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WEATHERING
Weathering is the action of elements of weather and climate over earth materials.
There are a number of processes within weathering that act either individually or together to affect the earth materials in order to reduce them to a fragmental state.
Weathering is defined as the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
CHEMICAL WEATHERING PROCESSES
A group of weathering processes viz; solution, carbonation, hydration, oxidation, and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface, and/or soil water, and other acids.
Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions.
Over and above, the carbon dioxide present in the air, decomposition of plants and animals increases the quantity of carbon dioxide underground.
SOLUTION
When something is dissolved in water or acids, the water or acid with dissolved contents is called a solution.
This process involves the removal of solids in solution and depends upon the solubility of a mineral (like nitrates, sulfates, and potassium) in water or weak acids.
So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones are soluble in water containing carbonic acid (formed with the addition of carbon dioxide in water) and are carried away in the water as a solution.
Carbon dioxide produced by decaying organic matter along with soil water greatly aids in this reaction.
CARBONATION
Carbonation is the reaction of carbonate and bicarbonate with minerals and is a common process helping the breaking down of feldspars and carbonate minerals.
Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave information.
HYDRATION
Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock.
Calcium sulfate takes in water and turns to gypsum, which is more unstable than calcium sulphate.
This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
Many clay minerals swell and contract during wetting and drying and a repetition of this process results in the cracking of overlying material.
Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing.
OXIDATION AND REDUCTION
In weathering, oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
In the process of oxidation, rock breakdown occurs due to the disturbance caused by the addition of oxygen.
The red color of iron upon oxidation turns brown or yellow.
When oxidized minerals are placed in an environment where oxygen is absent, reduction takes place.
Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
Red color of iron upon reduction turns to greenish or bluish-grey. These weathering processes are interrelated.
Hydration, carbonation, and oxidation go hand in hand and hasten the weathering process.
PHYSICAL WEATHERING PROCESSES
Physical or mechanical weathering processes depend on some applied forces.
The applied forces could be:
Gravitational forces such as overburden pressure, load and shearing stress;
Expansion forces due to temperature changes, crystal growth, or animal activity;
Water pressures controlled by wetting and drying cycles.
Many of these forces are applied both at the surface and within different earth materials leading to rock fracture.
Most of the physical weathering processes are caused by thermal expansion and pressure release.
These processes are small and slow but can cause great damage to the rocks because of continued fatigue the rocks suffer due to repetition of contraction and expansion.
UNLOADING AND EXPANSION
Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.
Fractures will develop roughly parallel to the ground surface.
Large, smooth rounded domes called exfoliation domes to result due to this process.
TEMPERATURE CHANGES AND EXPANSION
Various minerals in rocks possess their own limits of expansion and contraction.
Because of diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place regularly.
This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.
In rocks like granites, smooth-surfaced and rounded small to big boulders called tors form due to such exfoliation..
FREEZING, THAWING, AND FROST WEDGING
Frost weathering occurs due to the growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
This process is most effective at high elevations in mid-latitudes where freezing and melting are often repeated.
Glacial areas are subject to frost wedging In this process, the rate of freezing is important.
Rapid freezing of water causes its sudden expansion and high pressure.
The resulting expansion affects joints, cracks, and small intergranular fractures to become wider and wider till the rock breaks apart.
SALT WEATHERING
Salts in rocks expand due to thermal action, hydration, and crystallization. Many salts like calcium, sodium, magnesium potassium, and barium have a tendency to expand.
Salt crystals in near-surface pores cause the splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may result in granular disintegration or granular foliation.
Salt crystallization is the most effective of all salt-weathering processes.
Sodium chloride and gypsum crystals in desert areas heave up overlying layers of materials and as the result, polygonal cracks develop all over the heaved surface.
With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss, and granite, etc.
BIOLOGICAL ACTIVITY AND WEATHERING
Biological weathering is the contribution to or removal of minerals and ions from the weathering environment and physical changes due to the growth or movement of organisms.
