WEATHERING

INTRODUCTION

Weathering is the combined action of all processes, whereby rock is decomposed and disintegrated because of exposure at or near the earth’s surface. Weathering normally changes the hard, massive rock into finally fragmented, soft residual overburden, the parent matter of the soil. It depends both on the nature of the climatic elements involved and on the character of the rock; its chemical composition, its hardness, its texture and its permeability. The chief agents of weathering are insolation, frost, rainwater, gases of the atmosphere, and organisms.

WEATHER PROCESSES

Weathering processes may be subdivided into two large groups; Physical or Mechanical Weathering and Chemical Weathering. Mechanical Weathering tends to breakdown the rock into progressively smaller fragments while Chemical Weathering forms residual materials; the joint result is the production of a loose layer, which can readily be removed by the agents of transportation. The influence of man, plants and animals, although not strictly part of weathering, may be considered as a biological agency which directly contributes to the layers of rock waste (Biological Weathering).

Physical / Mechanical Weathering
Mechanical weathering involves the destruction of rocks through the imposition of certain stresses. This is carried out in deserts by rapid changes of temperature or in mountains by the action of frost. Mechanical weathering is also carried on, to a limited extent, by plants and trees. In regions of dry climates, wind is also a powerful agent of weathering. The physical or mechanical processes of weathering produce fine particles from massive rock, but do not change its chemical composition.

1. Frost Action
One of the most important physical weathering processes in cold climates is Frost Action, the repeated growth and melting of ice crystals in the pore spaces or fractures of rocks.
At night the temperature of water drops on cooling to 4oC (contraction). Then from 4oC to 0oC, water expands and freezes at 0oC. When the water freezes, it exerts a pressure of almost 2,000 lbs/in2.
As water in joints freezes, it forms needle-like ice crystals extending across the openings. As these ice needles grow (water increases in volume about 9% as it freezes, i.e., 1/10th of its volume), they exert tremendous force against the confining walls and can easily pry apart (move, push/pull) the joint blocks. Even massive rocks can be shattered by the growth of ice crystals created from water that has previously soaked into the rock. On mountain peeks, this process creates sharp pinnacles and angular outlines. Such peeks are described as Frost-Shattered Peaks.
Freezing water strongly affects soil and rock in all middle and high latitude regions having a cold winter season, but its effects are most striking in high mountains, above the timber line. Here the separation and shattering of joint blocks may produce an extensive ground surface littered with angular blocks. Such a surface is termed a Block Field or Felsenmeer (rock sea), or Boulder Field. Where cliffs of bare rocks exist at high altitudes, fragments fall from the cliff face, building up piles of loose blocks into conical forms, termed Talus Cones or a Scree Slope. [Strahler 398]
Example of frost action is Millstone Grit Areas of the Pennines (England).

2. Growth of Salt Crystals
Closely related to the growth of ice crystals is the weathering process of rock disintegration by growth of salt crystals (process called salt wedging). This process operates extensively in dry climates or arid regions and is responsible for many of the niches (holes, corners), shallow caves, rock arches, and pits in sandstone formations.
During long drought periods, ground water is drawn to the surface of the rock by capillary force. As evaporation of the water takes place in the porous outer zone of the sandstone, tiny crystals of salt are left behind. The growth force of these crystals is capable of producing granular disintegration of the sandstone, which crumbles into sand and is swept away by wind and rain. Specially susceptible are zones of rock lying close to the base of a cliff, for here the ground water tends to sweep outward.

3. Swelling and Shrinking of Soil
An important but little appreciated process of physical weathering is the continual swelling and shrinking of soils as the particles of fine silt and clay absorb or give up soil water in alternate periods of rain and drought. Shrinkage forms soil cracks in dry periods, making the infiltration of rainfall much more rapid in early stages of an ensuing (resultant) rain. In clay rich sedimentary rocks, such as shale, the swelling is largely responsible for a spontaneous breakup known as Slaking, in which the shale crumbles into small chips or pencil-like fragments when exposed to the air.
In tropical regions like Malaysia, short downpours saturate the rocks while the hot sun quickly dries them again. In coastal areas, the wet rocks may be quickly dried by sun and wind between the tides. Water expands the outer layers of the rock while drying causes shrinkage.

4. Diurnal Range of Temperature
Most crystalline solids, such as minerals of the rocks, tend to expand when heated and to contract when cooled. When rock surfaces are exposed daily to the intense heating of the sun alternating with nightly cooling, the resulting expansion and contractions exerts powerful forces upon the rock. Given sufficient time (tens of thousands of such daily alternations), even the strongest rock may develop fractures. Breakage can take the form of exfoliation (onion peeling) or granular disintegration.

