ENDOGENIC &

EXOGENIC FORCES

by TarunGoyal

The surface of the earth is not a plain platform. It is distributed unevenly

with a variety of landforms like mountains, hills, plateaus, plains, ravines,

cliffs, etc.

Why is the surface of the earth uneven?

What make changes in the earth’s surface?

What process makes mountains and hills?

The answer to all the questions above – Geomorphic Processes.

Geomorphic Process

The formation and deformation of landforms on the surface of the earth

are a continuous process which is due to the continuous influence of

external and internal forces.

The internal and external forces causing stresses and chemical action

on earth materials and bringing about changes in the configuration of the

surface of the earth are known as geomorphic processes.

Endogenic Forces

These are those internal forces which derive their strength from the earth’s interior and play a

crucial role in shaping the earth crust. E.g. : mountain building forces, continent building

forces, earthquakes, volcanism etc.

The endogenic forces are mainly land building forces.

The energy emanating from within the earth is the main force behind endogenic geomorphic

processes. This energy is mostly generated by radioactivity, rotational and tidal friction and

primordial heat from the origin of the earth.

Exogenic Forces

These are those forces which derive their strength from the earth’s exterior or are originated

within the earth’s atmosphere. E.g. : wind, waves, water, etc.

Examples of exogenic processes – weathering, mass movement, erosion, deposition.

Exogenic forces are mainly land wearing forces.

Weathering is the breaking of rocks on the earth’s surface by different agents like rivers, wind,

sea waves and glaciers.

Erosion is the carrying of broken rocks from one place to another by natural agents like wind,

water, and glaciers.

The actions of exo. forces result in wearing down (degradation) of relief/elevations and filling

up (aggradation) of basins/ depressions, on the earth’s surface. The phenomenon of wearing

down of relief variations of the surface of the earth through erosion is known as gradation.

Geomorphic Agents

Running water, groundwater, glaciers, the wind, waves, and currents, etc., can

be called geomorphic agents.

Geomorphic Processes vs. Geomorphic Agents

A process is a force applied on earth materials affecting the same.

An agent is a mobile medium (like running water, moving ice masses, the wind,

waves, and currents etc.) which removes, transports and deposits earth

materials.

Earth Movements

They are the movements in the earth’s crust caused by the endogenic or

exogenic forces. These movements are also termed as Tectonic movements.

The term ‘Tectonic’ derived from the Greek word tekton which means builders.

As the word means, these movements are mainly builders and have been

responsible for building up of different types of landforms.

ENDOGENETIC MOVEMENTS

Though the surface of earth appears to be static, the interior of the earth

is in a dynamic state and this dynamism of the earth results in

endogenetic movements.

The Movements of the Earth

Even the surface of the earth is dynamic. It has been moving verticallyand horizontally.

Since the origin of the earth, there have been major changes in thedistribution of continents and oceans.

The earth has experienced innumerable earth movements which havebrought about vast changes in its surface.

The lithosphere is broken into a number of plates known as theLithospheric Plates.

The movement and interaction of these plates cause changes on thesurface of the earth.

The forces which act in the interior of the earth are called

as Endogenetic forces and the forces that work on thesurface of the earth are called as Exogenic forces.

In general terms, the endogenetic forces are mainly land

building forces and the exogenic processes are mainly landwearing forces.

The actions of exogenetic forces result in wearing down

(degradation) of relief. But, the endogenetic forces

continuously build up parts of the earth’s surface and

hence the exogenetic processes fail to even out the reliefvariations of the surface of the earth.

So, variations remain as long as the opposing actions of

exogenetic and endogenetic forces continue.

SLOW MOVEMENTS

The movement which bring about changes in the Earth’s crust

gradually taking hundreds or thousands of years and which cover a

period much longer than a human life span are called slow movements.

These movements act on earth’s crust either vertically or horizontally.

Diastrophism

Diastrophism is included within slow movements.

