Rock Weathering

Mechanical Weathering

Mechanical weathering involves physical forces (compressive, tensile, and shear) that cause rocks and minerals to fracture (i.e., exceed the limit of plastic deformation). A number of physical forces are present in terrestrial environments.

Cryostatic pressure involves the compressive stress due to the expansion of water ice. Sometimes referred to in many writings as “frost” weathering, the process is not limited to the presence of frost crystals. Water ice is unique in its tendency to expand on the change of state from liquid to solid. Typically, liquid water works into the voids of existing rock weaknesses such as cracks, sutures, bedding, voids, or mineral or grain boundaries. In the confined spaces of the rock weakness, ice crystal growth exerts considerable pressure, forcing the rock or mineral apart. As the rock weakness void grows, confined pressures decrease, and the ability to continue cryostatic weathering decreases. Intense or prolonged stress will eventually completely separate the fragments. The products of cryostatic weathering are angular rock fragments, frequently observed in alpine, subarctic, and arctic regions.

Cryostatic pressures can be emphasized in environments with diurnal freeze-thaw cycles, in which ice crystal growth, melting, and regrowth allow repeated physical force. Freeze-thaw cycles are expected in temperate or midlatitude climates, though in polar areas rock surfaces exposed to sunlight can reach above-freezing temperatures even in winter. Cryostatic weathering is suspected on Mars in locations where water is or has been present.

Like ice, expansion and growth of other crystals in confined spaces also exerts compressive pressure to cause or enlarge fractures. Iron minerals, calcite, and expansive clays are examples of crystal growth weathering. However, salt (any of the alkaline minerals, including NaCl) is the chief and most common example cited and observed. Salt can enter the [Page 2478]rock or mineral by way of solution in water. As water evaporates, the salt is left behind and will expand as it crystallizes. Similar to ice, salt weathering will produce angular shards of by-product rock. Salt weathering is prevalent in arid and semiarid climates, where rapid evaporation is more apt to leave salt precipitates within the surface rocks rather than flushed by solution into deeper soil or ground-water. Salt weathering may be prevalent in polar deserts, where lack of liquid precipitation allows salts to accumulate at or near the surface. Thus, what is often assumed to be ice weathering may in fact be attributed to salt. Salt weathering is also seen in coastal areas, where sprays from waves can move inland up to several kilometers by way of atmospheric transport. Stone conservationists have noted salt weathering on building stones. Salt sources in urban areas include atmospheric aerosols, streets and sidewalks, and mortars.

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