Lecture 10: Intro to Geomorphology
Goals of Glacial Geomorphology
Lectures
1. Answer question: How do glaciers modulate landscape
development?
2. To introduce Earth’s glacial history.
3. To discuss the formation and movement of different types
of glaciers.
4. To discuss glacial erosion of bedrock as well as, material
deposition, and transport.
5. To review landforms developed by glaciers.
How do glaciers modulate landscape development?
Up to now, we have thought about the landscape as having developed in the absence of
external forcing. Periodically, changes in the climate cause the growth and development
of glaciers, which cover the landscape, smearing sediment everywhere, disrupting mass
movement and fluvial processes, and enhancing bedrock erosion.
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Lecture 10: Intro to Geomorphology
∆
∂z
=U-E∂t
· qs
All landscapes must obey this
fundamental statement about
sediment transport!
Geomorphic transport laws
In order to make predictions of landscape change
Geomorphologists need to parameterize (E and qs):
E includes:
•
Sediment production by weathering (P)
•
Bedrock erosion by glaciers, wind, water (W)
qs includes erosion and deposition by:
•
Mass wasting transport processes
•
Fluvial transport processes
The whole
landscape
in one
equation!
Photo courtesy of Bill Dietrich
Glaciers
Glaciers are masses of ice and granular snow formed by
compaction and recrystalization of snow, lying largely or
wholly on land and showing evidence of past or present
movement – Easterbrook, 1999.
Key concept: Ice must be on land and move, so sea ice
and snow fields are not considered glaciers.
There are two basic types of glaciers: alpine (confined) and
continental (unconfined).
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Lecture 10: Intro to Geomorphology
Alpine glacier with clear evidence of flow and confinement
Dirk Beyer
Greenland continental
ice sheet without
confinement
Edge of Greenland Ice Sheet
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Lecture 10: Intro to Geomorphology
Earth’s glacial history
Through most of geologic time, Earth
has been free of glaciers, but at
intervals of ~150Ma, ice ages occur.
Fig 16.1 easterbrook
Selby, 1985
They last tens of Ma with some portion
of the land surface covered by glaciers
or ice sheets.
It is widely held that the changes in
climate that bring about an ice age are
due to the changes in seasonality and
location of solar energy around the
Earth that occur due to Milankovitch
Cycles, which are variations in earth’s
orbit.
Milankovitch Cycles: Eccentricity
Eccentricity: the shape of the Earth's orbit around the Sun.
• Fluctuates between more and
less elliptical (0 to 5% ellipticity)
on a cycle of about 100ka.
• Important for glaciation because
it alters the distance from the
Earth to the Sun  changes the
distance the Sun's short wave
radiation must travel to reach
Earth.
• Thus reduces or increases the
amount of radiation received at
the Earth's surface in different
seasons.
• Eccentricity currently 3%  so 6% more
solar radiation in January than in July
• At maximum eccentricity (~5%)  23%
more solar radiation in Jan. than in July
http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
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Lecture 10: Intro to Geomorphology
Milankovitch Cycles: Axial Tilt
Axial Tilt: the inclination of the Earth's axis in relation to its plane of
orbit around the Sun (obliquity)
• There is a 2.4o change in the
axial tilt with a periodicity of
41ka.
• This changes the amount of
incoming solar radiation in
higher latitudes
21.5o
24.5o
• At high tilt, summers are warmer
and winters are cooler.
• Important for glaciation because
at low tilt, cooler summers are
suspected of encouraging the
start of an ice age by melting
less of the previous winter's ice
and snow.
Axial tilt is currently 23.44o
http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
Milankovitch Cycles: Precession
Precession: Earth's slow wobble as it spins on its axis.
• Wobble has a period of 23ka
• This wobble causes oscillations
between periods when NH is
closest to the sun in summer
and furthest from the sun in
summer.
• Important for glaciation because
when the NH is closest to the
sun in summer, there is a
greater temperature contrast
between summer and winter.
• When the NH is furthest from
the sun, cooler summers may
occur.
Presently, the NH is closest to
the sun in winter.
http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
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Lecture 10: Intro to Geomorphology
Milankovitch Cycles and Glaciation
Land masses dominate northern
hemisphere, so when the northern
pole is turned away from the sun, the
winters are longer and snow and ice
remain on the surface for longer
periods of time.
