Weathering

Weathering Lectures 1.Weathering vs. Erosion 2.Joints: Setting the Stages 3.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)

1 st - Weathering = decay in place 2 nd - Detach = break off 3 rd Erode = move

1 st – decayed 2 nd – detached 3 rd – 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.

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2. Joints: Setting the Stage Joints – fractures (that allow water to penetrate and weather the rock) Not what you are thinking!

2. Joints: Setting the Stage Joints 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 links Click here Click here to see a fun movie clip about frost weathering Click here 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

Erosion removes overburden & shells pop off as pressure is released

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

A bit of moisture & sudden heat makes the rock pop – typically flakes (fingernail thick) but sometimes scales (thicker)

“flaking” Wyoming rock art site

Vermillion Cliffs Wetting/Drying

Root Pressure creates weaknesses

Simple Model of the Process

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.