Planet Earth is in constant transformation through a series of external processes that modify its surface. These processes are driven by what is known as external geological agents, which are natural phenomena that act on the Earth's crust, changing its shape and structure. These agents do not create mountains or depressions as do the planet's internal agents, but they play a crucial role in the transformation of landscapes over time.
In this article, we will delve into what external geological agents are, how they act and what types exist, describing their characteristics and the impact they have on terrestrial ecosystems.
What are external geological agents
External geological agents, unlike internal geological processes that form mountains or volcanoes, are phenomena that wear out y model the surface of the planet. Its main function is modify the relief through processes such as erosion, transport and sedimentation. These processes are continuous and often silent, gradually transforming landscapes.
The most important external geological agents are:
- Weathering: Physical and chemical wear of rocks.
- Erosion: Wear and tear and transportation of materials by elements such as wind and water.
- Sedimentation: Deposit of materials transported by erosion.
All these agents act mainly on the natural ecosystems, where meteorological and natural variables directly affect the terrain. In urbanized areas, these processes can be altered or mitigated by human intervention, although they do not disappear completely.
Types of external geological agents
There are different types of external geological agents, which can be classified into two large groups: Physical y chemicalEach of them plays a key role in changing the landscape. Physical processes are those that change the shape of the land without altering its chemical composition, while chemical processes modify its internal structure through reactions with elements in the environment.
Among the main physical external geological agents are: wind, water and ice. These agents erode, transport and deposit sediments, transforming the relief significantly. On the other hand, chemical agents such as water and the oxygen play an essential role in the chemical weathering, breaking down the minerals in rocks and altering their structure.
Weathering
Weathering is a fundamental process within external geological agents. It refers to the wear and fragmentation of rocks caused by various natural factors. This process is classified into two main types:
Physical weathering
Physical weathering involves the rock fragmentation into smaller pieces without altering their chemical composition. This process is generated by changes in temperature, freezing and thawing of water, the action of the wind and the growth of plants.
Thermoclast It is an example of physical weathering, and occurs when extreme temperature variations between day and night generate stresses in rocks until they fragment. This phenomenon is common in arid climates where temperature differences are more marked. Another mechanism is the gelifraction, whereby water that freezes in rock cracks increases its volume and causes the rocks to break.
In addition, there is biological weathering, which involves the action of living organisms such as lichens, mosses and plant roots, which, as they grow, exert pressure on the rocks, facilitating their fragmentation.
Chemical weathering
Chemical weathering is the process by which rocks undergo changes in their chemical composition due to interaction with elements such as water, oxygen and other gases present in the atmosphere. These agents cause reactions that alter the minerals in rocks, weakening them and facilitating their decomposition.
One of the most common examples of chemical weathering is the oxidation, where oxygen dissolved in water reacts with minerals such as iron, forming oxides that give rocks a reddish or orange colour. This phenomenon is visible in many landscapes where the chemistry of iron has transformed rocks, giving them an eroded appearance.
Furthermore, the carbonation It is a chemical weathering process that mainly affects rocks containing carbonates, such as limestone, which dissolve in water rich in carbon dioxide.
Erosion, transport and sedimentation
Erosion is another of the main external geological agents and one of the most visible in many landscapes. It is a process in which wind, water, ice and other factors drag particles of earth and rock by breaking the earth's surface. This process of wear is continuous and can occur either gradually or rapidly, depending on the intensity of the factors involved.
El Services This is the next step after erosion. As particles are eroded, they are carried to different locations by water, wind or ice, until they are eventually deposited. This movement of particles can vary in distance, from a few centimeters to hundreds of kilometers. For example, rivers and streams constantly transport sediments that erode in mountainous areas and deposit them in low-lying areas.
La sedimentation refers to the deposition of materials that have been eroded and transported. Sediments accumulate mainly at the mouths of rivers, seas and oceans, or on the beds of lakes and lagoons. Over time, the accumulation of these sediments can form new geological formations such as deltas, estuaries and alluvial plains.
Understanding how these external geological agents act is key to studying relief modelling. Although these processes often go unnoticed due to their slowness, they are essential for the transformation of landscapes over millions of years.