Chapter 4 - Weathering

Geologi Kejuruteraan - BFC 3013 Weathering of Rocks

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Chapter 4 Weathering of Rocks INTRODUCTION Weathering is a general term describing all changes that result from the exposure of rock materials to the atmosphere. It is one of the most important geologic processes that leads to the disintegration or decomposition of geologic deposits. Weathering occurs because most rocks are in equilibrium with higher temperatures and pressure deep within the Earth. Rocks which are deeply buried lies in a different environment physically and chemically than those exposed on the earths surface and therefore changes will take place to accommodate these new conditions. If they are exposed to the much lower temperatures and pressures at the surface, to the gases in the atmosphere, and to the elements in water, they become unstable and undergo various chemical changes and mechanical stresses. As a result, the solid bedrock breaks down into loose, decomposed products. Rock fragments produced by weathering are removed by erosion and the general term for both weathering and erosion is known as denudation.

There are two classification of weathering processes which is physical and chemical weathering.

Figure 4.1 Sedimentary rocks in the Valley of the Gods in southern Utah. Notice that the layering in the rocks is horizontal and that erosion has exposed them in their present form. The red color results from iron cement in the rocks, which are mostly sandstone.


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4.1 Physical Weathering (Mechanical) Physical weathering is the mechanical breakdown of the rocks into smaller fragments without undergoing a change in chemical composition. No chemical elements are added to or subtracted from the rock. Physical forces that contribute to this type of weathering are frost action, unloading, saline crystal growth, alternate heating and cooling, and organic activities. 4.1.1 Frost Action Frost action works best in jointed rock or rocks with fractures in mountainous area with cool climates. Water that freezes in cracks and pores of rocks at temperature which drops below 0C will result in an increment of 9% in volume that will create pressure (compressive forces) against the wall of the fracture eventually widened the cracks.

Figure 4.2 Ice Wedging

4.1.2 Unloading / Exfoliation This is a process of reduction of pressure on underlying rocks by erosion that takes place on the overburden. The rocks expand as pressure is released and this process is known as unloading. The response to unloading may cause large joints (sheeting) to develop. The joints tend to be oriented parallel to the slope of the terrain. Natural erosion of overlying rocks has already induced unloading stresses in any exposed rocks. Further removal of material by man can create rapid strain.


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Figure 4.3 Sheeting Figure 4.4 Joint block separation

(a) (b) Figure 4.5 (a) Exfoliation occurs when solid rock mass comes apart in series of shells or plates (b) Jointing causes the rock to break up into large blocks 4.1.3 Saline Crystal Growth Combination of moisture and salts (halite, gypsum, etc.) has been found to cause scaling or decay of building stones. Stresses due to growth of salt can cause the rock to break apart physically. This process is particularly effective in porous rocks subjected to alternate wetting and drying. Further disintegration of rock may occur due to expansion of salt crystals which have grown in former voids.


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4.1.4 Alternate Heating and Cooling Happens in mountainous regions and deserts where rocks are subjected to drastic change of temperature. The rock will expand as they are heated during daytime and contract due to freezing temperature at night. This will lead to cracks and crevices 4.1.5 Organic Activities The activities of plants and animals also promote rock disintegration. Burrowing animals such as worms, ants and rodents mechanically mix the soil and loose rock particle. Pressure from growing roots widens cracks and contributes to the rock breakdown.

Figure 4.6 Ophiomorpha burrow, JKR Quarry, Bintulu - Miri Road Sarawak


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Figure 4.7 A tree that caused a growing crack in the rocks and thus contributes to mechanical weathering. 4.2 Chemical Weathering Chemical weathering reactions are exothermic and produced minerals of increased volume. Decomposition produces a chemical breakdown of rocks, which may destroy the original minerals and produce new ones while expansion will result in the physical disintegration or break up of rock. Common processes of chemical weathering reactions are oxidation, hydration, hydrolysis and dissolution. 4.2.1 Oxidation Occurs when oxygen in air assisted by water combines with minerals to form oxides. Oxidation normally occurs to rock or minerals such as olivine pyroxene and amphibole that contain high iron content and therefore produce rusty, red, yellow and brown rocks and soils.


