Physical Geology Notes
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Physical Geology Notes
Chapter Seven
Weathering: Chemical alteration and mechanical breakdown of rock and sediment.
* depth of air/water penetration
* as distance of formation from surface rises, so does susceptibility to weathering
* physical weathering: The physical breakup of rocks.
* chemical weathering: The decomposition of rocks and minerals caused by stability-increasing chemical reactions.
-rate of weathering positively linked to presence or absence of high mountains, and thus to plate tectonics
Physical Weathering-Mechanical Processes (water, fire, roots)
* development of joints- rock moved upward after erosion, causes fractures
-columnar joints in igneous rock
* crystal growth- salts precipitated from seeping groundwater
* frost wedging- frozen seeping water
* effects of heat- rocks don't conduct, spall (outer shell) breaks away
* plant roots- seeds germinate in cracks
Chemical Weathering-Chemical Reactions (stabilize to surface temp)
* carbonic acid- rain + CO2 makes carbonic acid, frees H+ (comp. chg.)
* hydrolysis- water ions replace mineral ions
-decomposed potassium feldspar, kaolinite replaced
* leaching- water removes soluble materials from bedrock/regolith
-seeps into water, bad taste
* oxidation- post-weathering iron release, goethite prod w/ hydration (O)
-hematite produced w/ dehydration
-intensity of color tells time/severity of weathering
* dissolution- carbonic acid dissolves calcite (previously unsoluble)
Results of Weathering
* common rocks- granite's high silica hydrolysis=* clays & soluble min.
-basalt oxidizes, limestone dissolves
* conc of stable minerals- resistant to attack, @ streambeds/beaches b/c spec. grav. (cw)
* weathering rinds- light colored rind surrounding a darker core of rock
-composed of solid byproduct of chemical weathering
* exfoliation- spalling off of outer rock shells caused by stress during weathering
* spheroidal weathering- rock tendency to be rounded by weathering
-weathering most effective w/ *surface area, & subdivision
Factors of Weathering
* rock type/structure- minerals: quarts & granite resistant
-differential weathering based on composition & structure
* slope- steep =* rapid, gradual =* slow
* climate- moisture and heat promote reactions (chem. react. prev. @ =* eq.)
-carbonate rocks (calcite diss.) in different regions
* time- hundreds/thousands years for a few mm
Soils: The part of the regolith that can support rooted plants. Formed by the weathering of bedrock: mineral component mixed wit...
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