External Geological Cycle: Weathering, Erosion, Transport, Sedimentation

External Geological Cycle

The external geological cycle describes how land areas, primarily the land surface of the continents, are subject to the continuous action of the atmosphere, hydrosphere, and biosphere. This leads to the destruction or denudation of topographic relief in the so-called cycle of denudation of the continents. This cycle comprises phases of erosion, transport, and sedimentation of the products resulting from the fragmentation and breakdown of surface rocks.

Weathering

Weathering is a set of processes due to the action of the atmosphere, hydrosphere, and living things, which cause the decomposition of rocks and the minerals that constitute them. Depending on the type of mechanisms that act on the rock, there are two types of weathering: mechanical (or physical) and chemical.

Mechanical or Physical Weathering

Mechanical or physical weathering is the process of fragmentation of rocks into ever smaller pieces, which retain the original characteristics. There are several mechanisms of physical weathering, among which we highlight:

• Gelifraction: The fragmentation of rocks by the action of ice. Water increases in volume when frozen, increasing the size of rock cracks. The repetition of the freeze-thaw process eventually breaks the rock into angular blocks. This phenomenon is especially notable in the mountains, where freeze-thaw cycles occur daily. The detached blocks form accumulations called scree or talus.
• Differential Expansion: This results from the different capacities of expansion of the mineral grains that make up rocks. As a result, the cohesion between the grains decreases, and the rock eventually fractures if the heating and cooling cycles are repeated. It is very important in places where the daily temperature oscillation is about 30 to 40 °C, such as hot deserts.
• Bioclastic Weathering: The fragmentation of rock by mechanical actions exerted by living things, such as the pressure exerted by roots as they grow, expanding cracks, or by excavating animals.

Chemical Weathering

Chemical Weathering is the set of chemical processes that break down minerals in rocks and transform them into other substances. We can highlight the following mechanisms of chemical weathering:

• Dissolution: This is due mainly to the action of water and is particularly important in rock salt (halite) and gypsum. The dissolving action of water is greatly enhanced by the presence of H⁺. The most notable example is the dissolution of limestone. Acidified water, where CO₂ reacts with it, decomposes limestone easily into water-soluble ions.
• Hydrolysis: A chemical reaction between minerals and water to form a new mineral. An example is the reaction of potassium feldspar, a mineral of granite, to give a clay mineral (kaolinite), potassium ions, and silicic acid, soluble in water.

Erosion

Erosion is the process that tends to level the Earth's surface relief. It is often simultaneous with the weathering process of rupture and disintegration of the rocks on the surface, causing debris. Erosive agents are fluids in motion that dislodge rock fragments or dissolve them and carry them away from their initial locations.

Transport

The transport of sediment is the result of the dynamic interaction between detrital particles and fluid flow in a given medium. The forms of transport are varied. Some of them are:

• Dissolution: The dissolved particles travel in the medium.
• Suspension: When the particles are able to remain in a stream, they are transported over long distances and for long periods.
• Saltation: If the particles trapped in the current rise at a distance of one to a thousand times their diameter.
• Bottom Trawling: When the particles are difficult to lift, so they move off the ground only a value less than their diameter or not at all.

The physical means of transport are rivers, lakes and seas, winds, glaciers, and mudflows.

Sedimentation

Sedimentation is the process of accumulation of sedimentary material in a particular morphology, caused by a loss of fluid velocity, which can no longer carry the fragments equal to or greater than a certain weight and size. These fall to the bottom under gravity to form a deposit or sediment. The loss of speed usually occurs in sedimentation basins, where rivers reach the sea or a lake, and there are backwaters. The debris settles. Except in glaciers, transport and deposition are selective: fine fragments are transported further than large ones and generally settle in different places. This will produce the size classification of the sediments.