Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Earth's External Processes and Landform Development
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Earth's External Processes and Landform Development
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Geography
Class: Senior Secondary 2
Term: 1st Term
Week: 1
Theme: The Earth And The Solar System
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This topic introduces learners to the dynamic processes occurring on the Earth's surface that continuously shape and modify landforms. It focuses on the external (exogenic) forces, primarily weathering, erosion, and mass movement, which are driven by atmospheric, hydrologic, and biological agents. Understanding these processes is crucial for Senior Secondary 2 Geography learners as they directly impact daily life in Nigeria, influencing agricultural practices, infrastructure development, natural disaster management (e.g., gully erosion, coastal erosion), and overall environmental sustainability.
Wind-borne particles colliding and breaking into smaller pieces.
Landforms: Sand Dunes: Mounds or ridges of sand piled up by wind. Different types (barchans, seif dunes). Nigerian
Example: Found in the Sahelian regions of Borno, Yobe, Jigawa, and Katsina states, contributing to desertification.
Deflation Hollows: Depressions formed by the removal of loose material by wind.
Desert Pavement: A surface covered by closely packed, interlocking angular or rounded rock fragments, too large to be moved by wind, left behind after finer particles are deflated.
3. Waves and Currents (Marine Erosion): Active along coastlines, particularly where exposed to strong wave action.
Processes: Hydraulic Action: Force of breaking waves compressing air in cracks, leading to rock disintegration.
Abrasion (Corrasion): Pebbles and sand carried by waves grinding against cliffs and shore platforms.
Solution: Dissolving of soluble rock materials by seawater.
Landforms: Cliffs: Steep rock faces formed by wave attack at the base.
Wave-Cut Platforms: Flat, rocky platforms formed at the base of cliffs by wave erosion. Sea Caves, Arches, Stacks, and Stumps: Formed by differential erosion of cliffs, especially along lines of weakness.
Beaches: Accumulations of sand and shingle, mainly depositional but can be eroded.
Spits and Bars: Elongated ridges of sand or shingle extending from the coast, formed by longshore drift.
Lagoons: Shallow bodies of water separated from the sea by a barrier (e.g., sandbar, coral reef). Nigerian
Example: Lagos Lagoon, Lekki Lagoon, coastal erosion affecting communities and infrastructure along the Nigerian coastline.
D. Mass Movement (Mass Wasting): Mass movement is the downslope movement of rock, soil, and regolith under the direct influence of gravity, without the aid of a moving agent like water, wind, or ice. Water often acts as a lubricant, reducing friction.
Types: Creep: Very slow, imperceptible downslope movement of soil and loose rock debris. Nigerian
Example: Often observed on gentle slopes, leading to tilted fences or trees. Flows (e.g., Mudflow, Earthflow): Saturated masses of clay-rich soil and regolith moving downslope as a viscous fluid. Nigerian
Example: Can occur during heavy rainfall in hilly areas with loose, saturated soils, such as parts of the Enugu escarpment or Jos Plateau, leading to significant damage. Slides (e.g., Landslide, Rockslide): Coherent blocks of rock or soil moving rapidly downslope along a distinct failure surface. Nigerian
Example: Landslides can be triggered by heavy rainfall, deforestation, or excavation in mountainous or hilly regions. Falls (e.g., Rockfall): Rapid, free-fall movement of detached rock fragments from a steep cliff or slope. Nigerian
Example: Roots of trees like Iroko or Ceiba penetrating cracks in granite outcrops or old buildings.
Animal Burrowing: Animals (e.g., rodents, worms) excavate burrows, exposing rocks to other weathering agents and weakening the soil/rock structure. Nigerian
Example: Termite mounds and burrows of rats/snakes destabilizing soil on farmlands or slopes.
Microbial Activity: Lichens and mosses produce weak organic acids that chemically etch rock surfaces.
Human Activities: Construction, mining, agriculture, deforestation all accelerate weathering processes. Nigerian
Example: Quarrying activities, road construction through hilly terrains, agricultural practices on slopes leading to soil loosening.
Factors Influencing Weathering: Climate: The most significant factor. Hot, wet climates (e.g., Southern Nigeria) favour rapid chemical and biological weathering. Hot, dry climates (e.g., Northern Nigeria) favour physical weathering (insolation weathering, salt crystal growth).
