Weathering, Erosion, and Deposition
Unit 1: Weathering
1. Concept of Weathering
Definition: Weathering describes all processes that break down rocks, soil, and other objects on the Earth's surface in one place.
Affected Items: All things on the Earth's surface are affected by weather action.
Stationary Process: Weathering occurs in a specific location with minimal movement of material.
Observable Effects: Effects of weathering can be seen on constructed features such as buildings:
Sun causes paint to peel.
Rain makes metal objects rust.
Hot and cold temperatures weaken surfaces.
Plants break up bricks.
Rate of Change: Weathering of rocks and soil is a very slow process compared to a human lifetime. Significant changes to rocks due to weathering are unlikely to be noticed within a single lifetime.
Primary Causes: Temperature and water are the main causes of weathering (e.g., rain, frost, ice slowly weather rocks).
2. Physical Weathering
Definition: Physical weathering occurs when a physical force breaks rocks into smaller pieces.
Causes: Temperature and water are the main drivers.
Conditions: Cold, wet conditions generally increase physical weathering.
Scale of Action:
Small Scale: Removes individual grains of rock.
Large Scale: Causes large pieces of rock to break away from mountains and crash down slopes.
Frost Shattering (Most Common Type of Physical Weathering)
Definition: The physical breaking of pieces of rocks by repeated freezing and thawing of water in cracks in the rocks.
Mechanism:
Water collects in cracks in rocks.
Water at the top of the cracks freezes first, forming an 'ice cap'.
Ice expands sideways because it takes up approximately more space than water.
The expanded ice puts significant pressure on the surrounding rock.
Repeated cycles of expanding (freezing) and contracting (thawing) weaken the rock.
Small pieces eventually shatter and fall off.
Location: Occurs mostly in mountainous areas that experience frequent temperature fluctuations around freezing point.
Resultant Landform: The small, shattered rock pieces collect at the bottom of a slope, forming piles of loose rock called scree.
Exfoliation
Definition: A type of physical weathering where outer layers of rock shed or cast off, caused by significant day and night temperature changes.
Location: Common in hot, dry climates like deserts and semi-desert areas, which have large temperature ranges.
Mechanism:
Extreme daytime heating causes the outer rock layers to heat up and expand away from the cooler internal parts of the rock.
At night, as temperatures drop quickly, the outer layers of rock cool rapidly and contract.
The constant cycle of expanding and contracting weakens the outer layers of the rock.
Eventually, these weakened outer layers flake off or 'shed'.
Analogous Example: Paint peeling off wood due to temperature changes demonstrates the principle of exfoliation (layers of paint expanding and contracting, eventually flaking off).
3. Chemical Weathering
Definition: Chemical weathering changes the chemical composition of minerals within rocks, thereby weakening them and causing them to break up and wear away.
Components: Rocks are made up of different minerals, each with its own chemical composition.
Examples: Carbonation and Oxidation.
Carbonation
Definition: A chemical weathering process primarily affecting limestone rocks.
Mechanism:
Rainwater passes through the atmosphere and mixes with carbon dioxide ().
This forms weak carbonic acid ().
Carbonic acid attacks calcium carbonate (), which is the main mineral in many limestones.
The reaction changes calcium carbonate into calcium bicarbonate ().
Calcium bicarbonate is soluble, meaning it can be dissolved and washed away in water.
Effects: This dissolving process forms caves and underground rivers, creating interesting cave features.
Experimental Illustration: Chalk (containing calcium carbonate) dissolving in vinegar (a weak acid) demonstrates carbonation.
Oxidation
Definition: A chemical weathering process occurring when oxygen from the air dissolves in water and combines with chemicals in rocks to form oxides.
Mechanism:
Oxygen () from the air dissolves in water.
This oxygen-rich water then reacts with certain chemicals (minerals) in rocks.
This reaction forms new chemical compounds called oxides.
Effects: If a rock contains significant iron, oxidation produces brownish iron oxide, which resembles rust. This process weakens rocks, causing them to crumble.
4. Biological Weathering
Definition: Weathering caused by the action of living organisms (plants and animals).
Plant Action:
Physical: Roots (e.g., weeds, trees) growing in cracks in roads, concrete, soil, and rocks physically expand the cracks, causing splitting and breaking.
