goals of today’s lecturejvenditt/geog213/lecture04_weathering_hill...geomorphology - lecture 4 1...

Geomorphology - Lecture 4

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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


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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)


3

dz

dt= U - E - ∙ qs



sediment transport!

The whole

landscape

in one


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…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



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.

More on this later!

Driving force of weathering:


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Types of weathering

Physical (Mechanical):

1. Pressure release

2. Freeze-thaw



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



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!


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+ H2CO3 (acid)

Typical Chemical Weathering Products

Olivine

Clay

+ H2CO3 (acid)


Feldspar

clay


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Calcite to …….

Nothing solid

+ anythingCalcite

+ anything

Quartz

Quartz



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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?


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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


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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


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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.


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.


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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.


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Canada’s Worst Natural Disaster

∂z

∂t= U - E - ∙ qs



sediment transport!

The whole

landscape

in one


Our discussion will focus on mass wasting

processes that cause erosion and deposition

at the Earth’s surface.


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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.


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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


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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


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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.


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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


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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

Debris flow track near

Bamfield


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Bamfield; looking

upslope


Bamfield; looking

downslope


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Anatomy of a debris flow deposit

Anatomy of a debris flow channel


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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.

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


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A Cool Earth Flow

http://blogs.agu.org/landslideblog/2015/04/20/bolshaya-talda-1/

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


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Churn Creek Earth Flow

Grinder Earth Flow


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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.

Toe of Drynoch

Earthflow along

Thompson River

June Ryder

Examples of flows: Earthflow


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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

Talus Slopes Fraser Canyon


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Talus Slopes Fraser Canyon

nr. Lillooet, southwestern B.C., Canada.

Fraser Canyon nr. Quesnel

Angle of repose

Examples of falls: Talus cones


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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.

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.


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Downslope creep of soil at surface of vertical shale beds

South of Dawson, Yukon, Canada Frank Nicholson

Examples of heave: Soil creep

Ian Alexander


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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

Deep-seated

landslide: Hope

Slide, BC


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Examples of slides: shallow-seated landslide, Briones Regional Park, CA

Examples of slides: shallow-seated rotational landslide, Marin Headlands, CA


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Carmen Krapf

Examples of slides: deep-seated landslide, Keetmanshoop, Southern Namibia

Christiane Mainzer

La Conchita slump.

March 4, 1995

Santa Barbara, California.

Examples of slides: Deep-seated rotational

landslide


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Examples of slides: Deep-seated rotational landslide, La Conchita

Ann Dittmer

Classification of Mass Movements


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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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