Weathering
Mar 19, 2016
Weathering Lectures
1. Weathering vs. Erosion2. Joints: Setting the Stages3. Physical (Mechanical) Weathering4. Chemical Weathering5. Products and Forms Made by Weathering6. Weathering Landscapes
1. Weathering vs. ErosionCommon Error Made in K-12 EarthScience Teaching: these are synonymsNo!
Weathering – chemically dissolving candy or physically crunching candy – breakdown in place
Erosion – moving pieces (dissolved or as fragments)
Humans can upset the balance and accelerate erosion. So when transport (detachment and erosion) becomes faster than weathering, landscapes are not sustainable.
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
• Can also be Foliated Metamorphic Rocks – where fractures occur along bedding planes or along foliations
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)
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
Pressure Release
Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage
pressure release shells
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.
Order from innermost crack out:calcrete – white calcium carbonateiron 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
A bit of moisture & sudden heat makes the rock pop –
typically flakes (fingernail thick)but sometimes scales (thicker)
• 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.