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Azhar Shahzad

Introduction to Agents of Weathering

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All rocks, when exposed for sufficient length of time to the atmosphere, undergo decay from disintegration and decomposition, together referred to as weathering.

Disintegration is the break down into small particles by the action of mechanical agents of weathering such as rain, frost etc, decomposition is the breakdown of mineral particles into new compounds by the action of chemical agents such as acid in air and in rain and river water.

Denudation is the general term used for the wearing down of land areas by the processes originating and acting at the earth’s surface.

It includes both weathering and erosion. In addition to the atmospheric processes, agents of erosion  (rivers, moving ice, water waves) contribute to the deduction of the land in their particular spheres of action, they also transport weathered and eroded material away from areas where it is derived, to from deposits of sediments elsewhere.

Agents of Weathering

1. Rain

The mechanical action of rain consist mainly in the washing of loose particles of soil and rock to lower levels. This phenomenon is known as rain-wash. It is the means by which rivers receive much of the sediments they carry in suspension. The chemical weathering effects of the rain are seen its solvent action on some rocks notably limestones. The process depends on the pressure of feeble acids, derived from gases such CO2 and SO2 which are present in air in small quantities and which enter into solution in rain water.

The denuding effects of heavy showers and rain-storms may be very sever, especially in regions where a covering of vegetation is lacking. It cuts gullies in the surface of the ground, some of considerable size and may cause great damage by the destruction of roads and livestock. Heavy rains also promote land slides. Vegetation protects the ground from the immediate disintegrating effects of rainfall.

2. Frost

In cold climates the action of the frost is to break off angular fragment from exposed rock surface, a process sometimes referred to as ice-wedge. Water enters rock along pores, cracks and fissures. On freezing it expands and occupies about 10% greater volume exerting a pressure of about 2000 lbs per square inch. This is therefore like a miniature blasting and brings about the disintegration of the rock. The loosened particles fall from the mass and accumulate as heaps of talus at lower levels and this material may later be consolidate into a deposit known as breccia.

3. Wind

It is one of the two natural agents which transport rock material against gravity. Its effect is three-fold. First it removes loose particles of rock decay as it blows over a surface, then charged with these grains the wind act as an abrading sand-blast driving the grains against rock surfaces which becomes worn and polished in course of time. Thirdly the blown grains are accumulated to from sand-dunes.

Lines of communication may be seriously affected by wind-blown sand in arid countries. It is on record  that the telegraphic  wire on the trans-Caspain railway was worm down to half of its diameter in eleven years, and renewal was then made. To avoid accumulation of sand alongside railway embankments in Sudan, culverts have been made to allow for easy passage of the wind and its load sediments.

4. Insolation

When a rock surface is exposed to a considerable daily range of temperature, as in arid and semi-arid regions, the expansion which occurs during the day and contraction at night, constantly repeated have a weakening effect on the texture of the rock over a period of time . The outer heated layers tend to pull away from the cooler rock underneath a process known as exfoliation. By the unequal expansion and contraction of its mineral constituents the strain is set up in a rock and its texture is loosened. This kind of weathering is prevalent in climates where high day and low night temperatures are prevalent.

Weathering by Organic Agents 

Plants retain moisture and any rock surface on which they grow is kept damp, thus aiding the solvent action of the water. The chemical decay of the rock is also promoted by the formation of vegetable humus organic product of the decay of plants. The mechanical break up of rocks is helped by the roots of plants which penetrates into cracks and crevices and tend to wedge apart the rock.


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