Weathering, Mass wasting and Erosion

CHAPTER – IV

WEATHERING, MASS WASTING AND EROSION

Weathering is the disintegration and decomposition of rock in place.  It is a general term applied for a group of processes which act at or near the earth’s surface and reduce solid rock masses by physical disintegration or by chemical decomposition.  It is a process of the breakdown of rocks.

This disintegrated rock debris are called as regolith are subjected to gravity and tend to fall or slip down the slopes especially when aided by the lubricating action of water, though water is not the transporting agent in this case.  This process of gravitational transfer of mass of rock debris down slopes is called Mass wasting.

Erosion is essentially concerned with various ways in which the mobile agencies acquire and remove rock debris.  The principal erosional agents are running water, ground water, wind, glaciers, waves and currents.  Each of the agents does erosion in distinctive processes and gives rise to distinctive landforms.  Basically there are five common aspects

i.                     The acquisition of rock fragments

ii.                   Wearing away the surface through impact of rock materials in transit

iii.                  Breaking down the rock particles by mutual wear while in transit

iv.                 Transportation of the acquired rock debris by moving medium

v.                   Ultimately its deposition somewhere either in transit or at the end.

Factors controlling the weathering:

Broadly speaking four factors influence the rate of weathering – rock structure, climate, topography, and vegetation.

Types of Weathering:

Weathering is of two types – Physical or mechanical and Chemical weathering.  In the physical weathering the rocks are disintegrated by temperature changes, frost action and organism.  While in chemical weathering the rock minerals are decomposed, dissolved and loosened by water, oxygen and carbon dioxide of the atmosphere and soil water and by organisms and the products of their decay.  The physical, chemical and biological agents co-operate with one another actively and both mechanical breaking up or disintegration and chemical decomposition of rocks proceed simultaneously in nature.

The physical or mechanical weathering takes place in four ways. 

i.                     Frost action and crystal growth

ii.                   Thermal expansion and contraction by temperature changes

iii.                  Organic activity

iv.                 Expansion by unloading.

Physical weathering:

Frost action and crystal growth:  This type is found in cold climatic areas.  When water fills the pores, cracks and crevices in rocks and then freezes it expands in volume and exerts a bursting pressure thereby the rocks are ruptured, fragmented and wedged apart.  It is the most effective weathering process in the areas where there is repeated freezing and thawing takes place.

                Closely related to the formation of ice crystals in rocks is the disintegration of rocks by the growth of salt crystals.  The salt crystals form in dry climates as a result of capillary action of water containing salts.  During the long dry period as the water rises to the surface and evaporates, tiny crystal of salt are left behind in the porous outer zone of the sandstone.  The force generated by these crystals lead to grain-by-grain breaking if the sandstone which disintegrates into sand.

Thermal expansion and contraction by temperature changes:  when the rock surfaces are exposed to marked diurnal changes of temperature as in the hot deserts, the alternate heating and cooling results in alternate expansion and contraction which exerts a powerful disruptive force on the rocks.  In the hot dry regions the difference between the day and night temperatures often exceeds 30 degrees centigrade.  The intense heat of the day causes the thin surface layer of the rock to expand and to pull it away from the cooler layer within.  This process of peeling of flakes and curved shells of rock just as in onion is called Exfoliation.

Organic activity:  This type referrers to the action of plants and animals.  As the plant roots grow, they wedge the rocks apart and cause the widening of joints and other fractures.  Organic activity is however, is of great importance in chemical than in physical weathering. Dead organisms produce acids as they decay and thus promote chemical weathering.  Earthworms, ants, termites and other burrowing animals move material to or near the surface where they are more readily subjected to chemical weathering processes.

Expansion by unloading:  unloading occurs when large igneous bodies are exposed through the erosional removal of overlying rock the resultant reduction in pressure.  Igneous and metamorphic rocks formed at great depths are in a compressed state because of the continuing pressure of the overlying rock.  On being exposed to the surface they expand slightly in volume.  Expansion or dilation accompanying the unloading causes thick shells of rock to break free from the parent mass below.  The process which produces thin onion like layers or concentric large scale fractures is called Exfoliation.

Chemical weathering:

In general, chemical weathering is probably more important than physical weathering, this is particularly in the warm and humid climates of the equatorial, tropical, sub-tropical zones, where heat and moisture are abundant.  Water is the main agent of chemical weathering.  Although water in a pure form is almost inactive, but when mixed with oxygen or carbon dioxide it becomes an active chemical agent.  Oxygen, carbon dioxide as well as watervapour are present in the atmosphere in abundant near the earth’s surface. Under this influence the rocks decompose, decay and break into smaller particle size and new secondary minerals are formed.  The chief chemical weathering processes are

i.                     Solution

ii.                   Oxidation

iii.                  Hydration and hydrolysis

iv.                 Carbonation

Solution:  it is a simple process in which the rock salt, gypsum and other minerals get dissolved in the water.  Limestone is not soluble in pure water but gets easily dissolved in rain water which contains carbon dioxide.  In fact atmospheric carbon dioxide is dissolved in all surface waters of the land, including rain water, soil water and river water.

Oxidation:  The presence of the dissolved oxygen in water in contact with mineral surfaces leads to oxidation.  Since some dissolved oxygen is always present in rainwater, surface water as well as sub-surface water oxidation is a universal phenomenon.  The effects of oxidation are most apparent in rocks containing Iron.  Iron rusts or oxidizes under the influence of moisture, as the dissolved oxygen changes the ferrous iron in mineral compounds to the more oxidized ferric state.  When iron combines with oxygen, the original mineral structure is destroyed and the mineral components are free to participate in other chemical reactions.

Hydration and Hydrolysis:  The process of hydration involves the absorption of water.  Most rock- forming minerals absorb rain water.  This not only increases their volume but also produces chemical changes resulting in the formation of new minerals which are relatively softer and more voluminous.  By the process of hydration a hydrite is converted to gypsum and haematite to limonite.  Both these reactions are however reversible upon application of heat, which indicates that there has been no fundamental chemical change.  In hydrolysis, on the other hand, there is a chemical change, and both the mineral and water molecules decompose and react to form new compounds. The significant result of hydration and hydrolysis is that the new minerals formed are more easily attacked by the chemical and physical weathering processes.  The hydrolysis of exposed granite surfaces results in grain-by-grain break-up of the rock, creating many interesting boulder and pinnacle forms by rounding of angular joint blocks.

Carbonation:  Carbonation and hydrolysis are closely linked.  Rain dissolves some carbon dioxide as it falls through the atmosphere, and additional amounts released by decaying organic matter are acquired as the water percolates through the soil.  Carbonic acid particularly attacks minerals which contain iron, calcium, magnesium, sodium or potassium.  These elements are soluble in carbonic acid and the minerals and the rocks containing them start decaying under its influence.