G1: Earth's Structure & Composition
The layered structure of the Earth — crust, mantle, outer core and inner core — and the composition and properties of each layer.
Earth's Structure & Composition
The layered structure of the Earth — crust, mantle, outer core and inner core — and the composition and properties of each layer.
Key Fact: The Earth has four main layers: crust, mantle, outer core and inner core
Key Fact: The crust is thin (5-70 km) and made of solid rock; continental crust is granitic, oceanic crust is basaltic
Key Fact: The mantle is the thickest layer (~2,900 km); the upper mantle is solid but the asthenosphere is partially molten and can flow slowly
Key Fact: The outer core is liquid iron and nickel; it generates Earth's magnetic field through convection currents
Key Fact: The inner core is solid iron and nickel at ~5,500 degrees C, kept solid by enormous pressure
Key Fact: We know Earth's internal structure from studying seismic waves: P-waves pass through all layers; S-waves cannot pass through the liquid outer core
📋 Key Vocabulary and Concepts
For Earth's Structure & Composition, you must know:
- Lithosphere: The rigid outer layer of Earth comprising the crust and uppermost solid mantle; divided into tectonic plates
- Asthenosphere: The partially molten, ductile layer in the upper mantle below the lithosphere that allows plates to move
- Seismic wave shadow zone: The region on Earth's surface where S-waves are absent (beyond ~104 degrees from an earthquake), proving the outer core is liquid
- P-wave: Primary/compressional seismic wave that travels through solids and liquids; fastest seismic wave
- S-wave: Secondary/shear seismic wave that travels only through solids; cannot pass through the liquid outer core
❓ Practice Questions
Q: Name the four main layers of the Earth from the surface inward.
Q: How do seismic waves provide evidence for Earth's internal structure?
Q: Why is the inner core solid despite temperatures of ~5,500 degrees C?
Q: What is the difference between continental and oceanic crust?
Q: What generates Earth's magnetic field?
✅ Answers
- Crust, mantle, outer core, inner core.
- P-waves travel through all layers (solids and liquids) but refract at boundaries. S-waves cannot pass through the liquid outer core, creating a shadow zone. The patterns of wave refraction and shadow zones reveal the depth and state of each layer.
- The immense pressure at that depth raises the melting point of iron and nickel above the actual temperature, keeping it solid.
- Continental crust is thicker (30-70 km), less dense (~2.7 g/cm3), and granitic in composition. Oceanic crust is thinner (5-10 km), denser (~3.0 g/cm3), and basaltic.
- Convection currents in the liquid iron-nickel outer core, acting as a geodynamo, generate Earth's magnetic field.
🎯 Exam Tips
- Seismic wave behaviour is the key evidence for internal structure
- S-wave shadow zone proves the outer core is liquid
- Continental vs oceanic crust: density and composition differences drive plate tectonics
- The lithosphere includes BOTH crust and uppermost mantle, not just the crust
📝 Exam Technique
GCSE Geology Exam Tips — Earth's Structure & Composition:
1. For Earth's Structure & Composition questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Earth's Structure & Composition to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Earth's Structure & Composition questions, use correct geological terminology and classify features precisely
2. Reference specific geological time periods and processes
3. When interpreting data, consider the quality and limitations of the evidence
4. Apply your understanding of Earth's Structure & Composition to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ The mantle is liquid magma ✓ The mantle is mostly solid rock; only the asthenosphere is partially molten and flows very slowly
✗ The crust is the lithosphere ✓ The lithosphere includes the crust AND the uppermost solid part of the mantle
✗ S-waves slow down in the outer core ✓ S-waves CANNOT travel through the liquid outer core at all, they are completely blocked
✍️ Model Answer
Full-Mark Response
Describe the internal structure of the Earth and explain how seismic waves provide evidence for it. [6 marks]
The Earth has four main layers. The crust is the thin outermost solid layer: continental crust is granitic, 30-70 km thick and density ~2.7 g/cm3; oceanic crust is basaltic, 5-10 km thick and density ~3.0 g/cm3. The mantle extends to 2,900 km depth; it is mostly solid rock but the asthenosphere (upper mantle) is partially molten and can flow plastically. The outer core (2,900-5,100 km) is liquid iron and nickel, generating Earth's magnetic field. The inner core (5,100-6,371 km) is solid iron and nickel at ~5,500 degrees C, kept solid by extreme pressure. Seismic waves provide this evidence: P-waves (compressional) refract at layer boundaries due to density changes, revealing layer depths. Crucially, S-waves (shear) cannot pass through liquids and are blocked by the outer core, creating a shadow zone beyond ~104 degrees from an earthquake epicentre. This proves the outer core is liquid. P-waves slow and refract sharply at the outer core boundary, confirming a denser liquid layer. Combined, these wave patterns map out each layer's depth, state and approximate composition.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of earth's structure & composition, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of earth's structure & composition to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to earth's structure & composition, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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