Lect 4: Intro to Geomorphology
10/5/2015
Goals of Today’s Lecture
1.
To answer the question: Where do landscape
materials come from?
2.
To examine weathering and bedrock erosion
processes at Earth’s surface.
3.
To consider how these ideas of soil and debris
production fit into our generalized continuity
equation of the landscape.
Change in
landscape
surface elevation
(rate)
∆
dz
=U-Edt
· qs
Bedrock erosion
rate (P+W)
Uplift rate of the
landscape
surface
All landscapes must obey this
fundamental statement about
sediment transport!
Sediment flux
divergence
(written in 3D)
The whole
landscape
in one
equation!
Photo courtesy of Bill Dietrich
1
Lect 4: Intro to Geomorphology
∆
dz
=U-Edt
10/5/2015
· 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
∆
∂z
=U-E∂t
· qs
All landscapes must obey this
fundamental statement about
sediment transport!
Our discussion today will focus on processes
responsible for the production of
transportable material in the landscape.
There are two components to E: soil
production (P) and bedrock wear (W).
The whole
landscape
in one
equation!
Photo courtesy of Bill Dietrich
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Lect 4: Intro to Geomorphology
10/5/2015
Where do landscape materials come from?
Granite
Sand
Enchanted Rock State Natural Area of Texas, USA
Where do landscape materials
come from?
There are two processes that are important:
1) Weathering or Soil Production: In situ disintegration or
breakdown of rock material
2) Bedrock Wear: Erosion of rock material by water, wind,
or ice
These are not mutually exclusive processes. Only where
rock is covered by soil does weathering operate as the
sole process. In many environments, both weathering
and bedrock erosion are occurring at the same time.
Enchanted Rock State Natural Area of Texas, USA
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Lect 4: Intro to Geomorphology
10/5/2015
Weathering in the Rock Cycle
Weathering in the Source to Sink Framework
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Lect 4: Intro to Geomorphology
10/5/2015
Types of weathering
Physical (Mechanical):
Chemical:
1. Pressure release
1. Solution
2. Freeze-thaw
2. Hydration
3. Salt-crystal growth
3. Hydrolysis
4. Thermal expansion
4. Oxidation
5. Biotic
5. Biological
Hematite
Mechanical Weathering: no change in
chemical composition--just disintegration
into smaller pieces
This increases the total surface area exposed to
weathering processes.
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Lect 4: Intro to Geomorphology
10/5/2015
Chemical Weathering: breakdown as a
result of chemical reactions.
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3Carbon
dioxide
Calcium
Carbonate
Limestone or
marble rock
Calcium
Water
Carbonic Acid
(H2CO3)
Bicarbonate
Rock that can be
carried in
solution!
Transformation/decomposition of one mineral into another!
Types of weathering
Physical (Mechanical):
Chemical:
1. Pressure release
1. Solution
2. Freeze-thaw
2. Hydration
3. Salt-crystal growth
3. Hydrolysis
4. Thermal expansion
4. Oxidation
5. Biological
5. Biological
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Lect 4: Intro to Geomorphology
10/5/2015
Mechanical Weathering:
1. Pressure Release
Exfoliation:
Rock breaks apart in layers that are parallel to
the earth's surface; as rock is uncovered, it
expands (due to the lower confining pressure)
resulting in exfoliation.
Weathering
Exfoliation: Formative mechanism
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Lect 4: Intro to Geomorphology
10/5/2015
Pressure release examples
Sheet Joints
(Exfoliation)
Pressure release examples
Half Dome,
Yosemite, CA
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Lect 4: Intro to Geomorphology
10/5/2015
Pressure release examples
Stone Mountain, GA
Mechanical Weathering:
2. Freeze-thaw
Frost Wedging: rock
breakdown caused
by expansion of ice
in cracks and joints
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Lect 4: Intro to Geomorphology
10/5/2015
Freeze-thaw example
Shattered rocks are common
in cold and alpine
environments where
repeated freeze-thaw cycles
gradually pry rocks apart.
