Today we
discussed
1.
Faults
2.
Types of fault
3.
Exogenetic
forces
4.
Weathering
5.
Mass
movements
Important Geographical Phenomenon
1.
As we know that there are several factors behind folded structure, in the
same way tensional and compressional forces causes displacement of rock along a
plane.
2.
The force may be vertical or horizontal and sometime it may be in both
ways.
3.
These crustal fractures depend on the strength of rock and intensity of
tensional force.
4.
Fractures are divided into two parts
(i)
Faults
(ii)
Joints
Faults
1.
A fault is a fracture is a fracture in the crustal rocks, due to
tensional movement caused by the endogenetic forces, wherein the rocks are
displaced along a plane, known as fault plane.
2.
The movement responsible for the formation of a fault may operate in
vertical or horizontal or any direction.
Types of fault
Normal fault
1.
These are formed due to displacement of both the rocks blocks in opposite
directions,
2.
Due to fracture consequent upon greatest stress, the fault plane is
usually between 45 degree and the vertical.
Reverse faults
1.
These are formed due to the movement of both the fractured rock blocks
towards each other.
2.
The fault plane, in a reverse direction is usually inclined at an angle
between 40 degree and the horizontal (0degree).
Lateral or strike faults
1.
These are formed when the rocks blocks are displaced horizontally along
the fault plane due to horizontal movement.
2.
When displacement of rock occurs to the left, they are called left – lateral
or sinistral fault.
3.
When displacement of rock occur to the right , they are called right –
lateral fault or dextral fault.
Step faults
1.
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 same direction.
Exogenetic forces
1.
The Exogenetic forces or processes also called denudational
processes or destructional forces.
2.
They are continuously engaged in the destruction of the relief features
created by the endogenetic forces.
3.
The exogenic processes derive their energy from atmosphere determined by
the ultimate energy from the sun and also the gradients created by tectonic
factors.
4.
Gravitational force acts upon all earth materials having a
sloping surface and tend to produce movement of matter in down slope direction.
5.
Force applied per unit area is called stress.
6.
Stress is produced in a solid by pushing or pulling. This induces
deformation.
7.
Forces acting along the faces of earth materials are shear stresses
(separating forces).
8.
It is this stress that breaks rocks and other earth materials.
9.
The shear stresses result in angular displacement or slippage.
10.
The gravitational stress earth materials become subjected to molecular
stresses
11.
Molecular stresses may be caused by a number of factors amongst which
temperature changes, crystallisation and melting are the most common
12.
Chemical processes normally lead to loosening of bonds between grains,
dissolving of soluble minerals or cementing materials
13.
The basic reason that leads to weathering, mass movements, erosion
and deposition is development of stresses in the body of the earth
materials.
14.
As there are different climatic regions on the earth’s surface the
exogenic geomorphic processes vary from region to region.
15.
Temperature and precipitation are the two important climatic elements
that control various processes
16.
All the exogenic geomorphic processes are covered under a general term, denudation
17.
The word ‘denude’ means to strip off or to uncover. Weathering,
mass wasting/movements, erosion and transportation are included in denudation.
18.
Within different climatic regions there may be local variations of the
effects of different climatic elements.
19.
Due to altitudinal differences, aspect variations and the variation in
the amount of insolation (amount of sun rays) received by north and south
facing slopes as compared to east and west facing slopes.
20.
Differences in wind velocities and directions, amount and kind of
precipitation, its intensity, the relation between precipitation and
evaporation, daily range of temperature, freezing and thawing frequency, depth
of frost penetration, the geomorphic processes vary within any climatic region.
21.
Climatic factors being equal, the intensity of action of exogenic
geomorphic processes depends upon type and structure of rocks.
22.
The term structure includes such aspects of rocks as folds, faults,
orientation and inclination of beds.
23.
Presence or absence of joints, bedding planes, hardness or softness of
constituent minerals, chemical susceptibility of mineral constituents.
24.
Different types of rocks with differences in their structure offer
varying resistances to various geomorphic processes
25.
