Folding And Faulting

Introduction • In the Earth’s crust there are different movements which cause changes in the crust of the Earth . • These movements are called Tectonic movements because they build major landforms on the Earth surface . • The tectonic movements are of

Vertical Movements • Vertical movements are known as Epeirogenic movements . • They are continent-building movements . • Are associated with Plate Tectonics . • Due to this movements a section of the Earth’s crust may rise or subside resulting in the formation of raised plateaus or extensive shallow depression . • The part that has risen is called the uplift & that which has sunk is called subsidence . • These movements are large scale and powerful , hence are called continent building

Horizontal Movements • They take place due to compressional or tensional forces. • Responsible for formation of fold mountains. • As a result of compressional forces acting in 2 opposite directions, the crustal rocks bend and form a series of arches & troughs. • This is known as folding of crustal rocks that usually occur in a zone where the crust is weak. • The lines of fracture are known as fault. • Due to this Earth’s surface tends to rise or sink along the plane of fracture. • Both process result in mountain building.

Folding and Fold Mountains • A fold is a bend in rock strata resulting from compression of an area of the Earth’s crust . • The severity of compression is indicated by the tightness of the folds. • In folding since the compressional force is acting towards the centre, the intervening land rises up in a series of folds. • The arch or anticline of the folds,

How folds are formed?

Types of folds • The folds are classified in the following types depending on the intensity of the

1.Symmetrical Folds • When the compression is too intensive the land gets folded into gentle, simple folds called as symmetrical folds. • Here both sides

2.Asymmetrical Fold • When the compressional force is more from one end one limb becomes steeper than the other . • Such a fold is called Asymmetrical

3.Recumbent Fold • When one side of the fold is pushed over the other side the landform is called Over fold . • When one side is pushed so much that it lies positioned over the other side , such a fold is called Recumbent fold.

4. Over thrust Folds • Due to excessive folding , a fracture is formed ,one side of the fold slides forward over this fault . • This is called Over thrust Fold . • The portion which slides forward is called Nappe .

5. Fan Fold • If the compression is from opposite sides , small arch shaped folds are formed . • Several arch shaped folds resembling a fan are formed . • This is a Fan Fold .

Fold Mountains • The compressional force exerted on the crust is a result of movement of earth’s plates. • When the plates converge, the weak rocks and sediments lying between the 2 plates get squeezed and get folded. • Parallel folds give rise to long chains of parallel mountain ranges with high peaks.

Characteristics of Fold Mountains

1) A fold mountain is characterized by peaks & valleys. The tops of anticlines become the peaks & the synclines become the valleys. 2) The fold mountain system has parallel ranges. 3) Intermont plateaus maybe found between high ranges. 4) This system has a large no. of swift flowing rivers, with many waterfalls & rapids.

Faulting • Faults are large scale internal movements causing large scale physical features especially mountains and valleys. • An elevated block is called block mountain. • When rocks are pulled apart, or in opposite direction, a tensional force is created. This results in a crack or fault line.

Types of Fault

• Sometimes a section of land slides down along the fault plane is called a normal fault. • When the Earth’s surface is subjected to severe compression along the fault line, a portion of rocks get

Contd…

• The second type of fault is the reverse or thrust fault. In this type, due to compressional forces, the beds on one side of the fault plain are thrust over the other. • As a result, a portion of the rocks is pushed up against the slope of the fault plane. • This causes the

Contd… • The third type of fault is the tear fault. • In this case the blocks on the either side of the fault are displaced horizontally or sideways. • Tear faults generally occur

Plate Tectonics • It is the theory according to which the outer shell of the Earth is made up of a number of relatively thin lithospheric plates composing the continental & oceanic crust . • Each plate is capable of moving over the semimolten asthesnosphere , carrying oceanic and continental crust a like . • At the mid-oceanic ridges ,the magma cools and forms a crust from below . This causes spreading of the sea floor and the movement of the of the plates away from the central ridges. This is known as constructive plate margin. • At the other places ,such as around the periphery of the Pacific ocean , the plates either pass or collide with each other

Contd…. • When a continental plate collides with the oceanic plate it causes the oceanic plate to move down into the mantle .This is destructive plate margin and is marked by a deep oceanic trench . • The six major plates are : (1) Pacific Plate (2)Eurasian plate (3) American Plate (4) African Plate (5) Indian Plate and (6) Antarctic Plate . • The movement of these plates with respect to each other throws up the fold mountains. According to this theory the collision of Eurasian and African –Indian continental plate about 60-30 Million years ago squeezed geosynclinal sediments of Tethys

Features of Faulting •

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The tensional features which cause faulting may be both due to compression & tension or tension alone. If the force is moderate only cracks may appear in the rocks. The displacement that occurs results in formation of faults. Such fractures are divided into- 1)Joints & 2)Faults. A joint is a fracture where a rock seems to be joined together without any movement

• The plane along which the rock blocks are displaced is called the Fault plane. • Like folds, faults are of various types depending on how they occur.

Main Features of Faulting • Faults rarely occur singly but a no. of these may be parallel. • Fault blocks: the elevated or subsided masses of rocks are called fault blocks. These fold blocks may be symmetrical or asymmetrical. • Horst: an individual fault block which is sharply defined with respect to the surrounding rock strata is called Horst. • Rift Valley: it is the reverse of a horst. A single fault in the same orientation is called a rift valley. A rift valley is a major relief feature of faulting. Rift valleys are long & narrow troughs bounded by one or

Landforms Of Faulting

Rift Valley Block Mountains

Rift Valley • •

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Major relief feature of Faulting Activity is Rift Valley. Formation : A Rift Valley may be formed in two ways : (1) When the middle portion between two normal faults is dropped downward the sides remain stable. (2) When the middle portion remains stable and the two sides are raised upward .

Contd…. • They are also known as Graben. • These valleys have flat bottoms and steep sides .E.g.- Jordan Valley , Great Rift Valley of East Africa . • Some Rift valleys have become lakes .E.g.- Lake Nyasa ,Lake Rudolf ,Dead Sea. • The Rhine Rift Valley in Europe is bounded by Vosages and Hardt Mountains .It is a fine example of the Rift Valley caused by Faulting. • In India the Narmada Valley, Damodar Valley, Tapi valley are example of the Rift valley.

Block Mountains • Block mountain is another important feature of Faulting caused by the Tensional features in the Earth’s crust . • The mechanism of formation of Block mountains is same as the Rift valley. • The Block mountains as in the case of the Rhine valley ,are often found on the sides of the Rift valley. It is because the land between two parallel

Contd…. • Characteristics of Block mountains – flat tops, flat bottoms on sides and steep fault scrap . • The Vosges in France and the Black Forest mountains in Germany are examples of the Block mountains . • Other Block mountains – Salt Rang in Pakistan, the Vindhyas and the Satpuras in India are examples of Block mountains .They enclose between them the Rift valleys. • The Fault Blocks with sharp features are

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Distribution of Folded and Faulted Landforms • The fold mountains of various periods are found almost in every continent .Almost all of them are arranges in long lines and interlinked extending through oceans . • The Alpine-Himalayan system extends from the strait of Gibraltar through the Mediterranean Basin , Asia Minor to India and South –East Asia. • The Circum –Pacific system comprises of Andes ,the North American cordillera & Island arcs .These two system meet north of Australia . • These two system are the main systems from where various branches extend in different directions .These are said to have existed as geosynclines some 50 – 60 million years ago .

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