Chemical: Tiny plants like lichens grow on rocks and produce a dilute, acidic solution that breaks down some rock minerals. Their tiny roots also physically dislodge small rock grains.
Experiment Note: Weathering on lichen-covered rocks has been observed to be to times greater than on bare rocks.
Organic Acids: Decaying remains of dead plants in soil form organic acids which, when dissolved in water, contribute to chemical weathering.
Animal Action:
Physical: Animals burrowing into the soil can weaken rocks or expose rocks and soil to enhanced physical and chemical weathering by creating pathways for water and air.
5. The Impact of Human Activities on Weathering
General Impact: Human activities can significantly increase physical, chemical, and biological weathering.
Contributing Activities:
Building settlements
Making farms
Digging mines
Building roads and railways
Impact on Physical Weathering
Exposure: Human activities expose rocks and soil to the elements, making them more vulnerable to weathering.
Examples:
Mine shafts and tunnels weaken rocks, leading to breakage and collapse.
Road construction involves moving large quantities of soil and rock, exposing new surfaces to physical weathering processes like frost shattering and exfoliation.
Impact on Chemical Weathering (Acid Rain)
Cause: Burning fossil fuels (coal, oil, natural gas) for energy releases chemicals into the air.
Chemical Release: Carbon dioxide (), nitrous oxide (), and sulfur dioxide () are common pollutants.
Acid Formation: These chemicals dissolve in water in clouds, forming stronger acids (e.g., sulfuric acid () and nitric acid ()).
Acid Rain: Rain falling from these clouds is called 'acid rain', a major human cause of chemical weathering.
Consequences:
Rock Damage: Damages constructed features like stone buildings and statues.
Environmental Damage: Kills plants and increases the acidity of lakes and rivers, harming fish and other aquatic organisms.
Impact on Biological Weathering
Exposure and Modification: Many human activities, such as creating vegetable gardens, expose rocks and soil to all three kinds of weathering (physical, chemical, biological).
Farming Practices:
Natural Vegetation Removal: Clearing land for fields exposes bare soil to weather.
Burning Vegetation: Causes chemical changes in the soil.
Chemical Additives: Agricultural chemicals can alter soil composition.
Ploughing: Regularly ploughed soil is disturbed and more prone to erosion.
Animal Movement: Farm animals walking over rocks and soil can dislodge material.
Burrowing Animals: Moles and worms attracted to farm food contribute to biological weathering.
Unit 2: Erosion and Deposition
1. Difference between Weathering, Erosion, and Deposition
Weathering: The breaking down of rocks, soil, and other objects by weather action in one place, with little to no movement of material (e.g., physical, chemical, biological weathering).
Erosion: The wearing away of the Earth's surface by the movement of weathered material (e.g., through moving objects scouring the surface).
Agents of Erosion: Forces in nature that cause erosion, primarily water, wind, and ice.
Deposition: The build-up of material that has been removed from the Earth's surface by weathering and erosion.
Process: Occurs when agents of erosion (rivers, ocean waves, wind) lose energy and can no longer carry their load.
Results: Produces loose stones, soil, and mud that can accumulate into large deposition features (e.g., scree at a slope base, sand dunes, river valley stones, beaches).
2. Rivers - Features of Erosion and Deposition along a River Course
River's Course: The length of a river from its source (where it starts) to its mouth (where it flows into the sea or a lake).
Influencing Factors: River erosion and deposition are influenced by the speed of the river flow and the amount of water it carries.
Sections of a River: Typically divided into three sections: upper, middle, and lower, each with distinct features.
Upper Course of a River (Dominated by Erosion)
Gradient: Steep gradient causes the river to flow fast.
Energy Use: Most of the river's energy is used to maintain its movement.
Load: Rivers transport a significant amount of loose weathered material (sand, stones, rocks, mud) and dissolved chemicals.
Origin of Load: Material washes in after rain or is eroded directly from river banks.
Definition: The entire material transported in a river is called its load.
Relationship: Faster flow allows for more load transport.
Abrasion: The flow of water and its load wears away the river banks and bed.
Vertical Erosion: In the upper course, the river's spare energy is used to transport large rocks, which in turn wear away the river bed. This dominant erosion is downwards.
