Geomorphology Lec 3 – Glacial Processes (Ch 12)
I.
Introduction
Responsible for sculpting terrane of large areas of Earth, especially at high lats
& elevations
II.
Formation
1. 1st step – transform snow to ice
1. density starts light – 0.04 to 0.5 g/cc
2. as snowpack builds, density incr to 0.4 to 0.8 g/cc (known as
“firn”)
3. transition depends on freeze-thaw….lots freeze-thaw, may take
only days…..low freeze-thaw (at poles), may take years
4. Ice develops elongated fabric with depth, C-axes (Fig 12-2) of
hexagonal xls orient in the direction of transport
2. Mech properties
1. Fig 12-2 again. Think of C-axis as the strong axis
2. Pressure meting point. Important concept. When things are
under pressure, they often respond by dropping to the highest
density, lowest volume state…for H20, this is NOT ice, but
water. So under pressure at 0oC, even though H20 should be
solid, it is liquid. This can result in an overpressured liquid at
the base of the glacier, which can facilitate ice flow.
3. Plastic deformation in ice (Fig 12-3) is temp-sensitive….the
higher the temp, the more a glacier can flow. So temperate
continental glaciers, like Pleistocene ice sheets near the
margin, can flow easily.
III.
Glacial Movement
A. Intro to movement
Movement of European glaciers has been studied for several hundred years.
Alpine glaciers can move 3 feet / day, but polar glaciers may move only few feet
/ year. Fastest speeds may be 100 ft / day, on steep slopes or where outlet
glaciers are next to large accumulation areas.
Internal movement – fastest speed is away from the edges, in the middle, on the
surface. (Fig 12-4). Fastest part of glacier is “equilibrium (=firn) line”, between
the accumulation and ablation zones.
B. Basal sliding (Fig 12-5) mechanism is in temperate glaciers, don’t usu see it
in polar glaciers, where the bed is frozen to the glacier all year long.
Fastest glaciers are thin, on steep slopes.
Percussion marks (Fig 12-7) are crescentic fracs that record jerky motion along
the bed.
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Basal sliding also produces grooves, striations, and polishing.
Pore pressure in water at base of glacier can rise, decrease the effective weight
of glacier, making it easier to move.
Often see deformation of sediments at the base of glaciers, much thrust faulting,
etc.
C. Plastic flow – accommodated by variety of mechanisms.
Mechanisms of motion involve
 grain-grain shearing,
 internal shearing (shearing parallel to the a-axes (Fig 12-2 a) of the xl).
Analogy is like shearing a deck of cards.
 Rexlization – here, ice melts at areas of pressure, water moves away and
re-freezes when pressure drops.
Why does the ice flow? Internal shear stress is high, higher than the elastic limit.
Also important – basal ice can flow up and over obstacles, because the ice flow
is controlled by the slope of the surface of the ice (like the GW table slope
controls GW flow…)
Compressive & extensional flow…this is a really fascinating 3D picture (Fig 129) - zone of accumulation causes ice to move downslope, but ice tends to pile
up as the bedrock slope flattens. When slope starts to pick up again, the glacier
is in extension.
D. Internal shearing – brittle deformation in the upper 100 feet of the glacier,
near the terminus
The shear planes act like thrust faults; debris can get caught up in fault planes,
visible in vertical walls (fig 12-10).
E. Crevasses – tension fractures that accommodate movement
F. Ogives – alternating bands of light & dark ice
IV.
Types of Glaciers (or glacier classification schemes)
1. Thermally classified
2. Morphologically classified
3. Dynamically classified
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thermal classification– 3 types
 temperate (“wet-based”)– ice is near pressure-melting temperature
 polar (“cold-base”) ice below pressure melting temp most of the time
 subpolar - intermediate
 temperate – temp throughout glacier close to pressure-melting temp, so
meltwater found throughout glacier. Meltwater facilitates fast conversion of
snow & firn to ice, and facilitates plastic deformation and flow. Basal sliding a
major factor in mvmnt. Also find large outwash plains in front of these glaciers
 polar – conversion to water rarely occurs, so firn / snow conversion to ice takes
a LONG time. Ablation only occurs by means like calving (not melting).
Velocities are slow, ice frozen to bedrock surface. Glacial motion is plastic flow
in the ice, NOT basal sliding. No outwash plains.
 Subpolar – ice frozen to substrate (like polar), but melting occurs in summer
Latest thoughts – a glacier may have mixes of these regimes…i.e., a polar
glacier may have parts that are wet-based
morphological classification – 3 types (shape, size, relation to topography):
 Alpine
 Piedmont
 Ice sheet / ice cap
 Alpine – small, confined to mts (as in the ALPS…)
o Cirque glaciers – limited to N and E-facing amphitheaters in N hemi (Fig 1214)
o Valley glaciers – move out of cirque, down valleys (Fig 12-15)
 Piedmont – formed by ice discharge of valley glaciers (Fig 12-16, 17)
 Ice sheets (Greenland, Antarctica) / caps (Barnes cap on Baffin Island, Canada) –
huge size, overwhelm topography (Fig 12-18)
Dynamic classification scheme - based upon mass balance (fig 12-23) – can be
determined by conditions at terminus
3 types according to this scheme:
 Advancing
 Neutral
 stagnating
Mass balance defined: relationship between accumulation and ablation. So this is a
balancing act between gain and loss
Feeding mechanism (“accumulation”) is snow…
Loss mechanism (“ablation”) is melting, calving, etc.
