Geological process
The solidification of molten rock into an igneous rock. How fast it happens sets the grain size: slow cooling at depth gives coarse crystals, rapid cooling at the surface gives fine-grained or glassy rock.
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Cooling rate sets grain size: magma trapped at depth cools over thousands to millions of years and grows large crystals, giving a coarse-grained texture, while magma erupted at the surface solidifies almost instantly and gives a very fine-grained or glassy texture.
Evidence support: Supported by the cited evidence one source
Evidence: U.S. Geological Survey n.d. Primary authority
Igneous rocks form when hot molten rock crystallises and solidifies, and divide into intrusive rocks that solidify at depth and extrusive rocks that solidify at or very near the surface.
Evidence support: Supported by the cited evidence one source
Evidence: U.S. Geological Survey n.d. Primary authority
Record last updated 2026-09-06. Corrections follow the published corrections policy; superseded claims are retained, never silently edited.