In general…High-temperature minerals + water = weathering products + dissolved ions

• Hydration/hydrolysis reactions. Depend on pH – acid vs. alkaline conditions

2 NaAlSi3O8 + 2H+ + H2O Al2Si2O5(OH)4 + 2 Na+ + 4 SiO2

Na-feldspar (albite) kaolinite (clay)

• “Carbonic acid” reactions. Involve dissolved atmospheric CO2 or CO2 respired by plants

CO2 + H2O H2CO3 H+ + HCO3-

“carbonic acid” “bicarbonate”

2 KAlSi3O8 + CO2 + 3H2O Al2Si2O5(OH)4 + 2 K+ + 2HCO3- + 4 SiO2

K-feldspar kaolinite (clay) (consumes CO2)

In general…High-temperature minerals + water = weathering products + dissolved ions

• Oxydation reactions. “Oxidation” is removal of an electron from an ion (e.g., Mn2+ Mn3+ + e-)

2 Fe2+ + ½ O2 + 2H2O Fe2O3 + 4 H+

Fe2+ in silicate dissolved O2 hematite (Fe3+) 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 CO2 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 HCO3

- (bicarbonate) and H2CO3 (carbonic acid) and are effective in weathering.

About weathering…

• Weathering and climate

- Chemical weathering of silicate minerals consumes atmospheric CO2, 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 CO2 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+, Ca2+, Mg2+… are leached away- Insoluble species Al3+, Fe3+, Ti4+ are concentrated in the residue

- Where do you expect Ca2+ 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)