Lesson Notes By Weeks and Term - Senior Secondary School 2

SUBJECT: GEOGRAPHY

CLASS:  SS2

DATE:

TERM: 1ST TERM

REFERENCE MATERIAL

• Essential Geography for Senior Secondary Schools, O.A. Iwena.

 
WEEK THREE

TOPIC: ACTION OF WIND IN THE DESERT

The action of wind is dominant in desert and other semi arid regions of the world. Deserts are places with little or no vegetation. Deserts which are associated with aridity (dryness) are caused by low rainfall, high temperature, cold currents and high evaporation rate. Examples of Deserts are:

(i)    Sahara desert (West Africa).

(ii)    Kalahari and Namib deserts (South Africa).

(iii)    Arabian, Iranian and Thar deserts (Middle East).

(iv)     Australian desert (Australia).

(v)    Atacama desert (South America).

(vi)    Mohave desert (U.S.A).

CHARACTERISTICS OF DESERTS

(i)    Extreme of Temperature.              (ii)    Low rainfall.

(iii)    Absence of vegetation cover.      (iv) High evaporation rate.

(v)    Wind action is dominant.            (vi) Presence of cold currents.

TYPES OF DESERTS

There are five distinct kinds of deserts

1. Erg or Sandy desert                        2. Hamada or rocky deserts
2. Reg or stony deserts                       4. Badlands
3. Mountain deserts

ACTION OF WIND EROSION

Wind erosion is carried out in the following ways:-

1. Deflation: This is the lifting and blowing of loose sand and pebbles by wind. Deflation results in the lowering of the land surface to form large depression called Deflation hollows.
2. Abrasion: This is the process whereby sand particles carried by wind are used to blast or wear away rock surfaces.                
3. Attrition: This is the process whereby materials carried by wind collide with one another thereby wearing away each other.

DIFFERENCES BETWEEN DEFLATION AND ABRASION

1. Deflation involves blowing while abrasion involves hauling rock against rock surface by wind.
2. Deflation involves rolling of loose materials along the ground whereas in abrasion, rock surface are polished, scratched and worn away.
3. Deflation usually results in lowering of land surface while abrasion is most effective at the base of rocks.
4. Deflation is associated with wind while abrasion can be caused by wind, water and wave.

EVALUATIONQUESTIONS

1. What are the causes of desert?
2. Mention the five types of desert.

FEATURES OF WIND EROSION IN THE DESERT

1. Rock Pedestals

Characteristics: Rock pedestals are irregular in shape e.g. mushroom shape with alternate horizontal layers of hard and soft rocks. They are like pillars in structure. They are desert land forms which range from 10 – 15 meters in height.

Mode of formation: Rock pedestals are formed by wind abrasion on alternate horizontal layers of hard and soft rocks. Abrasion is greater at ground level, resulting in serious under cutting, to produce irregular shaped feature called rock pedestals in deserts.

2. Zeugen

Characteristics: These are tabular masses with a layer of soft rocks lying beneath a surface layer of hard rock. It has a long ridge and furrow landscape. They are also formed by wind abrasion in deserts. 

Mode of formation: Zeugen is formed when a tabular mass of rock which has a layer of soft rocks lying under a layer of a more resistance hard rock islying horizontally to the direction of wind. The mass of rock is then attacked by wind abrasion and then wears the mass into a ridge and furrow landscape, leading to the formation of Zeugen. Mechanical weathering starts the formation by opening up joints of the surface of hard rocks.

3. Yardang

Characteristics: Yardages have vertical bands of hard and soft rocks, ridge and furrow of landscape of about 10 – 15 meters long.

Mode of formation: Yardangs are formed when hard and soft rocks in vertical bands are aligned in the direction of prevailing wind. Wind abrasion wears off the softer rocks into long narrow corridors which separate the steep-sided ridges of the hard rocks. These hard rocks are called Yardangs.

4. Mesas and Buttes

Characteristics: Mesa is a flat, table-like landmass. It has a resistant horizontal top layer with steep sides and is made up of soft and hard layers.

