In general… High-temperature minerals + water = weathering products + dissolved ions Hydration/hydrolysis reactions. Depend on pH – acid vs. alkaline.

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Presentation transcript:

In general… High-temperature minerals + water = weathering products + dissolved ions Hydration/hydrolysis reactions. Depend on pH – acid vs. alkaline conditions 2 NaAlSi 3 O 8 + 2H + + H 2 O  Al 2 Si 2 O 5 (OH) Na SiO 2 Na-feldspar (albite)  kaolinite (clay) “Carbonic acid” reactions. Involve dissolved atmospheric CO 2 or CO 2 respired by plants CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 - “carbonic acid” “bicarbonate” 2 KAlSi 3 O 8 + CO 2 + 3H 2 O  Al 2 Si 2 O 5 (OH) K + + 2HCO SiO 2 K-feldspar  kaolinite (clay) (consumes CO 2 )

In general… High-temperature minerals + water = weathering products + dissolved ions Oxydation reactions. “Oxidation” is removal of an electron from an ion (e.g., Mn 2+  Mn 3+ + e - ) 2 Fe 2+ + ½ O 2 + 2H 2 O  Fe 2 O H + Fe 2+ in silicate dissolved O 2  hematite (Fe 3+ ) acid

About weathering… Weathering and biology: - Biological activity (modification of chemical micro-environments near root fibers, colonization of mineral surfaces by micro-organisms, accumulation of organic decay products) modifies soil chemical environment, enhancing weathering. - Atmospheric CO 2 is ~380 ppm, but respiration by plant roots can result in concentrations up to 100,000 ppm (10%) in soil gases! Soil waters acquire high concentrations of HCO 3 - (bicarbonate) and H 2 CO 3 (carbonic acid) and are effective in weathering.

About weathering… Weathering and climate - Chemical weathering of silicate minerals consumes atmospheric CO 2, which is ultimately deposited in the oceans in Ca and Mg carbonates (marine limestones). Silicate weathering is the most important long-term regulator of atmospheric CO 2 level (multi-million yr timescales) - Weathering requires flux of water for reactions, and weathering rates are fastest under warm and humid (tropical) climate. - Where on Earth does weathering rate approach zero? - What happens if you uplift a mountain range and weather it intensely?

About weathering… Weathering and soils - Weathering reactions produce hydrated and/or oxidized minerals (clays, hydroxides, oxides). - Soluble species K +, Na +, Ca 2+, Mg 2+ … are leached away - Insoluble species Al 3+, Fe 3+, Ti 4+ are concentrated in the residue - Where do you expect Ca 2+ to be concentrated? Ocean or surface of an old continent? - How might chemical weathering and ore deposits be related?

Summary: Rates of soil production controlled by - Flux of water - Contact time - Reactivity of mineral assemblage - pH (acidity vs. alkalinity) - temperature (tropical vs. glacial) - biota

Physical weathering processes: Changes in rock volume (expansion from chemical changes like hydration; relaxation of confining stress; thermal expansion)

Physical weathering processes: joints

Physical weathering processes: Changes in rock volume (expansion from chemical changes like hydration; relaxation of confining stress; thermal expansion)

Physical weathering processes: Changes in shape or volume of voids (frost wedging, salt weathering)

Physical weathering processes: Changes in shape or volume of voids (tree roots, animal burrows)