Weathering

Example of a meteorized rock. It is observed that its exterior has been oxidized due to chemical meteorization and has also been left in two probably due to physical meteorization.

The weathering or weathering is the decomposition of minerals and rocks that occurs on or near the Earth's surface when these materials come into contact with the atmosphere, hydrosphere and the biosphere. However, there are several other definitions, which has made the term mean different things to different authors. Examples of other definitions are:

Meteorization represents the response of minerals that were in balance to varying depths in the liosphere to conditions of the Earth's surface or near it. In this place the minerals come into contact with the atmosphere, hydroosphere and biosphere originating changes, generally irreversible, that turn them into a more clestic or plastic state so that it increases the volume, decreases the density and size of the particles in addition to forming new minerals that are more stable under the interface conditions.

Chorley et al.

The meteorization is the disintegration and decomposition of the rocks, which originate, in situ, a mass of ruins.

E.J. Monkhouse

It is the process or group of destructive processes by which terrifying or rocky materials change color, texture, composition, firmness or form when contacting atmospheric agents, all this with little or no transport of the loosened or altered material.

Glossary of Geology

There are mainly two types of weathering: chemical weathering and physical weathering. Biological weathering is sometimes included as a third type. Weathering is considered as an exogenous process and is important among other things for the study of landforms and also to understand soils and their nutrients.

100 °C and 1 kbar can be considered as the maximum temperature and pressure under which weathering occurs.

Physical weathering

Canchal possibly formed by gelifraction in almost horizontal limestone strata on Flowerpot Island, Ontario, Canada.

Exfoliation of granite giving rise to rock spheres whose thickness is diminished by thermocholastia. Exfoliation in concentric layers is what has motivated the English name of onion weathering (meteorization in onion).

Physical weathering produces disintegration or rupture in the rock, without affecting its chemical or mineralogical composition. In these processes, the rock fractures, that is, it breaks up into smaller materials and this facilitates the process of erosion and subsequent transport. Rocks do not change their chemical characteristics but their physical ones do. It is caused by environmental conditions (water, heat, salt, etc.). The agents that cause it are:

• Decompression: The reduction of litostatic pressure produces the expansion and cracking in rocks that have formed to a great depth. Because of this dilation they experience the development of subhorizontal diaclase, which in compact and homogenous rocks, such as granite batolites, induce the formation of large horizontal slabs (lanchares).
• Termoclastia is the fissure of the flowering rocks as a result of the temperature difference between the interior and the surface. The day-night thermal difference is the cause: during the day, when warming, the rock is dilated; however, at night, when cooling, it is contracted. After a while it ends up breaking up. This type of meteorization is important in extreme climates with great thermal oscillation between day and night (as in the desert). Thermocholastia gives rise to a typical form of mechanical meteorization in granite rocks that is called ball exfoliation, in English onion weathering (meteorization in onion layers) because the solar radiation penetrates very superficially into the granite, heating just one or more centimeters from the surface, which is the area that is dilated, while cooling the innermost time, it keeps the same.
• Gelifraction: is the breakage of the rocks that bloom because of the pressure that the ice crystals exert on them. Water, when frozen, increases its volume by 9 %. If it is inside the rocks, it exerts great pressure on the inner walls that ends, after repetition, by fragmenting them. This type of meteorization is important in humid climates and with repeated alternations ice-dehydration (+0 °C/-0 °C), such as mountainous.
• Haloclastia: is the breakage of the rocks by the action of the salt. In certain environments there is a great presence of salt. This is in the arid environments, since the rains wash the ground taking with it the salt, which is precipitated on the ground by evaporating the water. The salt is embedded in the pores and fissures of the rocks and, by recristalizing and increasing volume, increases the pressure they exert on the inner walls (similar to the gelifraction) with which the rupture can be caused. The result is very narrow and smaller rocks, which usually results in erosion processes.