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Mineralogical examples of iron oxidation include the weathering of pyrite tolimonite, or weathering of siderite to hematite. 4FeCO3 + O2 + 4H2O = 2Fe2O3 + 4H2CO3 (Siderite) (Hematite) 4.2.2 Hydration Hydration is the process whereby a mineral combines with water to form a hydrated mineral especially hydrated silicates and hydroxides. e.g. The hydration of anhydrite to gypsum. .

CaSO4 + 2H2O = CaSO4. 2H2O (Anhydrite) (gypsum) The most important of aspect of hydration is that the hydrated mineral is larger in volume than the parent mineral to exert pressure on its surrounding space and contribute to rock disintegration. 4.2.3 Hydrolysis The chemical union of water and a mineral is known as hydrolysis. This is the reaction of mineral with water to produce a new mineral or minerals. An example is the weathering of feldspar by reacting with water to form clay. Feldspar is an abundant mineral in a great many igneous, sedimentary, and metamorphic rocks, so it is important to understand how feldspars weather and decompose into clay minerals, which form the most abundant sedimentary rock, shale. Two substances are essential in the weathering of feldspars; carbon dioxide and water. The atmosphere and the soil contain carbon dioxide, which unites with rainwater to form carbonic acid. If K-feldspar comes in contact with carbonic acid, the following chemical reaction occurs.

2KALSi3O8 + H2CO3 + H2O (K-feldspar) (carbonic acid) (water) K2CO3 + Al2Si2O5(OH)4 + 4SiO2 (potassium carbonate) (clay mineral) (soluble hydrated silica)


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The rock increases in volume due to the reaction of the feldsphatic minerals by hydrolysis process. The decay of igneous rocks such as granite is largely attributed to the hydrolysis of the granitic minerals such as feldspars and biotite. Example: Decay of Granite Composition of granite: orthoclase quartz plagioclase small amount of ferromagnesian minerals

Quartz and feldspar are two most abundant components of granite. On weathering of granite: feldspar decay to form clay minerals quartz which is resistant to decay accumulates as quartz sand 4.2.4 Dissolution Process whereby rocks and minerals are dissolved in solution, like salt in water. Quantitatively, the most important minerals involved in dissolution are the carbonate minerals, calcite and dolomite. Some rock types can be completely dissolved. Rock salt is perhaps the best known example. Gypsum is less soluble than rock salt but also easily dissolved by surface water. Limestone dissolves due to its reaction with percolating water which contains dissolved carbon dioxide.

H2O + CO2 + CaCO3 = Ca ++ 2H2CO3 Carbon dioxide present in air, water and soils when unite chemically with certain rock minerals will alter the rock composition. Carbonic Acid an effective agent in. altering minerals such as calcite and dolomite.

CO2 + H2O = H2CO3 (Carbonic Acid)


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Figure 4.8 Solution features in thinly bedded limestone, Ipoh, Perak

Once granite is exposed, its mineral components start to react with the atmospheric components namely air and water. The first sign of chemical reaction is the indication of rusty color covering the rock surface. The disintegration of ferromagnesian minerals of granite is due to its instability under atmosphere conditions. When masses of granite have been exposed for long period of time the accumulated weathered product can be seen as clayey granular residue known as grus. 4.3 Spheroidal Weathering

In this type of weathering, a rounded shape is produced because weathering attacks an exposed rock from all sides at once, and therefore decomposition is more rapid along the corners and edges of the rock (Figure 4.10). As the decomposed material falls off, the corners become rounded and the block eventually is reduced to an ellipsoid or a sphere. Exfoliation is a special type of spheroidal weathering, where the rocks break apart by separation along a series of layers.