Average number of freezethaw cycles per year in
Canada (Fraser 1959)
Trenhaile, 2010
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Lect 4: Intro to Geomorphology
10/5/2015
Mechanical
Weathering:
Weathering
3. Thermal Expansion
Thermal expansion
due to the extreme
range of temperatures
can shatter rocks in
desert environments.
Repeated swelling and
shrinking of minerals
with different
expansion rates will
shatter rocks.
Frost shattered
rock, Fraser
Canyon, 2009
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Lect 4: Intro to Geomorphology
10/5/2015
Mechanical Weathering:
4. Salt Crystal Growth
French
Mediterranean
coast near St.
Tropez
F.J.P.M. Kwaad
Often thought to be most effective at
granular disintegration on exposed
rock faces. Can it split rock?
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Lect 4: Intro to Geomorphology
10/5/2015
Salt weathering process
Developing salt
crust through
evaporation of
salt brine in the
surf zone.
Photos by F.J.P.M. Kwaad
Crystallized salt on the rock surface and in
rock fissures in granite
Rock particles released by salt weathering of granite
Tafoni weathering is common in arid coastal areas where brine
is abundant and salt crystallization is possible.
Yehliu, Taiwan
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Lect 4: Intro to Geomorphology
10/5/2015
Tafoni Salt Weathering, near
San Francisco California
Juha Huuskonen (www.juhuu.nu)
Honeycombs, Artillery Rocks, Victoria, Australia (Trenhaile, 2010)
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Lect 4: Intro to Geomorphology
10/5/2015
Mechanical
Weathering:
5. Biotic weathering
Root Splitting: At large scales,
seedlings sprouting in a crevice
and plant roots exert physical
pressure.
Burrowing animals and insects
disturb the soil layer adjacent to
the bedrock surface, increasing
water infiltration and exposure to
other processes.
Photo courtesy USGS DDS21
Role of Physical Weathering
1) Reduces rock
material to smaller
fragments that are
easier to transport
2) Increases the
exposed surface area
of rock, making it more
vulnerable to further
physical and chemical
weathering
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Lect 4: Intro to Geomorphology
10/5/2015
What controls rate of physical weathering?
Resistance to weathering: Rock strength, composition, fracture pattern.
Joints in a rock are a
pathway for water –
they can enhance
mechanical weathering.
The form and density of
fractures is controlled
by the rock type.
What controls rate of physical weathering?
Driving force of weathering:
1) Exfoliation requires erosion which requires  water
2) Ice crystalization requires  water
3) Salt crystalization requires  water
4) Biota growth requires  water
Physical weathering systems are controlled by the
availability of water and thus are climatically controlled.
More on this later!
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Lect 4: Intro to Geomorphology
10/5/2015
Chemical Weathering
Sandstone found in glacial drift near Angelica, New York
Types of weathering
Physical (Mechanical):
Chemical:
1. Pressure release
1. Solution
2. Freeze-thaw
2. Hydration
3. Salt-crystal growth
3. Hydrolysis
4. Thermal expansion
4. Oxidation
5. Biotic
5. Biotic (?)
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Lect 4: Intro to Geomorphology
10/5/2015
Chemical Weathering:
1. Solution
Solution: process by which rock is
dissolved in water
H2O + CO2 + CaCO3  Ca+2 + 2HCO3water + carbon dioxide + calcite dissolve
into calcium ion + bicarbonate ion
Biological activity in soils generates
substantial CO2
Bicarbonate is the dominant ion in
surface runoff (rivers).
Solution
Strongly influenced by pH and
temperature (as are all chemical
reactions)
evaporite
Rock particularly vulnerable to
solution weathering:
1. Calcium carbonate
(calcite, limestone)
2. Sodium chloride (salt)
3. Calcium sulfate (gypsum)
When water becomes saturated,
chemicals may precipitate out
forming evaporite deposits.
Salt Crystal growth on playa lakes in Death Valley, CA
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Lect 4: Intro to Geomorphology
10/5/2015
This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown,
which occurred over the night of 9-10 July.
‘Karst’ landforms develop in areas
underlain with limestone
Chemical Weathering: 2. Hydration
Hydration: attachment of water molecules to crystalline structure of a
rock, causing expansion and weakness.
