weathering processes. weathering 1.weathering vs. erosion 2.joints: setting the stage 3.physical...
TRANSCRIPT
Weathering Processes
Weathering
1. Weathering vs. Erosion
2. Joints: Setting the Stage
3. Physical (Mechanical) Weathering
4. Chemical Weathering
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
s
2. Joints: Setting the StageJoints – fractures (that allow water to
penetrate and weather the rock)
Many ways to make joints:
Cooling & contraction Tectonic Stresses
Very different!
Columnar Jointing
From contraction
after lava flow cools
or faulting/folding
stresses the rock
Joints can even fracture sedimentary strata from regional tectonic pressures
3. Physical (Mechanical) Weathering
• Frost weathering
• Pressure release weathering
• Salt Weathering
• Thermal Expansion/Contraction
• Wedging (Fissuresols & Calcrete)
• Wetting/Drying
• Root pressure
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
Caverns (tafoni – larger
cavernous forms)
(Goudie and Viles, 1997:168)
Caverns (alveoli – smaller cavernous forms)
Weathering along bases of rocks
Wedging
Sequence in Rock Fractures
Generalized Process
Fissuresol wedging can pry apart bedrock
Fissuresol wedging can pry apart
mountain sides and
giant boulders
Thermal Expansion/Contraction
• Thermal Expansion/Contraction
A bit of moisture & sudden heat makes the rock pop
Vermillion Cliffs
Wetting/Drying
Root Pressure
Online Animations
Physical Weathering Visualizations
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/physical_weathering.html
Physical weathering “sets up” chemical weathering
Why do you ground coffee?to increase surface area
4. Chemical WeatheringEgyptian Obelisk – chemically weathered
when brought to wetter environment, so water matters!
DissolutionSugar & Salt Dissolves – so do rocks
Best example: limestone
Other rocks dissolve too, but slower than limestone
Acid Rain Accelerates Decay
Crosses political boundaries …
Taj Mahal Athens
Oxidation
Hydrolysis
Water molecules at the mineral surface dissociate into H+ and OH- and the mobile H+ ions penetrate the crystal lattice, creating a charge imbalance, that causes cations (important nutrients) such as Ca2+ , Mg2+, K+ and Na+ to diffuse out. For example, the feldspar reacts to decay and leaves a residue of clay mineral.
H+
OH-
H+
Hydration
Water alters structure
Complexation Metals released from primary minerals such as iron and manganese build complexes with organic components, such as fulvic acids and humic acids, causing an imbalance between cations and anions – that leads to mineral decay
Online Animations
Chemical Weathering Visualizations
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/chemical_weathering.html