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Weathering: chemical and physical

• The destruction of rocks at the Earth’s surface by weathering has two fundamental modes of operation:– Chemical weathering is dissolution or

alteration of the original minerals, usually by reactions with aqueous solutions

• Chemical weathering puts ions from the source minerals into solution for subsequent erosion by transport in flowing water as dissolved load.

– Physical weathering is fragmentation into progressively smaller particles, from intact outcrop to boulders and on down to mineral fragments and sand grains.

• Physical weathering makes loose pieces of rock available for downslope movement by mass wasting or transport in flowing water as suspended or bed load.

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Weathering feedbacks: chemical and physical• Physical weathering and

chemical weathering generally proceed in parallel in most environments.

• Physical and chemical weathering promote one another:– Formation of cracks by

physical weathering increases reactive surface area, promoting chemical weathering.

– Chemical weathering replaces intact interlocking minerals with weak clays or void space, making the rock easier to physically disaggregate, promoting physical weathering

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Chemical Weathering

• Chemical weathering is driven by thermodynamic energy minimization, just like chemical reactions at high temperature.– The system seeks the most stable

assemblage of phases.– The differences are that (1) kinetics are

slow and metastability is common; (2) the stable minerals under wet, ambient conditions are different from those at high T and P; (3) solubility in water and its dependence on water chemistry (notably pH) are major determinants in the stability of minerals in weathering.

• A fresh rock made of olivine and pyroxenes will end up as clays and iron oxides, with other elements in solution

• A fresh rock made of feldspars and quartz will end up as clays, hydroxides, and quartz in most waters.

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Chemical Weathering

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Rates of Chemical Weathering

• Many factors affect the rate at which a rock will weather, as summarized here.

• Some of these variables are local (e.g., source rock), some are global. These include temperature and pCO2, leading to the CO2-weathering feedback cycle.

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Soil formation

• A weathered surface develops a stratified structure, with intact rock at the bottom (or inside) and maximum weathering at the top .

• Leachable ions are transported downwards by groundwater flow, possibly redeposited as water chemistry adjusts towards equilibrium with the developing soil profile.

• Chemically and physically weathered rock that is not eroded or transported but remains in place becomes soil.

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Soil formation• The mineralogy and thickness of soil layers depends on

source rock, climate (temperature and rainfall), and age.

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