Geographical Folds: Elements, Types and Main Characteristics

In the field of geology, the geographic folds they are the deformations present in the rocks produced by the permanent wear that occurs naturally with time. This phenomenon is common and occurs when the rock (usually sedimentary type) does not break under pressure, but adapts and takes another form 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 are commonly found in sedimentary rocks. The recurrence of the folds in the sedimentary rock is due to the"soft"nature of the rock, given that in soft sediments it is where 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. Folds exist that can be microscopic and that are generated in a quite tight disposition, just as there are others whose wave is rather extensive.

Since they are produced by compressions in the structure, the dimension of each fold will depend, among other factors, on the force that this compression has generated. Sometimes they are presented 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 a deformation that can accommodate itself by a reverse geological fault or by a fold.

When a fault occurs in the field, it is also common that a fold is generated from it. The shape it will have depends on the way the rock moves.

When there are large concentrations of recently sedimented rock, a fold is likely to be caused by the low strength of the rock and the high pressures that can be generated around it.

If the sedimented rock has a sandy origin and quickly loses its hydration, a slight seismic movement could shake the sediment and cause a fold in it.

The rock layers tend to slide with each other and often the failure in the flexural rigidity creates enough pressure to change the structure of the rocks. When the rock does not yield under conventional methods, it is forced to move away from the pressure area in a metamorphic process called pressure dissolution.

These causes are usually common in the sedimented rock folds; however, igneous rock is also prone to creases. In general, fiery folds are associated with the high temperatures at which the rocks are exposed.

Characteristics 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 can be located horizontally, vertically, or tilted 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 recurrent basic characteristics in all their forms, regardless of their size.

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

Form of the fold

If a fold occurs upward, in the form of a wave, the highest part of it is called a ridge. If it occurs down, in the form of"U", the lowest part of this is known as valley.

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

Other concepts

The hinge and horizontal plane create an imaginary line calculated mathematically as an angle, and referred to as immersion. In addition, 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 to which it is directed is known as vergence.

The 8 main types of geographic folds

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

1- Anticlinals

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

2- Synchlinales

The strata have less wear near the nucleus of the fold and this usually goes in the direction of the nucleus; therefore, a valley shape is created.

3- Antiformes

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

4- Non-conform

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

5- Dome

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

6- Cuenca

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

7- Monoclinal

Linear fold where the strata fall in the horizontal layers on both sides.

8- Cheurón

Angular fold with straight slopes and small inclinations.

Associations

It is common to find folds connected to one another. When two folds are together, it is an association of folds.

It is possible to find various 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 are connected flush with their axial planes, an association of isoclinor mode is created. If they do not connect in a directly linear manner, they can also connect below or above the axial of the fold.

Those that connect with each other above are called synclinorios, and those that do it below are called anticlinorios.

References

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