Geomorphology - Lecture 4 1 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 start answering the question: How do landscape materials get from mountain tops to valley floors? 4. To discuss different types of mass movements Where do landscape materials come from? Enchanted Rock State Natural Area of Texas, USA Granite Sand
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Geomorphology - Lecture 4
1
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 start answering the question: How do
landscape materials get from mountain tops to
valley floors?
4. To discuss different types of mass movements
Where do landscape materials come from?
Enchanted Rock State Natural Area of Texas, USA
Granite Sand
Geomorphology - Lecture 4
2
Where do landscape materials
come from?
Enchanted Rock State Natural Area of Texas, USA
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.
dz
dt= U - E - ∙ qs
∆ All landscapes must obey this
fundamental statement about
sediment transport!
The whole
landscape
in one
equation!Photo courtesy of Bill Dietrich
Change in
landscape
surface elevation
(rate)
Uplift rate of the
landscape
surface
Sediment flux
divergence
(written in 3D)Bedrock erosion
rate (P+W)
Geomorphology - Lecture 4
3
dz
dt= U - E - ∙ qs
∆ All landscapes must obey this
fundamental statement about
sediment transport!
The whole
landscape
in one
equation!Photo courtesy of Bill Dietrich
Our discussion today will focus on
Sediment production by weathering (P)
component of the bedrock erosion rate.
Weathering in the Rock Cycle
Geomorphology - Lecture 4
4
Weathering in the Source to Sink Framework
Types of weathering
Physical (Mechanical):
1. Pressure release
2. Freeze-thaw
3. Salt-crystal growth
4. Thermal expansion
5. Biotic
Chemical:
1. Solution
2. Hydration
3. Hydrolysis
4. Oxidation
5. Biological
Review different types in Textbook…review from
GEOG 111/EASC 101…will appear on exam!
Disintegration of rock into
smaller pieces in situ
Transformation/decomposition of one minerals to another in situ
1. Pressure release
2. Freeze-thaw
3. Salt-crystal growth
4. Thermal expansion
5. Biological
1. Solution
2. Hydration
3. Hydrolysis
4. Oxidation
5. Biotic
Geomorphology - Lecture 4
5
Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces
This increases the total surface area exposed to
weathering processes.
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
Geomorphology - Lecture 4
6
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?
1) Exfoliation requires erosion which requires water
3) Salt crystalization requires water
4) Biota growth requires water
2) Ice crystalization requires water
Physical weathering systems are controlled by the
availability of water and thus are climatically controlled.
More on this later!
Driving force of weathering:
Geomorphology - Lecture 4
7
Types of weathering
Physical (Mechanical):
1. Pressure release
2. Freeze-thaw
3. Salt-crystal growth
4. Thermal expansion
5. Biotic
Chemical:
1. Solution
2. Hydration
3. Hydrolysis
4. Oxidation
5. Biological
Review different types in Textbook…review from
GEOG 111/EASC 101…will appear on exam!
Disintegration of rock into
smaller pieces in situ
Transformation/decomposition of one minerals to another in situ
1. Pressure release
2. Freeze-thaw
3. Salt-crystal growth
4. Thermal expansion
5. Biological
1. Solution
2. Hydration
3. Hydrolysis
4. Oxidation
5. Biotic
Chemical Weathering: breakdown as a result of chemical reactions.
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-
Calcium
Carbonate
Carbon
dioxide
Water
Calcium
Bicarbonate
Limestone or
marble rock
Carbonic Acid
(H2CO3)Rock that can be
carried in
solution!
Transformation/decomposition of one mineral into another!
Geomorphology - Lecture 4
8
+ H2CO3 (acid)
Typical Chemical Weathering Products
Olivine
Clay
+ H2CO3 (acid)
Typical Chemical Weathering Products
Feldspar
clay
Geomorphology - Lecture 4
9
Calcite to …….
Nothing solid
+ anythingCalcite
+ anything
Quartz
Quartz
Typical Chemical Weathering Products
Geomorphology - Lecture 4
10
What controls rate of chemical weathering?
Resistance to weathering is controlled by rock type.
Water is main driving force:
– 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
What controls rate of chemical weathering?
Geomorphology - Lecture 4
11
Why is sand so prevalent at Earth’s surface?
It is composed of
quartz, a relatively
stable mineral!
