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.

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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.