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…