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examBoard: Cambridge
examType: IGCSE
lessonTitle: Metamorphic Rocks Formation and Characteristics
Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical processes without completely melting. The word "metamorphic" comes from Greek, meaning "to change form". These transformations happen deep within the Earth's crust, creating rocks with unique textures and mineral compositions that tell fascinating stories about Earth's geological processes.
Key Definitions:
Heat provides energy for atoms to move and rearrange themselves into new mineral structures. Sources include:
Temperatures typically range from 200°C to 850°C, below the melting point of most rocks.
Pressure forces minerals to become more compact and can cause them to align in specific directions. Sources include:
Pressure can reach several thousand times atmospheric pressure.
Different geological settings create different types of metamorphism, each producing distinctive rock types.
Cause: Heat from magma intrusions
Features:
Examples: Marble from limestone, hornfels from shale
Cause: Mountain-building processes
Features:
Examples: Slate from shale, schist from various rocks
Cause: Friction along fault zones
Features:
Examples: Mylonite, cataclasite
Metamorphic rocks are primarily classified based on their texture, particularly whether they show foliation or not.
Foliated rocks have a layered or banded appearance due to the alignment of platy or elongated minerals under directed pressure. The degree of foliation often indicates the intensity of metamorphism.
Parent rock: Shale or mudstone
Features: Fine-grained with perfect cleavage, splits into thin sheets
Uses: Roofing, flooring, blackboards
Grade: Low-grade metamorphism
Parent rock: Slate
Features: Silky sheen, visible mica flakes, wavy foliation
Uses: Decorative stone, construction
Grade: Low to medium-grade metamorphism
Parent rock: Various rocks including phyllite
Features: Medium to coarse-grained, visible mineral crystals, strong foliation
Uses: Construction, decorative stone
Grade: Medium-grade metamorphism
Parent rock: Various rocks including schist
Features: Coarse-grained, distinct banding of light and dark minerals
Uses: Building stone, countertops, decorative purposes
Grade: High-grade metamorphism
Parent rock: Various high-grade metamorphic rocks
Features: Mixed appearance with parts that look metamorphic and parts that look igneous
Uses: Decorative stone
Grade: Very high-grade metamorphism (partial melting)
Non-foliated rocks lack the layered appearance of foliated rocks. They typically form when the parent rock is composed of minerals that don't align under pressure or when metamorphism occurs primarily due to heat rather than directed pressure.
Parent rock: Limestone or dolostone
Features: Coarse crystalline texture, often with colourful patterns
Uses: Sculpture, architecture, decorative purposes
Formation: Recrystallization of calcite or dolomite
Parent rock: Sandstone
Features: Very hard, interlocking quartz grains, often white or light-coloured
Uses: Construction, road building, decorative stone
Formation: Recrystallization of quartz grains
Parent rock: Various fine-grained rocks
Features: Fine-grained, hard, dense texture
Uses: Construction, road metal
Formation: Contact metamorphism
Foliation - You can see the distinctive banded or layered appearance, especially in the larger specimens
Identifying metamorphic rocks involves examining several key characteristics:
The Moine Thrust in northwest Scotland is one of the world's most famous geological features showing the power of metamorphism. Formed about 430 million years ago during the Caledonian mountain-building event, it represents a massive sheet of rock that was pushed eastward over younger rocks for about 200 km.
Along this thrust zone, intense pressure transformed sedimentary rocks into metamorphic rocks including mylonites (intensely deformed rocks). The area now provides geologists with a perfect natural laboratory to study how rocks change under extreme pressure. The Moine Thrust has been crucial in developing our understanding of mountain-building processes and plate tectonics.
Metamorphic rocks play a crucial role in the rock cycle, representing the transformation stage between different rock types.
From igneous rocks:
From sedimentary rocks:
To igneous rocks:
To sedimentary rocks:
To other metamorphic rocks:
Metamorphic rocks are not just geologically interesting; they have significant environmental and economic importance.
When examining rocks in the field, look for these tell-tale signs of metamorphic origin:
Remember to bring a hand lens, a small bottle of dilute acid (for testing carbonate minerals) and a steel knife to test hardness.
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