Mean Lifetime of a 1mm crystal
at surface (in years)
Quartz 34,000,000
Kaolinite 6,000,000
Muscovite 2,600,000
Microcline (Alk. Feldspar) 921,000
Albite (Sodium Plagioclase) 575,000
Sandine (Alk. Feldspar) 291,000
Enstatite (Pyroxene) 10,100
Diopside (Pyroxene) 6,800
Forsterite (Olivine) 2,300
Nepheline (Amphibole) 211
Anorthite (Calcium Plagioclase) 112
Linkage between climate and weathering
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
Bierman and Montgomery Textbook
Geomorphology - Lecture 4
12
Yukon
Altiplano, Andes
Amazon
Vancouver
How does weathering fit into our generalized continuity
equation of the landscape?
Soil-mantled landscape
Bedrock landscape
Bill Dietrich
Bill Dietrich
Weathering is P
dz
dt= U - P -
∙∆
qs
dz
dt= U - P - W -
∙∆
qs
P>Transport
P<Transport Capacity
Geomorphology - Lecture 4
13
How can we predict P?
Bierman and
Montgomery
Textbook
Soil Production
Function
H
P
G. K.
Gilbert,
1870
Heimsath, et al., 1997, Nature.
HePP 0
An exponential decay in the
soil production rate with soil
depth for a given climate and
rock type.
Geomorphology - Lecture 4
14
A practical reason to know
the soil production rate
Bierman and Montgomery Textbook
Lynn Betts, USDA-NRCS
USDA-NRCS
How do landscape materials get from
mountain tops to valley floors?
The processes that move materials into stream, creeks, and rivers are
collectively called mass movements or mass wasting.
This includes all sorts of landslides, debris flows, and rock falls.
Geomorphology - Lecture 4
15
1.Introduction to mass movements
Impacts of mass movements
Types of mass movements
3. Slope stability analysis (next week)
4. Geomorphic transport laws for mass
wasting processes (next week)
Goals of Mass Movement Lectures
Frank Slide, Turtle mountain, Alberta.
Geomorphology - Lecture 4
16
Canada’s Worst Natural Disaster
∂z
∂t= U - E - ∙ qs
∆ All landscapes must obey this
fundamental statement about
sediment transport!
The whole
landscape
in one
equation!Photo courtesy of Bill Dietrich
Our discussion will focus on mass wasting
processes that cause erosion and deposition
at the Earth’s surface.
Geomorphology - Lecture 4
17
Mass Movement
Mass movements are important processes in all types of landscapes, in
all climatic settings, and even in the ocean.
Mass Movement
Simply put, mass movement will occur when the resisting forces holding rock in place are overcome by the gravitational forces.
This generally happens when the resisting forces are reduced due to water pressure.
We will formalize this idea mathematically when we consider how to predict when a slope will be unstable through slope stability analysis.
Geomorphology - Lecture 4
18
Rates of mass movement
Conceptually, mass movement can be though of as
working at two levels:
1. The obvious – we can see the evidence very
clearly (ie: houses falling down a cliff in North
Vancouver).
2. The hidden – movements that of themselves
are so small that they cannot be seen very
easily, but over time can be significant.
The Obvious
https://www.youtube.com/watch?v=23NZTzpw6cY
Geomorphology - Lecture 4
19
Photo by: Joan Miquel
Borce, France
http://www.panoramio.com/photo/57803435
The Hidden
From: Wolman, M. G. & Miller, J. P. (1960). Magnitude
and frequency of forces in geomorphic processes.
Journal of Geology, 68, 54-74.
Frequency and magnitude
of geomorphic processes
The most frequent events
(hidden) do not do the greatest
amount of work (not surprising)
The largest events (most
obvious) do the most work, but
they are infrequent.
Moderately sized transport
events (often hidden) do the
most geomorphic work in the
landscape as a consequence of
the frequency of moderate sized
events
Geomorphology - Lecture 4
20
Classification of Mass Movements
FALLSFLOWS
SLIDES
•Results in creep
•Debris Flows
•Earth Flows
•Slump
•Spread
•Rock Falls
•Rock topples
HEAVES
Flows
Spatially continuous movement in which surfaces of
shear are short lived, closely spaced and usually not
preserved. The distribution of velocities resembles
that in a viscous fluid.
Geomorphology - Lecture 4
21
Scars formed by debris flow in greater Los Angeles
during the winter of 1968-1969.
Examples of flows: Debris flow tracks
USGS
Some Cool Debris Flows
llgraben, Switzerland, 28 July 2014
Badakshan District of Varduj, Afghanistan, June 2007