The Lithosphere: Soil and Rock Resources

What you'll learn

This revision guide covers the lithosphere component of the CIE IGCSE Environmental Management syllabus, focusing on soil formation, properties, degradation, and rock resources. You'll understand how soil develops through weathering processes, the characteristics of different soil types, and the environmental impacts of extracting and using rock resources. This topic is essential for understanding sustainable land management and resource use.

Key terms and definitions

Lithosphere — the rigid outer layer of the Earth consisting of the crust and upper mantle, containing all soil and rock resources

Weathering — the breakdown of rocks in situ (in place) by physical, chemical, or biological processes without transportation of material

Soil profile — a vertical section through soil showing distinct layers (horizons) from the surface to the bedrock

Humus — dark, organic material formed from the decomposition of plant and animal matter, essential for soil fertility

Leaching — the downward movement of dissolved minerals and nutrients through soil layers due to percolating water

Soil erosion — the removal and transport of topsoil by wind, water, or human activity, leading to soil degradation

Quarrying — the extraction of rock, sand, or minerals from an open pit on the Earth's surface

Ore — naturally occurring rock or sediment containing sufficient metal or valuable minerals to make extraction economically viable

Core concepts

Weathering processes and soil formation

Weathering breaks down parent rock material into smaller particles, initiating soil formation. Three main types occur:

Physical weathering breaks rock mechanically without changing its chemical composition:

• Freeze-thaw action: water enters cracks, freezes, expands by 9%, and fragments rock
• Temperature fluctuations: repeated heating and cooling causes expansion and contraction, weakening rock structure
• Salt crystallization: salt deposits grow in rock pores, creating pressure and causing disintegration
• Biological action: tree roots penetrate cracks and exert pressure as they grow

Chemical weathering alters rock composition through chemical reactions:

• Carbonation: rainwater (weak carbonic acid) reacts with limestone (calcium carbonate), dissolving it
• Oxidation: oxygen reacts with iron-bearing minerals, forming iron oxides (rust) which weaken rock
• Hydrolysis: water reacts with minerals like feldspar, breaking them down into clay minerals
• Solution: minerals dissolve directly in water, particularly halite (rock salt)

Biological weathering combines physical and chemical processes:

• Organic acids from decomposing vegetation dissolve minerals
• Lichen and moss produce acids that chemically attack rock surfaces
• Burrowing organisms like earthworms mix soil and break down organic matter

Soil develops through five key factors working together:

1. Parent material — the underlying rock type determines mineral composition and initial texture
2. Climate — temperature and rainfall influence weathering rates and organic matter decomposition
3. Organisms — plants, animals, bacteria, and fungi add organic matter and create soil structure
4. Topography — slope angle affects drainage, erosion, and soil thickness
5. Time — soil development requires hundreds to thousands of years; mature soils show well-developed profiles

Soil composition and properties

Soil consists of four main components:

Mineral particles (typically 45% by volume):

• Sand (0.05-2.0 mm): large particles, visible to naked eye, feels gritty
• Silt (0.002-0.05 mm): medium particles, feels smooth when wet
• Clay (<0.002 mm): microscopic particles, feels sticky when wet, high nutrient retention

Organic matter (typically 5%):

• Living organisms: bacteria, fungi, earthworms, insects, plant roots
• Dead material: leaf litter, plant residues, animal remains
• Humus: fully decomposed organic matter, dark-coloured, improves structure and fertility

Water (typically 25%):

• Fills pore spaces between particles
• Dissolves and transports nutrients
• Essential for plant uptake and organism survival

Air (typically 25%):

• Occupies pores not filled with water
• Provides oxygen for root respiration and soil organisms
• Contains carbon dioxide from decomposition

Soil texture describes the proportion of sand, silt, and clay. Loam soils contain balanced proportions of all three, providing optimal conditions for most plants: good drainage, water retention, nutrient availability, and workability.

