Why study Weathering? Weathering process of disintegrating solid rock & producing loose debris To understand geol process (like hydrologic systems) and how landscapes evolve (topo maps, landforms)
Weathering Chemical weathering The decomposition of rocks and minerals as chemical reactions alter them into new minerals stable at the Earth s surface Physical weathering The disintegration or disaggregation of rocks by physically breaking them apart
Physical Weathering Physical breakage of rocks into smaller pieces; no change in chem composition Ice Wedging from freeze(water expands 9% when freezes)/thaw cycles; occurs best in preexisting fractures, joints where water can enter; salt water when dried can produce rock busting crystals Sheeting when confining pressure is released by erosion; rock body can expand or heave; fractures parallel to Earth s surface In high mt areas, talus cones form (lg volumes angular rock frags)
Fig. 10.2a. Ice wedging
Fig. 10.4. Sheeting in granite
Chemical Weathering Minerals/rocks are destroyed or altered by chemical reactions Dissolution, acid formation/dissolution, oxidation Dissolution: some rocks soluble in water ex. halite, gypsum Formation of acids (naturally & manmade): ions are leached, deposited with other ions to form new minerals. Ex.: feldspars to clays
Sources of Acids CO 2 mixes with water to produce carbonic acid, H 2 CO 3 Decaying organic matter produces acid Sulfuric acid naturally & burning coal (acid rain) Nitric acid nitrogen oxides as byproduct of gasoline burning
Oxidation Since oxygen is so abundant in atmosphere, common for chemical changes to occur from ferrous to ferric iron Iron is usually found as the Fe +2 ion in silicate minerals (olivine, pyroxenes, amphiboles) Exposed to the atmosphere it will oxidize to the Fe +3 ion (hematite, limonite); Common example???
Weathering by Rock Type Granites: feldspars, qtz, micas: weather to? Basalt: feldspar, pyrox, olivine: weather to? Sandstone: quartz: weathers to? Limestone:? ; weathers to? Shale:? ; weathers to?
Fig. 10.8. Relative stability of minerals
Products of Weathering 1) Rock bodies are modified. -first along pre-existing planes of weakness (joints, cleavage, foliation) ex. Devil s Post Pile, CA; Bryce Canyon, UT -spheroidal weathering rounded surfaces form regardless of original shape of rock -exfoliation rock weathers like layers of onion; usually granites; ex. Yosemite Dome, Llano Granite
Weathering Surface Area is increased by fracturing The increase in surface area, increases the rate of weathering Both physical and chemical Surface area increases exponentially
Products of Weathering 2) Formation of regolith blanket of loose, decayed rock debris and soil - Covers the solid, unaltered bedrock below - Few centimeters to 100 s of meters - Soil: uppermost layer of regolith; economically important
Soil Soil is earth material (weathered rock) that it is capable of sustaining rooted plant growth Soil = mineral matter + organic matter from biological processes + air + water Soils are classified into 12 soil orders by soil scientists; Houston: expanding clays create havoc on building foundations, roads
A Horizon Soil Horizons A dark gray to black horizon, rich in organic matter. Leaching carries dissolved ions and fine particles B Horizon Accumulates leached material from above Enriched in clay minerals and oxides; reddish C Horizon Parent rock in various stages of weathering
Fig. 10.14 Common soil profile
Climate & Weathering Climatic conditions strongly influence weathering reactions Amount of rainfall Most reactions need water Average temperature Increase of 10 o C doubles reaction rate
Climate & Weathering 1) Humid, tropical: extreme chemical wthg. -thick soils (70 m; Brazil: 150m thick!) -most insoluble mat l stays in soil (Si, Al, Fe) produces RED soil not good for ag 2) Deserts: physical w thin soils; exposures of fresh, unaltered bedrock: Geologists J 3) Temperate: both chem & phys w; precip. is humid to subarid;temps are cool to warm; soil depths: 1-10 m; nutrients stay; best ag areas 4) Polar: physical w thin, unproductive soils
Fig 10.19. Climate and weathering
End of Chapter 10