Rock-weathering is a phenomenon of the interface between the atmosphere and lithosphere. Rocks which are exposed to the surface are brought to a new environment that is quite different from those under which the rocks were formed and they are acted upon by various natural agencies as a result of which the rocks undergo several changes.
Sometimes the rock masses are mechanically disintegrated with the physical forces associated with the natural agencies and sometimes they are decomposed and altered due to chemical reactions.
The general outcome is a layer of degraded rock materials resting upon the unaltered rock. Generally the term weathering is applied to the combined action of all processes causing rocks to be disintegrated physically and decomposed chemically because of their exposure at or near the earth’s surface.
In general, weathering occurs, where rocks and minerals come in contact with the atmosphere, surgical water and organic life under conditions that are normal to the surface of the earth.
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Weathering is the initial stage in the process of denudation and is considered as the static part of the general process of erosion. An essential feature of the process is that it affects rocks institute and does not involve transport of the degraded rock materials.
The products of rock- weathering, therefore, tend to accumulate as a soft-surface layer, to form a mantle of waste or regolith. The regolith grades downward into the solid, unaltered rock, which is commonly known as bed-rock. Weathering helps erosion to a considerable extent.
Factors Affecting Weathering
A number of natural factors affect weathering in various ways and control the degree of weathering to a major extent. Some of the major factors that affect weathering are as follows
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(a) Climate,
(b) Topography,
(c) Structure, texture and mineral composition of rocks,
(d) Vegetation cover,
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(e) Time,
Climate :
Climate is more generally defined as the sum-total of the meteorological elements like temperature, moisture including both humidity and precipitation, atmospheric-pressure, wind etc.
Climate determines whether mechanical disintegration or chemical decomposition will predominate and the speed with which these processes will operate.
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The climates of various parts of the world are mainly classified on the basis of temperature as well as pressure.
According to the temperature prevailing in a region, it is termed either as:
(i) Tropical
Where the temperature is uniform throughout the year and there is no winter.
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(ii) Temperate
Where temperature is highly variable and there is summer and winter.
(iii) Polar
Where temperature is always low mostly below 10°C.
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On the basis of precipitation i.e. rainfall the climate of a region may be termed as:
(i) Arid climate
Where there prevails high temperature and low rainfall.
(ii) Semi-arid climate
Where the annual rainfall is 25 to 50 cm.
(iii) Semi-humid climate
Where the annual rainfall is between 50 to 105 cm.
(iv) Humid climate
Where the annual rainfall is between 105 to 210 cms.
It is commonly observed that physical disintegration predominates in the arid or tropical climatic conditions i.e. in the drier, higher and colder regions of the world. Similarly, rock decomposition is more active in humid climates i.e. in moist, warm and low-lying areas.