Soil structure refers to how particles aggregate together:

• Granular structure: rounded clusters, excellent for root growth and drainage
• Blocky structure: angular aggregates, moderate drainage
• Platy structure: horizontal layers, restricts drainage and root penetration
• Single-grain: no aggregation, common in sandy soils

Soil profile horizons

A mature soil develops distinct horizontal layers:

O Horizon (organic layer):

• Surface layer of fresh and partially decomposed organic matter
• Leaf litter, twigs, dead organisms
• Absent in cultivated soils

A Horizon (topsoil):

• Dark colour from humus content
• High biological activity and fertility
• Most important for plant growth
• Vulnerable to erosion
• Depth typically 10-30 cm

E Horizon (eluviation layer):

• Light-coloured due to leaching
• Minerals and nutrients washed downward
• Present in forest soils with high rainfall
• Often absent in agricultural soils

B Horizon (subsoil):

• Accumulation zone for leached minerals
• Less organic matter than A horizon
• Often reddish or yellowish from iron oxides
• Denser and more clay-rich

C Horizon (parent material):

• Partially weathered bedrock
• Few organisms or organic matter
• Source of mineral particles for upper layers

R Horizon (bedrock):

• Unweathered solid rock
• Impermeable to roots and water

Soil degradation processes

Soil erosion removes fertile topsoil through:

Water erosion:

• Sheet erosion: thin layer removed uniformly across slopes
• Rill erosion: small channels form in bare soil
• Gully erosion: large, deep channels cut into land
• Splash erosion: raindrop impact dislodges particles

Wind erosion:

• Removes fine particles (silt and clay)
• Most severe in dry regions with sparse vegetation
• Dust storms transport soil thousands of kilometres

Factors accelerating erosion:

• Deforestation removes protective vegetation cover
• Overgrazing by livestock destroys grass cover and compacts soil
• Poor farming practices leave soil bare between crops
• Steep slopes increase water velocity and erosive power
• Intensive rainfall exceeds soil infiltration capacity

Soil degradation also occurs through:

Compaction:

• Heavy machinery crushes soil structure
• Reduces pore space, restricting air and water movement
• Impedes root growth and drainage

Salinization:

• Accumulation of salts in surface layers
• Common in irrigated arid regions
• Occurs when irrigation water evaporates, leaving salts behind
• High salt concentration damages plant roots and reduces yields

Nutrient depletion:

• Continuous cropping without fertilizer replacement
• Harvesting removes nutrients from the system
• Leaching washes nutrients beyond root zones

Acidification:

• Excessive use of ammonium-based fertilizers
• Acid rain deposition
• Decomposition of organic matter in waterlogged conditions
• Low pH reduces nutrient availability and microbial activity

Conservation and sustainable soil management

Protecting soil resources requires multiple approaches:

Contour ploughing: ploughing across slopes rather than up and down creates ridges that slow water flow and reduce erosion

Terracing: cutting step-like levels into hillsides reduces slope angle and water velocity on each terrace

Strip cropping: alternating strips of different crops, combining dense-growing crops with row crops to maintain soil cover

Cover cropping: planting crops (like clover or rye) during fallow periods to protect soil and add organic matter

Crop rotation: growing different crops in sequence on the same land prevents nutrient depletion and breaks pest cycles

Windbreaks: planting trees or shrubs in lines perpendicular to prevailing winds reduces wind speed and erosion

Reduced tillage: minimizing soil disturbance maintains structure, reduces erosion, and preserves organic matter

Organic matter addition: incorporating compost, manure, or crop residues improves structure, water retention, and fertility

Drainage management: preventing waterlogging through ditches or subsurface drains reduces leaching and maintains aeration

Rock resources and extraction

The lithosphere provides essential mineral and rock resources:

Types of rock resources:

Igneous rocks:

• Granite: used for building stone, monuments, worktops
• Basalt: crushed for aggregate in concrete and road construction
• Formed from cooling magma or lava

Sedimentary rocks:

• Limestone: cement production, building stone, aggregate, chemical industry
• Sandstone: building material, grindstones
• Coal: fossil fuel for energy generation
• Formed from compressed sediments

Metamorphic rocks:

• Marble: decorative stone, sculpture
• Slate: roofing tiles, flooring
• Formed from pre-existing rocks altered by heat and pressure

Metallic ores:

• Iron ore: steel production for construction and manufacturing
• Bauxite: aluminium extraction for transport and packaging
• Copper ore: electrical wiring, plumbing, electronics

Extraction methods and impacts:

Quarrying process:

1. Vegetation and topsoil removal (overburden stripping)
2. Rock extraction by blasting or mechanical excavation
3. Crushing and grading material
4. Transportation to processing facilities or end users

Environmental impacts of quarrying:

Negative impacts:

• Habitat destruction and biodiversity loss from vegetation clearance
• Visual pollution creating scars on landscapes
• Noise pollution from blasting, machinery, and vehicle movements
• Dust generation affecting air quality and nearby vegetation
• Vibration from blasting disturbing local communities and structures
• Water table disruption affecting local hydrology
• Heavy vehicle traffic increasing road congestion and accidents
• Soil erosion from exposed surfaces

Positive impacts:

