources of glacial debrisSources of glacial debris
– Glacial debris transportGlacial debris transport– Character of glacial debrisCharacter of glacial debris– The glacier terminusThe glacier terminus
Glacier Glacier (summary)(summary)
Cirque glacier-Cirque glacier-Heap Steep, WYHeap Steep, WY
Snowfield/glacier Snowfield/glacier (bergshrund)(bergshrund)
Firn/iceFirn/ice Debris around, on, Debris around, on,
in, below, beyondin, below, beyond Flow/structuresFlow/structures
Tributary FlowTributary Flow
Blue Glacier Blue Glacier (WA)(WA)
Multiple Multiple cirquescirques
IcefallIcefall
Tributary FlowTributary Flow
Crevasse typesCrevasse types
ChevronChevron LongitudinalLongitudinal TransverseTransverse SplayingSplaying BergschrundBergschrund RandkluftRandkluft
Mechanics of crevassingMechanics of crevassing
Results from rapidly-applied stressResults from rapidly-applied stress Form many distinctive patternsForm many distinctive patterns Observed patterns relate the strain directly to the mechanics of stress Observed patterns relate the strain directly to the mechanics of stress
couplescouples
Basic Crevasse FormationBasic Crevasse Formation
(Sharp, 1960)
Crevasse Crevasse examplesexamples
Depth <40 m ?Depth <40 m ? Tensional and Tensional and
marginalmarginal Terminal splaysTerminal splays Complex Complex
systemssystems
Crevasse Crevasse examplesexamples
CrevassesCrevasses
Crevasses are principal Crevasses are principal points of input of water & points of input of water & debris into glaciersdebris into glaciers– moulin (glacier mill) = a moulin (glacier mill) = a
crevasses open across a crevasses open across a glacial streamglacial stream
– randkluft randkluft – bergschrundbergschrund
CrevassesCrevasses
Input of water & debris into glaciersInput of water & debris into glaciers– moulinmoulin– randkluft = break between ice and rock randkluft = break between ice and rock
at valley wallat valley wall– bergschrund = deep crevasses in ice, bergschrund = deep crevasses in ice,
near valley wallnear valley wall
Subsurface Crevasse FormationSubsurface Crevasse Formation Nath and Vaughn (2003) wanted to Nath and Vaughn (2003) wanted to
investigate the formation of crevasses at investigate the formation of crevasses at depths of ~10–30 metersdepths of ~10–30 meters
Used ground penetrating radar (GPR) to Used ground penetrating radar (GPR) to show that crevasses occur several meters show that crevasses occur several meters below the surface even where there are below the surface even where there are none at the surfacenone at the surface
Used linear elastic fracture mechanics Used linear elastic fracture mechanics (LEFM) to investigate feasibility of fracture (LEFM) to investigate feasibility of fracture at depthat depth
LEFMLEFM Assumes all materials have small cracks Assumes all materials have small cracks
and defects, near which stresses are and defects, near which stresses are concentratedconcentrated
LEFM describes the initiation and LEFM describes the initiation and propagation of fractures in brittle propagation of fractures in brittle materialsmaterials
If initial cracks are more than a few If initial cracks are more than a few centimeters long then they can propagate centimeters long then they can propagate into a crevasseinto a crevasse
GPR DataGPR Data
(Nath and Vaugn, 2003)
InitiationInitiation
Starter cracks are generally initiated in Starter cracks are generally initiated in brittle layersbrittle layers– Re-frozen meltwaterRe-frozen meltwater– Sun crustsSun crusts
These cracks propagate during plastic These cracks propagate during plastic flowflow– Varying dynamic tensile strength with Varying dynamic tensile strength with
depthdepth– foldingfolding
ResultsResults
They found very significant evidence They found very significant evidence for the feasibility of crevasse for the feasibility of crevasse initiation at depthinitiation at depth
More work is currently in progress to More work is currently in progress to determine if these cracks must determine if these cracks must propagate upward to eventually form propagate upward to eventually form surface crevassessurface crevasses
IcefallsIcefalls
““Ogives are one of Ogives are one of the most enigmatic the most enigmatic indicators of indicators of glacier flow and glacier flow and are of two main are of two main types: wave ogives types: wave ogives and band ogives” and band ogives”
(Goodsell (Goodsell et al.)et al.)