(a) (b)

Figure 4.9 (a) Granular disintegration common in coarse - grained igneous rocks (b) Granular disintegration in sandstone


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Figure 4.10 Spheroidal Weathering 4.4 Rates of Weathering There are few factors which determine the rate at which the exposed bedrock being weathered by various agents of weathering. (a) Composition of rocks

Mineral and chemical composition is one of the most important factors. Cementing materials (substance holding rock together)

Igneous rocks are resistant to mechanical weathering but more

susceptible to chemical weathering

Sedimentary rocks e.g. dolomites and limestones are decomposed by carbonation and solution

(b) Physical Condition of rock Crevices, cracks, holes will allow weathering agents to penetrate and eventually destruct the rock.


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(c) Topography Weathering is rapid where land slopes steeply. Increases in altitude have high rainfall and temperature will be low. This will eventually increase in the rate of weathering. (d) Climatic Condition Climates which have abundance rainfall and moist will accelerate the weathering process especially chemical weathering. Dry or cold weather are usually apt to physical weathering.

Figure 4.11 Type and extent of weathering vary with climate


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4.5 Classification of Soils (Product of Weathering) Most of the soils that cover the earth are formed by the weathering of various rocks. Soil is defined as weathered material that will support the growth of rooted plants (defined by geologist). It consist of minerals and weathered rock fragments (regolith), organic matter (humus), gases, water and living organisms. To a civil engineer, soil is simply unconsolidated material which typically disintegrates in water. Soil and surficial deposits can either be residual or transported in origin. Residual Soil Residual soils develop in situ, and their characteristics depend on the kind of bedrock from which they are derived. Residual soil deposits are common in humid tropical countries Transported Soil Transported soils are surficial deposits which accumulate due to the erosion, transportation, and deposition of weathered residual soil or bedrock. Four common surficial processes and their resulting deposits are: Colluvium: results from process of creep, whereby soil and weathered bedrock slowly move downslope due to gravity. Alluvium: includes all sediment deposited by streams. The deposits are stratified into layers of silt, sand, gravel and clay. Glacial drift: includes all deposits formed by glaciers. The weathering grades of in-situ material can be determined by using the weathering classification system. The weathering classification system is convenient in subdividing each unit to six distinct groups. The specified classification system adopted for residual soil is based upon material decomposition grades for weathered granite and volcanic rocks by Hencher and Martin (1982) and weathering classification for mudrocks by Anon (1977).


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Table 4.1 Weathering classification system for granite and volcanic rocks (Hencher and Martin, 1982)

GRADE DESCRIPTION TYPICAL DISTINCTIVE CHARACTERISTIC

VI Residual soil A soil formed by weathering in place but with original texture of rock completely destroyed.

V Completely weathered rock

Rock wholly weathered but rock texture preserved. No rebound from N Schmidt hammers. Slake readily in water. Geological pick easily indents surface when pushed.

IV Highly

weathered rock

Rock weakened so that large pieces can be broken by hand.

Positive N Schmidt rebound value of up to 25. Does not slake readily in water. Geological pick cannot be pushed into surface. Hand penetrometer strength index greater than 250 kPa. Individual grain may be plucked from surface.

III Moderately

weathered rock

Completely discolored. Considerably weathered but possessing strength such that

pieces 55mm diameter cannot be broken by hand. N Schmidt rebound value of 25 to 45. Rock material not friable.

II Slightly

weathered rock

Discolored along discontinuities. Strength approaches that of fresh rock. N Schmidt rebound value greater than 45. More than one blow of geological hammer to break

specimen. I Fresh rock No visible signs of weathering, discolored.


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Table 4.2 Weathering classification system for sedimentary rocks (Anon, 1977)

Grade Zone Description Remarks

Residual soil VI All rock material in degraded condition and original rock structure destroyed

No rock texture completely destroyed

Completely Weathered

V

All rock material in a degraded condition but original mass structure still discernible.