Rock Type: Mineral Composition: Some minerals are more resistant (e.g., quartz) than others (e.g., feldspar, calcite).
Structure: Jointed, fractured, or porous rocks weather faster due to increased surface area for agents to act upon.
Topography: Steep slopes allow for faster removal of weathered material, exposing fresh rock. Flat areas accumulate weathered material, slowing down the process.
Vegetation: Can protect rocks from direct impact (physical) but also promotes biological weathering (root action, organic acids). Dense vegetation cover can reduce erosion.
Time: Weathering is a continuous process, and its effects accumulate over longer periods.
C. Erosion: Erosion is the process by which weathered rock and soil materials are removed from one location and transported to another by natural agents such as running water, wind, waves, and ice. It always involves movement. Agents of Erosion and Associated Landforms:
1. Running Water (Fluvial Erosion): The most dominant agent of erosion in many parts of Nigeria due to high rainfall.
Processes: Hydraulic Action: Force of water dislodging loose material.
Abrasion (Corrasion): Particles carried by water grinding against the bed and banks.
Attrition: Colliding particles breaking each other into smaller, rounder fragments.
Solution: Dissolving soluble minerals.
Landforms: Sheet Erosion: Uniform removal of topsoil in thin layers over a wide area. Nigerian
Example: Common on gently sloping farmlands after heavy rainfall, leading to loss of fertile topsoil.
Rill Erosion: Small, finger-like channels formed by concentrated flow of water.
Gully Erosion: Enlargement of rills into deep, wide channels, often irreversible without major intervention. A severe environmental problem in Nigeria. Nigerian
Example: Extensive gully systems in Anambra, Imo, Enugu, Abia, Edo, and Cross River States, destroying farmlands, homes, and infrastructure.
River Valleys: V-shaped in upper courses (due to downward cutting), wider in lower courses.
Waterfalls: Formed where a river flows over a resistant rock layer onto a softer one. Nigerian
Example: Gurara Falls (Niger State), Farin Ruwa Falls (Nasarawa State), Erin-Ijesha Waterfalls (Osun State).
Flood Plains: Flat areas adjacent to rivers, built by deposition during floods. Nigerian
Example: The extensive flood plains of the Benue and Niger Rivers.
Meanders: Bends in a river channel, especially in its middle and lower courses.
Ox-bow Lakes: Crescent-shaped lakes formed when a meander neck is cut off during a flood.
Deltas: Triangular or fan-shaped landforms at river mouths, formed by deposition of sediment as river velocity decreases. Nigerian
Example: The Niger Delta, one of the world's largest deltas.
2. Wind (Aeolian Erosion): Significant in arid and semi-arid regions (Northern Nigeria, especially Sahelian belt).
Processes: Deflation: Lifting and removal of loose, fine-grained particles (sand, dust) by wind.
Abrasion: Sand particles carried by wind grinding against rock surfaces.
Attrition: Wind-borne particles colliding and breaking into smaller pieces.
Landforms: Sand Dunes: Mounds or ridges of sand piled up by wind. Different types (barchans, seif dunes). Nigerian
Example: Found in the Sahelian regions of Borno, Yobe, Jigawa, and Katsina states, contributing to desertification.
Deflation Hollows: Depressions formed by the removal of loose material by wind.
Desert Pavement: A surface covered by closely packed, interlocking angular or rounded rock fragments, too large to be moved by wind, left behind after finer particles are deflated. 3. *Waves and Currents (Marine This section provides a detailed explanation of the core concepts related to Earth's external processes and their role in landform development.
A. External (Exogenic)
Processes: These are geological processes that originate at or near the Earth's surface. They are driven by forces such as solar energy (heating of the atmosphere, oceans, and land), gravity, and biological activity. Exogenic processes are primarily destructive, leading to the denudation (wearing away) of the Earth's surface. Contrast with Internal (Endogenic)
Processes: Endogenic processes originate within the Earth (e.g., volcanism, plate tectonics, earthquakes) and are primarily constructive, building up the Earth's surface. Exogenic processes then modify these elevated features.
B. Weathering: Weathering is the in-situ (on-the-spot) breakdown and decomposition of rocks and minerals at or near the Earth's surface by agents of the atmosphere (e.g., temperature, moisture, gases) and biological organisms. It does not involve the removal of weathered material.
