G7: Igneous Rocks
Formation, classification and identification of igneous rocks, intrusive (plutonic) and extrusive (volcanic), and their textures.
Igneous Rocks
Formation, classification and identification of igneous rocks, intrusive (plutonic) and extrusive (volcanic), and their textures.
Key Fact: Igneous rocks form from the cooling and crystallisation of magma (intrusive) or lava (extrusive)
Key Fact: Intrusive igneous rocks cool slowly underground, producing large visible crystals (coarse-grained): granite, gabbro, diorite
Key Fact: Extrusive igneous rocks cool rapidly at the surface, producing fine-grained or glassy textures: basalt, rhyolite, andesite, obsidian
Key Fact: Granite (felsic, intrusive) = quartz + feldspar + mica; Basalt (mafic, extrusive) = pyroxene + plagioclase + olivine
Key Fact: Bowen's reaction series shows the order of mineral crystallisation from cooling magma: olivine/pyroxene first (high temperature), then quartz last (lowest temperature)
Key Fact: Silica content determines rock classification: felsic (>65% SiO2), intermediate (52-65%), mafic (45-52%), ultramafic (<45%)
📋 Key Vocabulary and Concepts
For Igneous Rocks, you must know:
- Phaneritic texture: Coarse-grained igneous texture with crystals visible to the naked eye; formed by slow cooling underground (intrusive)
- Aphanitic texture: Fine-grained igneous texture with crystals too small to see without a microscope; formed by rapid cooling at the surface (extrusive)
- Porphyritic texture: A mixed texture with large crystals (phenocrysts) in a fine-grained groundmass; indicates two-stage cooling
- Felsic: Igneous rock rich in feldspar and silica (>65% SiO2), light in colour (e.g. granite, rhyolite)
- Mafic: Igneous rock rich in magnesium and iron (45-52% SiO2), dark in colour (e.g. basalt, gabbro)
❓ Practice Questions
Q: How does cooling rate affect the texture of igneous rocks?
Q: What is the mineral composition of granite?
Q: Describe Bowen's reaction series.
Q: What is a porphyritic texture and how does it form?
Q: Compare granite and basalt in terms of composition, texture and origin.
✅ Answers
- Slow cooling (underground) allows time for large crystals to grow, producing coarse-grained texture (e.g. granite). Rapid cooling (at the surface) produces small crystals, giving fine-grained texture (e.g. basalt). Very rapid cooling produces glass (e.g. obsidian).
- Quartz, orthoclase feldspar, plagioclase feldspar, and mica (biotite and/or muscovite). It is a felsic, intrusive rock.
- As magma cools, minerals crystallise in a predictable order. The discontinuous branch: olivine, then pyroxene, then amphibole, then biotite. The continuous branch: calcium-rich plagioclase becoming sodium-rich plagioclase. Finally, potassium feldspar, muscovite and quartz crystallise at the lowest temperatures.
- Large crystals (phenocrysts) set in a finer-grained groundmass. It forms in two stages: initial slow cooling underground produces large crystals, then rapid cooling at the surface freezes the remaining magma as fine-grained groundmass.
- Granite: felsic (quartz + feldspar + mica), coarse-grained, intrusive (slow cooling underground). Basalt: mafic (pyroxene + plagioclase + olivine), fine-grained, extrusive (rapid cooling at surface). Granite is light-coloured; basalt is dark.
🎯 Exam Tips
- Cooling rate = texture: slow = coarse, fast = fine, instant = glassy
- Granite (intrusive) and Rhyolite (extrusive): same composition, different texture
- Basalt (extrusive) and Gabbro (intrusive): same composition, different texture
- Bowen's series: discontinuous branch + continuous branch, know the mineral order
📝 Exam Technique
GCSE Geology Exam Tips — Igneous Rocks:
1. For Igneous Rocks 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 Igneous Rocks to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Igneous Rocks 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 Igneous Rocks to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ All igneous rocks are the same colour ✓ Igneous rock colour depends on composition: felsic rocks are light (granite), mafic rocks are dark (basalt), intermediate rocks are medium-coloured (andesite)
✗ Magma and lava are the same thing ✓ Magma is molten rock BELOW the surface; lava is molten rock that has reached the surface, they are the same material at different locations
✗ All intrusive rocks are granite ✓ Intrusive rocks range from granite (felsic) to diorite (intermediate) to gabbro (mafic) to peridotite (ultramafic), composition varies independently of cooling rate
✍️ Model Answer
Full-Mark Response
Explain how igneous rocks are classified by composition and texture, and describe how Bowen's reaction series relates to their formation. [6 marks]
Igneous rocks are classified by two main criteria: composition (silica content) and texture (grain size from cooling rate). By composition: felsic rocks (>65% SiO2) are light-coloured and include granite (intrusive) and rhyolite (extrusive). Intermediate rocks (52-65% SiO2) include diorite and andesite. Mafic rocks (45-52% SiO2) are dark-coloured and include gabbro (intrusive) and basalt (extrusive). Ultramafic rocks (<45% SiO2) include peridotite. By texture: intrusive rocks cool slowly underground, producing phaneritic (coarse-grained) textures with visible crystals. Extrusive rocks cool rapidly at the surface, producing aphanitic (fine-grained) textures. Two-stage cooling creates porphyritic texture (large phenocrysts in fine groundmass). Bowen's reaction series explains the order of crystallisation from cooling magma. The discontinuous branch shows mafic minerals crystallising first at high temperature: olivine, then pyroxene, then amphibole, then biotite. The continuous branch shows plagioclase feldspar becoming progressively more sodium-rich. At the lowest temperatures, potassium feldspar, muscovite and quartz crystallise. This explains why mafic minerals form early and are found in hot, mafic magmas, while quartz is the last to form and is only in felsic rocks.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of igneous rocks, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of igneous rocks to both familiar and unfamiliar geological contexts, using field evidence and theoretical principles to explain phenomena.
AO3 (Analysis & Evaluation): Analyse geological data related to igneous rocks, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Geology exams?
Get the best revision books and guides to boost your grades.