Look at glacier overall – Fig 12-23 (a little confusing)
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Accumulation zone – annual net gain
Ablation zone – annual net loss
Equilibrium line on the glacier: annual net gain = annual net loss
“firn” line Fig 12-24 marks limit of new snow accumulation, and usually is
approx equal in position to equilibrium line
complex balance relationships shown in Figs 12-25, 12-26
Main points to consider re: ice movement:
 Ice discharge (rate of flow x cross-sectional area – just like Q….river or GW
discharge) increases from head of glacier to the equil. line
 Ice discharge decreases from equil. line to the terminus
Also, re: ice flow geometry… Fig 12-23
 Moves forward and downward above the equil line
 Moves forward and upward below the equil line
Now some key points about the terminus…
“position and activity of the terminus…are determined by the ratio of accumulation to
ablation”
so, this gives rise to some generalizations we can make:
if accum > ablation:
 Positive net mass balance
 Glacier front advances
 Terminus is steep, well-defined, with clean ice (Fig 12-27)
If accum = ablation:
 Terminus stable
 Ice lost is replaced from above by plastic flow, basal sliding, internal shearing
If accum < ablation:
 Terminus retreats or stagnates
 Glacier front low sloping, poorly defined, debris on surface of ice (Fig 12-28) Note
that in fig 12-28, there is a FOREST growing ON the glacier!!
 Terminus retreats upvalley but ice body keeps moving down valley
 Glacier also melts in place, leading to “downwasting”
V.
Glacial erosion
A. Erosional processes – 2 principle processes
1. abrasion
 rock debris in base of glacier acts to scour and polish, like
sandpaper – ice itself does not have the hardness to scour
 lot of “rock flour” (silt & clay size qts & fspar) in glacial meltwater is
testimonial to how much scour occurs
 hardness of 6-7 is adequate to scratch bedrock
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2.
VI.
plucking / quarrying
 mechanism: ice freezes around loosened blocks, pulls them away
often on fractured rx
 crescentic chattermarks thought to be caused by rock in the ice
pressing against bedrock
 no rule of thumb about direction of flow being deciphered by
geometry of “horns” of the crescent….they can point either up-ice
or down-ice
Glacial transportation / deposition
A. Transportation –
1. debris added from above or below
2. transported material is of many types:
 debris carried by ice itself
 meltwater, proglacial lakes
 wind carries glacially-derived rock flour (“loess”)
 material falls off floating icebergs
there is much diversity in transport & deposition, so there is much diversity in
landforms produced and the sedimentary character of deposits
differences include:
 sed from basal ice ≠ sed from margin ice
 sed from active ice ≠ sed from stagnant ice
 sed from meltwater ≠ sed directly deposited from ice
these diffs are apparent when you look at actual landforms (Chap 13)
B. deposition of material – several general deposit types
 till – poorly sorted, not stratified (not layered)
 outwash (streams) – stratified
 glaciolacustrine (lakes) – stratified
 erratics – large boulders diff than the local bedrock
all types of glacial deposits referred to as “drift”…WHY??
C. Diamicton (poorly sorted, non-stratified; aka “till”)
 Bimodal particle size distribution (“bi” = 2)
as in 2 peaks (vs typical single-peak Gaussian distribution)
wind-blown loess
till
% of sample
river
clay
silt
sand pebble
Particle Size
cobble
boulder
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
Till types:
1. Lodgment till – implies some forward movement of glacier in
conjunction with deposition
– subglacially deposited (melted out of the basal ice), then
“plastered” along the ground by the overlying ice as it moves
forward
– creates structural features such as folds, thrust faults, and
shear planes
– creates “preferred orientation” of elongated pebbles in the
matrix, both parallel and perpendicular to flow (can measure
orientations in the field with Brunton compass and plot on a
stereonet back in the office to determine direction of ice flow)
Ice flow
-
also, shearing motion and the weight of overlying ice tend to
compact lodgement till more than other diamictons, leading to
“overconsolidated” condition important to structural failure
analysis you will see in discussion of WWM PhD
2. ablation till (aka meltout till) – implies no forward mvmt of glacier in
conjunction with deposition
- sed released from ice as the ice melts, deposited on land surface
as the ice melts away
- when till is supraglacial, till can be angular rock debris
- ablation tills are frequently saturated, can often flow or slide
(sometimes known as “flow tills”)
3. glaciomarine drift – result from icebergs calving off a glacier into
ocean or large lake
- deposits poorly sorted, can look like lodgment till
- but not as compacted as lodgment till
- and lacking some of the pebble preferred orientation of lodgment
till
D. Stratified & well-sorted sediments
1. glacio-fluvial (steam deposits, aka glacial outwash) see Fig 12-39
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can distinguish as glacial origin only near the terminus….further downstream,
they look like normal river deposits (because they ARE river deposits…!)
-
deposits may be deposited on top of stagnating ice
deposits may bury blocks of ice, which as they melt produce
collapse structures:
Ice block
Ice block melts, leaves
a “kettle depression”
-
deposits well-sorted, unimodal, not bimodal (see graph above)
large cobbles & boulders have edges rounded off, vs angular
fragments deposited from ice
near margin, outwash often interbedded with flow & ablation till
2. glaciolacustrine (lake) deposits
- deltas found at the lake margin, where glacial outwash gets
dumped into lake
- varved deposits (banded pairs of silt/sand plus clay) further
offshore are diagnostic
o winter – lake iced over, very quiet water, caly settles out of
suspention
o summer – influx at the delta brings sand/silt out onto the
lake floor
clay-winter
silt-summer
clay-winter
silt-summer
3. glacioeolian (wind-blown) deposits
- loess occurs in great thickness in Miss R. Valley and China
- downwind from large glaciers
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