Equator
Snow and ice have a much higher albedo than ground and vegetation, thus ice
masses tend to reflect more radiation back into space, thus cooling the climate
and allowing glaciers to expand.
The specific feedbacks amongst various climatic variables that lead to a
glaciation are still being studied.
Earth's Current Ice Cover
Areal Extent: 14.5 x 106 km2 : 10% of Earth’s surface
Volume: 32.1 x 106 km3 (70% of Earth's freshwater reserve)
Area
Volume
Antarctica
12.1x106 km2
83%
29 x 106 km3
Greenland
1.7 x 106 km2
12%
2.95 x 106 km3
Other ice caps
0.7 x
106
km2
5%
0.18 x
106
km3
90%
9%
1%
Max. thickness of Antarctic ice sheet is 4300 m, Greenland 2700 m.
Change in global sea level when the world’s glaciers melt:
Antarctica:
Greenland:
All Ice:
68.0 m
7.4 m
75.9 m
But when will this occur?
(IPCC, 2001)
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Lecture 10: Intro to Geomorphology
Earth’s ‘recent’ ice cover
Fig 14-2 easterbrook
Easterbrook Text
During the
Quaternary Period
(last 1.8Ma),
glaciers have
covered as much as
30% of the Earth’s
surface (44 x 106
km2).
Earth’s ‘recent’ ice cover
Fig 14-2 easterbrook
USA:
Ice extended south to
Great Lakes, northern
Washington
Easterbrook Text
Europe:
England, Scandinavia,
Netherlands, central
Germany, Ukraine, and
northern Siberia.
Ice Free Areas in
Canada: northern
Yukon, and isolated
peaks in the Cordillera
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Lecture 10: Intro to Geomorphology
Glaciers and Geomorphology: Legacy
The current extent of glaciers is not
that great, but glaciers have
significantly modified the
landscape by eroding, depositing,
and generally reworking landscape
materials.
Since most of Canada was covered by
ice sheets, we need to understand
the legacy of this glaciation and the
dynamics of the ice that modified
the landscape.
Drumlin Field, near Prince George, BC (courtesy B. Menounos)
1 km
Glaciers and Geomorphology: Sedimentation
Kaskawulsh River: Sandur
formed by deposition of glacial
sediments as the glacier
retreats up valley
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Lecture 10: Intro to Geomorphology
Formation of glacial ice
• Glacial ice begins as light fluffy
snow with a density of 0.07 to
0.18 g/cm3
Pure Ice
(0.9 g cm-3)
Depth
• With further compaction and
recystalization, firn becomes
ice with a density of 0.8 to 0.9
g/cm3
Luis María Benítez
• As snow accumulates, the
weight on snow from above
melts basal snow that
recrystalizes in a more
compact form called firn or
neve (density of 0.4 to 0.8
g/cm3)
Density
gradient
though a
glacier
Density (g cm -3)
Increase in density of
snow/firn with depth
Seward Glacier, Alaska
Greenland Ice Sheet
Antarctic Ice Sheet at Bryd
Station, Antarctica
Easterbrook Text
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Lecture 10: Intro to Geomorphology
Morphologic
of glaciers
Types oftypes
Glaciers
Glaciers are classified according to:
A) size and morphology
B) thermal regime (cold vs. warm based ice)
C) mass balance climate-characteristics (maritime, transitional, continental)
Selby,
1985
Definitions of Glacier types
Type
Ice Sheet
Ice Cap
Ice Dome
Outlet Glacier
Ice Shelf
Iceberg
Description
>50,000 km2 that buries landscape
< 50,000 km2 ice sheet
Central portion of sheet or cap
Ice stream off of a sheet or cap
Floating ice anchored to land mass
Floating ice not anchored to land mass
Ice Field
Flat body of ice
Valley Glacier
Cirque Glacier
Niche Glacier
Ice flowing down a valley
Ice occupying a hollow in the bedrock that it formed
Ice occupying a hollow in the bedrock that it did not form
Confluent and diffluent glaciers either converge or diverge as they move
downstream
Pediment glaciers are formed at the outlet of a valley onto a valley floor
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Lecture 10: Intro to Geomorphology
Ice sheet
Edge of Greenland Ice Sheet
North Polar region of Mars
The central part is
covered by an ice
sheet that is cut by
spiral-patterned
troughs exposing
layered terrain. The
cap is surrounded
by broad flat plains
and large dune
fields.
