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 Chemical weathering decomposition operate simultaneously & 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)
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)
Sandstone, Arches National Park, Utah unloading Granite, Poudre drainage, Colorado Granite, Yosemite National Park, California
Granite, Missouri Granite, central Australia Sandstone, central Australia
Physical weathering dominates where mean annual T < 0 C, & there is precipitation
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
Roadcut in highly weathered granite, Sequoia National Park, California
As precipitation percolates into exposed rock & regolith, it chemically reacts with minerals & alters the atomic structure of the minerals & the ph of the water mottled laterite, India Reactions involved in chemical weathering: 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 mottling, nw Australia
solution atoms are dissolved from a mineral & the structure becomes unstable CaCO 3 + H 2 CO 3 = Ca 2+ + 2(HCO 3 ) - 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 2KAlSi 3 O 8 + 2H + + 9H 2 O = H 4 Al 2 Si 2 O 9 + 4H 4 SiO 4 + 2K + orthoclase kaolinite
solution Sandstone, Zion Canyon National Park, Utah Sandstone, Uluru, central Australia
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: Ca 2+, Mg 2+, Na + > K + > Fe 2+ > Si 4+ > Ti 4+ > Fe 3+ > Al 3+ increasing lichens, Arizona Havasu Creek, AZ
western Arizona Death Valley, California
joint-controlled weathering Joints in sandstone, Canyonlands NP, Utah Sandstone, Escalante basin, Utah Sandstone, Rainbow Arch, northern Arizona
joint-controlled weathering Sandstone, Zion National Park, Utah Rhyolite, Chiricahua National Monument, Arizona
joint-controlled weathering Granite, Sinai peninsula, Egypt Granite, Wyoming
joint-controlled weathering Granite, Half Dome, Yosemite Granite, Wind River Range, Wyoming Granite, Loch Vale, Colorado
joint-controlled weathering Sandstone, Zion National Park, Utah Granite,Yosemite, California
desert varnish Sandstone, northern Arizona Basalt, northern New Mexico
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