Hydration of Anhydrite
Ca2SO4 + 2H20  Ca2SO4 . 2H20
Anhydrite
Water
Gypsum
But, many of these reactions are reversible!
Dehydration of gypsum by heating: Ca2SO4 . 2H20  Ca2SO4 + 2H20
So, is this a chemical weathering process?
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Lect 4: Intro to Geomorphology
10/5/2015
Chemical Weathering: 3. Hydrolysis
Hydrolysis: combination of hydrogen and oxygen in water with rock to
form new substances. Carbonation is essentially the same reaction,
but with CO2 instead of H+.
Feldspar Weathering to Kaolinite:
Feldspar
Hydrogen
Ion
Water
2K Al Si3 O8 + 2H+ + 9H2O 
H4 Al2 Si2 O9 + 4H4 Si O4 + 2K+
Kaolinite
Disolved
silica
(acid)
Potassium
ion
Most common mechanism for clay formation.
Chemical Weathering: 4. Oxidation
Oxidation: Oxygen dissolved in water oxidizes sulfides,
ferrous oxides, native metals
Olivine
Water
Oxygen
2Fe Si O4 + 4H2O + O2
2Fe2 O3 + 4H4 Si O4
Hematite
Disolved
silica (acid)
Hematite
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Lect 4: Intro to Geomorphology
10/5/2015
Oxidation Weathering?
or
Aqueous
Mineralization?
“Blueberries” (hematite spheres) on
a rocky outcrop at Eagle Crater.
Images from Mars rover Opportunity, NASA
Oxidation, Phang
Nga Bay, Thailand
Trenhaile, 2010
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Lect 4: Intro to Geomorphology
10/5/2015
Weathering rind
formed by oxidation.
http://www.marlimillerphoto.com
Chemical Weathering: 5. Biotic
Plants and animals cause
chemical weathering through
release of acidic compounds
or hydrogen (H+).
Thus, biota provide the
reactant that causes chemical
weathering and may also
provide a conduit that feeds
water and reactants into
fissures and cracks, but does
not dissolve rock in any
constitutive way.
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Lect 4: Intro to Geomorphology
10/5/2015
Chemical Weathering
Water is the main operator:
– Dissolution  Many ionic and organic compounds
dissolve in water (Silica, K, Na, Mg, Ca, Cl)
– Hydration and Hydrolysis  both require water
– Acid Reactions  Require water
• Water + carbon dioxide  carbonic acid
• Water + sulfur  sulfuric acid
• Water + silica  silica acid
Typical Chemical Weathering Products
Olivine
+ H2CO3 (acid)
Clay
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Lect 4: Intro to Geomorphology
10/5/2015
Typical Chemical Weathering Products
Feldspar
+ H2CO3 (acid)
clay
Typical Chemical Weathering Products
+ anything
Quartz
Quartz
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Lect 4: Intro to Geomorphology
10/5/2015
Calcite to …….
Calcite
+ anything
Nothing solid
Resistance to Chemical Weathering
First to
Crystallize
Last to
Crystallize
Crystallization temperature
Bowen’s
Reaction
Series
Fast
Weathering
Goldrich
Stability
Series
Slow
Weathering
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Lect 4: Intro to Geomorphology
10/5/2015
Why is sand so prevalent at Earth’s surface?
Mean Lifetime of a 1mm crystal
at surface (in years)
Quartz
Kaolinite
Muscovite
Microcline (Alk. Feldspar)
Albite (Sodium Plagioclase)
Sandine (Alk. Feldspar)
Enstatite (Pyroxene)
Diopside (Pyroxene)
Forsterite (Olivine)
Nepheline (Amphibole)
Anorthite (Calcium Plagioclase)
34,000,000
6,000,000
2,600,000
921,000
575,000
291,000
10,100
6,800
2,300
211
112
It is composed of
quartz, a relatively
stable mineral!
Martian
Dunes!
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Lect 4: Intro to Geomorphology
10/5/2015
Meanwhile…Back in the real world
Mechanical and chemical weathering work together to break down
the landscape into materials that can be easily transported.