Ogives
Ogives on Juneau icefield
Two major types : Two major types : wave and bandwave and band
Occur down-ice Occur down-ice from icefallsfrom icefalls
Useful in velocity Useful in velocity calculations and to calculations and to identify basal identify basal featuresfeatures
(aka ~ Forbes or (aka ~ Forbes or Alaskan bands)Alaskan bands)
Ogive BasicsOgive Basics
Alternating crests, Alternating crests, convex down iceconvex down ice
Velocity is a Velocity is a function of function of wavelength and wavelength and amplitudeamplitude
Wave (swell-and-swale) OgivesWave (swell-and-swale) Ogives
Ogives are formed annually, Ogives are formed annually, alternating crest = 1 year alternating crest = 1 year advancementadvancement
Icefall travel time < 6 mo.Icefall travel time < 6 mo.
James Forbes (mid 19James Forbes (mid 19thth century) century) indicator of velocityindicator of velocity
Wave (swell-and-swale) OgivesWave (swell-and-swale) Ogives
Alternating convex Alternating convex bands of dark and bands of dark and lightlight
Color can come Color can come from debris or ice from debris or ice densitydensity
Band OgiveBand Ogive
Ogives are alternating colors or Ogives are alternating colors or ridges on glaciersridges on glaciers
Can form on surging glaciersCan form on surging glaciers Used to determine velocities or surge Used to determine velocities or surge
intervalsintervals Can be used to predict crevasse Can be used to predict crevasse
formation by identifying crevasse formation by identifying crevasse scarsscars
ConclusionConclusion
Deformation FabricsDeformation Fabrics
Common fabrics found in ice and Common fabrics found in ice and metamorphic rockmetamorphic rock
Layering (stratification)Layering (stratification) Foliation surfacesFoliation surfaces LineationsLineations FoldsFolds
FoliationFoliation Defined in rocks (Yardley 1989) = Defined in rocks (Yardley 1989) =
preferred orientation, caused by preferred orientation, caused by recrystallization of minerals into a recrystallization of minerals into a planar fabricplanar fabric– Oriented perpendicular to maximum Oriented perpendicular to maximum
compressive stresscompressive stress Defined in Defined in iceice by alternating fine- by alternating fine-
grained, granulated ice and coarse-grained, granulated ice and coarse-grained bubbly ice (Rigsby, 1960)grained bubbly ice (Rigsby, 1960)– Developed parallel to edges and bottom of Developed parallel to edges and bottom of
glacier – induced shear coupleglacier – induced shear couple
Foliation orientationFoliation orientation
LineationsLineations Defined in rocks (Yardley 1989) = Defined in rocks (Yardley 1989) =
elongation of recrystallized minerals elongation of recrystallized minerals – Induced under tensional stress Induced under tensional stress
environments – long axes parallel to environments – long axes parallel to stretching directionstretching direction
Elongation of polycrystalsElongation of polycrystals– Elongation axes perpendicular to c-axis Elongation axes perpendicular to c-axis
(optic and crystallographic) (optic and crystallographic) – Rapid growth encourages elongation Rapid growth encourages elongation
(Owston, 1951)(Owston, 1951)
Stereographic projectionStereographic projection
FoldsFoldsAs observed in rocksAs observed in rocks
Classically have been interpreted as Classically have been interpreted as having formed during contractional having formed during contractional and extensional tectonismand extensional tectonism
As observed in iceAs observed in ice Folding is expressed by alternating Folding is expressed by alternating
dirty bands and clean, hummocky ice dirty bands and clean, hummocky ice (Malaspina Glacier)(Malaspina Glacier)– Results from differential shearing along Results from differential shearing along
foliation planes and not compression of foliation planes and not compression of ice itself (Rigsby, 1960)ice itself (Rigsby, 1960)
Glacier ice foldingGlacier ice folding
Recumbent Recumbent folding (Tien folding (Tien Shan)Shan)
Thrusting Thrusting (no photo)(no photo)
Sources of Glacial DebrisSources of Glacial Debris
SupraglacialSupraglacial– (dust, tephra, meteorites, bugs) (dust, tephra, meteorites, bugs) – rockfallrockfall
EnglacialEnglacial– crevasse fillcrevasse fill– thrustingthrusting
SubglacialSubglacial– pluckingplucking
RockfallRockfall
Penny Ice Cap (Canada) – outlet glacierPenny Ice Cap (Canada) – outlet glacier Rock wallsRock walls MarginalMarginal
debrisdebris Lateral/Lateral/
medial medial morainesmoraines
Rockfall IIRockfall II
Mer de GlaceMer de Glace (France)(France)
Holocene Holocene trimlinetrimline
TrimlinesTrimlines
Big Timber CreekBig Timber Creek MorainesMoraines and trimlineand trimline
1964 M 8.9 “Good Friday EQ”1964 M 8.9 “Good Friday EQ”
Sherman Sherman Glacier rock Glacier rock avalancheavalanche
Glacier Glacier outcomes?outcomes?
