We have tried our best to make things simpler, so that
anyone can understand these topics.
These are basic concepts, upon which you can understand
much more complex geographical phenomenon,
Today we almost finished endogenetic forces only one
section is left i.e. faults, which will be discussed in upcoming module along
with a minor topic Exogenetic forces.
After that one more basic conceptual topic will be
discussed, from there onwards you will be in the position of digesting the
complex one.
Important
geographical phenomenon
1. Today
we are going to discuss the forces affecting the crust of the earth, because
these forces and resultant movements are involved in the creation, destruction,
recreation and maintenance of geomaterials.
2. The
geological changes are generally of two types
(i)
Long period changes – it is so slow
that we are unable to notice it.
(ii)
short- period changes – these are
noticed within few second to few hours (seismic events, volcanic eruptions,
etc)
3. The
forces which affect the crust of the earth, are divided into two broad
categories on the basis of their source of origin
(i)
Endogenetic Forces
(ii)
Exogenetic Forces
Endogenetic
forces
1. The
forces coming from within the earth are called as endogenetic forces.
2. It
causes two types of movements in the earth; (i) horizontal movements, (ii)
vertical movements
3. There
is lack of precise knowledge regarding the mode of origin of the endogenetic
forces and movements.
4. On
the basis of intensity, these are divided into two categories
(i)
Sudden forces
(ii)
Diastrophic forces
Sudden
forces
1. It
causes rapid events and massive destruction at and below the earth’s surface.
2. These
force work very quickly and there results are seen within minutes.
3. It’s
the result of long period preparation deep within the earth.
4. Because
these forces create certain relief features on the earth’s surface, they are
also termed as constructive forces.
Diastrophic
forces
1. These
forces operate slowly and their effects become discernible after thousand and
millions of years.
2. They
affect larger areas of globe and produce meso – level reliefs (e.g.) mountains,
plateaus, plain, lakes, big faults etc.)
3. It
is divided into two groups
(i)
Epeirogenetic movements
(ii)
Orogenetic movements
Epeirogenetic
movements
1. It
is continent forming
2. Acts
along the radius of earth, therefore also called radial movement
3. Direction
may be towards (subsidence) or away (uplift) from the centre.
4. Results
are seen in relief i.e. sea beaches (Kathiwar, Orissa), elevated wave cut
terraces, sea caves.
Example of
Emergence
1. Evidence
of marine fossils above sea level in parts of Britain Norway is examples of epeirogenetic
uplift
Example of
Submergence
1. Downward
movements cause subsidence of continental masses.
2. In
1819, Rann of Kachchh was submerged as a result of earthquake.
Orogenetic
movement
1. It
means mountain building
2. It
is caused due to endogenetic forces working in horizontal manner.
3. Thus
these forces create rupture, cracks, fracture and faults in the crustal parts
of the earth.
4. When
these forces work face to face these are called compressional forces or
convergent force.
5. It
causes crustal bending leading to the formation of folds
6. The
crustal rocks undergo the process of crustal bending in two ways
(i)
Wrapping
(ii)
Folding
Wrapping
1. It
affects the larger areas of the crust, wherein the crustal parts are either
warped (raised) upward or downward.
2. When
it affects larger areas, the resultant mechanism is called broad wrapping.
Folding
1. Waves
like bands are formed in the crustal rocks due to tangential compressive force.
2. It
is the result of horizontal movement caused by the endogenetic force
3. Some
parts of bent are in upward direction, they are called anticlines, and they are upfolded rock strata in arch like form.
4. The
down folded structure forming through – like features is called syncline.
Fig – Anticline & Syncline
5. Anticlinorium
represents a folding structure, where extensive anticline having numerous minor
anticline and synclines.
6. Synclinorium
represents a folding structure, where extensive syncline having numerous minor
anticline and synclines.
7. The
two sides of folds are called limbs of the fold, the limb shared between
anticline and syncline is called middle limb.
8. The
plain line which bisects the angle between the two limbs of the anticline or
middle limb of the syncline is called axis
of fold or axial plane.
9. It’s
also necessary to understand dip and strike, in order to understand the
structural form.
10. The
inclination of rock beds with respect to horizontal plane is called dip.
11. The
angle of dip is measured with an instrument called clinometers.
12. The
strike of an inclined bed is the
direction of any horizontal line along a bedding plane.
13. The
direction of dip is always at right angle to the strike.
Types of
folds
1. The
elasticity of rocks largely affects the nature and the magnitude of folding
process.
2. The
softer and more elastic rocks are subjected to more intense folding, while
rigid and less elastic rocks are moderately folded.
3. Difference
in intensity and magnitude of compressive forces also causes variations in the
characteristics of folds.
4. Normally
both the limbs of folds are having equal limbs, but mostly they are not.
5. Based
on the inclination of the limbs, folds are divided into five types.
(i)
Symmetrical
folds – The limbs (both) of which incline uniformly,
compressive force work regularly with moderate intensity.
(ii)
Asymmetrical
folds – characterized by unequal and irregular limbs, both
the limbs incline at different angles, one limb is relatively larger than
other.
(iii)
Monoclinal
folds – one limb inclines moderately with regular slope,
while other limb inclines steeply at right angle and slope is almost vertical.
(iv)
Isoclinal
folds – when both the limbs of fold become parallel but not
horizontal.
(v)
Recumbent
folds – when the compressive forces are so strong that both
the limbs of the fold become parallel as well as horizontal.
(vi)
The
other types of fold are overturned fold (one limb of
the fold is thrust upon another), plunge fold (when axis of fold become
tilted), fan fold (extensive and broad fold consisting of several minor
anticline and syncline), open folds (in which angle between the two limbs of
the fold is more than 90 degree but less than 180 degree) and lastly closed
folds (angle between two limbs of a fold is acute angle).
Nappes
1. Nappes
are the result of complex folding mechanism caused by intense horizontal
movement and resultant compressive force.
2. Both
the limbs of a recumbent fold are parallel and horizontal.
3. Due
to further increase in the continued compressive force one limb of the
recumbent folds slides forward and overrides the other fold.
4. This
process is called thrust and plane along which one part of the fold is thrust
is called thrust plane.
5. The
upthrust part of the fold is called overthrust fold.
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