454 lecture 2

WEATHERING

the interacting physical, chemical & biological processes that

progressively alter the original lithologic character of rocks to

produce secondary minerals (e.g. clays) & unconsolidated

regolith

Weathering – breakdown of rock materials

Erosion – transport of broken-down materials

Physical weathering – disintegration operate simultaneously &

Chemical weathering – decomposition affect each other

Physical weathering: the transformation of rocks into

unconsolidated debris; occurs when stress is exerted along

zones of weakness within the original material (bedding,

fractures, mineral boundaries)

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Stress field originates via expansion of rocks or minerals, or

pressure caused by growth of foreign substance in voids:

thermal expansion: low thermal conductivity of rocks allows them

to expand at surface, but not at depth, creating differential

stress & spalling

unloading: confining pressure decreases when overlying rocks

are removed, & sheets form perpendicular to the direction of

pressure release, mirroring surface topography

hydration: hydration of salts or mineral expansion causes

exfoliation at the large scale, spheroidal weathering at

small scale

growth in voids: openings may expand at different rates or in

different directions, creating differential pressure

frost action plant roots

minerals (percolating fluids evaporate, supersaturate, & precipitate;

e.g. sulfates & carbonates in arid/semiarid regions)

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unloading

Granite, Yosemite National Park,

California

Sandstone, Arches National Park, Utah

Granite, Poudre drainage, Colorado

454 lecture 2Sandstone, central AustraliaGranite, central Australia

Granite, Missouri

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Physical weathering dominates where mean annual T < 0° C,

& there is precipitation

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Chemical weathering

Rainwater is mildly acidic; chemical weathering is a neutralizing

process in which minerals assimilate H ions and/or water, &

release cations to the soil liquid

Other anions, cations, gases & organic acids are introduced by

chemical impurities in rainwater, groundwater, & organic

processes (metabolism of microrganisms & decay of vegetation)

Two stages of chemical weathering

Inorganic processes: geochemical weathering produces

saprolites (“rotten rock”)

Organic processes: biologically controlled, pedochemical

weathering produces soils

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Roadcut in highly weathered granite,

Sequoia National Park, California

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As precipitation percolates into exposed rock & regolith, it

chemically reacts with minerals & alters the atomic structure of

the minerals & the pH of the water

Reactions involved in chemical weathering:

mottling, nw Australia

mottled laterite, India

oxidation & reduction

in oxidation, an element loses an electron to an oxygen ion

(Zn + O = ZnO)

occurs spontaneously above the water table, where atmospheric oxygen

is readily available

in reduction, an element gains an electron; occurs below the water table

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solution

atoms are dissolved from a mineral & the structure becomes unstable

CaCO3 + H2CO3 = Ca2+ + 2(HCO3)-

the most common elements & minerals are soluble to some extent in

groundwater – exceptions are aluminum oxide and ferric iron

hydrolysis

reaction between mineral elements & H ion of dissociated water

works on silicates by removing metallic cations from the mineral structure

2KAlSi3O8 + 2H+ + 9H2O = H4Al2Si2O9 + 4H4SiO4 + 2K+

orthoclase kaolinite

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solution

Sandstone, Zion Canyon National Park, Utah

Sandstone, Uluru, central Australia

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ion exchange

substitution of ions in solution for those held by mineral grains

most effective in clay minerals

relative mobility (ease of removal) of cations:

Ca2+, Mg2+, Na+ > K+ > Fe2+ > Si4+ > Ti4+ > Fe3+ > Al3+

increasing

lichens, Arizona Havasu Creek, AZ

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Death Valley, California

western

Arizona

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joint-controlled weathering Joints in sandstone, Canyonlands NP, Utah

Sandstone,

Escalante basin,

Utah

Sandstone, Rainbow Arch,

northern Arizona

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joint-controlled weathering

Rhyolite, Chiricahua National

Monument, Arizona

Sandstone, Zion National Park,

Utah

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joint-controlled weathering

Granite, Wyoming

Granite, Sinai peninsula, Egypt

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joint-controlled weathering

Granite, Wind River Range,

Wyoming

Granite, Half Dome, Yosemite

Granite, Loch Vale, Colorado

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joint-controlled weathering

Sandstone, Zion National Park, Utah

Granite,Yosemite, California

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desert varnish

Sandstone, northern Arizona Basalt, northern New Mexico

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Weathering stability of common silicate minerals

1 olivine, anorthite least stable

pyroxenes, Ca-Na plagioclase

amphiboles, Na-Ca plagioclase

biotite, albite

K-feldspars

muscovite

7 quartz most stable

1 high-temperature minerals, first to crystallize, least stable

in surface environment, weather rapidly

7 last to crystallize, form under lowest temperature conditions,

most stable at surface

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