2002 M 7.9 Denali EQ2002 M 7.9 Denali EQ
Black Rapids Glacier panoramaBlack Rapids Glacier panorama Rock avalanches – effects?Rock avalanches – effects?
By USGS; from AK DNR - http://wwwdggs.dnr.state.ak.us/earthquake.html
Debris in / on IceDebris in / on Ice
Tulsequah Tulsequah Glacier (BC)Glacier (BC)
Surface areaSurface area Debris Debris
introduction to introduction to iceice
Glacial TransportGlacial Transport
Mooneshine Gl. Mooneshine Gl. (Canada)(Canada)
Note 5’9” Bill Locke Note 5’9” Bill Locke for scalefor scale
Estimate shear Estimate shear strength?strength?
Rock wall source – Rock wall source – angularangular
Note fines in Note fines in foreground and foreground and meltwatermeltwater
Supra- and Englacial Supra- and Englacial Processes II:Processes II:
the glacier terminusthe glacier terminus
Sources of Glacial DebrisSources of Glacial Debris
SupraglacialSupraglacial– rockfallrockfall
EnglacialEnglacial– crevasse fillcrevasse fill– thrustingthrusting
SubglacialSubglacial– pluckingplucking
Medial MorainesMedial Moraines
Mooneshine Glacier (Canada)Mooneshine Glacier (Canada) Ridge ~3 m tall – how much is debris?Ridge ~3 m tall – how much is debris?
Multiple Medial MorainesMultiple Medial Moraines
Muldrow Glacier (Alaska Range)Muldrow Glacier (Alaska Range)
Tributary FlowTributary Flow
Medial Moraine Medial Moraine EvolutionEvolution
Penny Ice CapPenny Ice Cap– Outlet glacierOutlet glacier– Concentration of Concentration of
debrisdebris– Supraglacial Supraglacial
drainagedrainage– Debris-covered Debris-covered
terminusterminus
The Glacier TerminusThe Glacier Terminus Black Rapids Black Rapids
GlacierGlacier– active iceactive ice– stagnant icestagnant ice– (surges)(surges)– local local
reworkingreworking
Ablation ZoneAblation Zone
Chugach MountainsChugach Mountains– Debris accumulationDebris accumulation– Surplus of water and Surplus of water and
debrisdebris– Dynamics of flow of Dynamics of flow of
ice and debrisice and debris– Evolution of local Evolution of local
topographytopography
Sources of Terminal DebrisSources of Terminal Debris
Ice-cored MorainesIce-cored Moraines
Melt-out of ice over timeMelt-out of ice over time f (climate)f (climate) Last for decades to centuries (+?)Last for decades to centuries (+?)
Melt-out TillsMelt-out Tills
Surface meltSurface melt– supraglacialsupraglacial– character of character of
till?till? Basal meltBasal melt
– subglacialsubglacial– character of character of
till?till?
FlowtillsFlowtills
Redistribution of supraglacial debrisRedistribution of supraglacial debris– Character?Character?
Character of Glacial DebrisCharacter of Glacial Debris
Pangnirtung Pangnirtung Pass (Canada)Pass (Canada)
Note figure Note figure (6’3” Pete (6’3” Pete Birkeland) for Birkeland) for scalescale
No real limit to No real limit to debris caliberdebris caliber
Till we meet again…Till we meet again…