WEATHERING: TYPES AND PROCESSES - GEOGRAFÍA - 2024

The weathering is the breakdown of rocks by mechanical disintegration and chemical decomposition. Many form at high temperatures and pressures deep in the earth's crust; when exposed to lower temperatures and pressures at the surface and encountering air, water and organisms, they decompose and fracture.

Living things also have an influential role in weathering, since they affect rocks and minerals through various biophysical and biochemical processes, most of which are not known in detail.

Devil's Marbles, a weather-cracked rock, Australia. Source:

There are basically three main types through which weathering takes place; this can be physical, chemical or biological. Each of these variants has specific characteristics that affect rocks in different ways; even in some cases there may be a combination of several phenomena.

Physical weathering or

Mechanical processes reduce the rocks into progressively smaller fragments, which in turn increases the surface area exposed to chemical attack. The main mechanical weathering processes are the following:

- The download.

- The action of frost.

- Thermal stress caused by heating and cooling.

- The expansion.

- Shrinkage due to wetting with subsequent drying.

- The pressures exerted by the growth of salt crystals.

An important factor in mechanical weathering is fatigue or repeated stress generation, which decreases tolerance to damage. The result of fatigue is that the rock will fracture at a lower stress level than a non-fatigued specimen.

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When erosion removes material from the surface, the confining pressure on the underlying rocks decreases. The lower pressure allows the mineral grains to separate further and create voids; rock expands or expands and can fracture.

For example, in granite or other dense rock mines, pressure release from mining cuts can be violent and even cause explosions.

Exfoliation Dome in Yosemite National Park, USA. Source: Diliff, from Wikimedia Commons

Freeze fracture or gelling

The water that occupies the pores within a rock expands by 9% when frozen. This expansion generates internal pressure that can cause physical disintegration or fracture of the rock.

Gelling is an important process in cold environments, where freeze-thaw cycles occur constantly.

Physical weathering of a concrete "cairn". Source: LepoRello., from Wikimedia Commons

Heating-cooling cycles (thermoclasty)

Rocks have low thermal conductivity, which means that they are not good at conducting heat away from their surfaces. When rocks are heated, the outer surface increases in temperature much more than the inner part of the rock. For this reason, the external part suffers greater dilation than the internal one.

Additionally, rocks composed of different crystals show differential heating: crystals with a darker color heat up faster and cool more slowly than lighter crystals.

Fatigue

These thermal stresses can cause rock disintegration and the formation of huge flakes, shells and sheets. Repeated heating and cooling produces an effect called fatigue that promotes thermal weathering, also called thermoclasty.

In general, fatigue can be defined as the effect of various processes that decrease a material's tolerance to damage.

Rock scales

Thermal stress exfoliation or sheeting also includes the generation of rock flakes. Likewise, the intense heat generated by forest fires and nuclear explosions can cause rock to fall apart and eventually break.

For example, in India and Egypt fire was used for many years as an extraction tool in quarries. However, daily fluctuations in temperature, found even in deserts, are well below the extremes reached by local fires.

Wetting and drying

Clay-containing materials - such as mudstone and shale - expand considerably upon wetting, which can induce the formation of micro-faults or microfractures (microcracks), or the enlargement of existing cracks.

In addition to the effect of fatigue, expansion and shrinkage cycles - associated with wetting and drying - lead to rock weathering.

Weathering by growth of salt crystals or haloclasty

In coastal and arid regions, salt crystals can grow in saline solutions that are concentrated by evaporation of the water.

The crystallization of salt in the interstices or pores of the rocks produces stresses that widen them, and this leads to the granular disintegration of the rock. This process is known as saline weathering or haloclasty.

When the salt crystals formed within the rock pores are heated or become saturated with water, they expand and exert pressure against nearby pore walls; this produces heat stress or hydration stress (respectively), both of which contribute to rock weathering.

Chemical weathering

This type of weathering involves a wide variety of chemical reactions, acting together on many different types of rock across the range of climatic conditions.

