You must have seen mountains and different kinds of patterns in them. Sometimes wave-like pattern or sometimes V-shaped or zig-zag patterns as well. These patterns are called folds which are formed because of bending due to some stress. There can be 2 fold, 3 fold, four-fold, or multifold in the mountains and these folds form different patterns. In this article, we will be talking about folds, it's meaning or factors responsible for folds, types of folds, etc.
What is Fold?
It is a structure of wave-like pattern which is formed by bending of the rocks instead of breaking under any compression. These waves can be seen in stratified rocks which were formed because of sedimentary deposits on the flat horizontal sheets. These horizontal sheets are no longer flat at some places and have been warped with time. Sometimes warping can be seen in the rocks as the appearance of the structure changes and sometimes the warping is so much that two layers become parallel to each other. The rocks fold in and fold down and form a pattern.
The size of these folds can be varied. Somewhere they can be hundreds to thousands of kilometres and somewhere they can be only of few centimetres or even less. Sometimes there can be seven fold or 10 fold mountains or even more. The size and number of folds can be varied.
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The classification of various folds depends upon the appearance and attitude of their axes. The axial plane can be horizontal, vertical, or inclined and it divides the fold symmetrically as possible. An axis is the point of intersection of the axial plane with one of the starta. The Important two-fold which we usually see in the mountains are anticline and syncline. One fold is convex-shaped upwards and another has downwards.
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Factors Responsible for Folds
Following are the factors which can lead to the classification of folds:
Folding Tightness
It depends upon the tightness of the folding of the layers as shown in the figure below. It can be open, tight, or isoclinal.
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Orientation of the Axial Plane
Relative to the horizontal plane along with the orientation of gold limbs, the orientation of the axial plane also leads to different kinds of folds which can be seen in the diagram below. It can be upright, overturned, or recumbent. You clearly can see the difference in folding between all of them in the diagram.
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Thickness
It depends on the thickness of the folded beds as well. The thick beds form concentric folds whereas thin beds form a foliation parallel to the axial plane.
Types of Folds
Anticline:
It has an arc-like shape and its core has the oldest beds. The layers of the rocks become older towards the centre and they are more convex upwards.
Syncline:
It is the opposite of anticline. It is a fold that is convex downwards which is also termed as synformal syncline and has younger rocks at the centre. Some synclines can point upwards due to overturned or formation of an anti-formal syncline.
Symmetrical:
If the axial plane is vertical, then it is a symmetrical fold.
Asymmetrical:
if the axial plane is not vertical but inclined, then it is an asymmetrical fold.
Isoclinal:
these are those kinds of folds in which limbs are parallel to each other and approximately to the axial plane as well.
Overturned:
When the folds are highly inclined they are called overturned or overfold.
Monoclines:
These are kind of folds in which all the layers incline in one and same direction.
Chevron:
In chevron, folds look like zig-zag and it has repeated patterns with straight limbs and sharp hinges. They formed V-shaped beds which are formed of regional compressive stress.
Slump:
It is a fold that is formed because of a landslide where sediments were softer. It forms during sedimentation or lithification.
Ptygmatic:
It is a type of slump fold where the material which is folding is more vicious than the surrounding material. These are random and disconnected.
Disharmonic:
These folds are those in which different layers are having different kinds of folds. There can be a combination of various kinds of folds.
Fold Mountains
On the nature of folds, mountains can be divided into two parts which are mentioned below:
Simple Fold Mountains:
These are said to be the mountains with open folds where patterns of anticlines and synclines can be found and have wave-like patterns.
Complex Fold Mountains:
These are those mountains in which rock strata are compressed so much that it forms a complex pattern.
Features of Fold Mountains
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These are said to be the most widespread as well as important mountains in the world.
They are young mountains.
They can extend up to a great length but they have a small width.
These are great sources of minerals as well such as tin, copper, gold, etc.
They are mostly found in the margins of a continent and ocean.
Fossils here suggest that sedimentary rocks were formed because of the accumulation and consolidation of sediments and silts in the marine environment.
Usually, they have both concave and convex types of slopes.
FAQs on Fold
1. What is a fold in the context of geography?
In geography and geology, a fold is a wave-like or bend-like structure that forms when originally flat rock layers are subjected to immense compressional forces. Instead of fracturing, the rocks bend in a process known as ductile deformation. These structures are most visible in sedimentary or metamorphic rocks and are fundamental to the formation of fold mountains.
2. What are the two most common types of folds?
The two primary types of folds are the anticline and the syncline. An anticline is an arch-like fold where the rock layers are bent upwards, with the oldest rocks at its core. A syncline is the opposite, forming a trough-like structure where the layers are bent downwards, with the youngest rocks found at its core.
3. What are the key factors that cause rock layers to fold?
The formation of folds is primarily influenced by several factors:
Compressional Stress: This is the main driver, where tectonic plates push against each other, squeezing the rock layers.
Temperature and Pressure: High temperature and confining pressure make rocks more ductile (plastic-like), allowing them to bend rather than break.
Rock Type: Some rocks, like shale or salt, are more pliable and prone to folding than brittle rocks like sandstone or granite.
Time: The compressional forces must be applied slowly over millions of years for large-scale folding to occur.
4. What is the difference between a symmetrical and an asymmetrical fold?
The key difference lies in the pressure applied. A symmetrical fold has limbs (the sides of the fold) that dip at roughly the same angle, which indicates that the compressional forces were relatively even from both sides. In contrast, an asymmetrical fold has one limb that is steeper than the other, suggesting that the pressure from one direction was significantly stronger than from the other.
5. Can you give some real-world examples of fold mountains?
Certainly. Fold mountains are some of the most prominent mountain ranges on Earth. Prime examples include the Himalayas in Asia, which formed from the collision of the Indian and Eurasian plates; the Alps in Europe, formed by the African and Eurasian plates colliding; and the Andes in South America, created by the subduction of the Nazca plate beneath the South American plate.
6. Why do some rock layers fold while others break to form a fault?
Whether a rock folds or faults depends on its response to stress. This is governed by temperature, pressure, and the rate of strain. Rocks deep within the Earth's crust are under high temperature and pressure, which makes them behave in a ductile (pliable) manner, leading to folding. In contrast, rocks closer to the surface are cooler and under less pressure, making them brittle. When subjected to the same stress, these brittle rocks are more likely to fracture and create a fault.
7. How does the study of folds help us understand Earth's history and find natural resources?
The study of folds is crucial for several reasons. Geologically, the orientation and type of folds reveal the direction and intensity of ancient tectonic forces, helping us reconstruct past plate movements. Economically, certain types of folds, particularly anticlines, are excellent structural traps for oil and natural gas. These resources, being less dense than water, migrate upwards and get trapped in the crest of the anticline fold.
8. What is a recumbent fold and how does it form?
A recumbent fold is an extreme type of fold that has been pushed over so much that its axial plane (the imaginary plane that divides the fold in half) is nearly horizontal. These folds form under intense and prolonged directional pressure, causing an initially upright or asymmetrical fold to be continuously pushed over onto its side. They are often found in the core of major mountain ranges where compressional forces were greatest.