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Lecture 10: Intro to Geomorphology
Summit Ice Cap of Mont Blanc, French Alps
BGRG Website
Columbia Icefield
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Lecture 10: Intro to Geomorphology
Columbia Icefield
Icelandic outlet glacier
Olafur Ingolfsson (www.hi.is)
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Lecture 10: Intro to Geomorphology
The Ross Ice Shelf
Anchored
to land
mass
Iceberg west of Ilulissat inlet, Greenland
Nathaniel B. Palmer
NOT
Anchored
to land
mass
Uta Wollf
Alpine valley glacier: Aletsch Glacier, Switzerland
Dirk Beyer
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Lecture 10: Intro to Geomorphology
Lower Curtis Cirque Glacier: North Cascades, WA
Wikipedia
Cirque is an amphitheatre-like valley (or valley head)
of glacial origin, formed by glacial erosion at the head
of the glacier.
Iceberg Cirque, Glacier National Park, Montana
http://www.nps.gov/glac/gallery/parkpics.htm
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Lecture 10: Intro to Geomorphology
Piedmont Glaciers, Taylor Valley Antarctica
www.sethwhite.org
Piedmont Glaciers, Taylor Valley Antarctica
www.sethwhite.org
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Lecture 10: Intro to Geomorphology
Ice dynamics
The dynamics of ice, and ultimately, glaciers ability to
modify the landscape and impact landscape
development processes is dependent on the properties
of the ice and its movement, which is governed by:
1) Ice rheology
2) Type of ice movement
3) Temperature
Ice rheology: recall from previous lectures
Definitions: Stress is a force applied to a surface area and strain is any
deformation or change in shape or volume of a material caused by
application of stress
Force
Force
Strain
(deformation)
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Lecture 10: Intro to Geomorphology
Rheology: Physical Characteristics of Materials
Elastic behavior follows these general rules:
1. The same stress always produces the same strain (deformation)
2. Sustaining a stress produces a constant strain (deformation)
3. Removing the stress always results in recovery
Plastic behavior is when a material can no longer recover from a stress
Viscous behavior is when a material is fluid and flows in response to
stress. Newtonian fluids flow at rates proportional to applied stresses
(i.e. water) while non-Newtonian fluids do not (i.e. ketchup)
Physical Characteristics of materials
Stress
Perfect Elastic
Most earth materials exhibit mixed behavior. Yield
stress is sometimes referred to as the plastic limit
and the breaking stress is sometimes referred to
as liquid limit (Atterberg limits).
Yield
Stress
Strain
Failure
Breaking
Stress
Stress
Perfect Plastic
Stress
Strain
Strain
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Lecture 10: Intro to Geomorphology
Physical Characteristics of materials
Newtonian Fluid: strain rate is a
linear function of stress
Perfect Plastic: an applied stress
always causes the same
deformation rate.
Stress
Perfect Plastic
Pseudo-Plastic Deformation:
material experiences no
deformation until the stress is
increased to the yield stress.
Deformation will occur as long as
the stress is applied.
Strain rate
Ice typically exhibits behavior that is characteristic of pseudoplastic deformation.
Shear stress in a glacier
At any level in a glacier:
   ice gz sin 
 = shear stress
ρice = density of ice
g = gravitational acceleration
z = depth of the ice
sin θ = slope of the substrate
below the ice
θ
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Lecture 10: Intro to Geomorphology
Basal Shear Stress
If z is the full depth of ice:
 b   ice gh sin 
τb = basal shear stress
h = depth of the ice
θ
Thicker ice or ice on steeper slopes typically exerts
greater stress on the bed of the glacier than thinner ice
or ice on lesser slopes
Internal Motion of Ice
Consider an element within the ice column
Start
z
After a Short Time
dz
dx
x
Strain is

1 dx
2 dz
Strain Rate is

1 dx 1 1 du

2 dz dt 2 dz
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Lecture 10: Intro to Geomorphology
Glen’s Law: governs plastic ice deformation
The relation between stress and strain is given
by Glen’s Law:
  k n
ε = strain rate
τ = shear stress
k = temperature dependent constant
n = is a constant that varies between 1.3
and 4.5 (average ~3)
Small changes in the basal shear stress can produce
major changes in deformation (ice becomes softer with
greater imposed stress)
Ice Flow
By combining the shear strain rate definition with
Glen’s law we can find a relation for the velocity of ice
within a glacier.