This great variety can be grouped into six main types of chemical reactions (all involved in the decomposition of rock), namely:

- Dissolution.

- Hydration.

- Oxidation and reduction.

- Carbonation.

- Hydrolysis.

Dissolution

Mineral salts can be dissolved in water. This process involves the dissociation of the molecules into their anions and cations, and the hydration of each ion; that is, the ions surround themselves with water molecules.

Dissolution is generally considered a chemical process, although it does not involve actual chemical transformations. As dissolution occurs as an initial step for other chemical weathering processes, it falls into this category.

Dissolution is easily reversed: when the solution becomes supersaturated, some of the dissolved material precipitates as a solid. A saturated solution does not have the ability to dissolve more solid.

Minerals vary in their solubility and among the most soluble in water are the chlorides of the alkali metals, such as rock salt or halite (NaCl) and potash salt (KCl). These minerals are only found in very arid climates.

Gypsum (CaSO ₄.2H ₂ O) is also quite soluble, while quartz has a very low solubility.

The solubility of many minerals depends on the concentration of free hydrogen ions (H ⁺) in the water. H ⁺ ions are measured as the pH value, which indicates the degree of acidity or alkalinity of an aqueous solution.

Hydration

Hydration weathering is a process that occurs when minerals adsorb water molecules on their surface or absorb it, including them within their crystal lattices. This additional water creates an increase in volume that can cause the rock to fracture.

In humid climates of mid-latitudes, the colors of the soil present notable variations: it can be observed from brownish to yellowish. These colorations are caused by the hydration of the reddish iron oxide hematite, which turns into an oxide-colored goethite (iron oxyhydroxide).

The uptake of water by the clay particles is also a form of hydration that leads to the expansion of the same. Then, as the clay dries, the crust cracks.

Oxidation and reduction

Oxidation occurs when an atom or ion loses electrons, increasing its positive charge or decreasing its negative charge.

One of the existing oxidation reactions involves the combination of oxygen with a substance. Dissolved oxygen in water is a common oxidizing agent in the environment.

Oxidative wear mainly affects iron-containing minerals, although elements such as manganese, sulfur, and titanium can also rust.

The reaction for iron - which occurs when dissolved oxygen in water comes into contact with iron-containing minerals - is as follows:

4Fe ²⁺ + 3O ₂ → 2Fe ₂ O ₃ + 2e ^-

In this expression e ^- represents electrons.

Ferrous iron (Fe ²⁺) found in most rock-forming minerals can be converted to its ferric form (Fe ³⁺) by altering the neutral charge of the crystal lattice. This change sometimes causes it to collapse and makes the mineral more prone to chemical attack.

Carbonation

Carbonation is the formation of carbonates, which are the salts of carbonic acid (H ₂ CO ₃). Carbon dioxide dissolves in natural waters to form carbonic acid:

CO ₂ + H ₂ O → H ₂ CO ₃

Subsequently, carbonic acid dissociates into a hydrated hydrogen ion (H ₃ O ⁺) and a bicarbonate ion, following the following reaction:

H ₂ CO ₃ + H ₂ O → HCO ₃ ^- + H ₃ O ⁺

Carbonic acid attacks minerals forming carbonates. Carbonation dominates the weathering of calcareous rocks (which are limestones and dolomites); in these the main mineral is calcite or calcium carbonate (CaCO ₃).

Calcite reacts with carbonic acid to form acidic calcium carbonate, Ca (HCO ₃) _2, which, unlike calcite, dissolves easily in water. This is why some limestones are so prone to dissolution.

The reversible reactions between carbon dioxide, water, and calcium carbonate are complex. In essence, the process can be summarized as follows:

CaCO ₃ + H ₂ O + CO ₂ ⇔Ca ₂ + + 2HCO ₃ ^-

Hydrolysis

In general, hydrolysis - the chemical breakdown by the action of water - is the main process of chemical weathering. Water can break down, dissolve, or modify susceptible primary minerals in rocks.