Under varying climatic conditions, particular rocks may exhibit different
degrees of resistance to geomorphic processes and hence they operate at
differential rates.
26.
The effects of most of the exogenic geomorphic processes are small and
slow and may be imperceptible in a short time span
WEATHERING
1.
Weathering is action of elements of weather and climate over earth
materials.
2.
There are a number of processes within weathering which act either
individually or together to affect the earth materials in order to reduce them
to fragmental state.
3.
Weathering is defined as mechanical disintegration and chemical
decomposition of rocks through the actions of various elements of weather and
climate.
4.
As very little or no motion of materials takes place in weathering, it is
an in-situor on-site process.
5.
Weathering processes are conditioned by many complex geological,
climatic, topographic and vegetative factors.
6.
Climate is of particular importance.
7.
There are three major groups of weathering processes
a. Chemical
b. Physical
c. Biological weathering processes
Chemical
weathering process
1.
A group of weathering processes, include in this
a. Carbonation
b. Hydration
c. Oxidation
d. Reduction
2.
They act on the rocks to decompose, dissolve or reduce them to a fine clastic
state through chemical reactions by oxygen, surface and/or soil water and other
acids.
3.
Water and air (oxygen and carbon dioxide) along with heat must be present
to speed up all chemical reactions
4.
Over and above the carbon dioxide present in the air, decomposition of
plants and animals increases the quantity of carbon dioxide underground.
Steps involved in chemical weathering
Solution
1.
When something is dissolved in water or acids, the water or acid with
dissolved contents is called solution.
2.
This process involves removal of solids in solution and depends upon
solubility of a mineral in water or weak acids.
3.
This process involves removal of solids in solution and depends upon
solubility of a mineral in water or weak acids.
4.
On coming in contact with water many solids disintegrate and mix up as
suspension in water.
5.
Soluble rock forming minerals like nitrates, sulphates, and potassium
etc. are affected by this process.
6.
So, these minerals are easily leached out without leaving any residue in
rainy climates and accumulate in dry regions.
7.
Minerals like calcium carbonate and calcium magnesium bicarbonate present
in limestones are soluble in water containing carbonic acid (formed with the
addition of carbon dioxide in water)
8.
Are carried away in water as solution. Carbon dioxide produced by
decaying organic matter along with soil water greatly aids in this reaction.
Carbonation
1.
Carbonation is the reaction of carbonate and bicarbonate with minerals
and is a common process helping the breaking down of feldspars and carbonate
minerals.
2.
Carbon dioxide from the atmosphere and soil air is absorbed by water, to
form carbonic acid that acts as a weak acid.
3.
Calcium carbonates and magnesium carbonates are dissolved in carbonic
acid and are removed in a solution without leaving any residue resulting in
cave formation.
Hydration
1.
Hydration is the chemical addition of water. Minerals take up water and
expand; this expansion causes an increase in the volume of the material itself
or rock
2.
Calcium sulphate takes in water and turns to gypsum, which is more
unstable than calcium sulphate
3.
This process is reversible and long, continued repetition of this process
causes fatigue in the rocks and may lead to their disintegration.
4.
Salts in pore spaces undergo rapid and repeated hydration and help in
rock fracturing.
5.
The volume changes in minerals due to hydration will also help in
physical weathering through exfoliation and granular disintegration.
Oxidation and Reduction
1.
Oxidation means a combination of a mineral with oxygen to form oxides or
hydroxides.
2.
Oxidation occurs where there is ready access to the atmosphere and
oxygenated waters.
3.
The minerals most commonly involved in this process are iron, manganese,
sulphur etc.
4.
In the process of oxidation rock breakdown occurs due to the disturbance
caused by addition of oxygen.
5.
Red colour of iron upon oxidation turns to brown or yellow.
6.
When oxidised minerals are placed in an environment where oxygen is
absent, reduction takes place.
7.
Such conditions exist usually below the water table, in areas of stagnant
water and waterlogged ground.
8.
Red colour of iron upon reduction turns to greenish or bluish grey.
9.