Chemical weathering

A yagrumo (Peeled cecropia) grows on the wall of the Monument to the Battle of the Gate, in Venezuela, and shows the action on the dissolution of the cement and of the limestone rock of the monument itself by the action of the acids of its roots.

It produces a chemical transformation of the rock causing the loss of cohesion and alteration of the rock. The most important processes are atmospheric, water vapor, oxygen and carbon dioxide that are involved in:

• Oxidation. It occurs by reacting some minerals with atmospheric oxygen. New minerals are formed with elements in one or more oxidized states (higher positive load)
• Dissolution. It is very important in soluble minerals such as chlorides, nitrates, calcareous rocks and karstic modeling.
• Carbonation. It occurs by combining carbon dioxide with water forming carbonic acid, which is combined with certain minerals such as calcium carbonate that is transformed into bicarbonate: the first is insoluble in water but the second is not, so it is dragged by it. It is a very important and harmful process for soils, especially drip irrigation.
• Hydrating. In this reaction, water is incorporated into the structure of some minerals increasing volume as happens with the hydrated calcium plaster or sulfate. This process is easy to see, for example, by mixing anhydrate with water, which produces an exothermal reaction (discovers heat) by turning into plaster (hydrated calcium sulphate).
• Hydrolysis. It is the breakage in the structure of some minerals by the action of H ions⁺ and OH^- of water, mainly in the meteorization of feldespato, which is transformed into clays and granite that can reach caolinization (transformation in clays, especially in caolin).
• Biochemistry. The action of organic acids from the decomposition of biological materials in the soil or by physical action - chemistry of the living vegetables themselves.

The nests made on the ground by the termites (Isoptera) in the Gran Sabana (Venezuela) generate a considerable alteration of the soil and subsoil minerals. This alteration favors the growth of some plants in most of the abandoned termiteros.

• Laterization. It is a process of widespread and deep chemical meteorization in which silica and bases are extracted, by the leaching (washing) of the mother rock, in which iron and aluminum concretions are produced. They are red residual deposits associated with flat surface reliefs. In fact, the process is not confined to the formation of soil (latosols) but is an authentic morphogenetic process. Formation of a soil (pedogenetic) that occurs in warm climates, with abundant rainfall, both in the jungle and savannah regions, where a large bacterial activity causes the humus to be consumed quickly. The clay minerals dissolve, while the iron and aluminum accumulate in the form of oxides and give rise to the formation of a hard crust, called laterite (from Latin later, brick). They're not fertile soils.

Biological weathering

Biological or organic weathering consists of the breaking of rocks by the activity of animals and plants. Burrowing and the action of tree roots can cause mechanical action, while the effects of the presence of water and various organic acids, as well as increased carbon dioxide, can complement weathering by altering the rock. Thus, the effects of biological weathering combine the processes of disintegration and alteration.

Vegetation plays a decisive role in chemical weathering processes, as they supply ions and dissolving acids to the water. Organic decomposition generates more or less acid humus that causes podsolization phenomena.

The weathering that produces soils

Weathering breaks down existing rocks and provides materials to form new ones. However, weathering also plays an important role in the creation of soils. A soil reflects, to some degree, the rock material from which it was derived, but bedrock is not the only factor determining soil type, since different soils develop on identical rocks in different areas when the climate varies from one region to another. area to another. Therefore, other factors exert important influences on soil development, such as relief, weather and type of vegetation. The composition of a soil varies with depth. The natural or artificial outcrop of a soil reveals a series of zones that differ from each other. Each of these zones constitutes a horizon, which represents, from the surface inwards, the most weathered or decomposed layers and with different mineral accumulations by leaching or washing from the soil, until reaching the mother or fresh rock, from which it is extracted. drifted the ground. These soil horizons have developed from the underlying parent material. When this material is first exposed at the surface, the upper part is subject to intense weathering and decomposition works rapidly. As the decomposition of the material progresses, the water that percolates downward begins to leach some of the minerals and deposits them in lower levels, which, over time, become thicker and reach greater depths.