5. Unloading
One process of physical disintegration of rocks is known as Unloading or Pressure Release. If overlying layers of rocks are removed by denudation (exposure), the release of this weight-caused pressure may allow the newly exposed rock to expand and form new curvilinear (well-developed round) joints, causing curved rock-shells to pull away from the mass, a process known as Sheeting. Granite appears to be particularly prone (liable) to this. The enormous domes of the Yosemite Valley in California are probably the result of this outwards expanding force of pressure release on little jointed granite.

6. Wedging of Plant roots
In physical weathering processes, the wedging of plant roots deserve consideration as a possible mechanism, whereby joint blocks may be separated. We have all seen, a tree whose lower trunks and roots are firmly wedged between two great joint blocks of massive rock. The growth of tiny rootlets in joint fractures are of great importance in loosening countless small rock scales and grains, particularly when a rock has already been softened by decay or fractured by frost action.

7. Wind
The atmosphere has a mechanical action through wind. Fantastic weathering results when wind blows fine grit or sand against the exposed rock. The winds act as a sand blast and cause uneven weathering according to the hardness and softness of the rock, e.g., Brimham Rocks, Yorkshire, England.
This action is most pronounced in sandy regions. In some towns on the West Coast of the United States, the windows facing the shore have become so worn that they appear to be made of frosted glass. Trains that cross desert have to repainted more frequently due to the action of dust-laden wind. In many sea side areas, cliffs are fantastically weathered as a result of the wind blown sand.
In dry regions, wind aids in disintegration by cracking and peeling away the layers of rocks in shell-like formations. This is known as exfoliation.

8. Rainfall
Rain also acts mechanically by washing away the loose particles of insoluble material. The carving out of Gullies or miniature Valleys can be seen very well on the pit heaps of mining areas. Each successive rain storm, beating on the loosely piled material cuts the gullies deeper and deeper. The material is then carried to the foot of the slope partly by the force of rain.
The sides of an active volcano show such a carving with narrow gullies radiating out of the mountain like the spikes of a wheel.
The activity of rain as an erosive/weathering agent can be seen in the formation of Earth Pillars. These are formed in regions of soft soil or clays containing scattered blocks of rock. The rain washes away the soft material, except where a small boulder occurs. The boulders protect the underlying soil from the action of rain. As a result, pillars of earth are formed, each one in the early stages, having a CAP of rock. These pillars are not long-lived, for in time the cap falls and the rain destroys the uncapped softer material. Most famous earth pillars are in Bolzano, Italy.

Chemical Weathering

Chemical weathering is chiefly carried on by rain water and the atmospheric gases. The atmospheric moisture in conjunction with various gases, brings about a chemical change in the rocks. The rocks begin to decompose and they decay. High temperature and humidity, specially favor this sort of activity and, therefore, it is most common in the hot wet regions of the world. Oxygen, CO2 and water affect the composition of the rocks.
Some minerals such as quartz resist this alteration quite successfully, but other dissolve easily, such as the calcium carbonate or lime stone. In any rock made up of a combination of minerals, the chemical breakdown of one set of mineral grains leads to the disintegration of the whole mass. In granite, for example, the quartz resist chemical decay much more effectively than the feldspar, which is chemically more reactive and weathers to become clay. So even a rock as hard as granite cannot withstand the weathering process forever.
In Malaysia, the surface of the exposed granite is found to be pitted and rough. This is because the granite is made of three main minerals, i.e., quartz, feldspar, and mica. The feldspar is more quickly weathered and worn away. The quartz crystals are eventually loosened and form a course sandy residue.
The weathered material (regolith; including soil, broken rock, volcanic ash and glacial material overlying the bedrock also called mantle rock) may be taken away by the erosive agents or it may stay in position forming basis of soil. That soil contains regolith (minerals) and organic materials.
The chemical weathering may be said to take the following forms:

1. Oxidation
The presence of dissolved oxygen in water in contact with mineral surfaces in the soil leads to oxidation, which is the combination of oxygen ions with metallic ions, such as calcium, sodium, potassium, magnesium and iron; abundant in the silicate minerals. When oxygen reacts with iron, the latter gets a coating of iron oxide or rust, which weathers it away and the piece of iron decades. Rocks, which contain a mineral content of iron, are oxidized during the rainy season and whole rock mass decomposes.
The products of oxidation are compounds of iron and aluminum, which account for the reddish color seen in so many rocks and soils. In tropical areas, oxidation is the dominant chemical weathering process; e.g. Grand Canyon.
The rock surfaces of old tombstones or buildings are decaying e.g., Oxford and Cambridge Colleges. The carvings on Cleopatra’s Needle have become far less distinct during the 80 years than the monument has been on Thames Embankment. It had stood in the dry climate of Egypt for 4000 years.