Valley Shape: Steep V-shaped valleys are typical, with loose stones and soil easily washing into the river, adding to its load.
Specific Upper Course Features:
Rapids: A series of small steps in a river, often caused by alternating bands of hard and soft rock being eroded differently, creating an uneven river bed.
Potholes: Deep holes worn into the river bed by stones in the river's load spinning around in strong currents. Occur during high flow (e.g., after heavy rain).
Waterfalls: Formed by a sudden drop in the river bed's gradient (like a cliff).
Mechanism: Occur where a river flows over a layer of hard rock that overlies softer rock. The softer rock erodes more quickly than the hard rock, creating a step.
Process: Pieces of the harder rock break off and fall into the river. Some of these pieces are then stirred by the forceful water, eroding a deep plunge pool at the base of the waterfall.
Middle and Lower Course of a River (Erosion and Deposition)
Land Gradient: The land is typically flatter than in the upper course.
Lateral Erosion: Erosion here is predominantly sideways (widening the valley) rather than vertical.
Specific Middle/Lower Course Features (Erosion & Deposition):
Meanders: Large, winding bends in the river's shape.
Erosion: Water flows fastest on the outside of a bend, where the force of the river and its load erodes the bank, forming a river cliff.
Deposition: On the inside of the bend, the current is much weaker, causing some of the river's load to be deposited, forming a slip-off slope.
Dynamic Nature: Meanders continuously change shape and position due to ongoing erosion and deposition.
Ox-Bow Lakes: Remains of a meander cut off by river erosion.
Formation: Repeated erosion on the outside of meanders narrows the land between bends (the meander neck). During a flood, the river may cut through this narrow neck, creating a shortcut.
Isolation: Deposition from the river then seals off the old meander channel, leaving an isolated, crescent-shaped ox-bow lake.
Eventual Fate: Ox-bow lakes eventually dry up.
Specific Middle/Lower Course Features (Deposition):
Levees: Natural walls formed on river banks.
Mechanism: Rivers in their middle and lower courses carry significant loads of mud and silt (which give the water a red/brown color).
Formation during Floods: When a river floods, its energy decreases outside the main channel. Heavier material is deposited on the river banks, forming natural embankments. Lighter silt and mud are deposited further onto the flat valley floor.
Floodplains: Wide, flat areas in a river valley adjacent to the river channel that are sometimes covered by floodwater.
Benefit for Humans: Often used for farming and settlements due to fertile soil deposited by floods.
Disadvantages for Humans: Prone to flooding, which can cause damage and loss of life.
Deltas: Semi-circular areas of sand and silt deposited at the mouth of a river.
Formation: Occurs when a river reaches a large, still body of water (like a lake or sea) and spreads out, losing energy and depositing its remaining load.
Development: Material (sand, silt, mud banks) builds up, often blocking the main river channel and causing the river to divide into smaller channels. Deltas can extend outwards, creating new land.
Favorable Conditions: More likely to form in lakes and still seas (e.g., Mediterranean Sea, Nile Delta).
South African Context: No large deltas are found along the South African coast because strong ocean waves and currents are too powerful to allow stable deposition.
3. Sea Features of Erosion and Deposition Associated with Wave Action
Wave Power: The sea is a powerful force of weathering and erosion, with approximately waves breaking on the coast every minute. Each wave, especially during storms, carries a load of sand and stones, capable of eroding land.
Features of Erosion Associated with Wave Action
Cliffs: Steep walls of rock, formed when waves erode a hollow at the base of the shore.
Factors: Size and shape depend on rock type, wave power, and removal rate of eroded material.
Mechanism: As the hollow enlarges, the unsupported rocks above collapse. This process causes the cliff to slowly retreat (move back from its original position).
Associated Feature: A wave-cut platform is often formed at the base of the retreating cliff.
Caves, Arches, and Stacks: Common coastal features produced sequentially by wave erosion.
Cave: Waves erode a weak point or joint in resistant rock to form a hollow.
Arch: If two caves on opposite sides of a headland meet, or if a single cave is enlarged sufficiently, the rock above the cave forms an arch.
Stack: Continued wave erosion and weathering further widen and weaken the arch. Eventually, the arch collapses, leaving an isolated column of rock known as a stack (e.g., 'Hole in the Wall' in the Eastern Cape).
