Plutonic rock — also called intrusive igneous rock — is igneous rock that formed as magma cooled slowly and solidified deep in the Earth’s crust, often at kilometres of depth. Because the cooling was so slow, large, visible mineral crystals had time to grow, giving plutonic rocks their coarse-grained appearance.
How does plutonic rock form?
Magma from the Earth’s interior sometimes rises but does not reach the surface. It remains as a magma chamber inside the crust and cools there. The deeper and slower this happens, the larger the mineral crystals can grow. Only millions of years later — through uplift and erosion — does the rock arrive at the surface.
Difference from other igneous rocks
| Type | Place of solidification | Grain size | Example |
|---|---|---|---|
| Plutonic (intrusive) | Deep in the crust | Coarse, visible to naked eye | Granite, gabbro, diorite |
| Hypabyssal (subvolcanic) | In cracks and dykes | Medium | Dolerite, porphyry |
| Volcanic (extrusive) | At the surface | Fine to glassy | Basalt, andesite, rhyolite |
Types and uses in construction
Common plutonic rocks include:
- Granite — light coloured (quartz, feldspar, mica), very hard, popular as cladding, flooring, worktops and headstones
- Gabbro — dark, low in quartz, used for setts and tombstones
- Diorite — intermediate in tone, limited in use
- Larvikite — Norwegian variety with a blue-grey shimmer, popular as decorative cladding
- Syenite — similar to granite, also decorative
Properties
Plutonic rocks are:
- Hard and abrasion-resistant — suited to heavy-traffic floors and paving
- Frost-resistant — due to low porosity
- Polishable — give a mirror finish with deep gloss
- Chemically resistant — barely attacked by acids
Because the blocks are often millions of years old and show little internal variation, they give a building a calm, durable appearance.
Related terms
- Granite
- Natural stone
- Hypabyssal rock
- Basalt
- Rock cycle
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