What is extrusive rock?
Extrusive rock is igneous rock that forms when magma reaches the earth’s surface during a volcanic eruption and cools and solidifies quickly as lava. It is also called volcanic rock or effusive rock. In Dutch it is called ‘uitvloeiingsgesteente’, literally ‘outflow rock’. Its counterpart is intrusive or plutonic rock, which solidifies deep underground.
Formation and features
Extrusive and plutonic rocks often share a similar chemical composition, because both come from magma. The difference lies in the cooling. Plutonic rock cools slowly deep in the crust, giving crystals time to grow large, as in granite. Lava at the surface cools fast, so the crystals stay small and the rock is fine-grained. With extremely rapid cooling, volcanic glass forms instead.
A typical feature is porphyritic texture: a few larger, well-formed crystals (phenocrysts) set in a fine-grained groundmass. Those crystals grew in the magma chamber before the rest solidified rapidly during the eruption. Escaping gas can leave pores, as seen in lava stone.
Examples
- Basalt: dark, heavy and hard; the best-known extrusive rock.
- Diabase (dolerite in the UK): dark and tough; geologists usually class it as a dyke rock, as it solidifies just below the surface.
- Porphyry: recognisable by clear crystals in a fine groundmass.
- Tuff: also volcanic, but formed from compacted volcanic ash rather than solidified lava.
Application
- Paving, steps, sills and cladding in basalt or diabase, thanks to their hardness and wear resistance.
- Basalt columns and rock armour to protect dykes, sea walls and riverbanks.
- Tuff in historic churches and buildings in the Netherlands and Germany; restoration often calls for matching replacement stone.
- Lava granulate as a drainage layer, green roof substrate or decorative gravel.
Tips and points of attention
- Weight: basalt is considerably heavier than many other stones. Check loads and fixings for cladding and floors.
- Working the stone: fine-grained hard rock needs diamond tools and wears saw blades quickly.
- Slip risk: polished basalt becomes slippery when wet. Outdoors, choose a flamed, honed or blasted finish.
- Tuff is soft and porous. Repair it with a soft lime mortar, not a hard cement mortar.
Related terms
- Igneous rock
- Plutonic rock
- Dyke rock
- Basalt
- Diabase
- Tuff stone
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