Types of Weathering:
1. Physical (Mechanical)
Weathering: Involves the disintegration of rocks into smaller fragments without any change in their chemical composition.
Processes: Exfoliation (Insolation Weathering): Occurs in areas with large diurnal temperature ranges (common in tropical and semi-arid Nigeria). Rocks expand when heated during the day and contract when cooled at night. This repeated stress causes outer layers to peel off like an onion skin. Nigerian
Example: The rounded granite domes of Idanre Hills (Ondo State), Zuma Rock (Niger State), and parts of the Jos Plateau exhibit significant exfoliation.
Granular Disintegration: Individual mineral grains become detached from the rock due to differential expansion and contraction of minerals with varying thermal properties.
Block Disintegration: Occurs in jointed rocks where blocks separate along pre-existing weaknesses due to expansion and contraction.
Frost Action (Freeze-Thaw): Water penetrates cracks, freezes, expands by about 9%, exerting pressure that widens the cracks. Less dominant in most of Nigeria due to high temperatures, but can occur in high-altitude cold environments if present.
Salt Crystal Growth: Evaporation of saline solutions in rock pores leaves salt crystals that grow and exert pressure, breaking the rock. Common in coastal and semi-arid regions. Nigerian
Example: Coastal rocks and structures exposed to sea spray, or rocks in drier northern regions with saline groundwater.
2. Chemical Weathering: Involves the decomposition of rocks and minerals through chemical reactions, leading to changes in their chemical composition. This is prevalent in hot, humid climates like much of Southern Nigeria.
Processes: Solution: Minerals dissolve directly in water (e.g., rock salt, gypsum).
Carbonation: Carbon dioxide dissolves in rainwater to form carbonic acid, which reacts with carbonate rocks (like limestone) to form soluble bicarbonates. Nigerian
Example: Formation of limestone caves and karst topography in areas like Ogbunike Cave (Anambra State) and parts of Cross River State.
Hydration: Minerals absorb water molecules into their crystal structure, causing them to expand and weaken the rock. Nigerian
Example: Anhydrite converting to gypsum; feldspar becoming clay minerals.
Hydrolysis: Water reacts with minerals, especially silicates, to form new compounds (e.g., feldspar reacting with water to form clay minerals like kaolinite). Nigerian
Example: Common in the formation of lateritic soils rich in clay, widespread in tropical Nigeria.
Oxidation: Reaction of rock minerals (especially iron-bearing minerals) with oxygen in the presence of water, leading to rust formation and rock weakening. Nigerian
Example: The reddish-brown coloration of lateritic soils and many iron-rich rocks across Nigeria.
3. Biological Weathering: Involves the breakdown of rocks by living organisms (plants, animals, microorganisms) through physical or chemical means.
Processes: Root Action: Tree roots grow into cracks, expanding them and breaking the rock. Nigerian
Example: Roots of trees like Iroko or Ceiba penetrating cracks in granite outcrops or old buildings.
Animal Burrowing: Animals (e.g., rodents, worms) excavate burrows, exposing rocks to other weathering agents and weakening the soil/rock structure. Nigerian
Example: Termite mounds and burrows of rats/snakes destabilizing soil on farmlands or slopes.
Microbial Activity: Lichens and mosses produce weak organic acids that chemically etch rock surfaces.
Human Activities: Construction, mining, agriculture, deforestation all accelerate weathering processes. Nigerian
Example: Quarrying activities, road construction through hilly terrains, agricultural practices on This section outlines practical activities for the teacher and learners to facilitate understanding and engagement.
Teacher Activities: Introduction (10 minutes): Begin by asking students to describe any distinct landforms they have observed in their local environment (e.g., a nearby hill, a river, a gully, sandy beach).
Ask: "How do you think these features came to be?" or "Do these features change over time, and if so, how?" Introduce the topic: "Earth's External Processes and Landform Development," explaining that these processes are responsible for shaping the landscape around them. Briefly state the learning objectives for the lesson. Concept Explanation and Illustration (30 minutes): Systematically explain the key concepts: External vs. Internal processes, Weathering (physical, chemical, biological), Erosion (fluvial, aeolian, marine), and Mass Movement. Use clear definitions and provide numerous local Nigerian examples for each process and resulting landform.
