- 2069 Chaitra · 5 marks
Metamorphic rock formation; igneous rock texture
Short answer
Metamorphic rocks form from protoliths altered by heat, pressure, and fluids without melting, producing foliated or non-foliated textures. Igneous rock textures depend on cooling rate: phaneritic (slow, coarse), aphanitic (fast, fine), porphyritic, glassy, and vesicular.
Write down the formation process of metamorphic rock. Describe the texture of igneous rock.
Formation of metamorphic rock
Metamorphism is the change of an existing rock (parent rock, or protolith) into a new rock in the solid state, by heat, pressure and chemically active fluids, without melting.
- A parent rock is buried deep, comes near hot magma, or is squeezed during mountain building.
- Heat causes recrystallisation and growth of new, stable minerals.
- Pressure (confining and directed) compacts the rock and aligns platy minerals, forming foliation, slaty cleavage and schistosity.
- Fluids (water, CO₂) circulate and promote reactions.
Types of metamorphism:
- Contact (thermal): near intrusions, mainly heat; non-foliated rocks (marble, hornfels, quartzite).
- Regional: widespread, with heat and directed pressure during mountain building; foliated rocks (slate, phyllite, schist, gneiss).
- Dynamic (cataclastic): along faults; crushed rock.
Example: shale to slate to phyllite to schist to gneiss with increasing grade.
Texture of igneous rocks
Texture depends on the cooling rate and the gas content of the magma.
- Phaneritic (coarse): slow cooling at depth, visible crystals (granite).
- Aphanitic (fine): quick cooling (basalt).
- Porphyritic: large phenocrysts in a fine matrix.
- Glassy: very rapid cooling (obsidian).
- Vesicular: gas holes (pumice).
- Pegmatitic: very coarse crystals.
By degree of crystallinity, textures are also holocrystalline (all crystals), hypocrystalline (crystals and glass) and holohyaline (all glass).
✓ Answer checked by Claude Opus 5.5 (high effort)
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