Igneous rocks are rocks formed when molten rock material cools and solidifies. Magma is molten or partially molten rock located beneath Earth's surface, while lava is magma that has erupted onto the surface. Igneous rocks are divided broadly into intrusive or plutonic rocks, which crystallise underground, and extrusive or volcanic rocks, which crystallise at or near the surface. Their textures and mineral compositions depend strongly on cooling rate, magma chemistry, pressure, volatile content and the environment of crystallisation.
Granite, gabbro, basalt, diorite, rhyolite and obsidian are familiar examples. Igneous rocks are fundamental to the structure of Earth's crust and are also important in understanding the origin and evolution of magma. Because they crystallise from melts, their minerals and textures can preserve information about temperatures, pressures, cooling histories and the chemical evolution of Earth's interior.
Intrusive igneous rocks form when magma cools beneath the surface. Because surrounding rocks insulate underground magma, cooling can be relatively slow, allowing mineral crystals time to grow. Granite is a common coarse grained intrusive rock containing minerals such as quartz, feldspar and mica. Gabbro is another intrusive rock, generally darker and richer in calcium rich plagioclase and pyroxene. Large intrusive bodies include batholiths, stocks, dykes and sills.
A batholith may form from multiple magma intrusions over long periods rather than one enormous pulse of magma. Dykes cut across existing rock layers, while sills generally intrude parallel to layering. Intrusive rocks can become exposed at Earth's surface only after uplift and erosion remove the overlying material, meaning rocks now visible at the surface may originally have crystallised kilometres underground.
Extrusive igneous rocks form when lava or volcanic ejecta cools at or near Earth's surface. Cooling is generally faster than underground, so crystals may be small or invisible to the unaided eye. Basalt is a common dark coloured volcanic rock formed from relatively low silica, iron and magnesium rich magma. Rhyolite is generally lighter and more silica rich. Very rapid cooling can produce volcanic glass such as obsidian because atoms do not have sufficient time to organise into crystalline structures.
Pumice forms when gas rich magma expands and cools rapidly, producing a highly vesicular rock with abundant cavities. Scoria is another vesicular volcanic rock, commonly darker and denser than pumice. Ash and other pyroclastic materials can also become welded or lithified into rocks such as tuff.
Igneous rock texture provides important evidence about cooling history. Coarse grained textures generally indicate slower cooling that allowed larger crystals to develop, whereas fine grained textures usually indicate rapid cooling. Porphyritic textures contain relatively large crystals called phenocrysts embedded in a finer groundmass and can indicate a two stage cooling history. The magma may have begun crystallising slowly underground before rising and cooling more rapidly near the surface. Vesicular textures indicate gas bubbles were present during solidification.
Glassy textures indicate extremely rapid cooling. The distribution and sizes of crystals are therefore not merely visual features. They record the physical conditions under which molten rock changed into solid rock. Geologists use these textures alongside mineral chemistry and field relationships to reconstruct igneous histories.
Magma composition strongly affects the properties of igneous rocks. Silica rich magmas tend to be more viscous, while basaltic magmas generally have lower viscosity and can flow more readily. Temperature, dissolved gases and crystal content also affect viscosity. Magmas can evolve through fractional crystallisation, in which early forming minerals are removed from the remaining melt, changing its composition. Magma can also assimilate surrounding rocks or mix with other magmas.
These processes can produce a wide range of igneous compositions from related source materials. Partial melting of mantle or crust does not necessarily melt the source rock completely, and the resulting melt can have a composition different from the original source. Igneous petrology therefore examines both the physical crystallisation process and the chemical evolution of magma.
Igneous rocks are important for understanding plate tectonics. Basalt is abundant in oceanic crust and is produced at mid ocean ridges through partial melting of mantle material. Granite and related rocks are widespread in continental crust and can form through complex histories involving magma generation, crystallisation, crustal melting and tectonic processes. Volcanic arcs develop where subduction generates magma, while continental rifts can produce magma as lithosphere stretches and mantle rises. Hotspots can generate volcanic chains that record plate movement. Igneous rocks therefore provide evidence about processes occurring deep within Earth and about the movement and interaction of tectonic plates. Their mineral chemistry can also reveal the pressure, temperature and source characteristics of the magmas from which they formed.
Igneous rocks have many practical uses. Granite and related rocks are used as building and decorative stone because of their durability and attractive textures. Basalt is used as aggregate, road material and construction stone. Pumice and scoria have uses in lightweight construction, horticulture and abrasive applications. Certain igneous formations can host valuable mineral deposits, including ores associated with magma crystallisation, hydrothermal activity or later alteration.
Igneous rocks can also form important groundwater reservoirs where fractures provide pathways for fluid movement, although their primary mineral structure may be relatively impermeable. Volcanic rocks can influence soil fertility after weathering because they release minerals and elements into developing soils. Their economic importance therefore extends from construction materials to mineral resources and agricultural landscapes.
Igneous rocks participate continuously in the rock cycle. They can be uplifted and weathered into sediment, buried and transformed through metamorphism, or remelted to form new magma. Some magma crystallises completely underground, while some erupts explosively or effusively. A single igneous body may therefore have a complicated history involving magma generation, transport, storage, crystallisation, deformation and erosion. A common misconception is that all igneous rocks are volcanic.
Volcanic rocks are only the extrusive subset, whereas a large proportion of igneous rock forms underground. Another misconception is that cooling always happens instantly when lava reaches the surface. Thick lava flows and intrusive margins can cool over substantial periods. Igneous rocks are best understood as records of crystallisation from molten material under diverse geological conditions.
