# Types of Fold

## What is a Fold?

A Fold is nothing but Wave like bends that are formed in crustal rocks due to tangential compressive forces resulting from horizontal movement caused by the endogenetic force originating deep within the earth. Some parts are bent up and some parts are bent down. A fold consists of two sides which are known as limbs of the fold.

A set of folds distributed on a regional scale constitutes a fold belt. Fold belts are typically associated with convergent plate boundaries and directed compressive stress.

## What are Anticline and Syncline?

The upfolded part of rock strata in arch-like form or we can say upfolded rock beds are called Anticlines.

When the slopes of both the limbs or sides of anticline are uniform, the anticline is called symmetrical anticline but when the slopes are unequal, the anticline is known as an asymmetrical anticline.

Down folded structure forming trough like feature or downfolded rock beds is called as Syncline. They are formed due to compressive forces caused by horizontal tangential forces.

## What are the types of folds?

There are many types of folds the nature of which depends on many factors, for example, the nature of rocks, the nature and intensity of compressive forces, duration of the operation of compressive forces, etc.

## Types of Folds:

Symmetrical Folds:  These are simple folds, the limbs (both) of which incline uniformly. These folds are an example of open fold.  These kinds of folds are formed when compressive forces work regularly but with moderate intensity.

Asymmetrical Folds: These are characterized by unequal and irregular limbs. Both the limbs incline at different angles.   One limb is relatively larger and the inclination is moderate and regular while the other limb is relatively shorter with steep inclination. Thus, both the limbs are asymmetrical in terms of inclination and length.

Monoclinal Folds: These are those folds in which one limb inclines moderately with regular slope while the other limb inclines steeply at a right angle and the slope is almost vertical. It may be pointed out that vertical force and movement are held responsible for the formation of monoclonal folds. There is a possibility for the splitting of the limbs of such folds because of intense folding. Splitting of limbs gives birth to the formation of faults. It is opined that these folds are formed due to unequal horizontal compressive forces coming from both sides.

Isoclinal folds: These are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.

Recumbent fold: These are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.

Overturned folds: These are those folds in which one limb of the fold is thrust upon another fold due to intense compressive forces. Limbs are seldom horizontal.

Plunge folds: These are formed when the axis of the fold instead of being parallel to the horizontal plane becomes tilted and forms plunge angle which is the angle between the axis and the horizontal plane.

Fan folds: These folds represent an extensive and broad fold consisting of several minor anticlines and synclines. Such fold resembles a fan. Such a feature is also called anticlinorium or synclinorium.

Open folds: These are those folds in which the angle between the two limbs of the fold is more than 90 degrees but less than 180 degrees (i.e. obtuse angle between the two limbs of a fold). Such open folds are formed due to wave-like folding because of the moderate nature of compressive force.

Closed Folds: These are those folds in which the angle between the two limbs of a fold is an acute angle. These folds are formed due to an intense compressive force.

Drape folds: These are generally open curvatures in a sedimentary layer that conforms passively to the configuration of underlying structures and geological bodies. A fold formed by differential compaction is an example.

## What is an open fold?

See types of the fold above.

## How does a fold form?

Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature as evidenced by their presence in sediments, sedimentary rocks, the full spectrum of metamorphic rocks, and in some igneous rocks. Folds may result from a primary deformation, which means that folding occurred during the formation of the rock, or a consequence of a secondary, i.e. tectonic deformation. Slumps in soft sediments and flow folds in lavas are examples of primary folds.

## What Causes Folding?

Formation of Rocks and Tectonic deformation(Tectonic pressure and stress).

## How are folds created?

When two forces act towards each other from opposite sides, rock layers are bent into folds. The process by which folds are formed due to compression is known as folding. Folding is one of the endogenetic processes; it takes place within the Earth’s crust.

## What are examples of folded mountains?

Example of Folding includes Himalayas mountains in Asia, The Alps in Europe, The Andes in South America, The Rockies in North America, The Urals In Russia. etc.

## Where does folding occurs?

Folding usually occurs in an area where there are tectonic deformations or converging plate boundaries.

## How are folded mountains formed?

Folded mountains are formed due to two converging plate boundaries which result in folded mountains. They are usually formed of sediments which are accumulated at margins along the continents.

## Conclusion:

A fold is a bend in a layered rock caused by compressive stress (buckling) or passive draping of layers over a lower structure or around a resistant object. Folds represent a large-scale flow of material. Folds display a wide range of shapes and result from a wide range of processes that both largely reflect the rock behavior. It is therefore common to observe that geometrical characteristics change within the same fold from layer to layer.

Folds host ore deposits in hinge areas due to flow of material to those localities. Folds form associated with faults and thus can signal earthquake hazards. Folds record periods of rock deformation, therefore generations need to be distinguished and dated.