Physical or mechanical weathering is the breakdown of rocks into smaller pieces, fragments, or grains without much chemical or mineral composition change.
It differs from chemical weathering since the disaggregation or breakdown doesn’t alter the composition or characteristics of rocks.
Also, it doesn’t form new material. What happens is that only the rock shape changes and sizes progressively become smaller.
Like other weathering forms, physical weathering happens in situ, i.e., involves little or no movement of the broken rocks. However, erosion agents like water, wind, or ice can transport rocks that disintegrate.
While some authors consider abrasion by wind, ice, or water a form of physical erosion, it is not.
Why? Because erosion involves mainly the removal of weathered materials, irrespective of how their breakdown happens.
How does mechanical weathering happen?
Mechanical weathering happens when induced shear, expansion, contraction forces, or stress within a rock exceed the rock’s tensile strength. Any additional stress will propagate these cracks at their tip, causing rocks to split or disintegrate.
These stresses can be due to changes in the volume of the rock itself or by materials deposited in voids and/or volume change of these materials in voids.
Also, repeated stress can cause fatigue, which will eventually cause rocks to break down.
Usually, in mechanical weathering, rocks and minerals will disintegrate along zones of weakness. These include existing fractures (cracks or microcracks), cleavage or bedding planes, or metamorphic foliation.
Also, it can happen between intergranular boundaries of crystalline metamorphic and igneous rocks. Intergranular boundaries are zones of weakness between individual locking crystals or grains.
Some mechanical weathering products include boulders, gravel, sand, or mud with granular disintegration, forming mineral crystals or fragments.
Lastly, the most pronounced physical weathering occurs in places with strong directional contrasting pressure.
Example of physical or mechanical weathering
Mechanical or physical weathering types or processes are the various mechanisms by which rocks break down physically.
These methods include:
1. Unloading or sheeting weathering
Sheeting or unloading weathering happens when initially deeply buried rocks expand and break into concentric or nearly onion-like layers parallel to the rock body when exposed to the surface.
While deeply buried, plutonic rocks like granites, diorites, or gabbro, some metamorphic rocks or thick sandstones are under immense compressive pressure from overlaying materials or overburden. Also, those under thick glaciers are under some confining compressive pressure.
Uplift, erosion, or glacier melting/flow can remove the overburden, i.e., unloading happens. This will release compressive pressure, causing these rocks to expand. The rebound or expansion force will fracture rocks and enlarge existing cracks.
Usually, the portion closer to the surface will expand more than one beneath. Thus, slabs or layers nearly parallel to the surface will peel off or separate from the rest of the rock body. This is essentially exfoliation.
Also, granular disintegration can happen as the gap between grains increases due to unloading.
Unloading or exfoliation weathering causes exfoliation domes like Liberty Cap and Half Dome in Yosemite National Park in California or Moxham Mountain in New York, USA.
Also, this physical weathering process can cause catastrophic bursting of rocks in deep mining tunnels. Rocks will similarly crack in quarries when large pieces of overlaying rocks are removed.
Lastly, unloading weathering is also known as offloading or jointing. It can occur to any exposed rocks that were previously buried or spent some time under pressure from an overburden. It doesn’t matter where.
2. Ice wedging or frost weathering
Ice wedging, freeze-thaw, or frost weathering results from the unique property of water to expand by about 9% when it crystallizes.
This mechanical weathering happens when it seeps into fractures and pores; upon crystallizing, it expands or increases in volume. The increase in volume will cause stress on the walls of these voids that will widen cracks and pores or cause rocks to disintegrate or split.
Later, the ice will thaw when temperatures rise, and more water will go into the now larger crack or pore. Again, freezing will occur, causing the voids to enlarge further and rocks to break down.
Repeated freeze-thaw cycle will progressively increase fracture and pore sizes and cause more rock to disintegrate.
Besides disintegrating rocks, ice wedging damages roads and concrete slabs. Also, it causes frost heaving that misshapen lawns visible after the spring thaw.
Where is it common? Frost weathering occurs in high mountains in the tropics with snow on their peaks. Also, it is common in temperate or colder regions, especially those near glaciers.
Moisture is abundant in these places, and freeze-thaw cycles happen often. For instance, during the day, snow will melt on high mountain peaks, and at night, it will freeze.
Factors influencing the extent of rock weathering from ice wedging include temperature and freezing speed. Also, the moisture content in rocks and the frequency of the freeze-thaw cycle has an impact.
Lastly, frost wedging is also known as congelifraction or gelifraction. Hydrofracturing and ice lens formation or frost heaving are kinds.
3. Wedging, salt crystal growth, or weathering
Salt weathering, salt crystal growth, or haloclasty happens when rocks break down from stresses exerted by salt crystals growing in voids or cracks. Also, the expansion or hydration of these salt crystals in voids causes rock disintegration.
