Relief Modeling Agents (continued)


Weathering, also known as weathering, is the set of mechanical, chemical and biological processes that cause the disintegration and decomposition of rocks.

In the case of mechanical (or physical) disintegration, rocks may break without changing their composition.

In the deserts, temperature variations end up breaking the rocks, as well as in cold areas, where water seeps into the cracks in the rocks.

We cannot confuse weathering with erosion as it implies material transport.

Weathering differs from metamorphism (transformations experienced by rocks when subjected to heat / temperature, pressure, fluids, and time) because they affect ambient pressure and temperature, while metamorphic transformations occur at higher pressure and temperature.

The products of weathering are very variable. In general, rocks and minerals are weathered from the surface downwards. Thus, in the same place we can have materials at very different levels of change, which gives the set a different aspect.

On the surface we have a material in advanced state of disintegration and decomposition, unlike the deepest material, where a mixture of unchanged material with altered material can be found. The changed material set, regardless of its state, is called regolith or decomposition cloak.

The superficial material, in advanced state of alteration and leaching, associated with organic matter, we call the soil. Since weathering depends on climate and relief, soil and regolith are always the product of climate-rock interaction. The same rock in different climates will produce different soils.

For study purposes, we can divide weathering into two types: physicist and chemical.

In practice this division is problematic, as the two processes occur together, although often overlapping each other, depending on the factors climate and relief.

Physical weathering

  • This process causes the rock to break down into smaller and smaller fragments, but with the same chemical structures. Therefore, there is no rock decomposition.
  • Temperature - minerals undergo dilation.
  • Salt crystallization - salt brought by the salt spray.
  • Biological activities - tree roots that penetrate the soil.

Types of Physical Weathering

Thermal weathering: It occurs due to varying temperatures in the rocks and is very common in places with dry climates, whether hot or cold places. When the temperature is too hot, the rocks tend to expand and when it gets cold they contract. This weakens the rock structures and helps to break them apart. We also have to consider that the minerals that make up the rocks have different expansion coefficients, which facilitates the fragmentation of the rocks.

Mechanical weathering: occurs due to several factors, including the dissolution of water in glaciers and also its crystallization in fractures that cause the rock blocks to crumble by increasing the volume of water. The salts then precipitate and cause the volume of rock cracks as well as minerals to increase.

Physical Weathering (mechanical)

Chemical Weathering (water / humidity)

  • The decomposition of the rock occurs and its constitution changes.

Chemical weathering depends on the region in which it is occurring. That is, factors such as climate, vegetation, rainfall, among others, are directly linked to how much weathering will occur. The intertropical regions are more favorable for the occurrence of this phenomenon.

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Types of Chemical Weathering

Within the large group of chemical weathering, we can make subdivisions according to the agents of this process:

Oxidation Weathering: It is the change of the oxidation state of an element through reaction with oxygen. This type of reaction destroys the crystalline structure of the mineral.

Hydration Weathering: In this process, by incorporating water into the structure, a new mineral is formed.

Dissolving Weathering: It is the complete solubilization of minerals by acids.

Hydrolysis Weathering: Because the rocks are largely composed of silicates, when they come into contact with water, they undergo hydrolysis and result in an alkaline solution.

Acidolysis weathering: is the decomposition reaction of minerals in cold environments. In these cases, the decomposition of organic matter is incomplete, and therefore forms organic acids that significantly decrease the pH of the water and end up complexing and solubilizing the water. Faith it's the Al.

Biological weathering: happens by the action of bacteria that produce the biotic decomposition of organic materials. The great importance of this type of weathering is that it produces one of the world's two most fertile soils. It is a fairly common process in Russia and Ukraine.

Chemical weathering

Factors that influence weathering

Some factors make the conditions for weathering more favorable. Are they:

  • Climate: can be classified as the main agent of weathering, because it determines the amount of rainfall that will hit the rocks, as well as the temperature of the place where they are. Rain and temperature chemically modify the rocks, and winds, also due to the weather, cause physical changes.
  • Relief: plays a very important role as an agent of weathering, as its inclination will decide how intense the contact of rainwater with the rocks. In the case of steeper terrain, water infiltration into the soil will be low. In flatter terrain, it will be bigger. This is important because the longer in contact with water, the more chemical reactions will occur in the rocks.

Difference between weathering and erosion

  • Weathering: causes changes in the rocks.
  • Erosion: Wears the surface and makes material transport and deposition.