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Weathering
Weathering Lectures
1. Weathering vs. Erosion
2. Joints: Setting the Stages3. Physical (Mechanical) Weathering
4. Chemical Weathering
5. Products and Forms Made by Weathering
6. Weathering Landscapes
1. Weathering vs. Erosion
Common Error Made in K-12 Earth
Science Teaching: these are synonyms
No!
Weathering – chemically dissolving candy or physically crunching candy – breakdown in place
Erosion – moving pieces (dissolved or as fragments)
1st - Weathering = decay in place
2nd - Detach = break off
3rd Erode = move
1st – decayed
2nd – detached
3rd – eroded
The balance between weathering and erosion defines the landscape
In deserts – transport is faster
Humans can upset the balance and accelerate erosion. So when transport (detachment and erosion) becomes faster than weathering, landscapes are not sustainable.
s
2. Joints: Setting the StageJoints – fractures (that allow water to
penetrate and weather the rock)
Not what you are thinking!
2. Joints: Setting the StageJoints dependent on lithification
(sedimentary & foliated metamorphic rocks) – where fractures occur along bedding planes or along foliations
Joints independent of lithification (all rock types) – all other fractures
Joints dependent on Lithification
• Sedimentary Rocks – where fractures occur along bedding planes
Joints dependent on lithification
• Can also be Foliated Metamorphic Rocks – where fractures occur along bedding planes or along foliations
Joints dependent of lithification
Rocks can be cracked by cooling, leading
To columnar jointing
Columnar Jointing
From contraction
after lava flow cools
Joints independent of lithification
Rocks can be cracked by Tectonic Stresses
or faulting/folding
stresses the rock
Joints can even fracture sedimentary strata from regional tectonic pressures
Sandstone fractured
across bedding planes
Can have both together• Along bedding (& foliation) weaknesses
and cutting across
3. Physical (Mechanical) Weathering
• Abrasion
• Frost weathering
• Pressure release weathering
• Salt Weathering
• Thermal Expansion/Contraction
• Wetting/Drying
• Root pressure
Abrasion• Often see abrasion by sediment or soil
transport by water over rock… this both weathers (decays) and erodes by abrasion (like sand paper)
Anthropogenic Abrasion
• carving chiseling, bullet impacts
Frost Weathering
Check out these linksClick here to see a
fun movie clip about frost weathering
Click here to see a 360 degree view of fractured rock in Antarctica by frost weathering
Breaks rock along small fractures
Breaks rock along large fractures& produces jagged alpine topography
Aesthetic, so used in commercials
Pressure Release
Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage
pressure release shells
Erosionremovesoverburden &shells pop offas pressure isreleased
Glacial Erosion Great Way to Generate Pressure Release
Salt Weathering
Salt is common in deserts
Salt is common along coasts
Mechanics of Salt Weathering• Salt Crystal Growth: Extreme
pressures in cracks and rock pores are
caused by salt crystal growth from
solution. There are varying causes of
and extents to which salt growth occurs.
• Hydration: The hydration of various salts
causes expansion & contraction, pushing
apart the silicate host minerals
• Thermal Expansion: During
temperature fluctuations, salts
trapped in pores may expand to a
greater degree than the
surrounding rock minerals.
Caverns (tafoni)
Base of rock notching (basal weathering)
Wedging
tafoni – larger cavernous forms
(Goudie and Viles, 1997:168)
Alveoli – smaller cavernous forms: alveolization is the process
Alveolization can occur from more than salt
weathering, including
insects & bird activity!
Weathering along bases of rocks:basal notches
Salt Efflorescence
Salt subflorescence(salt under the surface)
Salt subflorescence(salt under the surface – usually see when
look at undersides of eroded flakes)
fissuresol wedging & calcrete wedging
Order from innermost crack out:calcrete – white calcium carbonate
iron film – orange clays and iron
rock varnish- outer edge black clays & manganese/Iron
Wedging can split small rocks and giant boulders – carbonate & dust washes
away – leaving iron film and rock varnish
Thermal Expansion/Contraction
• Thermal Expansion/Contraction
A bit of moisture & sudden heat makes the rock pop –
typically flakes (fingernail thick)but sometimes scales (thicker)
“flaking”Wyomingrock art
site
Vermillion Cliffs
Wetting/Drying
Root Pressure creates weaknesses
SimpleModel of
theProcess
• More than roots can affect rock. Plants growing next to rock art can burn and wind movement can abrade
Imagery seen in this presentation is courtesy of ASU faculty, Paradise Valley and Mesa CC faculty, students and colleagues in other academic units, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, city, state governments, other countries government websites and U.S. government agencies such as NASA, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.