Physical weathering

• the disintegration of rock by mechanical forces concentrated along rock fractures
• the separation of rock into concentric layers is called exfoliation in rock masses and spheriodal or onion skin weathering in boulders: 

frost shattering

the force of water in rock fractures as it freezes and expands, or is forced into the rock by the pressure of freezing water

• the most common physical weathering process, given the widespread distribution of frost (even in the tropic at high elevations) 
• most effective in coastal arctic and alpine environments where there are hundreds of frost cycles per year
• the specific volume (vol./unit mass) of water increases by 9% upon freezing producing stress that is greater than the strength of all common rocks

 

pressure (stress) release

exfoliation of a rock mass as it expands in response to the removal of adjacent rock

• the most common mechanism of stress release is the erosion of overlying rock by erosion
• the rock disintegrates along dilation (expansion) fractures that conform to the surface topography and increase in spacing with depth 

 

salt weathering

growth of salt crystals in rock fractures with the evaporation of saline groundwater

• causes mostly granular disintegration
• the salt is in solution and precipitate as soil water and groundwater evaporate
• it is most effective in dry landscapes where water tables are near the surface (e.g., saline seeps)
• the salt is derived from sea water, the chemical weathering of marine or evaporite sediments, snow and rain

 

hydration

wetting, swelling and disintegration of soil aggregates and fine grained rocks

• caused by the expansion and contraction with wetting and drying
• also the pressure of air drawn into pores under dry conditions and then trapped as water advances into soil and rock
• e.g., biotite expands 40% by volume contributing to the weathering of granite

 

insolation (thermal) weathering

expansion and contraction with wetting and drying

• the surface temperature of dark colored rock can vary from 0-50^o C between day and night, since rock (especially jointed rock) has low thermal conductivity
• the differential stresses of expansion and contraction of the outer 1-5 cm of rock causes separation of concentric shallow layers called spalling or spheroidal weathering in boulders

Chemical weathering

• the decomposition of rock by chemical reactions
• occurs in water, especially soil water and groundwater are rich in dissolved carbon dioxide produced during the decomposition of plants

 

carbonation

dissolving of calcium carbonate (limestone) in acidic groundwater

• similar to hydroloysis but the all the products are ionic, there is no residue
• bicarbonate (HCO₃^-) is a product of carbonation and a major part of the dissolved load of most rivers
• the carbonation of limestone results in karst topography: caves, sinkholes, etc.

 

chelation

bonding of mineral cations and organic molecules produced by plants

• these chelates are stable at a pH at which the cation would normally precipitate and thus they are leached in seeping soil water
• H⁺ released during chelation from organic molecules is available for hydrolysis
• thus plants, such as the lichens on bare rocks, contribute to the decomposition of soil and rock 

 

hydrolysis

mineral cations (e.g., Ca⁺, Fe⁺, Na⁺, K⁺, Al⁺) are replaced by hydrogen ions (H⁺) from acidic water

• the most common weathering process
• pure water is a poor H+ donor, however biogenic CO² dissolves in water to produce carbonic acid:
• the weathering products are in solution or a residue is clay, that is, the first stage of soil development
• the soil water solution becomes more basic as H+ is consumed

 

oxidation

loss of an electron to dissolved oxygen 

• iron is the most commonly oxidized mineral element Fe⁺² (ferrous iron) ——> Fe⁺³ (ferric iron) or 2FeO + O₂ ——> Fe₂O₃
• other readily oxidized mineral elements include magnesium, sulfur, aluminum and chromium