Physical Weathering

Physical weathering - also called mechanical weathering - is the process by which rocks are broken into smaller fragments through a variety of causes, but without significant change in their chemical or mineralogical composition. ^[1] The distinction from chemical weathering is important: physical processes alter the size and shape of rock material, not its chemical identity. A grain of quartz produced by freeze-thaw fragmentation of a granite has exactly the same composition as the quartz that was bonded in the original rock.

Except in extremely cold or very dry climates, physical and chemical weathering act together, and it is difficult to separate their effects. ^[1] In practice, physical processes enhance chemical ones by creating more surface area for chemical attack. A boulder broken into gravel exposes far more mineral surface to acidic water than the intact boulder did, accelerating the chemical decomposition of each fragment.

Freeze-Thaw Weathering

Freeze-thaw weathering is an important physical weathering process wherever recurring short-term changes from freezing to thawing temperatures take place. ^[1] Water expands by about 9 percent when it freezes, generating enough pressure in confined rock fractures to crack most rock types. ^[1] For this process to be effective, water must be trapped within the rock body - sealed by freezing - and repeated freeze-thaw cycling is required, because each cycle opens fractures a little further. ^[1] Freeze-thaw weathering typically produces large, angular blocks of rock, though it may also cause granular disintegration in coarse-grained rocks such as granites. ^[1] Mechanically weak rocks such as shales and sandstones tend to disintegrate more readily under this process than hard, strongly cemented rocks such as quartzites and igneous rocks. ^[1]

Insolation Weathering

Insolation weathering is driven by the repeated expansion of rock surfaces heated by sunlight followed by contraction as temperature falls. ^[1] A thermal gradient develops between the surface and interior of a heated rock; the surface expands more than the interior, creating internal stresses that can lead to small cracks and granular disintegration. ^[1] Once a crack opens at a rock’s surface, silt or sand particles can sift into the crack and prevent it from closing when the rock cools. Repeated heating and cooling then causes the crack to widen progressively, causing small-scale disruption of the rock surface. ^[1] These physical changes are caused mainly by solar heating but may also result from fires. ^[1]

Salt Weathering

In desert environments, high temperatures promote weathering caused by the crystallization of salts in pore spaces and fractures. ^[1] Evaporation of water concentrates dissolved salts in saline solutions that have penetrated rock fractures and pores. Growth of salt crystals generates internal crystallization pressures that can force cracks apart or cause granular disintegration in weakly cemented rocks. ^[1] An additional mechanism operates when salts in fractures become hydrated - absorbing water and expanding - generating expansion pressures that further widen cracks. ^[1] Salt weathering is most common in semiarid regions but also occurs along seacoasts where salt spray is blown onto sea cliffs. ^[1]

Wetting and Drying

Alternate wetting and drying of soft or poorly cemented rocks such as shales causes fairly rapid breakdown, with most disintegration occurring during the drying cycle. ^[1] The exact causes are not entirely understood, but drying may produce negative pore pressures and consequent tensile stresses that pull the rock apart, while absorption of water during wetting creates swelling pressures that push cracks open. ^[1] Disintegration by wetting and drying is particularly effective on well-exposed, steep cliff faces where loosened fragments fall off and expose fresh surfaces to the next cycle. ^[1]

Stress-Release Weathering and Sheeting

A rock buried beneath a land surface experiences high compressional stresses from the weight of overlying rock. When erosion removes some of that overlying material, the compressional stresses are reduced and the rock rebounds upward. That upward expansion creates tensile stresses - the rock is effectively being pulled apart - causing fractures to develop that are oriented nearly parallel to the topographic surface. ^[1] These parallel fractures divide the rock into a series of layers or sheets, which is why the process is called sheeting. The layers increase in thickness with depth and may extend several tens of meters below Earth’s surface. ^[1] Sheeting is most conspicuous in homogeneous rocks such as granite but may also occur in massive sandstone. ^[1]

Exfoliation and Spheroidal Weathering

Exfoliation is the peeling off of large, curved sheets or slabs of rock from the weathered surfaces of an outcrop. It is an example of two or more physical processes working together: stress release may create initial fractures, which then allow water to enter and widen the fractures further through freeze-thaw or chemical processes. ^[1]

Spheroidal weathering is a smaller-scale version of the same kind of process. It affects roughly cubic rock masses cut by intersecting joints, and causes successive thin layers - or skins - to spall off, producing rounded spheroidal cores. ^[1] The fractures that separate the weathering rinds may form in response to stress release or thermal changes; entry of water into those fractures then promotes additional physical stresses from freeze-thaw or chemical processes. ^[1] The rounded boulders produced by spheroidal weathering can look superficially like stream-transported pebbles but can be distinguished by context - they sit within a weathering profile rather than within a sedimentary deposit.

Other Contributing Processes

Several other factors can contribute to physical weathering under the right conditions. These include volume increases caused by absorption of water by clay minerals or other minerals; volume changes caused by alteration of minerals such as biotite and plagioclase to clay minerals; growth of plant roots in rock cracks; plucking of mineral grains from rock surfaces by lichens as they expand and contract; and burrowing and ingestion of soil materials by worms or other organisms. ^[1] These processes are individually less important than freeze-thaw or salt weathering in most settings, but in combination and over geological timescales they contribute meaningfully to rock disintegration.

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