Features of Deposition Caused by Wave Action
Longshore Drift: The direction in which waves move loose material (eroded from cliffs, caves, arches, stacks, or brought by rivers) along the coast.
Mechanism: Waves typically wash sand and stones onto the beach at an angle, while gravity pulls the material directly back down the beach. This creates a zigzag movement of sediment along the coast.
Beaches: Sandy or stony areas on the coast where waves deposit material.
Location: Found between the lowest low tide and the highest high tide marks.
Formation: Material eroded from other coastal parts and transported by longshore drift accumulates to form a beach.
Spits: Ridges of sand and stones that extend out from the coast into the sea.
Formation: Formed by waves and longshore drift at points where the coastline changes direction, but the longshore drift continues to move material in its original direction.
Bars: A ridge of sand that closes off a bay or river mouth.
Formation: Occurs when a spit grows large enough to extend across a river mouth or a small bay.
Associated Feature: Water trapped behind a bar forms a lagoon.
Permanence: All coastal features formed by erosion and deposition are not permanent due to continuous wave action and changing conditions.
4. Moving Ice - Features of Erosion and Deposition Associated with Glaciated Landscapes
Ice Ages: Periods lasting several thousand years when large areas of the Earth are covered by ice. The last Ice Age ended about years ago, covering approximately one-third of the Earth.
Glaciers: Large masses of ice that slowly move over land.
Movement: Glaciers move very slowly (typically a few meters per year).
Load: They carry a significant load of loose rocks and stones.
Glaciation: The process by which the movement of glaciers and their load erodes and shapes the land.
Features of Glaciated Landscapes Caused by Erosion
Cirques: Deep, semi-circular hollows with a steep back wall, forming near the origin of a glacier (also known as corries or cwms).
Arête: A steep-sided, knife-edge ridge that forms when two or more cirques develop back-to-back or side-by-side.
Horn: A sharp, pyramid-shaped mountain peak formed when cirque erosion carves back into a mountain from multiple sides.
U-Shaped Valleys: Glaciers transform normal V-shaped river valleys into distinctive U-shaped valleys by eroding and removing large masses of material from the valley sides and floor.
Hanging Valleys: After a main glacier melts, tributary river valleys (which were once at the same level as the main glacier) are left high above the floor of the main valley.
Features of Glaciated Landscapes Caused by Deposition
Climate Change: Around years ago, the world's climate changed, causing glaciers to melt and deposit the stones, rocks, and sand they carried.
Moraines: Lines or mounds of unsorted mixtures of stones and sand.
Origin: Material can fall onto the glacier from frost-shattered peaks or be worn away from the valley floor.
Location: Melting glaciers dump moraines on the sides of valleys (lateral moraines) and across the end of a valley (terminal moraines).
Eskers: Long, winding ridges of deposited gravel or stones, formed by rivers that flowed underneath a glacier.
Drumlins: Small, elongated hills made of stones and clay, shaped when a glacier begins to melt and reshapes previously deposited till.
5. Wind - Features of Erosion and Deposition Associated with Wind
Location: Prominent in desert and semi-desert areas (e.g., Karoo and Kalahari Desert in South Africa) where loose sand and soil are prevalent.
Transport of Material by Wind:
Suspension: Very fine, light material is carried in the air.
Saltation/Creep: Heavier sand and stones move along the ground by 'jumping' (saltation) or rolling (creep).
Features Produced by Wind Erosion
Abrasion (Sand Blasting): The action of wind blowing sand and soil against rocks, causing erosion.
Active Zone: Most wind erosion occurs between the ground and m above the ground, where the concentration of transported material is highest.
Influencing Factors: Rock hardness and distance above ground significantly influence the shape of erosion features.
Results of Wind Erosion: Wind erodes softer rock more quickly than hard rock.
Specific Landforms: Rock pinnacles (or rock fingers), arches, and mushroom-shaped rocks are commonly produced.
Features Produced by Wind Deposition
Mechanism: Deposition occurs when the wind speed drops, and it can no longer carry its load. Deserts are dynamic environments, with sand constantly shifting.
Dune Movement: Grains of sand roll down dunes due to gravity. Combined with wind action, this causes sand dunes to move slowly forward in the direction of the wind.
