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GENERAL GEOLOGY

Unit - I

External agencies

WeatheringWind

River

Sea

Landslide

Internal agencies

Earthquakes

Plate tectonics

Groundwater

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Wind

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Introduction

Atmosphere is composed chiefly of gases

collectively known as air.

Air in motion is called wind

Wind is one of the major geological agents of

change on the surface

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[ORIGIN from Greek anemos wind] [ORIGIN from Greek baros weight]

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Creates temporary or permanent changes on the

land surface depending on

wind volume,

wind velocity,

nature of the surface,

duration for which the wind blows etc. Strong winds blowing over loose ground, dry soils

and deserts may create many features within short

span of time

Wind acts as agent of erosion as a carrier fortransportation of particles and then deposit huge

quantitiesat different places.

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Geological Work of Wind

1. Erosion Deflation

Abrasion

Attrition

2. Transportation Source of sediments

Methods of transport

The transporting power of wind

3. Deposition Dunes

Loess

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1. Wind erosion Deflation

Abrasion

Attrition

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Wind Deflation( To blow away)

Wind possesses not much erosive over the

ground covered with vegetation

Wind moving with sufficient velocity over dry

sands or bare ground covered with dust, its

remove huge quantity of the material from the

surface known as deflation

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Wind Deflation

In some desert deflation may remove the sand from a particular

location and creates depression - virtually touching the water tableand such depression is known asBLOW OUTS.

Water filled depression known asOASIS is a place where vegetation

grows in desert

SLACK is another term used for depression created by

deflation

Ex :Quattara in western Egypt (Biggest slack)

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Oasis

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Wind Abrasion

Wind is a powerful agent for rubbing and

abrading the rock surfaces when naturally

loaded with sand & dust particles.

This type of erosion involves rubbing,

grinding, abrading and polishing of rock

surface by a natural agent known asAbrasion

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Responsible for features of erosion

Yardangs

Pedestal rocksVentifacts

Desert pavements

Wind Abrasion

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Yardangs It is an elongated low lying ridges forming

overhangs above depression. Trend is parallel to the prevailing wind

Yardangs are formed in area where rocks of

alternate hard and soft rock lying one over otherwith gentle slope.

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Pedestal Rocks It is known as mushroom rocks

They flat topped rock masses

that are characterized with

slender lower region The top is commonly referred

as overhang and the support as

pedestal. They are generally

few meters in height

Overhang

Pedestal

Same rock

but affected

unevenly

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Small sized rock fragments having

one, two, three or more polished

faces The polishing of the sides of the

originally rough fragments is

carried out by prolonged wind

abrasion on the surface of each

fragment.

Ventifacts

After one faces is plashed a

second face is made available tothe wind action by overturning of

the fragments during gushes of

wind

These wind polished & facetted

fragments are called ventifacts.

http://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpg

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It is flat rock surfaces covered by rounded or sub

rounded pebbles spread all over them & are typicalfeatures of rock deserts.

Pebbeles - distinguishing feature of desert

pavements. These pebbles may get further polished with time

and even acquire thin filmy oxides of Iron &

manganese.

This coated pebbles give a characteristic brow, dark

red or even black appearance to the desert

pavements.

http://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpghttp://upload.wikimedia.org/wikipedia/en/c/c7/Desert_Pavement_Mojave_2000.jpg

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Wind Attrition

The sand grains and other particles that arelifted by the winds and carried away do not

travel in a straight path

Their path is determined by their density andvelocity of the wind

It moves in Zig-Zag paths

The wear & tear of load sedimentssuffered by wind due to mutual impacts

during transport is Attrition

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The above processes depends on

i. Nature of the Region

ii. Velocity of Windiii. Duration

Wind Attrition

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2. Transportation Source of sediments

Methods of transport

The transporting power of wind

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Source of Sediments

Sand, Silt, Clay (huge volumethousands of tons a day)

Sand deserts and freshly ploughed fields

Abrasive action over the rock surface

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Methods of transport

Suspension Thelight and silt grade particlesmay be lifted high up in

the upper regions of the wind and moved along with it is

known as transport insuspension

Because the particles once lifted are not allowed to reston ground again and again unless they are finally

deposited.

Saltation:

The heavier and coarse sediments such as pebbles,gravels and sand grains are lifted up only for short

distance (up to 2m).

This type of sediment transport by wind issaltation.

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Transporting power of Wind

Depends on velocity of wind

Particle size, shape and density.

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Dunes

Variously shaped deposits of sand-gradeparticles accumulated by winds.

Gentle windward and steep leeward

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A sand dune is defined as a broadly

conical heap of sand characterized with

two slopes on either side of a medial

ridge or crest

Sand dunes show great variation in their

shape, size & gropingtype of dunes.

1. The Cresentic Dunes

2. The Sigmoidal Dunes

3. Transverse Dunes

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1. Crescent dunes:

It is characterized with two slopes in such a

way that the windward slope is convex andrise gently between 7 and 20

The slip face starts from the crest and is quite steepand concave in outline.

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Simple type of cresentic dunes

typically half moon shape Developed by wind blowing in

the same direction for

considerable length of time

Barchans

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2. The sigmoildal dune:

It is characterized with the absence of horns or

cups and a curved outline A sigmoidal dune is a s`teep sided ridge that

extends in a s-sheped outline

Both the sides of the sigmoidal dune are steeply

inclined at angle b/w 25to 32

Longitudinal dunes are called as SEIFS.

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Dune classification

Dunes are classified in to simple, compond

and complex

Simple : single dune

Compound : two of more dunes of same type

Complex : group of different types of dunes

L

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Loess It is used for wind blown deposits of silt and clay grade

particles.

Unconsolidated, Unstratified and Porous

Size range (0.01 0.05mm in diameter) 40 %

Rest 60 % by silt and finer clay

Occur in China, North America and Europe.

Particles get accumulated due to precipitation

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Engineering considerations

Protecting the bui l t-up areas and agr icul tur al

lands

movements of sand dunes

M ethods for combating and advancing sands:

1. Establ ishi ng f r ontal tr acts or belt of vegetation that can

resist the advancing sands by checking the velocity of wind

2. Constru ction of win d breaks or wall saround the area to

be protected.

3. Treating the sands locall y with cr ude oil whereby theirsusceptibil ity for transpor t by wind is consider ably

reduced

L oess soi ls ar e ver y danger ous f or

foundation

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