weathering of rock

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CHAPTER 4

WEATHERING OF ROCKS

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LEARNING OUTCOME (CO2:PO1)

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- Identify the types of weathering and their respective agents

- Sketch the weathering grade in accordance to BS 5930

- List out the factors that influence the rate of weathering

- Identify the products of weathering

WHAT IS WEATHERING?

Weathering is the process which rocks disintegrated and decomposed by the action of external factors such as wind, water, glacier, rain, temperature, climate changes, chemical soluble, gravity, plants, turbidity flow, bacteria and etc.

Disintegration – physical breakdownDecomposition – chemical alteration

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WEATHERING

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(i) PYHSICAL WEATHERING (MECHANICAL)

Physical weathering is the mechanical breakdown of the rocks into smaller and smaller pieces without changing the mineral composition of the rocks by the physical forces. Physical forces that contribute to this type of weathering are:- 5

2 main types of weathering :

1. Physical Weathering (mechanical)2. Chemical Weathering

TYPES OF WEATHERING

VIDEO PRESENTATION

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INTRODUCTION TO PHYSICAL WEATHERING

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AGENTS OF PHYSICAL WEATHERING

FROST ACTION

MECHANICAL UNLOADING

THERMAL IMPACTS

SALINE CRYSTAL GROWTH

ORGANIC ACTIVITIES

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A) FROST ACTION

Frost action works best in jointed rocks in mountainous area with cool climates. Water that freezes in cracks and pores of rocks at temperature 0ºC will result in an increment of 9% in volume that will create pressure against the wall of the fracture eventually widened the cracks

AGENTS OF PHYSICAL WEATHERING

FROST ACTION1

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AGENTS OF PHYSICAL WEATHERING CONT’D….

FROST WEDGING

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B) MECHANICAL UNLOADING

This is a process of reduction of pressure (load) on underlying rocks by erosion. 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 have already induced unloading stresses in any exposed rocks. Further removal of material by man can create rapid strain.

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Sheeting in granite

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Horizontal joints form by sheeting as overburden pressure is released

C) THERMAL IMPACT

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 cooled temperature at night. This will lead to cracks and crevices

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D) 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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The surface pattern on this pedestral rock is honeycomb weathering caused by salt crystallization. This example is at yehliu, Taiwan.

Salt weathering of building stone on the island of Gozo, Malta


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E) ORGANIC ACTIVITIES

The activities of plants and animals and insects also promote rock disintegration. Burrowing insects such as worms, ants and rodents mechanically mix the soil and loose rocks particle.Pressure from growing roots widens cracks and contributes to the rocks breakdown


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(ii) 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:-

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TYPES OF WEATHERING CONT’D….

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AGENTS OF CHEMICAL WEATHERING

OXIDATION

DISSOLUTION

HYDROLISIS

HYDRATION

REDUCTION

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A) OXIDATION

Oxidation process occur 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. The iron is oxidize because its loses electrons to oxygen

AGENTS OF CHEMICAL WEATHERING

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Oxidation turned these rocks into fire red

AGENTS OF CHEMICAL WEATHERING CONT’D….

B) DISSOLUTION

Dissolution is the 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. Dissolution of some minerals such as calcite dramatically increases if the water is slightly acidic. Dissolution of calcite is responsible for creating limestone cavern (eg. Gua Tempurung)

Rain is slightly acidic because atmospheric carbon dioxide dissolve in the drops as they fall to the ground.An acidic solution contains the active hydrogen ion.

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LIMESTONE CAVERN

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C) HYDROLYSIS

A mineral reacts with either the H+ or the OH- (hydroxide) from water to produce a new mineral. Aluminum silicates do not dissolve in water. Feldspars weather this way to form clay;

e.g.: plagioclase to clay:

NaAl3Si3O8 + H20 + H2CO3 Na+ + HCO3- + H4SiO4 + Al2Si2O5(OH)4

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Quartz and feldspar are two most abundant components of granite.

For example on weathering of granite - Feldspar decay to form clay minerals - Quartz which is resistant to decay accumulates as quartz sand.


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3000 year old building in NY 3000 year old building in NY after 100 years

Hydrolisis Cleopatrais Needle Building


D) HYDRATION

Hydration is the process whereby a mineral combines with water to form a hydrated minerals especially hydrated silicates and hydroxides. The most important aspect of hydration is that the hydrated minerals is larger in volume than the parent mineral to exert pressure on its surrounding space and contribute to rock disintegration

E) REDUCTION

Release of Oxygen from minerals32


VIDEO PRESENTATION

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CHEMICAL WEATHERING-FELDSPAR INTO CLAY

CHEMICAL WEATHERING cont’d..

SPHEROIDAL WEATHERING

• In this type of weathering, a rounded shape is produced because weathering attacks an exposed rock from all sides at once, therefore decomposition is more rapid along the corners and edges of rock.

• 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 spheroid weathering, where the rocks break apart by separation along a series of layers

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TYPES OF WEATHERING CONT’D….

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WEATHERING GRADES

Metamorphic CarbonateGranite

Schematic diagram of weathering profiles (Fookes,97)

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WEATHERING GRADES

METAMORPHIC ROCK

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WEATHERING GRADES CONT’D…..