All processes that move, elevate or build up portions of the earth’s

Crust come under diastrophism. They include :

1. Epeirogenic processes 2. Orogenic processes

Through these processes, there can be folding and faulting of the

Crust.

All these processes cause pressure, volume and temperature (PVT)

changes which in turn induce metamorphism of rocks.

Epeirogenic Processes (Vertical Movements)

Epeirogeny is a continental building process. The word Epeirogenetic

consists of two words viz., ‘epiros’ (continent) and ‘genesis’ (origin).

Due to epeirogeny, there may be mild deformations of the surface of the

Earth.

Epeirogenic processes involve uplift or warping of large parts of the

earth’s crust.

The broad central parts of continents are called cratons, and are subject

to epeirogeny.

Epeirogenic or continent forming movements act along the radius of the

earth; therefore, they are also called radial movements.

Their direction may be towards (subsidence) or away (uplift) from the

Centre.

The results of such movements may be clearly defined in the relief.

Uplift

Raised beaches, elevated wave-cut

terraces, sea caves and fossiliferous

beds above sea level are evidences of

uplift.

Raised beaches, some of them

elevated as much as 15 m to 30 m

above the present sea level, occur at

several places along the Nellore andTirunelveli coasts.

Several places which were on the sea

some centuries ago are now a few

miles inland. For example, Coringa

near the mouth of the Godavari,

Kaveripattinam in the Cauvery delta

and Korkai on the coast of

Tirunelveli, were all flourishing seaports about 1,000 to 2,000 years ago.

Subsidence

Submerged forests and valleys as well as buildings are evidences of

subsidence.

In 1819, a part of the Rann of Kutch was submerged as a result of an

earthquake.

Presence of peat and lignite beds below the sea level in Sunderbans is

an example of subsidence.

The Andamans and Nicobars have been isolated from the Arakan coast

by submergence of the intervening land.

On the east side of Salsette Island, trees have been found embedded in

mud about 4 m below low water mark.

A large part of the Gulf of Mannar and Palk Strait is very shallow and has

been submerged in geologically recent times. A part of the former town of

Mahabalipuram near Chennai is submerged in the sea.

Orogenic Processes (Horizontal Movements)

Horizontal forces act on the earth’s crust from side to side to causethese movements. Also called tangential forces.

Orogeny is a mountain building process.

Orogenic processes involve severe folding and affecting long andnarrow belts of the earth’s crust.

In contrast to epeirogenic movement, orogenic movement is a morecomplicated deformation of the Earth's crust.

Orogenic processes are associated with crustal thickening, notablyassociated with the convergence of tectonic plates.

If the orogeny is due to the colliding of the two continental plates,

very high mountains can result. E.g. Himalayas, Alps, etc.

They can be classified as Forces of Compression and Forces ofTension.

Forces of Compression

• Forces which push rock strata against a

hard plane from one side or from bothsides.

The compressional forces lead to the

bending of rock layers and thus lead tothe formation of Fold Mountains.

Most of the great mountain chains of the

world like the Himalayas, Rockies,

Andes, Alps, etc are formed in thismanner.

Forces of Tension

• Under the operation of intense

tensional forces, the rock stratum

gets broken or fractured which

results in the formation of cracks

and fractures in the Crust.

The displacement of rock upward

or downward from their original

position along such a fracture is

termed as faulting.

The line along which displacement

of the fractured rock strata take

place is called as the fault line.

Faulting results in the formation of well-known

relief features such as Rift Valleys & Block

Mountains (e.g. Vindhya & Satpura Mountains).

A rift valley is formed by sinking of rock strata

lying between two almost parallel faults (e.g.

Valley of Nile, Rift valley of Narmada and Tapti).

Rift valleys with steep parallel walls along the

fault are called as Graben and the uplifted

landmass with steep slopes on both sides are

called as Horst.

The very steep slope in a continuous line along

a fault is termed as Escarpment. It is a steep

slope or long cliff that forms as a result of faulting

or erosion and separates two relatively level

areas having different elevations.

