GEOGRAPHIC FOLDS: CHARACTERISTICS, ELEMENTS, TYPES - GEOGRAPHY - 2025

The geographical folds, in the field of geology , are the deformations present in the rocks produced by the permanent wear that occurs naturally over time. This phenomenon is common and occurs when the rock (usually of the sedimentary type) does not break under pressure, but adapts and takes another shape from this force.

The folds can occur in any area of ​​the earth's crust under different circumstances and in all types of rock, although they commonly occur in sedimentary ones. The recurrence of folds in sedimentary rock is due to the "soft" nature of the rock, since in soft sediments this phenomenon can be most appreciated.

They do not have a certain length: there are folds that extend for kilometers, while others do not measure more than five centimeters. There are folds that can be microscopic and that are generated in a fairly tight arrangement, as well as others whose wave is rather extensive.

Since they are produced by compressions in the structure, the size of each fold will depend, among other factors, on the force that has generated said compression. Sometimes they appear individually, although the most common is that they appear together, forming several undulations.

Why do folds occur?

There are several geological movements that cause the creation of folds. For example, when two contiguous rock layers move, they cause deformation that can accommodate itself by a reverse geologic fault or by a fold.

When a ground fault occurs, it is also common for a kink to be generated from it. The shape it will take will depend on the way the rock moves.

When there are large concentrations of recently settled rock, a crease is likely to develop caused by the low force of the rock and the high pressures that can be generated around it.

If the settled rock is sandy in origin and rapidly loses its hydration, a slight earthquake could shake the sediment and cause it to fold.

Rock layers tend to slide past each other, and often the failure of flexural stiffness creates enough pressure to change the structure of the rocks. When rock does not yield under conventional methods, it is forced out of the pressure area in a metamorphic process called pressure dissolution.

These causes are often common in sedimentary rock folds; however, igneous rock is also prone to kinks. In general, igneous folds are associated with the high temperatures to which rocks are exposed.

Features and elements

The folds are usually categorized by their size, shape, pressure between rocks and by the curve they have with respect to the axial plane.

The axial plane of a fold is the surface that divides the fold as symmetrically as possible, and it can be located horizontally, vertically or inclined at any angle.

However, in traditional folds the axial plane is usually horizontal or slightly inclined. As the axial plane is located in the middle of a fold, it is divided into two halves called flanks.

In addition to the axial plane, the folds present a series of recurring basic characteristics in all their forms, regardless of their size.

The area of ​​the fold where the bend curve is greatest is called the hinge, and the line that joins this area of ​​curvature with the surface is called the fold axis. The direction of a fold is that which the axis of the fold tends to go: either to the north or to the south.

Fold shape

If a fold occurs upwards, in the form of a wave, the highest part of it is called the crest. If it occurs downwards, in a “U” shape, the lowest part of it is known as the valley.

The area of ​​the fold where the pressure that leads to the movement of the rock is most strongly exerted, generally located deep within it, is the core.

Other concepts

The hinge and the horizontal plane create an imaginary line calculated mathematically as an angle, and is referred to as a dip. Furthermore, the flanks divided by the axial plane form a second angle with respect to the same axial plane, and this angle is called dip.

When a fold is not straight and the axial plane has a certain degree of inclination, the direction in which it is directed is known as vergence.

Main types of geographic folds

Based on each individual characteristic that folds present, they can be classified into several different categories. Among the main classifications are:

Anticlines

The fold layers always have more temporary wear near the core. The fold is usually outward from the core, that is, it creates a wave shape.

Synchlines

Strata have less wear near the core of the fold and this tends to go towards the core; therefore, a valley shape is created.

Antiforms

It is not possible to determine how old they are, like the symmetrical ones, but the strata move away from the center of the axial plane.

Symmetrical

The strata fall towards the center of the axial plane. However, it is not possible to determine its age.

Dome

They are not straight and the strata move away from the nucleus in all directions.

Basin

They are not straight, but the strata go toward the center in all directions.

Monocline

Linear fold where the strata fall into the horizontal layers on both flanks.

Chevron

Angular fold with straight slopes and small slopes.

Associations

It is common to find folds connected to each other. When two folds meet together, it is an association of folds.

It is possible to find different types of associations given the large number of folds studied, but there are some that tend to be more common, since the sedimentary movement of the rocks usually causes them contiguously. The name of each association is given depending on the way in which the folds are connected.

If the folds connect flush with their axial planes, an isoclinium-mode association is created. If they do not connect in a directly linear way, they can also do so below or above the axial of the fold.

Those that connect with each other above are called synclinoria, and those that connect below are called anticlinoria.

References

1. Fold (geology), (nd), January 18, 2018. Taken from wikipedia.org
2. Folding, (nd), November 23, 2017. Taken from wikipedia.org
3. Fold, (nd), 2018. Taken from brittanica.com
4. Geological Folds, (nd), December 26, 2015. Taken from geologypage.com
5. Fold Classification, University of Saskatchewan, (nd). Taken from usask.ca
6. Folds, Geological Society of London, (nd). Taken from geolsoc.org.uk
7. What are geological folds? (nd). Taken from eartheclipse.com