In this process, water dissociated into hydrogen cations (H ⁺) and hydroxyl anions (OH ^-) reacts directly with silicate minerals in rocks and soils.

The hydrogen ion is exchanged with a metal cation of the silicate minerals, commonly potassium (K ⁺), sodium (Na ⁺), calcium (Ca ^{2 +),} or magnesium (Mg ^{2 +}). The released cation then combines with the hydroxyl anion.

For example, the reaction for the hydrolysis of the mineral called orthoclase, which has the chemical formula KAlSi ₃ O ₈, is as follows:

2KAlSi ₃ O ₈ + 2H ⁺ + 2OH ^- → 2HAlSi ₃ O ₈ + 2KOH

So orthoclase is converted to aluminosilicic acid, HAlSi ₃ O _8, and potassium hydroxide (KOH).

This type of reaction plays a fundamental role in the formation of some characteristic reliefs; for example, they are involved in the formation of the karst relief.

Biological weathering

Some living organisms attack rocks mechanically, chemically, or by a combination of mechanical and chemical processes.

Plants

Plant roots - especially those of trees that grow on flat rocky beds - can exert a biomechanical effect.

This biomechanical effect occurs as the root grows, as the pressure exerted by it on its surrounding environment increases. This can lead to fracture of the root bed rocks.

Biological meteorization. Tetrameles nudiflora growing on a temple ruin in Angkor, Cambodia. Source: Diego Delso, delso.photo, CC-BY-SA License via

Lichens

Lichens are organisms made up of two symbionts: a fungus (mycobiont) and an alga that is generally cyanobacteria (phycobiont). These organisms have been reported as colonizers that increase rock weathering.

For example, it has been found that Stereocaulon vesuvianum is installed on lava flows, managing to enhance its weathering rate up to 16 times when compared to non-colonized surfaces. These rates can double in humid locations, such as in Hawaii.

It has also been noted that as lichens die, they leave a dark stain on rock surfaces. These spots absorb more radiation than the surrounding light areas of the rock, thus promoting thermal weathering or thermoclasty.

Mytilus edulis a rock-boring mussel. Source: Andreas Trepte, from Wikimedia Commons

Marine organisms

Certain marine organisms scrape the surface of rocks and bore holes in them, promoting the growth of algae. These piercing organisms include mollusks and sponges.

Examples of this type of organisms are the blue mussel (Mytilus edulis) and the herbivorous gastropod Cittarium pica.

The lichen Stereocaulon vesuvianum is a colonizer that is installed in lava flows, Canary Islands Fuerteventura and Lanzarote of Spain. Source: Lairich Rig via

Chelation

Chelation is another weathering mechanism that involves the removal of metal ions and, in particular, aluminum, iron, and manganese ions from rocks.

This is achieved by binding and sequestration by organic acids (such as fulvic acid and humic acid), to form soluble organic matter-metal complexes.

In this case, the chelating agents come from the decomposition products of plants and secretions from the roots. Chelation encourages chemical weathering and metal transfer in soil or rock.

References

1. Pedro, G. (1979). Characterization générale des processus de l'altération hydrolitique. Science du Sol 2, 93–105.
2. Selby, MJ (1993). Hillslope Materials and Processes, 2nd edn. With a contribution by APW Hodder. Oxford: Oxford University Press.
3. Stretch, R. & Viles, H. (2002). The nature and rate of weathering by lichens on lava flows on Lanzarote. Geomorphology, 47 (1), 87–94. doi: 10.1016 / s0169-555x (02) 00143-5.
4. Thomas, MF (1994). Geomorphology in the Tropics: A Study of Weathering and Denudation in Low Latitudes. Chichester: John Wiley & Sons.
5. White, WD, Jefferson, GL, and Hama, JF (1966) Quartzite karst in southeastern Venezuela. International Journal of Speleology 2, 309–14.
6. Yatsu, E. (1988). The Nature of Weathering: An Introduction. Tokyo: Sozosha.