Parts of a Sand Dune:
Windward Side: The side facing the wind, where sand is pushed up.
Slip Face: The sheltered, steeper side where sand rolls down due to gravity.
Types of Sand Dunes (Shaped by Wind)
Transverse Dunes: Sand dunes that form horizontally or perpendicularly to the prevailing wind direction.
Barchans: Arc-shaped sand dunes with two 'horns' pointing downwind.
Longitudinal Dunes: Sand dunes that form lengthways, in the same direction as the prevailing wind.
Unit 3: The Impact of People on Soil Erosion
1. Human Contributions to Soil Erosion Through Agriculture, Construction, and Mining
Definition of Soil Erosion: The removal of soil faster than natural processes can form new soil.
Natural Balance: In nature, soil formation and soil removal rates are typically balanced.
Human Impact: Human activities cause soil to be removed at a significantly greater rate than it is replaced. Scientists estimate human activity causes soil removal ten times faster than natural processes.
Major Contributors (by % of global soil loss):
Crop farming (largest contributor)
Deforestation
Over-grazing
Industry
Soil Erosion Caused by Agriculture (Greatest Cause)
Removing Natural Vegetation: Exposes bare soil to wind and water.
Ploughing: Loosens the soil, making it more easily washed or blown away. Ploughing down a slope creates channels that accelerate water runoff and soil removal.
Monoculture: Growing only one type of crop year after year depletes specific soil nutrients, weakening the soil structure and making it more susceptible to erosion.
Over-grazing: Keeping too many animals leads to excessive consumption of plant cover, resulting in bare soil patches that are highly vulnerable to erosion.
The Impact of Construction on Soil Erosion
Land Clearing: People clear land of vegetation and soil to build roads and settlements.
Soil Loss and Damage: Many large settlements are on flat, fertile lands near rivers. During construction, large amounts of this valuable soil are lost, damaged, or polluted.
Exposure: Road and building construction exposes soil to wind and water erosion, especially newly disturbed areas.
The Impact of Mining on Soil Erosion
Open Cast Mining: A surface mining method that has a significant impact.
Process: The first stage involves removing all vegetation and topsoil, leaving the remaining soil directly exposed to erosion.
Pollution: The exposed soil is also polluted by dust and chemicals generated from the mining operation.
2. Case Study: Agriculture as a Contributor to Soil Erosion in South Africa
Context: South Africa's dry climate, historical land-use policies (apartheid), and farming practices have led to severe soil erosion across large areas.
Scale of Loss: Between and million tons of soil are lost from South African farms annually.
Consequence: Soil erosion is a serious issue because it reduces agricultural productivity, affecting the ability to produce enough food for the population.
How Modern Commercial Farming Methods Contribute to Soil Erosion
Ploughing Practices:
Using machines (ploughs) for food production can create channels that enable rainwater to flow quickly down slopes, washing away soil.
Solution: Contour ploughing (ploughing in line with the contours of the land) can reduce soil erosion by up to compared to ploughing straight down slopes.
Limited Crop Range (Monoculture):
Many South African farmers grow a narrow range of crops (e.g., maize, sorghum, sunflowers, groundnuts).
These crops continuously draw the same specific minerals from the soil.
Depletion: Soils do not have sufficient time to recover and replenish these minerals before the next crop, leading to reduced fertility and increased susceptibility to erosion.
Statistic: South Africa loses approximately tons of soil for every ton of crops it produces.
How South Africa's Apartheid Policies Contributed to Soil Erosion on Farms
Land Distribution: White farmers controlled the best agricultural land and practiced commercial farming.
'Homelands': Black farmers were forcibly settled on only of the total land area, typically in less productive regions.
Consequences on Black Farms:
Small, Over-crowded Farms: Led to intense pressure on the land.
Over-grazing: Too many animals on limited land resulted in severe depletion of plant cover.
Over-farming: Continuous cultivation without proper rest led to soil exhaustion.
Outcome: Widespread and severe soil erosion, which further reduced the already limited land available for farming.
Example (Former Ciskei): Over-grazing by cattle (ate grass) and goats (ate grass roots) created large patches of bare soil. Subsequent heavy summer rainstorms washed away soil that had taken thousands of years to form, depositing it into local dams and rivers (e.g., the Great Fish River).
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