Goals of Today’s Lecture - SFU.cajvenditt/geog213/Lecture04_Weathering_Hill... · To examine...
Transcript of Goals of Today’s Lecture - SFU.cajvenditt/geog213/Lecture04_Weathering_Hill... · To examine...
Lect 4: Intro to Geomorphology 10/2/2017
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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
Lect 4: Intro to Geomorphology 10/2/2017
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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)
Lect 4: Intro to Geomorphology 10/2/2017
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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
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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…is 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
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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
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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.
Driving force of weathering:
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Chemical Weathering: breakdown as a result of chemical reactions.
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-
Calcium
Carbonate
Carbon
dioxide
Water
Calcium
Bicarbonate
Limestone or
marble rockCarbonic Acid
(H2CO3)
Rock that can be
carried in
solution!
Transformation/decomposition of one mineral into another!
+ H2CO3 (acid)
Typical Chemical Weathering Products
Olivine
Clay
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+ H2CO3 (acid)
Typical Chemical Weathering Products
Feldspar
clay
Calcite to …….
Nothing solid
+ anythingCalcite
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+ anything
Quartz
Quartz
Typical Chemical Weathering Products
What controls rate of chemical weathering?
Resistance to weathering is controlled by rock type.
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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?
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
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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
Yukon
Altiplano, Andes
Amazon
Vancouver
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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
How can we predict P?
Bierman and
Montgomery
Textbook
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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.
A practical reason to know
the soil production rate
Bierman and Montgomery Textbook
Lynn Betts, USDA-NRCS
USDA-NRCS
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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.
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
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Frank Slide, Turtle mountain, Alberta.
Canada’s Worst Natural Disaster
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∂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.
Mass Movement
Mass movements are important processes in all types of landscapes, in
all climatic settings, and even in the ocean.
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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.
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.
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The Obvious
https://www.youtube.com/watch?v=23NZTzpw6cY
Photo by: Joan Miquel
Borce, France
http://www.panoramio.com/photo/57803435
The Hidden
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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
Classification of Mass Movements
FALLSFLOWS
SLIDES
•Results in creep
•Debris Flows
•Earth Flows
•Slump
•Spread
•Rock Falls
•Rock topples
HEAVES
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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.
Scars formed by debris flow in greater Los Angeles
during the winter of 1968-1969.
Examples of flows: Debris flow tracks
USGS
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Some Cool Debris Flows
llgraben, Switzerland, 28 July 2014
Badakshan District of Varduj, Afghanistan, June 2007
Source area for debris
flow near Bamfield
Debris flows
typically have a
point source
Originate when poorly
consolidated rock or soil
masses are mobilized by
the addition of water by:
•Periods of extended rainfall
•Localized areas of intense
rainfall
•Ponding on surface upstream
of flow
•Snowmelt or rain on snow
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Debris flow track near
Bamfield
Debris flow track near
Bamfield; looking
upslope
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Debris flow track near
Bamfield; looking
downslope
Anatomy of a debris flow deposit
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Anatomy of a debris flow channel
Debris flow failure mechanisms
Most debris flows originate
on slope >15%
Stock and Dietrich (WRR, 2003)
Many debris flows originate at
channel headwaters (hollows)
But, they may also be formed by
other types of initial failure upstream
of the debris flow location.
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Martin Geertsema, 2002
Confluence of Muskwa
and Chisca rivers,
northern British Columbia.
Examples of flows: Earthflow
Typically high viscosity flows
formed from weathered volcanic rock
A Cool Earth Flow
http://blogs.agu.org/landslideblog/2015/04/20/bolshaya-talda-1/
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Anatomy of
an Earthflow
• Large slow moving flows
common in the western part of
the interior plateau of BC
• Several km in length and
typically composed of ~106 m3
of material.
• Form in weathered volcanic
rock that forms clay materials
• Often have a defined slide
plane and shear surfaces
• Movement and rotation of
blocks mean there is mixing
• Flows occur over several
thousands of years
• Have velocities up to 1 m/a.
Bovis, 1986
Churn Creek Earth Flow
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Grinder Earth Flow
Anatomy of
an Earthflow
Bovis, 1986Pavillion Earth Flow
• Large slow moving flows
common in the western part of
the interior plateau of BC
• Several km in length and
typically composed of ~106 m3
of material.
• Form in weathered volcanic
rock that forms clay materials
• Often have a defined slide
plane and shear surfaces
• Movement and rotation of
blocks mean there is mixing
• Flows occur over several
thousands of years
• Have velocities up to 1 m/a.
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Toe of Drynoch
Earthflow along
Thompson River
June Ryder
Examples of flows: Earthflow
Falls
Falls begin with the detachment
of rock from a steep slope along
a surface on which little or no
shear displacement takes
place. The material then falls
or rolls through the air.
Topple is a forward rotation, out
of the slope, of a mass of soil or
rock about a point or axis below
the center of gravity of the
displaced mass.
Rockfall in the Talkeetna Mountains, Alaska
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nr. Lillooet, southwestern B.C., Canada.
Fraser Canyon nr. Quesnel
Angle of repose
Examples of falls: Talus cones
Heaves
Periodic expansion and contraction of a soil or sediment mass
that is usually linked to clay swelling and dewatering or freezing
and thawing. Heave leads to downslope creep of hillslope
materials as the strength of the materials is decreased.
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Solifluction: downslope movement caused by
vertical heave as soil freezes and downslope
meovemtn when the soil thaws.
Gelifluction: Slippage of the soil
along a slide plane when it is
thawed
Both are simply a form of creep
induced by freeze-thaw heave
cycles.
Downslope creep of soil at surface of vertical shale beds
South of Dawson, Yukon, Canada Frank Nicholson
Examples of heave: Soil creep
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Ian Alexander
Slides
Downslope movement of
soil or a rock mass
occurring dominantly
along a surface of
rupture or relatively thin
zones of intense shear.
A) Pure slide (translational)
B) Rotational slide
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Deep-seated
landslide: Hope
Slide, BC
Examples of slides: shallow-seated landslide, Briones Regional Park, CA
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Examples of slides: shallow-seated rotational landslide, Marin Headlands, CA
Carmen Krapf
Examples of slides: deep-seated landslide, Keetmanshoop, Southern Namibia
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Christiane Mainzer
La Conchita slump.
March 4, 1995
Santa Barbara, California.
Examples of slides: Deep-seated rotational
landslide
Examples of slides: Deep-seated rotational landslide, La Conchita
Ann Dittmer