Fold Mountains Geography

Fold Mountains are the mountains formed from the folding of the earth’s crust. These movements are caused due to various factors like movements in the mantle, expansion and contraction of some parts of the Earth.

The 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.

Fold mountains form when two or more of the Earth’s tectonic plates collide.(This is why Fold Mountains, which are produced by sedimentary rocks, are usually found near continent borders.)
Rocks and debris are distorted and folded into rocky outcrops, hills, mountains, and entire mountain ranges as a result of compression.

They form at convergent plate boundaries, which are also known as continental collision zones or compression zones.

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.

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.

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.

Classification of fold mountains

On the basis of period of origin

On the basis of the period of origin, fold mountains are divided into (very old , old and Alpine fold mountains) .

Very Old Fold Mountains

They are more than 500 million years old.
They have rounded features (due to denudation).
They are of low elevation.
Some of the examples are Laurentian mountains, Algoman mountains, etc.

Old Fold Mountains

Old fold mountains had their origin before the Tertiary period (tertiary period started 66 million years ago).
The fold mountain systems belonging to Caledonian and Hercynian mountain-building periods fall in this category.
The Appalachians in North America and the Ural Mountains in Russia are the examples.
They are also called thickening relict fold mountains because of lightly rounded features and medium elevation.
Top layers are worn out due to erosional activity. Example: Aravalli Range in India.
The Aravalli Range in India is the oldest fold mountain systems in India.
The range rose in post-Precambrian event called the Aravalli-Delhi orogeny.

Alpine or young fold mountain

Alpine fold mountains belonging to the Tertiary period (66 million years ago to present) can be grouped under the new fold mountains category since they originated in the Tertiary period.
Examples are the Rockies, the Andes, the Alps, the Himalayas, etc.