• Employment opportunities in extraction and processing
• Economic development in rural areas
• Raw materials for construction and manufacturing
• Abandoned quarries can create wildlife habitats (wetlands, refuge sites)

Mitigation and restoration:

• Planning permission restrictions limiting hours and methods
• Dust suppression using water sprays
• Landscaping and screening with earth banks and vegetation
• Progressive restoration during extraction
• Final restoration converting sites to lakes, nature reserves, or recreational areas
• Topsoil storage and replacement supporting vegetation re-establishment
• Noise barriers reducing sound transmission

Sustainable resource management:

• Recycling aggregates from demolition waste
• Using alternative materials where possible
• Extracting resources closest to point of use (reducing transport)
• Complete extraction from approved sites before opening new ones
• Extending quarry life through careful planning

Worked examples

Example 1: Describe how climate affects soil formation. [4 marks]

Model answer: Temperature affects the rate of weathering, with higher temperatures increasing chemical weathering rates [1]. Rainfall influences leaching, with high rainfall washing nutrients from upper to lower horizons [1]. Climate determines vegetation type and density, affecting organic matter input to soil [1]. Temperature and moisture together control decomposition rates of organic matter, with warm, moist conditions accelerating humus formation [1].

Example 2: Explain why topsoil is more fertile than subsoil. [3 marks]

Model answer: Topsoil contains higher concentrations of humus from decomposed organic matter, providing nutrients for plants [1]. The topsoil has greater biological activity, with organisms such as earthworms creating good soil structure and mixing nutrients [1]. Plant roots concentrate in topsoil, cycling nutrients back when they decompose [1].

Example 3: A farmer notices gullies forming on a sloped field after removing hedgerows. Suggest and explain two soil conservation methods the farmer could implement. [6 marks]

Model answer: The farmer could use contour ploughing, ploughing across the slope rather than up and down [1]. This creates ridges perpendicular to water flow, slowing runoff and reducing its erosive power [1], allowing more water to infiltrate and reducing soil transport [1].

The farmer could plant windbreaks, establishing hedgerows or tree lines along field boundaries [1]. These reduce wind speed across the field, decreasing wind erosion [1], and the roots also help bind soil particles together, stabilizing the surface [1].

Common mistakes and how to avoid them

• Confusing weathering with erosion: Weathering breaks down rock in place; erosion involves transport of material. Always specify that weathering occurs in situ without movement.

• Misidentifying soil horizons: The A horizon is topsoil (organic-rich), B horizon is subsoil (accumulation zone), and C horizon is weathered parent material. Don't confuse their positions or characteristics.

• Incomplete answers on soil degradation: When asked about soil degradation, students often mention only erosion. Remember to include compaction, salinization, nutrient depletion, and acidification where relevant.

• Vague descriptions of quarrying impacts: Avoid general statements like "damages the environment." Be specific: habitat destruction, noise pollution, dust generation, visual scarring, water table disruption.

• Forgetting positive aspects: Questions about resource extraction often expect balanced answers. Include economic benefits (employment, raw materials) alongside environmental costs.

• Not linking processes to examples: When describing soil conservation methods, always explain the mechanism. Don't just state "contour ploughing helps"—explain that it slows water flow, reducing erosive power.

Exam technique for "The Lithosphere: Soil and Rock Resources"

• Command word precision: "Describe" requires characteristics or features; "explain" needs reasons or mechanisms with linking words (because, therefore, this causes). For a 4-mark "explain" question, provide four distinct points with clear cause-and-effect relationships.

• Use proper terminology: Replace casual language with technical terms. Say "leaching of nutrients through soil horizons" rather than "minerals washing away." This demonstrates subject knowledge and earns marks for terminology.

• Structure comparison questions: When asked to compare soil types or rock resources, create a clear point-by-point structure. State the similarity or difference, then provide specific details for each type being compared.

• Quantify where possible: Include specific measurements when known (clay particles <0.002 mm, topsoil depth 10-30 cm). This shows precise knowledge and distinguishes top-band answers.

Quick revision summary

The lithosphere provides soil and rock resources essential for human activity. Weathering (physical, chemical, biological) breaks down parent rock, initiating soil formation influenced by climate, organisms, topography, and time. Mature soils develop distinct horizons, with the A horizon (topsoil) most fertile due to humus content. Soil degradation occurs through erosion, compaction, salinization, and nutrient depletion. Conservation methods including contour ploughing, terracing, and cover cropping protect this vital resource. Quarrying extracts rocks and minerals but causes environmental impacts (habitat loss, pollution, landscape scarring) requiring careful management and restoration.