PHYSICAL GEOLOGY
WEATHERING
Weathering is defined as a process of decay, disintegration and decomposition of rocks under the influence of certain physical and chemical agencies.
Disintegration:
It may be defined as the process of breaking up of rocks into small pieces by the mechanical agencies of physical agents.
Decomposition:
It may be defined as the process of breaking up of mineral constituents to form new components by the chemical actions of the physical agents.
Denudation:
It is a general term used when the surface of the earth is worn away by the chemical as well as mechanical actions of physical agents and the lower layers are exposed.
The process of weathering depends upon the following three factors:
i) Nature of rocks
ii) Length of time
iii) Climate
Two Chief types of weathering are commonly distinguished on the basis of type of agency involved in the process and nature of the end product. They are:
i) Physical or mechanical weathering
ii) Chemical weathering
Physical weathering:
It is the physical breakdown of rock masses under the attack of certain atmospheric agents. A single rock block is broken gradually into smaller irregular fragments and then into particles of still smaller dimensions. It is the most active in cold, dry and higher areas of the earth surface Temperature variations are responsible to a great extent of physical weathering.
Thermal effects:
The effect of change of temperature on rocks is of considerable importance in arid and semi arid regions where difference between daytime and nighttime temperature is often very high. Such temperature fluctuations produce physical disintegration in a normally expected manner. Expansion on heating followed by contraction on cooling. When the rock mass is layered and good thickness additional disturbing stresses may be developed into by unequal expansion and contraction from surface to the lower regions. The rock sometimes is found to break off into concentric shells. This process is known as exfoliation.
When weathering occurs part of the disintegrated rock material is carried away by running water or any other transporting agent. Some of them are left on the surface of the bedrock as residual boulders. It is often seen that boulders have an onion like structure. This kind of weathering is called spheroidal weathering.
Chemical weathering:
The chemical decomposition of the rock is called chemical weathering which is nothing but chemical reaction between gases of the atmosphere and minerals of the rocks. The chemical changes invariably take place in the presence of water generally rainwater -in which are dissolved many active gases from the atmosphere like C02, nitrogen, Hydrogen etc.These conditions are defined primarily by chemical composition of the rocks humidity and the environmental surrounding the rock under attack.
Chemical weathering is essentially a process of chemical reactions between gases of the atmosphere and the surface rocks. For example:
1) 2CaCO3 + H2O + CO2 —————— 2 Ca (HCO3) 2
2) CaSO4 + 2H2O ——————– CaSO42.H2O
Engineering importance of rock weathering:
As engineer is directly or indirectly interested in rock weathering specially when he has to select a suitable quarry for the extraction of stones for structural and decorative purposes. The process of weathering always causes a lose in the strength of the rocks or soil.
For the construction engineer it is always necessary to see that:
To what extent the area under consideration for a proposed project has been affected by weathering and
What may be possible effects of weathering processes typical of the area on the construction material.
LANDFORM AND PROCESS ASSOCIATED WITH WIND
The earth is surrounded by an envelop of gases called the atmosphere. The movement of the atmosphere in a direction parallel to the earth surface is wind.i.e the air in motion is called wind whereas the vertical movement s of the atmosphere are termed as air currents.
Erosion by wind and developed features:
Wind erosion is generally caused by two erosion processes:
i) Deflation
ii) Abrasion.
Deflation:
Deflation is the process of simply removing the loose sand and dust sized particles from as area, by fast moving winds. Wind deflation can successfully operate in comparatively dry regions with little or no rainfall and where the mantle is unprotected due to absence of vegetation.
Such a removal of loose fine particles may at certain places leave a denuded surface consisting mostly of hard rocks or coarse materials like gravel and is called lag gravel. This gravel layer prevents further deflation.
Abrasion:
The wind loaded with such particles attains a considerable erosive power which helps a considerable er4osive power which helps in eroding the rock surfaces by rubbing and grinding actions and produce many changes. This type of wind erosion is known as abrasion.
Vertical column of rocks are thus more readily worm out towards their lower portions and a result pedestal rocks are formed which wider tops have supported on comparatively narrower bases. Such type of rock formations is called Pedestal or Mushroom rocks.
Transportation by wind:
The total sediment load carried by a wind can be divided into two parts. a) Bed load
b) Suspended load
The larger and heavier particles such as sands or gravels, which are moved by the winds but not lifted more than 30 to 60 cm of the earth surface constitute the bed load. Whereas the finer clay
or dust particles which are lifted by the moving winds by a distance of hundreds of meters above the earths surface constitute the suspended load.
Deposition of sediment by wind and the developed features:
The sediments get dropped and deposited forming what are known as Aeolian deposits. There are two types of Aeolian deposits;
a) Sand dunes b) Loess
Sand dunes:
Sand dunes are huge heaps of sand formed by the natural deposition of wind blown sand sometimes of characteristics and recognizable shape. Such deposits are often found to migrate from one place to another due to change in the direction and velocity of wind.
The active dunes can be divided into three types:
a) Barchans or Crescent shaped dunes b) Transverse dunes
c) Longitudinal dunes
Barchans:
These dunes that look like a new moon in plan are of most common occurrence. They are triangular in section with the steep side facing away from the wind direction and inclined
at an angle of about 300 to 330 to the horizontal.
The gently sloping side lies on the windward side, and makes an angle of about 10 to 150
with the horizontal. They may have variable sizes, with a generally maximum height of about 335 meters and horn to horn width of say 350 meters.
Transverse Dunes:
A transverse dune is similar to a barchans in section but in plan it is not curved like barchans such that its longer axis is broadly transverse to the direction of the prevailing winds.
Longitudinal dunes:
Longitudinal dunes are the elongated ridges of sand with their longer axis broadly
parallel to the direction of the prevailing wind. When seen in the side view they will appear to be triangular on an average they may be 3 m height and 200 m long.
Loess:
The finest particles of dust travelling in suspension with the wind are transported to a considerable distance. When dropped down under favourable conditions these have been found to accumulate in the different constituents the form of paper-thin laminae, which have aggregated together to form a massive deposit known as Loess.
Engineering considerations:
In general no site is selected for any type of important work on the moving dunes because such dunes are always a source of trouble to an engineer. It has been experienced that sometimes the moving dunes damage certain important works. But if an engineer is compelled to select such a site, special methods should be adopted to check the motion of the moving dunes. For ex:
Either to construct wind breaks or growing vegetation on the surrounding areas.