What is igneous rock?
Igneous rock is rock that forms when molten rock, magma or lava, cools and solidifies. It is one of the three main rock groups, alongside sedimentary and metamorphic rock, which pass into each other through the rock cycle. The Dutch term is stollingsgesteente, from stollen, to solidify.
Types
Where the magma solidifies decides the type:
- Plutonic (intrusive) rock: cools slowly deep in the Earth’s crust, growing large, visible crystals. Examples: granite and gabbro.
- Dyke rock: solidifies in cracks and fissures. Examples: diabase (dolerite) and porphyry.
- Volcanic (extrusive) rock: cools quickly as lava at the surface, so it is fine-grained. Examples: basalt and rhyolite. Very rapid cooling gives volcanic glass such as obsidian.
Composition varies too. Pale rocks such as granite are rich in silica (felsic); dark rocks such as basalt and gabbro contain less silica and more iron and magnesium (mafic).
Recognising it
Coarse grains visible to the naked eye point to a plutonic rock. An even, fine, dark stone is often basalt. Large crystals set in a fine groundmass show a porphyritic texture.
Application
- Choose it for wear. Igneous rocks are generally hard and low in porosity, suiting floors, stair treads, worktops and paving.
- Mind slip resistance. Polished granite is slippery outdoors and in wet areas. Specify a flamed or blasted finish there.
- Crushed material. Crushed basalt, porphyry and diabase are used as aggregate, railway ballast and in coastal and river defences.
Common mistake: assuming everything sold as “granite” is granite in the geological sense. The stone trade applies the name to other hard igneous rocks too, especially dark varieties. If it matters, ask for the stone’s technical data.
Related terms
- Plutonic rock
- Dyke rock
- Granite
- Basalt
- Porphyry
- Rock cycle
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