Definition · Plain-language
Igneous rock
Igneous rock is rock that forms when molten material — magma below ground or lava above it — cools and hardens into solid crystals.
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Rock born from molten material
Igneous rock forms by the cooling and crystallising of molten rock. Deep inside the Earth, intense heat melts rock into magma; when that magma loses heat, mineral crystals grow and lock together into solid igneous rock. The word “igneous” comes from the Latin ignis, meaning fire, reflecting this fiery origin. It is one of the three great families of rock. Because it crystallises from a melt rather than from layered sediment or from heat-and-pressure alteration, igneous rock generally lacks the layering seen in sedimentary rock.
Intrusive versus extrusive
How fast the melt cools shapes the rock. Intrusive (or plutonic) igneous rock forms when magma cools slowly deep underground; the slow pace lets large, visible crystals grow, as in granite. Extrusive (or volcanic) igneous rock forms when lava erupts at the surface and cools quickly in air or water; the rapid pace gives tiny crystals or a glassy texture, as in basalt or obsidian. So crystal size is a clue to history: coarse grains suggest a slow, deep birth; fine grains suggest a fast, surface one.
Its place in the rock cycle
Igneous rock is often described as the starting point of the rock cycle, because the first solid crust formed from cooling melt. Once exposed, igneous rock can be weathered and eroded into sediment that later compacts into sedimentary rock, or buried and altered by heat and pressure into metamorphic rock. If any rock is heated enough to melt again, it returns to magma and can crystallise once more into new igneous rock. Common examples include granite, basalt, pumice, obsidian and gabbro.
Key facts
At a glance
- Definition: rock formed when molten magma or lava cools and solidifies
- Name origin: Latin “ignis”, meaning fire
- Two types: intrusive (cools slowly underground) and extrusive (cools fast at the surface)
- Crystal size: slow cooling = large crystals; fast cooling = small or no crystals
- Examples: granite (intrusive), basalt and obsidian (extrusive)
- Rock family: one of three — igneous, sedimentary and metamorphic
Common misconceptions
What people often get wrong
Often heard: All igneous rock comes from erupting volcanoes.
Actually: Only extrusive igneous rock forms from erupted lava. Intrusive igneous rock, such as granite, forms from magma that cools slowly deep underground and is exposed only later by uplift and erosion.
Often heard: Igneous rocks all look the same because they come from melt.
Actually: They vary widely. Cooling speed and chemistry produce textures from coarse, crystalline granite to glassy obsidian and frothy pumice, and colours from pale to almost black.
Often heard: Once igneous rock forms, it stays igneous forever.
Actually: No rock is permanent. Through the rock cycle, igneous rock can be weathered into sediment, compressed into sedimentary rock, altered into metamorphic rock, or remelted into magma.