– Fracturing, disintegration caused by mechanical weathering
exposes more surface area.
– Greater surface area, means more places for chemical
action to occur.
– Disturbances by biota are one of the primary ways in which
weathered bedrock is churned up to form soil.
Linkage between climate and weathering
Boreal Forest
Chemical weathering
Most effective in areas of warm, moist climates – decaying
vegetation creates acids that enhance weathering
Least effective in polar regions (water is locked up as ice)
and arid regions (little water)
Mechanical weathering
Enhanced where there are frequent
freeze-thaw cycles
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Lect 4: Intro to Geomorphology
10/5/2015
Yukon
Vancouver
Amazon
Altiplano,
Andes
How does weathering fit into our generalized continuity equation
of the landscape?
Bill Dietrich
P>Transport
Weathering is P
Bedrock landscape
Detachment or supply-limited
P<Transport Capacity
dt
=U-P-
·∆
dz
qs
Soil mantled landscape
(Transport limited)
Bill Dietrich
dt
=U-P-W-
·∆
dz
qs
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Lect 4: Intro to Geomorphology
10/5/2015
Can we predict rates of soil production (P)
from bedrock for inclusion in our generalized
continuity equation of the landscape?
∆
dz
=U-Edt
· qs
Some things that we know:
1) Soil profile development varies with climate
2) The production rate can be predicted from
observed soil depth
Soil Production
Function
P  P0 e H
G. K.
Gilbert,
1870
P
H
There is an exponential decay
in the soil production rate with
soil depth for a given climate
and rock type.
Heimsath, et al., 1997, Nature.
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Lect 4: Intro to Geomorphology
10/5/2015
How about the bedrock wear (W)
contribution to E?
∆
dz
=U-Edt
· qs
Wear (G.K. Gilbert’s ‘corrasion’) is a function of the
imposed stress and the resistance of the bedrock to
this stress (Erodibility).
Erodibility
Resistance to Erosion:
There is at least a 5 order
of magnitude range in
bedrock erodiblity
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Lect 4: Intro to Geomorphology
10/5/2015
Erosion varies inversely as the square of the
rock strength.
Driving force: Water
Near Banff Springs Hotel, Alberta
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Lect 4: Intro to Geomorphology
10/5/2015
Driving force: Water
J David Rogers
Wind sculpted
arches?
J David Rogers
As with water, it
is the sediment
in the sand that
does the work,
not the wind
itself!
Arches National Park near Moab, Utah
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Lect 4: Intro to Geomorphology
10/5/2015
Yardangs
Arizona East of Cameron, Little Colorado River Valley
Peru (Coastal), Ica Valley
Ica Valley, Cerro Yesera Yardang Field
Egypt, Northwest of Bir Tarfawi, Western Desert
Photos by USGS, Desert Studies Group
Driving force: Wind
~17km
Yardangs formed on Mars south of Olympus
Mons at 60 N latitude and 220 E longitude,
which was probably formed by the action of
the wind.
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Lect 4: Intro to Geomorphology
10/5/2015
Driving force: Ice
Glacial grooves caused by the
Wisconsin glaciation at Kelleys
Island, Ohio
Note here too
that it is
sediment and
rock embedded
in the ice that
actually scours
the bedrock
Glacial grooves in rock panels,
Churchill, Manitoba, Canada.
Lassen NP, Northern California.
Polished
bedrock
Yosemite NP
http://www.yosemite.ca.us/library/geologic_story_of_yosemite/final_evolution.html
photo by Ann Dittmer 2002
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Lect 4: Intro to Geomorphology
10/5/2015
Driving force: Ice (Avalanche)
Driving force: Sediment flow (debris flows)
Landslide and debris flow tracks
debris flow track
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Lect 4: Intro to Geomorphology
10/5/2015
Leslie Hsu’s Bedrock
Erosion Wheel
We will talk more
about the stress
imposed by
sediment flow
(mass wasting),
water flow, and
ice flow in future
lectures.
∆
dz
=U-Edt
· 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
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