du
 2 k n
dz
Using a little calculus, assuming n=3, and inserting the
equation for the shear stress, we arrive at an equation
for the ice velocity:
3
4
4
u ( z )  2k
g sin  
4
h
z

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Lecture 10: Intro to Geomorphology
Ice Velocity Profile
u ( z )  2k
g sin  3 h 4  z 4 
4
Ice Velocity Profile

g sin  3 4 4
h  z 
u ( z )  2k
4
If we integrate this equation over the full depth of ice
(sum velocities at all heights) and divide by the depth,
we get an equation for the mean velocity of the ice:
u  2 5 k g sin   h 4
3
Internal ice velocity is dependent upon
slope, ice thickness, and temperature.
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Lecture 10: Intro to Geomorphology
Ice movement
Topography exerts a first order control on flow rates.
u  2 5 k g sin   h 4
3
Compressive
Flow
Compressive flow thickens ice and
extending flow thins ice.
Extending
Flow
Calculate in excel
to see variation!!!
Ice movement
Bird's Eye View
Longitudinal
(lateral extension)
Ice movement
causes tensional
cracks termed
crevasses that
reveal the types of
movement occurring
in the ice
Transverse
(downvalley extension)
Chevron
(friction along valley)
Radial
(spreading)
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Lecture 10: Intro to Geomorphology
Courtesy: B. Menounos
Courtesy: B. Menounos
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Lecture 10: Intro to Geomorphology
Thermal regime classification
Internal Thermal Regime
‘Temperate’ or warm glaciers are at approximately the pressure
melting point throughout their depth and contain water
‘Polar’ or cold glaciers are well below the pressure melting point at
depth
The temperature at which water freezes is 0 oC at
atmospheric pressure and declines by ~1 oC for every 14
MPa (megapascal) of pressure.
Under 20 MPa of pressure, ice at the base of the Antarctic
ice sheet freezes at -1.6 oC.
Role temperature plays in glacial movement
Warm glaciers may slip at their
boundary because of basal
water.
Cold glaciers freeze to the
boundary, causing internal
deformation
u  2 5 k g sin   h 4  ub
u  2 5 k g sin   h 4
3
3
Animations courtesy: B. Menounos
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Lecture 10: Intro to Geomorphology
Mass balance Classification:
Anatomy of a glacier
Although it is convenient to break glaciers into two groups (warm and cold),
most glaciers exhibit different behaviors based on different zones.
Accumulation Area
Ablation Area
ELA:
Equilibrium
line altitudes
Rock
Ablation and
accumulation areas
Upper Grindelwald Glacier and
Schreckhorn, Switzerland
Accumulation includes all the
processes responsible for
adding snow to the glacier
Ablation includes all the
processes responsible for
snow or ice loss from the
glacier
Firn line
• Locally, melting by
evaporation, sublimation,
calving or wind erosion may be
important.
• However, melting is largely
facilitated by low albedo, dark
rock material on the surface of
the glacier
Adrian Pingstone
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Lecture 10: Intro to Geomorphology
Zonation of Laurentide ice sheet
Menzies (2002)
Mass balance of a glacier
A glacier’s mass balance is simply the difference between
the annual accumulation and ablation volumes.
If the mass balance is positive, glaciers are growing
If mass balance is negative, glaciers are disappearing
Most of the world’s glaciers currently have a negative mass
balance, but locally very few glaciers are monitored!
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Lecture 10: Intro to Geomorphology
Goals of Glacial Geomorphology
Lectures
1. Answer question: How do glaciers modulate landscape
development?
2. To introduce Earth’s glacial history.
3. To discuss the formation and movement of different types
of glaciers.
4. To discuss glacial erosion of bedrock as well as, material
deposition, and transport.
5. To review landforms developed by glaciers.
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