THE geological fault is the splitting or rupture of a rock or rocky block along its structure, dividing it into two compartments that move vertically or horizontally, presenting a residual difference that varies from centimeters to kilometers. Its occurrence is more evident in areas of tectonic instability, but it can also manifest itself in other regions.
There are two causes of geological faults: atectonic or external processes and tectonic or internal processes. The first case, more rare, occurs when some event above the surface causes the fissure of the bodies rocks, such as the collapse of a mountainous layer or slope, causing impacts on the rocks that break up. Tectonic processes, on the other hand, are more common to originate faults and occur when the movement forces of the tectonic plates they accumulate to the point where they surpass the rocks' capacity for resistance and cohesion, causing them to gradually break and move. It is a relatively slow process and difficult to visualize directly.
According to the classification based on the behavior and specification of faults, there are three types of geological faults: normal, inverse and transcurrent.
THE normal failure - also called distensible failure – occurs when the offset block is positioned below the fault plane. As we can see in the diagram below, the displaced block “descends” in relation to the original plane, which is caused by the negative tension caused by the internal forces of the relief transformation.
Schematic example of normal failure
THE reverse failure – also called compression failure – manifests itself in the opposite way to the normal fault type, with the displaced block moving above the original plane. It occurs when tectonism exerts positive pressures on the rock block in question.
A reverse fault scheme
THE current failure – also called horizontal fault – happens when there is displacement in the horizontal plane between the two blocks, being more common in areas where two tectonic plates meet, when these also move horizontally. Thus, each block suffers from a different type of force and has different displacements.
Explanatory scheme of a transient failure
One of the main effects of geological faults is the earthquakes, which are more common and intense in meeting zones between two tectonic plates. In the long term, faults also shape the relief and form areas of valleys and relative depressions, in addition to producing cliffs, mountain ranges and other forms of relief.
An example of a geological fault is that of San Andreas (see photo below), which eventually causes earthquakes. There are also cases of geological faults in Brazil, such as the one that caused some transformations in the relief of the Recôncavo Baiano. Therefore, understanding the dynamics of this type of geomorphological process is also a way to better understand the processes of formation and transformation of features on the Earth's surface.
San Andreas Fault, United States, aerial view. Image released by NASA
By Rodolfo Alves Pena
Graduated in Geography
Source: Brazil School - https://brasilescola.uol.com.br/geografia/falhas-geologicas.htm