Transcript Supra/Englacial

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 glacierHeap 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
flow
cracks propagate during plastic
– 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

“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 on Juneau icefield
Ogive Basics
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)
Wave (swell-and-swale) Ogives
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 19th century)
indicator of velocity
Band Ogive
Alternating convex
bands of dark and
light
 Color can come
from debris or ice
density

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
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 finegrained, granulated ice and coarsegrained 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 - http://wwwdggs.dnr.state.ak.us/earthquake.html
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
– Character?
of supraglacial debris
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…