Wave Rock, Australia

GEOLOGICAL FOLDS

A wave-like geologic structure that forms when rocks deform by bendinginstead of breaking under compressional stress.

TYPES OF FOLDS

When there is continental collision, the intervening sedimentary rocks of

the continental margins come under strong forces of compression, and

begin to contract and the horizontal strata begin to fold into wavelikeundulations.

Depending on how the layers of earth get folded, the folds are classifiedas Anticlines (upward folds) and Synclines (downward folds).

Upward folds like arches are called anticlines. In an anticline, a fold

arches away from the earth. The top of the arch is called the crest of theanticline. In an anticline, the oldest rocks are at its core, or center.

Synclines are downward folds like troughs. In a syncline, the fold bends

down towards the earth. Synclines have their newer rocks at their centersand older rocks at the outsides.

Folds typically occur in anticline-syncline pairs.

The hinge is the point of

maximum curvature in a fold.

The two sides of length of a fold

are called its Limbs.

The plane that bisects the angle

between the two Limbs is called

the Axial Plane, and the line of

intersection of this plane with a

bedding plane (the hinge line)

gives the direction of the axis of

the fold at that place.

Depending on their orientation, the folds maybe symmetrical,

inclined or asymmetrical, one Limbed Vertical (or Overturned).

If the Axial Plane is vertical and the axis horizontal, the fold is

upright or symmetrical.

If the Axial plane is inclined, the fold is also said to be inclined.

With further intensity in compression, both the Limbs of the fold

may be almost parallel. Folding of this time is called isoclinal.

Deep within the intensely deformed core of an orogen, folds not

only become overturned or isoclinal but they are doubled over

in reclining or recumbent forms, both the Limbs lying one

above the other. These are called recumbent folds.

Further development of the structure results in

forward movement of rocks of the upper Limb along

the plane of shearing. The plane of shearing is a

thrust plane and the structure an Overthrust Fold.

With further increase in pressure, the recumbent

folds may be sheared or sliced by fault planes lying

almost horizontal; these are called OverthrustFaults.

The sheet of rocks that has moved forward along

the thrust plane and may be highly contorted in the

process, is called a Nappe (a French word for

‘cover sheet’ or ‘tablecloth’).

Thrust sheets are known to have moved

horizontally for several tens of kilometers.

The Himalayas are not only marked by complex

folds, but also contain overthrusts and nappes ofgreat dimensions.

Overturned Fold

Recumbent Fold

Overthrusting

Overthrust Fault

Orientation of Axial Plane

The orientation of the axial plane relative to the horizontal together with the

orientation of fold limbs allow subdivision into upright (axial plane vertical,

limbs symmetric), overturned (axial plane moderately inclined, one limb

overturned), or recumbent (axial plane near horizontal, one limb inverted).

Tightness of Folding

The shape & size of folds depend upon the intensity and direction of the compressive

forces and upon the nature of the rocks which compose the different strata.

It can be described as open (limbs dip gently), tight (limbs dip steeply) or isoclinal (limbs

are parallel).

Where the compression is relatively

mild or more or less equal from both

sides, the folds are simple or

symmetrical and are called Open

Folds.

Simple or symmetrical folds are

rather rare but a very well-known

example of open folds is provided by

the Jura mountains of France and

Switzerland.

Crustal Fracture or FAULT

A fault is a fracture or zone of fractures between two

blocks of rock.

Faults allow the blocks to move relative to each other.

This movement may occur rapidly, in the form of an

earthquake – or may occur slowly, in the form of creep.

Faults may range in length from a few millimeters to

thousands of kilometers.

Most faults produce repeated displacements over

geologic time.

During an earthquake, the rock on one side of the fault

suddenly slips with respect to the other.

The fault surface can be horizontal or

vertical or some arbitrary angle in

between.

A Fault Plane is the plane that represents

the fracture surface of a fault.