Slakes readily in water. Geological pick easily indents surface when pushed. Coring not possible by ordinary methods.

Highly Weathered IV

More than half of the rock material in a degraded condition

NX size core can be broken and crushed by hand. Rock material plastics does not readily slake in water

Moderately weathered III

Less than half of the rock material in a degraded condition

Hammer blow makes drumming sound possessing strength such that NX core (55mm) cannot be broken by hand. Rock material not plastic.

Slightly weathered

II

Discoloration of discontinuity weathered surfaces and some degradation material on discontinuity surface.

Hammer blows give a dull note. Needs more than one blow of the geological hammer to break specimen.

Faintly weathered IB

Discoloration of major discontinuity surfaces.

Fresh IA No visible evidence of weathering


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Table 4.3 Weathering profiling of sub surface (Martin and Hencher, 1986)


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Figure 4.12 Changing of strength, permeability and deformability of rock (Dearman, 1974) SOIL PROFILES - Soils are the most important products of weathering and consists of 3 main horizons.

Figure 4.13 Soil Profiles


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Table 4.4 Soil Profiles Description

Layer / Horizon Description O (Decaying vegetation) This is the layer of humus on the ground surface.

A Top soil. Rich in organic matter. Typically has dark color. Also called zone of leaching.

B

Subsoil. Also called zone of accumulation. May contain soluble minerals such as calcite in arid climates (caliche).

C Weathered bedrock or saprolite (rotten rock). Bedrock Lies below the soil profile.

Table 4.5 Major Types of Soil

Type Description

Pedalfer These soils are rich in Al and Fe. They form in humid climates, such as the southeastern U.S.

Pedocal

These soils are rich in Ca. They form in arid climates, such as the southwestern U.S. These soils commonly contain caliche (or hardpan), a calcium carbonate deposit which accumulates in the soil.

Laterite

These soils have been depleted of nearly all elements except iron and aluminum oxides. Laterites are derived from the weathering of basalt (mafic parent rock). They form in tropical climates with very high rainfall. The high rainfall has caused leaching of most of the elements and nutrients from the soil. This is the soil typical of a tropical rainforest. When used for agriculture, the small amount of nutrients is quickly depleted, and the soil dries to become as hard as a brick.


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Figure 4.14 Major Types of Soil SUMMARY 1. Weathering is the breakdown and alteration of rocks at Earths surface through

physical and chemical reactions with the atmosphere.

2. Physical and mechanical weathering fragments the rock by various physical stresses e.g. frost action, unloading, alternate heating and cooling.

3. Chemical weathering includes a variety of chemical reactions between

elements in the atmosphere and those in the rocks.

4. Rock disintegration is greatly influenced by patterns of joints, bedding, and other planes of structural weakness in the parent rock material.

5. Climate is the single most important factor in weathering.

6. Rates of weathering depend mostly on climate and the composition of the

rock.


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REVIEW QUESTIONS 1. Discuss tie processes involved in physical or mechanical weathering.

2. What is spheroidal weathering? 3. Explain how climate, rock types, topoghraphy and time influence the types of

soil produced by weathering. 4. Discuss the chemical reaction involved in hydrolysis. True (T) I False (F) Questions 1. The rate and degree of chemical weathering are influenced greatly by the

amount of precipitation. [ ] 2. Chemical weathering is the breakdown of rock into smaller, fragments by

various physical stresses. [ ]

3. The uppermost horizon of soil profile is the topsoil. [ ] 4. Climate with abundant rainfall and moist will accelerate chemical weathering. [ ] 5. Residual soils are soils that develop insitu. [ ] 6. Quartz is one of the most highly resistant minerals. [ ]

7. Physical weathering reactions are exothermic and produced minerals of

increase volume. [ ] 8. Both erosion and weathering process of rock fragments produced during

weathering is known as denudation. [ ]

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Chapter 4 - Weathering