GRANITE

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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 factor. Cementing materials

- 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 40

RATES OF WEATHERING

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RATES OF WEATHERING CONT’D….

b) Physical Condition of Rocks

Crevices, cracks, holes will allow weathering agents penetrate and eventually destruct the rock

c) Topography

Weathering is more rapid at slope area. At high altitude will increase the rate of weathering due to low temperature and high rainfall intensity

d) Climatic Condition

Climates which have abundance rainfall and moist will accelerate the weathering process especially chemical weathering. Dry or cold weather are usually accelerate the physical weathering

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RATES OF WEATHERING CONT’D….

Most of the soils that cover the earth are formed by the weathering of various rocks.

Geologist - Soil is defined as weathered material that will support the growth of rooted plants

Engineers – Soil is simply unconsolidated material which typically disintegrates in water.

Soil and superficial deposits can either be residual or transported in origin.

a) Residual SoilResidual soils develop insitu, and their characteristics depend on the kind of bedrock from which they are derived. Residual soil deposits are common in humid tropical countries

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PRODUCT OF WEATHERING

b) Transported Soil

Transported soils are superficial deposits which accumulate due to the erosion, transportation, and deposition of weathered residual soil or bedrock. Common superficial processes and their resulting deposits are:

i) Colluvium – Results from process of creep, whereby soil and weathered bedrock slowly move downslope due to gravity

ii) Alluvium – Includes all sediment deposited by streams. The deposits are stratified into layers of silt, sand, gravel and clay

iii) Glacial drift – Includes all deposits formed by glaciers

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PRODUCT OF WEATHERING CONT’D….

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PRODUCT OF WEATHERING CONT’D….

Colluvium and alluvium

• Erosion is one of the important agent of removal and transportation of superficial materials which is the product of weathering.

• Materials that are susceptible to erosion not necessarily from weathered soil but any unconsolidated surgical deposits. There are three agents of erosion

i) Waterii) Wind

iii) Glacial Ice 46

EROSION

EROSION BY WATER

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• The most important type of erosion in terms of the amount of sediment removed from land surface such as erosion in streams and river channels.

• Rivers are formed as a result of run-off and of many different types. There are few factors that determine the type of erosion:

a) Abrasion

Depends on load in river and amount of material carried by water. E.g Sandgrain, pebbles or boulders which act as cutting tools that can deepen or widen the river bed. Effective where velocity of river is rapid, river loads are heavy and rolled along the bottom

b) Solution or Corrosion

Water as solvent and have corrosive effect on rocks especially when water contains acid that could attack the rocks and dissolve the minerals in the bedrock. E.g. Carbonic Acid from vegetation may attack rocks such as limestone and dolomite

c) Attrition

Boulders and rock fragments are worn by friction and may be broken down as they are rolled and carried along in water. 48

EROSION BY WATER CONT’D….

Erosion Rates by WaterThere are three factors that affect the rate at which running water will erode;

i) Stream sizes – Large volume of water can carry larger loads and therefore very effective during flood stage

ii) Gradient and Velocity – Slope at which river flows will affect the velocity and usually is higher at the source and lower at estuary. Velocity is usually higher at rivers with straight channels, free of obstacles and at steep gradient. Straight and narrow channels will erode more effectively

iii) Nature of Load – River with many obstacles will reduce velocity and carrying power.

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Deposition

Load will be deposited when competency and capacity of river is decreased for example when the river:

- Decrease in volume- Loss of velocity- Obstacles in stream channel- Widening of stream bed- Overloading- Freezing- Reduction in stream gradient

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• The effect of erosion of soil by wind is similar as erosion by water where its capable to lifting and transporting loose sand and dust, but its ability to erode solid rock is limited

• Very fine to fine sand are most susceptible to wind erosion. It is responsible for the formation of the great desert like Sahara and other deserts.

• Few variables that contribute to the effectiveness of this type of erosion are velocity, duration, and length of open area without obstacles over which the wind blows. Erosion can be by;

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EROSION BY WIND

Vegetation provides physical protection for the soil as well as holding the moisture. Deflation basins commonly develop where calcium carbonate cement in sandstone is dissolved by groundwater. As the result, loose sand grains are remain and picked up and transported by wind

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EROSION BY WIND CONT’D….

a) Deflation (Blowing away process)

This is the process of lifting and removing of loose sand and dust particles from earth’s surface. Occurs where no vegetative cover at weak area. Unconsolidated sediment exposed - Wind may blow away much of the dust and fine sand and create large depression called deflation basin Bedrocks are exposed – Oasis are created

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b) DepositionLoad is deposited when velocity is decreased due to obstacle or rain. Types of wind deposits are;

i) Dunes Formed when there is a sudden drop of speed of wind forming hills of sand. Usually formed in areas where there is sufficient amount of loose, unprotected sand and strong winds


Many dunes originate when velocity is reduced due to the present of an obstacle such as large rock and a clump of vegetation. Therefore, the deposit settles to the ground

ii) Loess Deposit of windblown silt (dust) that accumulates slowly and ultimately blankets large areas. Well known for its ability to form fine textured, fertile, yellowish soils in areas of sufficient rainfall and important for agricultural purposes

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• Describe the following terms (8 marks)– Mechanical weathering– Chemical weathering

• Describe three (3) factors that determine the rate of rock weathering. (9 marks)

• How does weathering differ from erosion? (6 marks)

• How do organisms contribute to mechanical and chemical weathering? (3 marks)

• What is soil degradation? List three (3) types of soil degradation.(5 marks)

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REVIEW QUESTION