Utilize visual aids: Diagrams of exfoliation, gully formation, river meanders, sand dunes, coastal features. Pictures/posters showing Idanre Hills (exfoliation), Ogbunike Cave (carbonation), gullies in Anambra, sand dunes in Borno, Lagos coastline (marine erosion). If available, short video clips demonstrating erosion processes.
Guided Discussion and Q&A (15 minutes): Facilitate a class discussion on the factors influencing weathering in different parts of Nigeria (e.g., why chemical weathering is more dominant in the south, while physical weathering is prominent in the north).
Engage students with questions like: "How might human activities like farming or building construction accelerate gully erosion?" Address student questions and clarify misconceptions.
Activity Facilitation (25 minutes): Divide the class into small groups (4-5 students). Provide each group with charts, markers, and worksheets containing different scenarios or images of Nigerian landforms/processes.
Instruct groups to: Identify the dominant external process (weathering, erosion, mass movement) at play. State the specific type of process (e.g., gully erosion, exfoliation). Suggest potential causes or contributing factors in the Nigerian context. Discuss potential impacts on the local community. Circulate among groups, providing guidance and checking understanding.
Wrap-up and Preview (5 minutes): Summarize the main points of the lesson. Briefly introduce what will be covered in the next lesson if applicable (e.g., specific landforms in detail, or human responses to these processes). Assign homework.
Student Activities: Active Listening and Note-taking: Students listen attentively to explanations and take comprehensive notes.
Participation in Discussions: Students actively participate in class discussions by asking questions and sharing observations related to local landforms and processes.
Group Work: In assigned groups, students analyze provided scenarios, diagrams, or images. They work collaboratively to identify and explain different external processes and landforms. They brainstorm Nigerian examples and discuss the impacts of these processes. Groups present their findings briefly to the class. Observation and Sketching (Optional/Homework): Students observe local environments for evidence of weathering, erosion, or mass movement, and perhaps sketch what they see (e.g., a small gully, a tree root cracking a wall, rust on metal).
Understanding Earth's external processes has significant practical applications in Nigeria, influencing various aspects of community life, environment, and economy.
Agricultural Management and Food Security: Application: Knowledge of soil erosion (sheet, rill, gully erosion) is critical for sustainable agricultural practices. Farmers can apply techniques like contour ploughing, terracing, strip cropping, and afforestation to minimize soil loss, preserve fertile topsoil, and improve crop yields. Understanding weathering helps in assessing soil composition and fertility.
Local Context: In states severely affected by gully erosion (e.g., Anambra, Imo, Enugu), this knowledge helps inform government intervention projects and community-led initiatives to reclaim and protect farmlands, thereby safeguarding livelihoods and food security. It also guides the selection of appropriate crops for different soil types resulting from weathering. Infrastructure Development and Disaster Risk Reduction: Application: Civil engineers and urban planners use knowledge of weathering, erosion, and mass movement to design and site infrastructure (roads, bridges, buildings, pipelines) safely. This involves assessing soil stability, predicting landslide risks, and designing erosion control measures. Coastal engineers require this understanding for constructing sea walls, groynes, and other coastal protection structures.
Local Context: The construction of federal and state roads often traverses diverse geological terrains prone to erosion and landslides (e.g., areas around the Jos Plateau, Enugu escarpment). Coastal cities like Lagos and communities in the Niger Delta face severe coastal erosion; applying this knowledge aids in projects like the Eko Atlantic City or shoreline protection efforts to protect properties and human lives. Environmental Conservation and Resource Management: Application: This topic is fundamental to environmental conservation efforts, including desertification control, watershed management, and biodiversity preservation. Understanding the dynamics of wind erosion helps in planting shelterbelts to combat desertification in northern Nigeria. Managing fluvial erosion in river basins is vital for maintaining water quality and aquatic ecosystems.
Local Context: The Great Green Wall initiative in northern Nigeria aims to combat desertification (driven by wind erosion) through tree planting. The ongoing efforts to manage gully erosion in Southern Nigeria are crucial for protecting ecosystems, water resources, and preventing further land degradation. Understanding rock weathering helps in identifying mineral deposits (e.g., laterite for construction, bauxite for aluminium) and managing their extraction sustainably.