Metamorphic rock is rock whose texture and mineral content have been transformed deep in the earth’s crust by high pressure and/or high temperature, without the rock melting completely. The word means “transformed rock”. Together with igneous and sedimentary rock it forms the third main group in the rock cycle.
From parent rock to metamorphic rock
| Parent rock | Becomes | Use in construction |
|---|---|---|
| Limestone | Marble | Floors, worktops, ornament, sculpture |
| Clay/shale | Slate | Roofing slate, cladding, floor tiles |
| Sandstone | Quartzite | Floors, facades, highly wear-resistant |
| Granite/clay | Gneiss | Cladding, paving, retaining walls |
Characteristics
- Foliation — under pressure, platy minerals align in one direction; this is why slate splits so cleanly into thin sheets
- Hardness varies widely — quartzite is very hard and scratch-resistant, while marble is relatively soft and acid-sensitive
- Dense texture — most metamorphic rocks absorb little water, which helps their frost resistance
What to watch when specifying
- Marble is sensitive to acids: vinegar, lemon and many cleaning products etch dull marks. Use a pH-neutral cleaner and consider sealing
- Slate splits along the bedding; for roofing slate, thickness and format determine service life
- Quartzite and gneiss are very wear-resistant and therefore suited to heavily used floors and external paving
- For external use, always check the frost resistance and water absorption stated by the supplier
Related terms
- Natural stone
- Rock cycle
- Sedimentary rock
- Plutonic rock
- Slate
- Marble
- Gneiss
- Quartzite
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