It starts with salty water or saline solution percolating into rock voids and crystallizing or precipitating salt crystals. As crystals grow, they will apply stress to void walls. This causes most rocks to break.
Also, the hydration of the salt crystals will cause an increase in their volume. Again, this will exert stresses within rocks, causing cracks and pores to enlarge and some rocks to disintegrate.
Salt expansion is the last way that rock weathering occurs, but only to a small extent. Some salts may expand more than rocks. This will cause internal pressure that can cause rock disintegration.
Where does it occur? Salt weathering is common in arid, semi-arid, coastal shorelines and urban environments. These places have salts readily available.
Besides causing rocks to weather, salt wedging will also damage roads where salt is used as a deicing agent. Also, it can damage monuments and building stones in urban and coastal areas.
Lastly, rock properties, salt type, and availability will affect this mechanical weathering mechanism. Also, climatic conditions like humidity, temperature, moisture, and precipitation have an impact.
4. Thermal expansion weathering or insolation
Insolation or thermal expansion weathering (thermoclasty or thermal weathering) is a mechanical weathering process that occurs due to the heating or cooling of rocks. Heating and cooling are mostly caused by solar energy and drop in temperature. But it can be from nuclear explosions or wildfires.
In thermal expansion weathering, rocks spall, crack, or disintegrate when heated or cooled because:
- Rocks are low conductors of heat. Therefore, a few millimeters to inches of the rock near the surface will expand or contract more than the one beneath.
- Minerals in crystalline rocks transfer heat, expand, or contract at different rates.
This differential expansion in a layer on the surface and beath or minerals in crystalline rocks causes stress that breaks rocks.
Where is it common? Thermal expansion occurs anywhere with cyclic heating and cooling. However, arid and semi-arid areas with huge temperature fluctuations are more susceptible.
Also, it can happen in Arctic areas where the sky is clear. While ambient temperatures are low, cooling occurs rapidly.
Lastly, factors affecting insolation weathering include temperature, fluctuation, and rock properties.
5. Wetting and drying
In slaking or wetting and drying, rocks disintegrate due to expansion and contraction stresses associated with alternating wetting and drying.
Wetting adds water to rocks, making some minerals expand or increase in volume, causing expansion stress. In contrast, drying causes them to shrink. Shrinking causes contraction stress.
Repeated expansion, contraction, and stress can widen fractures (microcracks or cracks), cause granular disintegration, and cause rock breakdown.
Wetting and drying weathering are common in coastal shorelines where tidal cycles and waves splash water onto rocks. Also, it is common in places with frequent heavy rainfall and sunshine.
Also, dew, fog, and rainfall in deserts, followed by drying during the hot day, can cause wetting and drying weathering.
Factors influencing slaking include prevailing conditions like heat, wind, and humidity. They affect how quickly rocks dry after a wetting episode.
Also, wetting and drying frequency, rock composition, and properties influence wetting and drying weathering.
6. Biological activities
Biological organisms, including plants, lichens, algae, bacterial activities, burrowing animals, and humans, are agents of physical weathering, i.e., they can cause biophysical weathering.
For instance, human activities like mining, quarrying, laying the foundations for structures, or creating roadbeds can involve blasting, shattering, or removing rocks’ large rock volumes. Also, uprooting trees growing on rocks can expose the rocks to weathering.
Secondly, burrowing animals can loosen and push sediments closer to the surface.
Thirdly, root wedging can also widen cracks as plant roots grow into fractures, applying a force that will push, crack, and split the rocks. You can notice this on the sidewalk pavement near trees.
Furthermore, lichen hyphae can exert pressure that exceeds tensile stress as they grow into microcracks grain boundaries.
Lastly, thalli, hyphae polymer sheet in algae in microcrack or grain boundaries will expand when they take water, exerting pressure that will enable them to split already weakened rocks.
Mechanical weathering significance
Physical weathering increases the surface area of rocks. This aids chemical weathering. It is like dissolving an ice block vs. a ground one.
Also, it creates landscapes like cliffs, steep-walled canyons, escarpments, and talus on the slope of the lower slopes of a mountain.
Lastly, it plays a role in the rock cycle by producing debris, forming sediments, and forming sedimentary rocks.
References
- Elorza, M. G. (2013). Geomorphology. Taylor & Francis.
- Dixon, J. C. (2004). Weathering. In Goudie, A(ed.) Encyclopedia of geomorphology (vol. 1, pp 1108-1109). Routledge
- Huggett, R. J. (2011). Fundamentals of geomorphology (3rd ed.). Routledge.
- Turkington, A. (2004). Mechanical weathering. In Goudie, A. (ed.) Encyclopedia of geomorphology (vol. 1, pp-657-659). Routledge.
- Rothery, D. A. (2017). Geology: A complete introduction (Rev. ed). Teach Yourself.
- Bierman, P. R., & Montgomery, D. R. (2014). Key concepts in geomorphology. W.H. Freeman and Company Publishers
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.