weathering

geography chapter 9

define weathering.

weathering is described as the disintegration or decomposition of a rock in size by natural agents at or near the surface of the earth.

what are the chief characteristics of weathering?

1. it involves disintegration and decay of solid rocks.

2. depends on climatic elements and on characteristics of rocks.

3. it affects the surface of the earth.

4. it causes formation of soil.

5. it involves not one but several processes for disintegration of rocks.

6. changes massive hard rocks into finer materials.

7. prepares rock materials for transportation by agents of gradation.

what are the 3 types of weathering?

physical, chemical and biological.

define physical weathering.

physical weathering involves rock disintegration without any changes in the chemical constituents of the rock.

what are the main factors responsible for physical weathering?

temperature changes, moisture, frost action and wind.

weathering is more rapid in hot deserts because temperature changes are the sharpest there.

what are the 3 types of mechanical weathering?

block disintegration, granular disintegration, exfoliation.

explain block disintegration.

• sudden rise and fall of temperature causes expansion and contraction of rocks.

• the repeated action causes their breakdown.

• temperature decreases abruptly at night and rises abruptly in day.

• leads to tension and splitting of rocks known as block disintegration.

explain granular disintegration.

• as rocks are made up of different minerals, weathering may reduce the rocks to pieces and fragments - this is called granular disintegration.

• may take place at cold places at high altitudes.

• cracks and joints of rocks get filled with water during day and freeze during night leading to rupture of rocks —> frost action.

what is exfoliation?

• sudden temperature changes may also cause fissures into which water penetrates which in turn stimulates chemical weathering.

• most rocks are not good conductors of heat.

• the sudden contraction and expansion peels off the outer layer known as exfoliation.

describe chemical weathering.

• some or all of the minerals of the rocks suffer decay or alteration by agents like water, oxygen, carbon or various organic acids.

• leads to decomposition of rocks and minerals.

• 4 main processes - oxidation, carbonation, hydration, solution.

explain oxidation.

• it means reaction of minerals in rocks to atmospheric oxygen.

• rainwater also contains atmospheric oxygen.

• on coming in contact with iron-compounds in rocks, the iron starts rusting or crumbling.

• colour of the rock changes to red, yellow or brown.

explain carbonation.

• many mineral constituents of rocks like CO₂ while coming in contact with water produce acidic effect on rocks.

• this action dissolves most calcareous rocks like gypsum, limestone etc.

• rainwater converts calcium carbonate into calcium bicarbonate which is soluble and taken away by the ground water.

explain hydration.

• expansion of minerals occurs on coming into contact with rainwater.

• minerals become heavy and start disintegrating.

• rocks like feldspar —> kaolin.

explain solution.

• minerals in the rocks dissolve in water.

• all rocks are subjected to solution at some extent.

• the rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure.

what is biological weathering?

• AKA organic weathering.

• main agents - animals, insects, plants and man.

• both physical disintegration and chemical decomposition are involved.

what are the 3 agents of biological weathering?

humans, plants and animals & insects.

how are humans an agent of biological weathering?

• man is the most active agent of both physical and chemical weathering of rocks.

• mining, excavation, constructing roads, buildings etc. cause physical disintegration of rocks.

• agriculture, dumping of chemicals and wastes etc. promote both physical and chemical weathering.

how are plants an agent of biological weathering?

• roots of large trees reach deep into rocks and cause physical disintegration due to pressure.

• most vegetation, however, prevents disintegration of rocks because it binds the surface layer and does not allow exposure of rocks beneath.

how are animals and insects agents of biological weathering?

• animals like rabbits, worms, moles bring a lot of fine material to the surface.

• burrowing animals helps loosen the surface materials around the rocks, facilitating their physical disintegration.