When the rocks are subjected to horizontal compression forces, the rocks get folded . This is called Folding .
Causes of the folding-
- Folding occurs through compression.
- Folding is generally seen in mountain region.
- Folding often takes place in sedimentary rocks as they are more flexible as to compared to igneous or the metamorphic rocks.
- As a result of folding certain parts of the rocks are bent upward while the others are bent downward.
- The upfolds in the rocks are called anticlines while the downfolds are called synclines.
- In anticline, the rock strata inclines in opposite direction or away from common centre.
- In synclines, it bends towards a common centre.
- The upfold form fold mountain . E.g. Alps in Europe, Himalaya in Asia , Andes of South America and Rockies in North America.
The downloads form longitudinal valleys.
|Lithosphere folding and faulting|
Types of folds :
The degree of compressional force leads ro different types of folds. The type, shape and size of folds depends upon the intensity and direction of the comprehensive force and nature of the sedimentary rocks.
Based on the shape, size and inclination of limbs , folds have been classified under the following categories : –
Symmetrical fold or monoclines –
These are simple folds where both the limbs or the side of the fold remain equally bent and the is steeper.
Asymmetrical fold –
When a limb of the fold is pushed too far , making one limb steeper than the other, an asymmetrical fold is formed.
Recumbent folds –
When one limb is pushed over the other , it is known as overfold. But sometimes one limb is pushed so much that it lies on the other limb then it is called Recumbent fold.
Overthrust fold –
When, because of too much folding , the pressure is so great that a fracture occurs in the fold and one limbs slides over the other along the thrust plane. This is called overthrust fold. The overriding part of the plane is called nappe. The Nappes were first noticed in Alps.
Sometimes the compressional force is generated from the opposite sides. This two sided compressional forces cause the development of several folds from both sides. As a result, an arch shaped fold is developed in the centre, whose structure resembles a fan . This is known as Fanfold.
Effects of folding –
The main effect of folding is the formation of fold mountains. Some fold mountains are the highest and most expensive mountains and are found in almost all the continents. E.g. Rockies in North America, Andes in South America, Himalayas in South Asia and Alps in Europe.
The main characteristic features of fold mountains include –
- These fold mountain comprises great thickness of sedimentary strata and that have been formed due to intense compressional forces e.g. Himalayas.
- A period during which major mountains were formed is called Orogeny.
- Himalayas and Alps are the youngest fold mountain of the world.
- These mountains have many sharp peaks and valleys .
- The tops of anticlines become the mountain peaks and synclines becomes the valleys.
- Between the high ranges , there are also intermontane plateaus e.g , Colorado and Mexican plateaus.
- A fold mountain region is generally a region of glaciated activity. The rivers flowing from these mountains generally flow through narrow and deep valleys called gorges and make waterfalls.
- Some of the fold mountains of the world were formed in the ancient times.
- These are called old fold mountain, e.g, The Aravalli hills in India, the Urals in Russia and the Applachiana in North America.
- Most of the areas of young fold mountain suffers crustal instablity.
When the rocks are subjected to horizontal compressional forces, they do not get folded but break up along the lines of weakness. Due to this rocks get fractured. Such fracture of the rocks is called faulting.
|Lithosphere Folding and Faulting|
- The lines of fracture in the rocks is called a Fault.
- The plane along which the rocks are fractured is called fault plane
Types of faults :
There are four main types of faults .
- Normal fault – when the faulting is a result of tensional forces, the rock on the two sides of the fault plane are pulled away from each other . Such a fault is called a Normal fault.
- Reverse fault – when the crust is subjected to the force of compression from both the sides , a portion of the rock gets pushed up against the slope of the fault plane . Such a fault plane is called a Reverse Fault.
- Slip fault – when the faulted block slip is horizontally along the fault plane , it is called a slip fault.
Tear fault – when the faulted blocks move away from each other due to horizontal displacement, a gap or valley is formed between the two blocks. This type of valley is called fault valley and such a fault is called Tear fault.
Importance of fault or Effect of faults –
- Faults have great geomorphological , ecological and socio-economic significance . They are the producer of earthquakes.
- From the economic points of view, petroleum deposits are found in porous sedimentary rocks that have been faulted against impervious shale beds.
- Areas of faulted sedimentary strata are favourite areas of oil exploration.
- Most of the hot and cold springs are situated along the fault lines.
- It also give rise to waterfalls.
- Faults have tremendous importance in the weathering of rocks .