2. Carbonation
Various circumstances may convert water into a mild acid solution, thereby increasing its effectiveness as a weathering agent. With a small amount of CO2, for example; water forms Carbonic Acid, which in turn reacts with carbonate minerals such as Lime Stone and Dolomite (a hard relative of limestone; a carbonate of calcium and magnesium). This form of chemical weathering, carbonation, is specially vigorous in humid areas where percolating waters contain CO2, derived from its passage through the atmosphere, it act as a dilute acid upon calcareous rocks such as lime stone and chalk dissolving and removing them in the form of calcium bicarbonate.

3. Hydrolysis
Water itself combines with certain mineral compounds in a reaction known as Hydrolysis. This process in not merely a soaking or wetting of minerals, but a true chemical change producing a different compound and a different mineral. It may also lead to volume expansion which in turn contribute in the breakdown of rocks. The reaction is not readily reversible under atmospheric conditions, so that the products of hydrolysis are stable and long lasting. The hydrolysis of granite with accompanying granular disintegration and exfoliation of thin scales, produces many interesting boulder and pinnacle (tower) forms by rounding of angular joint blocks. These forms are particularly conspicuous in arid regions because of the absence of any thick cover of soil and vegetation. There is ample moisture in most deserts for hydrolysis to act, given sufficient time. In warm, humid climates, hydrolysis of susceptible (exposed, vulnerable) rocks goes on below the soil and may result in the deep decay of igneous and metamorphic rocks to depths as much as 100-300 ft.

4. Hydration
Certain minerals have the property of taking up water and thus expanding, so stimulating the disintegration of the rock containing them; this is hydration.

5. Solution
The rain water is able to dissolve certain minerals and leach the soil. Through this process, many minerals are washed out of the soil and rocks so that their chemical composition changes. Some minerals, such as quartz are virtually unaffected; others such as Olivine, Augite, Hornblende, Biotite, Orthoclase, and Muscovite are very susceptible; and a few such as Rock Salt can be completely removed in solution.
Solution is the most potent weathering process in lime stone regions because the rain water dissolves the calcium carbonate (CaCO3). The removal of CaCO3 (marble and chalk), leaves joints and cracks which are slowly widened and the whole system of caves and passages is worn out. The chemical action of rain on the feldspar in granite leads to the formation of Kaolin or China Clay.
River Thames carries down to the sea in solution 250,000 tons of chalk and limestone everyday.

Biological or Organic Weathering

Plants assist in surface weathering by both chemical and mechanical means. Algae, mosses, lichens (fungus) and other vegetation retain water on the surface of the rock, and various organic acids help to decay the rock beneath, so that a tuft of moss may lie in a small and growing hollow in the rock. The presence of vegetation increases the acid content of the soil-water, which will be effective in chemical disintegration of calcareous rocks. Water containing bacteria can assist the decomposition of some rocks particularly Lime Stones. The mechanical disintegration effect of vegetation is mainly due to the penetrating and expanding power of roots, which exert considerable force as they grow and help to widen cracks and crevices (fissures), thus allowing water and air to enter.
Various forms of animal life such as warms, rabbits and moles, may have a contributory effect. Warms bring large quantities of fine material to the surface in the form of casts (inclination), while burrowing animals in some measure help to loosen the surface material.
Technically, humans are also agents of biological weathering. Human activity contributes to various forms of weathering in a number of ways; by polluting the air with substances that greatly accelerate some chemical weathering, specially in and around large urban centers; by quarrying and mining, we accelerate mechanical, chemical and biological weathering through exposure of deep strata to these processes.

WEATHERING IN VARIOUS CLIMATIC REGIONS

The effects of the weather upon rocks vary accordingly to the potency of the different climatic elements.

1. Equatorial Latitudes
In equatorial latitudes, where both humidity and temperature are consistently high, chemical weathering is continuously active, and it is generally much more rapid and effective than the transport and removal of the weathered material.

2. Desert Areas
In desert areas there is little weathering by ordinary leaching, but considerable mechanical weathering. While chemical weathering takes place by the drawing of strong solutions to the surface by capillary action.

3. Mid Latitudes
In mid latitudes, frost is by far the most powerful agent, while solution, particularly in lime stone areas, exert great effects.

4. Polar Conditions
Under polar conditions, great areas of permanent snow prevent any ordinary weathering, but where nunataks project from ice-sheets, frost action is rampant. Chemical and organic agencies here seem to be negligible for their effects. CO2 is more soluble at low temperatures than at high, and as the melted water has therefore a higher carbonic acid content, chemical weathering may be quite active under a glacier or at the edge of an ice-sheet.

CONCLUSION

The weathering processes, both physical and chemical, work universally but produce few distinctive large land forms or spectacular activities that would draw the attention of the average person. Nevertheless, these processes are of enormous importance in slope development for that they prepare the bedrock for soil formation and for erosional removal by the agents of land sculpture. Without the weathering processes, vegetation could not thrive as we know it today, nor could the great continental land masses be easily reduced by the agents of denudation (exposure).
{Also see GOH’s Chapter 4}