G9: Metamorphic Rocks
Formation, classification and identification of metamorphic rocks, regional and contact metamorphism, foliation and index minerals.
Metamorphic Rocks
Formation, classification and identification of metamorphic rocks, regional and contact metamorphism, foliation and index minerals.
Key Fact: Metamorphic rocks form when existing rocks are changed by heat and/or pressure without melting
Key Fact: Regional metamorphism: large-scale, caused by tectonic compression and heat during mountain building; produces foliated rocks
Key Fact: Contact metamorphism: local, caused by heat from an igneous intrusion; produces non-foliated rocks
Key Fact: Foliation: alignment of minerals in bands, produced by directed pressure (e.g. slate to schist to gneiss with increasing grade)
Key Fact: Index minerals indicate metamorphic grade: chlorite (low), biotite, garnet, staurolite, kyanite, sillimanite (high)
Key Fact: Common metamorphic rocks: slate (from mudstone), schist (from shale), gneiss (from granite), marble (from limestone), quartzite (from sandstone)
📋 Key Vocabulary and Concepts
For Metamorphic Rocks, you must know:
- Foliation: The parallel alignment of platy minerals (e.g. mica) in a metamorphic rock, produced by directed pressure
- Metamorphic grade: The intensity of metamorphism, from low grade (minor changes) to high grade (extensive recrystallisation)
- Index mineral: A mineral that forms only within a specific range of temperature and pressure, used to indicate metamorphic grade
- Contact metamorphism: Localised metamorphism caused by the heat of an igneous intrusion baking surrounding country rock
- Regional metamorphism: Large-scale metamorphism caused by tectonic compression and elevated temperatures during mountain building
❓ Practice Questions
Q: How does regional metamorphism differ from contact metamorphism?
Q: Describe the metamorphic progression from mudstone to gneiss.
Q: What are index minerals and why are they useful?
Q: How does marble form and what are its characteristics?
Q: Why does foliation develop in some metamorphic rocks but not others?
✅ Answers
- Regional metamorphism is large-scale, caused by tectonic compression and heat during mountain building, producing foliated rocks over wide areas. Contact metamorphism is local, caused by heat from an igneous intrusion, producing non-foliated rocks in a narrow aureole around the intrusion.
- Mudstone to slate (low grade, fine-grained, slaty cleavage) to phyllite (low-medium, slightly coarser) to schist (medium grade, visible mica, schistosity) to gneiss (high grade, coarse, banded segregation of light and dark minerals).
- Minerals that only form within specific temperature-pressure ranges. Their presence in a rock indicates the metamorphic grade. For example, chlorite indicates low grade, garnet indicates medium grade, and sillimanite indicates high grade.
- Marble forms from the metamorphism of limestone. Calcite recrystallises into interlocking crystals, destroying any fossils and sedimentary textures. Marble is non-foliated and has a sugary texture.
- Foliation develops under directed pressure (differential stress) which aligns platy minerals like mica perpendicular to the maximum stress direction. It does not develop under uniform pressure (confining pressure only, as in contact metamorphism) or in rocks lacking platy minerals.
🎯 Exam Tips
- Metamorphic grade increases: slate, phyllite, schist, gneiss, know the sequence
- Foliated = directed pressure (regional); Non-foliated = heat only (contact)
- Index minerals: chlorite (low), garnet (medium), sillimanite (high)
- Parent rock matters: mudstone to slate, limestone to marble, sandstone to quartzite, granite to gneiss
📝 Exam Technique
GCSE Geology Exam Tips — Metamorphic Rocks:
1. For Metamorphic 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 Metamorphic Rocks to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
1. For Metamorphic 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 Metamorphic Rocks to fieldwork observations and map data
5. For evaluation, weigh competing geological theories with evidence
⚠️ Common Errors
✗ Metamorphic rocks form from melting ✓ Metamorphic rocks form from solid-state changes, heat and pressure alter the mineralogy and texture WITHOUT melting. If melting occurs, it becomes an igneous process.
✗ All metamorphic rocks are foliated ✓ Only rocks formed under directed pressure (regional metamorphism) develop foliation. Contact metamorphic rocks (e.g. marble, quartzite, hornfels) are non-foliated because only heat, not directed stress, was involved.
✗ Schist forms from sandstone ✓ Schist forms from shale/mudstone (a pelitic parent rock). Sandstone metamorphoses to quartzite, the parent rock composition determines the metamorphic product
✍️ Model Answer
Full-Mark Response
Explain how metamorphic rocks are formed and describe the differences between regional and contact metamorphism. [6 marks]
Metamorphic rocks form when pre-existing rocks (protoliths) are altered by heat and/or pressure in the solid state, without melting. New minerals grow that are stable at the new conditions, and the rock's texture changes. Regional metamorphism occurs over large areas during mountain building, where tectonic compression provides both directed pressure and elevated temperature. Directed pressure causes platy minerals (especially micas) to align perpendicular to the maximum stress, producing foliation, the hallmark of regional metamorphism. Increasing metamorphic grade produces: slate (fine-grained slaty cleavage), then phyllite, then schist (visible mica flakes with schistosity), then gneiss (coarse banding of light and dark minerals). Index minerals like chlorite (low grade), garnet (medium) and sillimanite (high) indicate the intensity. Contact metamorphism occurs locally around igneous intrusions, where heat from the magma bakes the surrounding country rock in a narrow metamorphic aureole. Only heat (no directed pressure) is involved, so rocks are non-foliated: limestone becomes marble (recrystallised calcite), sandstone becomes quartzite (fused quartz grains), and mudstone becomes hornfels (hard, fine-grained, non-foliated). The key difference is that regional metamorphism involves directed pressure (producing foliation) over wide areas, while contact metamorphism involves only heat (producing non-foliated rocks) in a narrow zone.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of metamorphic rocks, including key geological concepts, observational data and theoretical models relevant to OCR J357.
AO2 (Application of Knowledge): Apply knowledge and understanding of metamorphic 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 metamorphic rocks, evaluate evidence from observations and investigations, and construct reasoned arguments using scientific methodology.
📝 Exam Questions by Topic
🎬 Video Resources
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