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Definition: igneous rock from The Penguin Dictionary of Science

One of the three main types of rock: igneous rocks form when molten ➤magma cools and crystallizes. Extrusive igneous rock, such as basalt, forms at the Earth's surface; rapid cooling results in small crystals. Intrusive igneous rock, such as ➤granite, solidifies beneath the Earth's surface; slow cooling results in large crystals. Compare ➤metamorphic rock; sedimentary rock.

Summary Article: igneous rocks
From Encyclopedia of Environmental Change

Rocks formed by the cooling and solidification of molten magma, either below the Earth’s surface as intrusions, or at the surface through volcanic eruptions of lava and pyroclastic material. Their characteristic feature is a crystalline texture. Igneous rocks are subdivided into four categories according to their chemistry: (1) ultrabasic (silica content, SiO2<45 per cent), (2) basic (SiO2 45–53 per cent), (3) intermediate (SiO2 53–66 per cent) and (4) acidic (or silicic, SiO2 >66 per cent). Mineral content or percentage of dark minerals are proxies for chemistry. Crystal size ranges from fine (<1 mm-aphanitic; fast cooling as lava or high-level intrusion) through medium to coarse (>3 mm-phaneritic; slow-cooling as large, deep plutonic intrusion). volcanic glass is a product of very rapid cooling. Common igneous rocks are defined in the Table. Pyroclastic rocks are named according to different criteria.

Minerals in igneous rocks are important for radiometric dating. Studies of igneous rocks contribute to understanding the behaviour and evolution of volcanoes, which constitute a major natural hazard. Sites of igneous activity are closely related to plate tectonic activity (see hotspot, in geology), and igneous rocks in the geological record can help reconstruct past plate positions and movements. Pyroclastic beds form important , synchronous marker horizons of wide extent (see tephrochronology) and igneous rocks provide evidence of past episodes of volcanism, which may have contributed to environmental change on a variety of timescales.

Igneous rocks Characteristics of the principal igneous rock types.

Rock type

Silica content






Crystal size

Igneous setting

Igneous form





Fine (glassy)






Rhyolite (obsidian)



Dykes, sills





Deep intrusions, batholiths






Dominant mineral phases

Olivine, pyroxene

Pyroxene, plagioclase

Plagioclase, hornblende

Plagioclase, K-feldspar, quartz

K-feldspar, quartz


[See also metamorphic rocks, sedimentary rocks, volcanic impacts on climate]

  • Best, MG (2003) Igneous and metamorphic petrology. Oxford: Blackwell.
  • Cas, RAF; Wright, JV (1987) Volcanic successions: Modern and ancient. London: Allen and Unwin.
  • Francis, P; Oppenheimer, C (2003) Volcanoes. Oxford:Oxford University Press.
  • Gill, R (2010) Igneous rocks and processes: A practical handbook. Chichester: Wiley-Blackwell.
  • Maitre, Le RW (ed.) (1989) A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Oxford: Blackwell.
  • Middlemost, EAK (1997) Magmas, rocks and planetary development: A survey of magma/igneous rock systems. Harlow: Longman.
John B. Hunt
University of Gloucestershire
© by SAGE Publications Ltd.

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