Burrowing and wedging by organisms like earthworms, termites, rodents, etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
Human beings by disturbing vegetation, ploughing, and cultivating soils, also help in mixing and creating new contacts between air, water, and minerals in the earth material.
Decaying plant and animal matter help in the production of humic, carbonic, and other acids which enhance decay and solubility of some elements.
Algae utilize mineral nutrients for growth and help in the concentration of iron and manganese oxides.
Plant roots exert tremendous pressure on the earth materials mechanically breaking them apart.
SIGNIFICANCE OF WEATHERING
Weathering processes are responsible for breaking down the rocks into smaller fragments and preparing the way for the formation of not only regolith and soils but also erosion and mass movements.
Biomes and biodiversity are basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.
Weathering aids mass wasting, erosion, and reduction of relief, and changes in landforms are a consequence of erosion.
Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminum, copper, etc., which are of great importance for the national economy.
Weathering is also an important process in the formation of soils.
MASS MOVEMENTS
Mass movements transfer the mass of rock debris down the slopes under the direct influence of gravity.
The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide, and fall.
Mass movements are very active over weathered slopes rather than over un-weathered materials.
That means mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.
Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation, etc., favor mass movements.
They are:
1. Removal of support from below to materials above through natural or artificial means
2. Increase in gradient and height of slopes;
3. Overloading through the addition of materials naturally or by artificial filling;
4. Overloading due to heavy rainfall, saturation, and lubrication of slope materials;
5. Removal of material or load from over the original slope surfaces;
6. Occurrence of earthquakes, explosions, or machinery;
7. Excessive natural seepage;
8. heavy drawdown of water from lakes, reservoirs, and rivers leading to slow outflow of water from under the slopes or river banks;
9. Indiscriminate removal of natural vegetation.
Heave (heaving up of soils due to frost growth and other causes), flow, and slide are the three forms of
Mass movements can be grouped under three major classes:
Slow movements;
Rapid movements;
SLOW MOVEMENTS
1. CREEP can occur on moderately steep, soil-covered slopes.
The movement of materials is extremely slow and imperceptible except through extended observation. Materials involved can be soil or rock derbies.
Depending upon the type of material involved, several types of creep viz., soil creep, talus creeps, rock creeps, rock-glacier creep, can be identified.
2. SOLIFLUCTION which involves slow downslope flowing soil mass or fine-grained rock debris saturated or lubricated with water.
This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long-continued rain respectively, occur frequently.
When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.
RAPID MOVEMENTS
These movements are most prevalent in humid climatic regions and occur over gentle to steep slopes.
1. EARTHFLOW
The movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earthflow.
Quite often, the materials slump making step-like terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.
2. MUDFLOW
In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite channels.
When the mudflows emerge out of channels onto the piedmont or plains, they can be very destructive engulfing roads, bridges, and houses.
3. AVALANCHE
Avalanche is more characteristic of humid regions with or without vegetation cover and occurs in narrow tracks on steep slopes.
This debris avalanche can be much faster than the mudflow.
Debris avalanche is similar to snow avalanche.
LANDSLIDES
These are known as a relatively rapid and perceptible movement.
The materials involved are relatively dry.
The size and shape of the detached mass depend on the nature of discontinuities in the rock, the degree of weathering, and the steepness of the slope.
1. SLUMP is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place.
2. DEBRIS SLIDE – Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide.
Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
3. ROCKSLIDE – Sliding of individual rock masses down bedding, joint, or fault surfaces is rockslide.
Over steep slopes, rock sliding is very fast and destructive.
Slides occur as planar failures along discontinuities like bedding planes that dip steeply.
4. ROCKFALL -Rockfall is the free-falling of rock blocks over any steep slope keeping itself away from the
Rockfalls occur from the superficial layers of the rock face, an occurrence that distinguishes it from rockslide which affects materials up to a substantial depth.