A fault trace or Fault Line is the

intersection of a fault plane with the

ground surface.

Fault dip is the angle between the fault

plane and horizontal plane.

Fault scarp is the steep wall-like slope

caused by faulting of the crustal rocks.

Four Main Types of Faults

Faults are subdivided according to the

movement of the two blocks :

1. Normal Fault

A dip-slip fault in which the block above the fault has moved downward relative

to the block below.

This type of faulting occurs in response to extension.

It occurs when the “hanging wall” moves down relative to the “foot wall”.

The fault plane is usually between 45 degree and the vertical.

2. Reverse Fault

A dip-slip fault in which the upper block, above

the fault plane, moves up and over the lower

block.

This type of faulting is common in areas of

compression (shortening of the Crust).

When the dip angle is shallow, a reverse fault

is often described as a thrust fault.

It occurs where the “hanging wall” moves up or

is thrust over the “foot wall”.

On account of extreme compression, the rocks

snap and one strata is pushed over the

underlying strata, i.e., the upper side is

displaced above the fault plane relative to theside below.

3. Strike-Slip Fault

• A fault on which the two blocks slide past one another.

• The San Andreas Fault is an example of a right lateral fault.

• These are called left lateral or sinistral faults when the displacement of the rock

blocks occurs to the left on the far side of the fault and right lateral or dextral faults

when the displacement of the rock blocks takes place to the right on the far side of the

fault.

4. Step Fault

• When a series of faults occur in any area in such a way that the slopes of all the

fault planes of all the faults are in the same direction.

Rift Valley and Graben

They are formed due to displacement of crustal parts and subsidence of middle portion between

two normal faults. Rift valleys are generally called graben (a German word).

Rift valley may be formed in two ways :

1. When the middle portion of the crust between two normal faults is dropped downward when

the two blocks on either side of the downdropped block remain stable.

2. When the middle portion between two normal faults remains stable and the two side blocks on

either side of the middle portion are raised upward.

• Normally, a rift valley is long and narrow but very deep.

• Rhine rift valley is the best example of a well defined rift valley. It stretches for a distance of

320 km.

• The example of the longest river valley is the African Rift valley that

runs from the Jordan river valley through Red Sea basin to Zambezi

valley for a distance of 4,800 km.

• Death valley of the Southern California, Dead Sea, Narmada valley,

Damodar valley and Tapti valley are other examples of rift valleys.

SUDDEN MOVEMENTS

Contrary to the slow movements, there are certain movements which bring

about abrupt changes in the crust. The examples of such movements are

volcanic eruptions and earthquakes.

EARTHQUAKE

Occurs when the surplus accumulated stress in rocks in earth’s interior

is relieved through the weak zones over the earth’s surface in form ofkinetic energy of wave motion causing vibrations on the earth’s surface.

Such movements may result in uplift or subsidence in coastal areas.

An earthquake in Chile (1822) caused a 1 m uplift in coastal areas.

An earthquake in New Zealand (1885) caused an uplift of up to 3 m in

some areas while some areas in Japan (1891) subsided by 6 m after anearthquake.

Earthquakes may cause change in contours, change in river courses,

tsunamis (which may cause shoreline changes), spectacular glacialsurges (as in Alaska), landslides, soil creeps, mass wasting etc.

VOLCANISM

A volcano is a vent in the earth’s crust through which molten

material erupts suddenly toward the earth’s surface.

Molten magma is accompanied by steam, gases (hydrogen

sulphide, sulphur dioxide, hydrogen chloride, carbon

dioxide) and pyroclastic material.

Volcanism is responsible for formation of many intrusive and

extrusive volcanic forms.

Volcanoes are classified on the basis of nature of eruption and

the form developed at the surface

Major types of volcanoes include – Shield Volcanoes,

Composite Volcanoes, Caldera, Flood Basalt Provinces and

Mid-Ocean Ridge Volcanoes.