These weathering processes are interrelated. Hydration, carbonation and
oxidation go hand in hand and hasten the weathering process.
Physical
Weathering Processes
1.
Physical or mechanical weathering processes depend on some applied
forces.
2.
The applied forces could be
3.
Gravitational forces such as overburden pressure, load and shearing
stress
4.
Expansion forces due to temperature changes, crystal growth or animal
activity
5.
Water pressures controlled by wetting and drying cycles
6.
Many of these forces are applied both at the surface and within different
earth materials leading to rock fracture.
7.
Most of the physical weathering processes are caused by thermal expansion
and pressure release.
8.
These processes are small and slow but can cause great damage to the
rocks because of continued fatigue the rocks suffer due to repetition of
contraction and expansion.
Unloading and Expansion
1.
Removal of overlying rock load because of continued erosion causes
vertical pressure release with the result that the upper layers of the rock
expand producing disintegration of rock masses.
2.
Fractures will develop roughly parallel to the ground surface
3.
In areas of curved ground surface, arched fractures tend to produce
massive sheets or exfoliation slabs of rock.
4.
Exfoliation sheets resulting from expansion due to unloading and pressure
release may measure hundreds or even thousands of metres in horizontal extent.
5.
Large, smooth rounded domes called exfoliation domes
Temperature Changes and Expansion
1.
Various minerals in rocks possess their own limits of expansion and
contraction.
2.
With rise in temperature, every mineral expands and pushes against its
neighbour
3.
As temperature falls, a corresponding contraction takes place.
4.
Because of diurnal changes in the temperatures, this internal movement
among the mineral grains of the superficial layers of rocks takes place
regularly.
5.
This process is most effective in dry climates and high elevations
6.
Where diurnal temperature changes are drastic.
7.
As has been mentioned earlier though these movements are very small they
make the rocks weak due to continued fatigue
8.
The surface layers of the rocks tend to expand more than the rock at
depth and this leads to the formation of stress within the rock resulting in
heaving and fracturing parallel to the surface.
Freezing, Thawing and Frost Wedging
1.
This process is most effective at high elevations in mid-latitudes where
freezing and melting is often repeated.
2.
Glacial areas are subject to frost wedging daily. In this process, the
rate of freezing is important.
3.
Rapid freezing of water causes its sudden expansion and high pressure
4.
The resulting expansion affects joints, cracks and small inter granular
fractures to become wider and wider till the rock breaks apart.
Salt Weathering
1.
Expansion of these salts depends on temperature and their thermal
properties
2.
High temperature ranges between 30 and 50 o C of surface temperatures in
deserts favour such salt expansion.
3.
Salt crystals in near-surface pores
4.
Cause splitting of individual grains within rocks, which eventually fall
off
5.
Salt crystallisation is most effective of all salt-weathering processes
6.
Sodium chloride and gypsum crystals in desert areas heave up overlying
layers of materials and with the result polygonal cracks develop all over the
heaved surface.
7.
With salt crystal growth, chalk breaks down most readily, followed by
limestone, sandstone, shale, gneiss and granite etc.
BIOLOGICAL ACTIVITY
AND WEATHERING
1.
Biological weathering is contribution to or removal of minerals and ions
from the weathering environment and physical changes due to growth or movement
of organisms.
2.
Burrowing and wedging by organisms like earthworms, termites, rodents
etc., help in exposing the new surfaces to chemical attack and assists in the
penetration of moisture and air.
3.
Human beings by disturbing vegetation, ploughing and cultivating soils,
also help in mixing and creating new contacts between air, water and minerals
in the earth materials.
4.
Decaying plant and animal matter help in the production of humic,
carbonic and other acids which enhance decay and solubility of some elements.
5.
Decaying plant and animal matter help in the production of humic,
carbonic and other acids which enhance decay and solubility of some elements.
6.
Algae utilise mineral nutrients for growth and help in concentration of
iron and manganese oxides.
MASS MOVEMENTS
1.
Disintegrated and fragmented rock materials due to mechanism of
weathering process.
2.
That means, air, water or ice does not carry debris with them from place
to place but on the other hand the debris may carry with it air, water or ice
3.