Mode of formation: As a result of the action of denudation, the hard top layer of rock resists agents of denudation and protects the softer layers of rocks below from being eroded. At times, mesas may be formed in canyon regions. Canyon develops in the space between mesas and butte. Denudation, sometimes, may reduce mesas in areas to become isolated flat topped hills called buttes.

5. Inselberg

Characteristics: Inselberg is an arid land form. It is an isolated rocky out crop having steep sides, round top and composed of granite. It may rise up to a height of over 500metres and may occur singly or in group. Examples are found in Northern Nigeria, Kalahari desert, and Western Australia.

Mode of formation:Inselbergis formed from the existence of extensive old plateau. It is caused as a result of weathering and removal of weathered materials by water and wind. It is an exposure of rock out-crop.

6. Ventifacts and Dreikanters: These are pebbles sharpened or faceted by sand blasting. Wind abrasion shapes and polishes the pebbles and new facets develop when wind direction changes. Ventifacts with three wind faceted surfaces are called dreikanters.

7. Deflation Hollows or Depression:

Characteristics: They are formed by wind deflation. It is a desert landform with varying depths. There may be presence of sand dunes on the leeward side. It has a basin and a saucer shape. It is very extensive and can form oasis in deserts.

Mode of formation: As a result of wind deflation, large depression or hollows are produced by the scooping away of loose sand materials by wind, Sometimes below the water table. When this happens, water seeps out and oasis or swamps are formed.

FEATURES OF WIND DEPOSITION IN DESERTS

(1)    Dunes: Dunes are hills or ridges of sand formed by the piling up of sand into hill shape by the action of wind. They are initiated when an obstacle of some kind prevents free movement of the wind. Thus, creating certain shape against the obstacle until it covers the latter and falls over to the other side. There are two main types of dunes, These are:

(a)    Barchan

Characteristics: A barchan is a cresentic or moon shaped structure. They may occur in groups or singly. A barchan has a convex shape on windward side and a concave shape on the leeward side with horns of 15-30 metre long. It is formed by wind deposition in deserts. 

Mode of Formation: A barchan is formed when an obstacle like rock impedes or prevents the movement of wind, resulting in the accumulation of sand materials. Later the sand begins to accumulate on the other side of the obstacle, leading to a crescent or moon shaped structure with horns called Barchan. The windward side of a barchan is convex and gentle, while the leeward side, being sheltered is concave and steep.

(b)    Seifs or Longitudinal Dunes

Characteristics: Seifs are sword-shape. They have long and narrow ridges of sand usually of hundreds of kilometers. They are formed by wind deposition in the desert e.g. Sahara desert.

Mode of formation:Seifs are formed during wind deposition. They lie parallel to the prevailing wind which clear the corridors between dunes of sand. Eddies blow towards the side of the corridor and build up seif dunes.

(2)    Loess: These are fine soil particles carried by wind. They are deposited outside the desert as loess. Loess is a fine loam, very fertile and porous. 

EVALUATION QUESTION

1. Mention two features of wind erosion.
2. State two differences between Zeugen and Yardang.

GENERAL EVALUATION

1. What is a desert?
2. State three causes of a desert.
3. Mention four types of desert.
4. Explain action of wind erosion in desert regions.
5. Describe a rock pedestal.

READING ASSIGNMENT

Essential Geography, O.A. Iwena, Pages 58-62.

WEEKEND ASSIGNMENT

1. The largest desert is (a) Kalahari desert (b) Sahara desert (c) Atacama desert (d) Namib desert
2. Which of these deserts is sandy (a) Erg (b) Reg (c) Hamada (d) Mountain desert
3. The process of wind erosion which involves the lifting and blowing away of loose sand  is (a) Abrasion (b) Deflation (c) Attrition (d) Saltation   
4. Which of these is formed by the deposition action of wind? (a) Dunes (b) Yardang (c) Zeugen (d) Rock pedestals 
5. An Isolated rocky outcrop of granite is (a) Dunes (b) Inselberg (c) Seifs (d) Yardang

THEORY 

1. With the aid of a diagram, describe the mode of formation of yardeng.
2. List two differences between yardeng and zeugen.