2 unit-i geological agencies - weathering

8/11/2019 2 Unit-I Geological Agencies - Weathering

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GENERAL GEOLOGY

Unit - I


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Geological Agencies

External agenciesWeathering

Wind

River

Sea

Landslide

Internal agencies Earthquakes

Plate tectonics

Groundwater


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Geological Agencies

Exposed rocks on the earth subjected to

direct of indirect attack of Natural /

Geological agencies

Atmospheric gases Heat

Moisture

Surface and subsurface water

Wind

Sea water

Ice etc.,

They act alternatively season by season /

year by year.


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Geological Agencies

Thus, responsible for modifying the Physical

features.

Example: River (carries millions of tons of

sediments to sea)

Where from the sediments derived???

Valley slopes

Basement

Side of the channel

Modifiying the existing river valley and

drainage basin


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Work of geological agencies

Either Constructive or Destructive in naturein relation to the existing landform at any

given point of time

Example to explain this aspect: River

Rivers carve out valleys

Valleys are enlarged

Valleys are deepenedAlluvial plains

Flood plains

Deltas

Destructive work

Constructive work


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Weathering


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Weathering

The disintegration, or breakdown ofrock material

Types of weathering

Physical (Mechanical)

Chemical (reactions)

Biological (Bio organism)


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Physical weathering

No change in chemical composition--justdisintegration into smaller pieces


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Physical Breakup/Weathering

a) Frost action

b) Exudation

c) Thermal Effects - Thermal expansion and contraction

d) Unloading

All this increases the total surface area exposed

to weathering processes.


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a) Frost Action

Water on freezing undergoes an increase in

its volume by about ten percent.


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b)Exudation

It is a process similar to frost action but in this

case disintegration takes place due to

formation of crystals of salts like sodium

chloride etc. within the cavities.

The process is seen in rocks near shores.


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b) Thermal Effects

Direct slow heating (followed by cooling) of

the exposed rocks by the blazing sun.

This phenomenon is especially of great

significance in arid, desert, and semi arid

regions.

Rocks, like many other solids, expand on

heating and contract on cooling


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d) Unloading

Large scale development of fracturing in

confined rock masses---

removal of

overlying rocks due to erosional work of

other agencies.

Results in Sheet weathering

Decay along the joint planes


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Joints in a rock are a pathway

for water they can enhance

mechanical weathering

Rates of weathering


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Chemical Weathering

breakdown as a result of chemicalreactions

CaCO3+CO2+H2O ---> Ca2+

+ 2HCO3-


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Chief processes of chemical weathering

a) Solution,

b) Hydration and hydrolysis,

c) Oxidation and reduction,d) Carbonation

e) Colloid formation


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Solution: process by which rock isdissolved in water

Is strongly influenced by pH and temperature

When water becomes saturated, chemicals mayprecipitate out formingevaporitedeposits.

Calcium carbonate (calcite, limestone), sodiumchloride(salt), andcalcium sulfate(gypsum) areparticularly vulnerable to solution weathering.


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Hydration: attachment of watermolecules to crystalline structure of arock, causing expansion and weakness

CaSO4 or anhydrite, gets slowly converted to gypsum by hydration :

CaSO4 + 2H2O CaSO4 . 2H2O

Anyhydrite Water Gypsum

Hydrolysis: the process of exchange ofions.combination of hydrogen and oxygenin water with rock to form newsubstances


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OxidationOxygen dissolved in water promotesoxidation of sulfides, ferrous oxides,native metals

4 Fe + 3O2

2Fe2O2Iron + Oxygen Ferric oxide

Fe2O3 + H2O Fe2O3.H2O

2 Fe S2 + 7O2 + 2H2O 2FeSO4 + 2H2 SO4Pyrite + Oxygen + Water Ferrous Sulphate + Sulphuric acid


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Carbonation:It is the process of weathering of rocksunder the combined action of atmospheric

carbon dioxide and moisture.


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Colloid formation:

The process of hydration, hydrolysis and oxidation (andreduction) acting on rocks and minerals under various

atmospheric conditions may not always end in the

formation of stable end products.

Often they result in splitting of particles into still smaller

particlesthe colloidscharacterized by atoms with only

partially satisfied electrical charges.

Formation of colloidal particles is especially common in

the weathering ofclay minerals, silica and iron oxides.


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Spheroidal Weatering

It is a complex type of weathering observed in

jointed hard rocks and breaking of the original rockinto spheroidal blocks.

Both chemical and physical processes operate in

spheroidal weathering.

Creates domedmonoliths


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iii) Biological Weathering

Role of plants and organisms

Can be both chemical and mechanical

in nature.

roots split rocks apart

roots produce acids (H+)that dissolve rocks.

tree throw

burrowing animals
http://www.kesgrave.suffolk.sch.uk/Curric/geog/biolw2.html


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Factors Affecting Weathering

Nature of the Rock

(Chemical compositionstability)

Sandstone

Granite

Fractured? / Massive??

Climate

Same rock type exposed to different climate

leads to different trend of weathering

Physical Environment

Topography

Vegetation Cover


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Mineral constituents Early formed minerals easily get weathered

compared to minerals formed late.

Eg:

Feldspar, pyroxenes, ultra basic rocks-early formed

Quartz- formed later

Resistance to Weathering

Dark Colored Minerals:Olivin, Augite, Hornblende; Biotite

Light Colored Minerals:Feldspars, Quartz

Weathering increases in the following order


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Bowens

Reaction

Series

Goldrich

Stability

Series

First to

Crystallize

Last to

CrystallizeSlow

Weathering

Fast

Weathering

Resistance to Weathering


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Products of Weathering

Elluvium :

end product of weathering lie over the parent

rock

Delluvium :

end product of weathering, that has been moved

to some distance

Regolith :

Express all the weathered material, elluvium orDelluvium that covers the parent rock.

Weathering of rocks become slow after the

formation of weathered layers at the top.


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Soil Profile The record of behavior of the materials with

depth below the surface up to which the effect ofweathering can be easily established.

S il P fil F W th i Z


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Soil Profile Four Weathering Zones

Zone A Completely weathered soil

supporting veg cover

May or may not contain humus

Zone B Zone of mixed composition

Partly soil and partly weathered rock

Zone C Soil free zone

Under attack by weathering

Zone D Zone of the so far in tact rock (parent

rock)

Starts from the base of C Zone and

extends downwards infinitely

A

B

C

D


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Minerals and Rock formation

Clay minerals

Montmorrolonite

Kaolinite

Illite Ores of Aluminium

Bauxite

Laterite

Advanced stages of weathering of Clay rocks

weathering of pre-existing silicate rocks

E d


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Engineering Consideration

Engineering projects are built either on soil

or on rocks.

Soil,as we know, is the ultimate end product

of weathering of rocks.

As such, for a better understanding of the

engineering properties of soils the engineer will benefit a great deal if he has an

understanding of the genetic background of the soils.

E i i C id i


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Similarly, when foundations are to be carried

down to the bed rock,

the depth of the weathered cover,

degree of weathering and

trend of weathering - safety of the project.

For the construction Engineers

The extent of operational process of weathering

in the area

Likely effect of weathering on construction

materialto assess the durability


E i i C id i


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It is now well established that weathering is

the main cause of instability of slopes inmany areas

Slope rocks lose shearing strength and

become prone to failure ensure protection

of slope rocks.

Recommending special stones (Marbles,Limestone, Granites) in major construction

towards chemical environment of the area


2 unit-i geological agencies - weathering

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