WEATHERING Lesson Note
Learning Objectives
This SS3 Geography lesson note covers WEATHERING for First Term, Week 4.
Lesson Note
Learning Objectives
At the end of the lesson, students should be able to:
i. Define weathering.
ii. Identify and explain the types of weathering.
iii. Explain the processes of weathering.
iv. Describe the effects of weathering on soil formation and farming.
A. DEFINITION OF WEATHERING
Weathering is the breakdown or disintegration and decomposition of rocks in their original position (in situ) by physical, chemical, and biological agents without the movement of the weathered materials.
B. TYPES OF WEATHERING
1. Physical (Mechanical) Weathering
Physical weathering is the breakdown of rocks into smaller pieces without changing their chemical composition.
Examples include:
a. Temperature changes causing expansion and contraction.
b. Frost action where water freezes and expands in rock cracks.
c. Pressure release or exfoliation.
d. Abrasion by wind, water, or ice.
Characteristics
i. No chemical change occurs.
ii. Common in deserts and cold regions.
2. Chemical Weathering. Chemical weathering is the decomposition of rocks through chemical reactions that alter their mineral composition.
Examples include:
*Oxidation
*Hydrolysis
*Carbonation
*Hydration
*Solution
Characteristics
a. Changes the chemical composition of rocks.
b. Common in hot and humid climates.
3. Biological Weathering
Biological weathering is the breakdown of rocks by living organisms.
Examples include:
i. Plant roots growing into rock cracks.
ii. Burrowing animals.
iii. Human activities such as mining, road construction, and quarrying.
iv. Microorganisms producing acids that dissolve rocks.
Characteristics
a. May involve both physical and chemical processes.
b. Common where vegetation is abundant.
C. PROCESSES OF WEATHERING
1. Exfoliation (Onion-Skin Weathering). The outer layers of rocks expand during the day due to heat and contract at night due to cooling. Repeated expansion and contraction cause the outer layers to peel off like an onion.
2. Frost Action: Water enters cracks in rocks. When temperatures fall below freezing, the water freezes and expands, widening the cracks until the rock breaks apart.
3. Oxidation
Oxygen combines with minerals, especially iron, to form rust. This weakens and breaks down rocks.
4. Carbonation
Rainwater absorbs carbon dioxide from the atmosphere to form weak carbonic acid. The acid reacts with limestone and other carbonate rocks, dissolving them gradually.
5. Hydrolysis
Water reacts chemically with minerals such as feldspar, converting them into clay minerals and weakening the rocks.
6. Hydration
Certain minerals absorb water, expand, and become weak, causing the rocks to disintegrate.
7. Solution
Some minerals dissolve directly in water, especially rocks containing soluble minerals such as rock salt and gypsum.
8. Biological Activity
Plant roots penetrate cracks in rocks while burrowing animals loosen rocks and soil. Microorganisms also release acids that aid rock decomposition.
EFFECTS OF WEATHERING ON SOIL FORMATION
1. Produces soil from parent rocks.
2. Supplies essential minerals to the soil.
3.bDetermines soil texture and structure.
4. Forms fertile soils suitable for agriculture.
5. Contributes to the formation of different soil horizons.
6. Increases soil depth over time.
EFFECTS OF WEATHERING ON FARMING
Positive Effects
1. Produces fertile soils for crop cultivation.
2. Releases nutrients needed by plants.
3. Improves water infiltration into the soil.
4. Enhances root penetration.
5. Supports agricultural productivity.
Negative Effects
1. Excessive weathering may lead to nutrient leaching.
2. May result in soil erosion.
3.BCan produce infertile lateritic soils.
4. Weakens rocks, leading to landslides in hilly areas.
5. Reduces crop yield where soil fertility is lost.
Evaluation
OBJECTIVE QUESTIONS
Choose the correct option.
1. Weathering is the ________.
A. Movement of rocks by rivers
B. Transportation of sediments
C. Breakdown of rocks in their original position
D. Deposition of sediments
2. Which of the following is a type of weathering?
A. Transportation
B. Deposition
C. Biological weathering
D. Erosion
3. Physical weathering does not change the ________ of rocks.
A. Shape
B. Size
C. Chemical composition
D. Appearance
4. Which process involves peeling of rock layers?
A. Carbonation
B. Exfoliation
C. Solution
D. Oxidation
5. Oxidation mainly affects rocks containing ________.
A. Sand
B. Iron
C. Quartz
D. Salt
6. Carbonation is common in rocks made of ________.
A. Granite
B. Limestone
C. Basalt
D. Marble
7. Which of the following is an agent of biological weathering?
A. Wind
B. Rain
C. Plant roots
D. Sunlight
8. Hydrolysis mainly converts minerals into ________.
A. Sand
B. Clay
C. Gravel
D. Salt
9.One importance of weathering is that it ________.
A. Destroys all vegetation
B. Forms soil
C. Causes volcanic eruptions
D. Prevents farming
10.Excessive weathering may result in ________.
A. Nutrient leaching
B. Increased rainfall
C. Volcanic eruptions
D. Earthquakes
Theory Questions
1a. Define weathering.
1b. Explain the three major types of weathering.
1c. Describe any five processes of weathering.
2a. Discuss the effects of weathering on soil formation.
2b. Explain the importance of weathering to farming and state two negative effects.