The movements of mass may range from slow to rapid, affecting shallow to
deep columns of materials and include creep, flow, slide and fall.
4.
Gravity exerts its force on all matter, both bedrock and the products of
weathering. So, weathering is not a pre-requisite for mass movement though it
aids mass movements.
5.
Mass movements are very active over weathered slopes rather than over
unweathered materials.
6.
Mass movements are aided by gravity and no geomorphic agent like running
water, glaciers, wind, waves and currents participate in the process of mass
movements.
7.
That means mass movements do not come under erosion though there is a
shift (aided by gravity) of materials from one place to another.
8.
Materials over the slopes have their own resistance to disturbing forces
and will yield only when force is greater than the shearing resistance of the
materials.
9.
Several activating causes precede mass movements
a. Removal of support from below to materials
above through natural or artificial means
b. Increase in gradient and height of slopes
c. Overloading through addition of materials
naturally or by artificial filling
d. Removal of material or load from over the
original slope surfaces
e. Occurrence of earthquakes, explosions or
machinery
f.
Heavy drawdown of water from lakes, reservoirs and rivers leading to slow
outflow of water from under the slopes or river banks
g. Indiscriminate removal of natural vegetation
Mass Movements
1.
Mass movements can be grouped under three major classes
a. Slow movements
b. Rapid movements
c. Landslides
Slow Movements
2.
Movement of materials is extremely slow and imperceptible except through
extended observation. Materials involved can be soil or rock debris.
3.
Depending upon the type of material involved, several types of creep
viz., soil creep, talus creep, rock creep, rock-glacier creep etc., can be
identified.
4.
This process is quite common in moist temperate areas where surface
melting of deeply frozen ground and long continued rain respectively, occur
frequently.
5.
When the upper portions get saturated and when the lower parts are
impervious to water percolation, flowing occurs in the upper parts.
Rapid Movements
1.
These movements are mostly prevalent in humid climatic regions and occur
over gentle to steep slopes.
2.
Movement of water-saturated clayey or silty earth materials down low-angle
terraces or hillsides is known as earthflow.
3.
When slopes are steeper, even the bedrock especially of soft sedimentary
rocks like shale or deeply weathered igneous rock may slide downslope.
4.
Another type in this category is mudflow. In the absence of vegetation
cover and with heavy rainfall, thick layers of weathered materials get
saturated with water and either slowly or rapidly flows down along definite
channels.
5.
It looks like a stream of mud within a valley
6.
Mudflows occur frequently on the slopes of erupting or recently erupted
volcanoes.
7.
Volcanic ash, dust and other fragments turn into mud due to heavy rains
and flow down as tongues or streams of mud causing great destruction to human
habitations
8.
A third type is the debris avalanche, which is more characteristic of
humid regions with or without vegetation cover and occurs in narrow tracks on
steep slopes.
9.
This debris avalanche can be much faster than the mudflow. Debris
avalanche is similar to snow avalanche
10.
In Andes mountains of South America and the Rockies mountains of North
America, there are a few volcanoes which erupted during the last decade and
very devastating mudflows occurred down their slopes during eruption as well as
after eruption.
Landslides
1.
These are known as relatively rapid and perceptible movements. The
materials involved are relatively dry.
2.
The degree of weathering and the steepness of the slope. Depending upon
the type of movement of materials several types are identified in this
category.
3.
Slump is slipping of one or several units of rock debris with a backward
rotation with respect to the slope over which the movement takes place
4.
Rapid rolling or sliding of Earth Debris fall is nearly a free fall of
earth debris from a vertical or overhanging face.
5.
Sliding of individual rock masses down bedding, joint or fault surfaces
is rockslide.
6.
Over steep slopes, rock sliding is very fast and destructive.
7.
Landslide scars over steep slopes. Slides occur as planar failures along
discontinuities like bedding planes that dip steeply.
8.
Rock fall is free falling of rock blocks over any steep slope keeping
itself away from the slope
9.
Rock falls occur from the superficial layers of the rock
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