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Igneous rocks form from the cooling and crystallization of magma (molten rock underground) or lava (molten rock at the surface). Intrusive (plutonic) igneous rocks cool slowly deep in the crust, producing coarse-grained textures like granite; extrusive (volcanic) rocks cool rapidly at the surface, producing fine-grained or glassy textures like basalt or obsidian. Composition ranges from felsic (silica-rich, low density) to mafic (silica-poor, iron- and magnesium-rich, high density) and determines both mineralogy and physical properties. Bowen's Reaction Series describes the sequence in which minerals crystallize as a melt cools, explaining why different igneous rock types coexist.
Thin-section microscopy or hand-sample comparison of granite (coarse, felsic) versus basalt (fine, mafic) versus rhyolite (fine, felsic) makes texture-composition relationships tangible. Tracing Bowen's Reaction Series from olivine and pyroxene at high temperature to quartz and muscovite at low temperature connects thermodynamics to petrology.
From your study of rock-forming minerals and phase diagrams, you know that minerals have specific chemical compositions and that melts crystallize different minerals at different temperatures. Igneous rocks are the direct products of this crystallization process — they form when molten rock cools and solidifies. The two fundamental variables that control what an igneous rock looks like are where it cools (which determines texture) and what it's made of (which determines composition and mineralogy).
Texture is controlled almost entirely by cooling rate. When magma is trapped deep underground in large chambers, it loses heat slowly — over thousands to millions of years — giving atoms ample time to migrate through the melt and attach to growing crystal faces. The result is a coarse-grained (phaneritic) rock like granite, where individual mineral crystals are easily visible to the naked eye. When lava erupts at the surface and is exposed to air or water, it cools in days to weeks, and crystals have almost no time to grow. This produces fine-grained (aphanitic) rocks like basalt, where crystals are too small to see without a microscope. In extreme cases — obsidian, for instance — cooling is so rapid that no crystals form at all, and the result is volcanic glass. Sometimes magma begins cooling slowly at depth (growing large crystals) before being erupted rapidly, producing a porphyritic texture: large crystals (phenocrysts) embedded in a fine-grained groundmass, recording the two-stage cooling history in a single rock.
Composition ranges along a spectrum from felsic to mafic (and further to ultramafic). Felsic rocks like granite and rhyolite are rich in silica (65–75% SiO₂), aluminum, sodium, and potassium; their dominant minerals are quartz, potassium feldspar, and plagioclase, giving them light colors and relatively low densities. Mafic rocks like basalt and gabbro are lower in silica (45–55% SiO₂) but rich in iron and magnesium; their dominant minerals are pyroxene, olivine, and calcium-rich plagioclase, making them dark and dense. This compositional spectrum is not arbitrary — it is governed by Bowen's Reaction Series, which describes the order in which minerals crystallize from a cooling melt. High-temperature minerals like olivine and pyroxene crystallize first, removing iron and magnesium from the remaining liquid and enriching it in silica. If these early crystals are separated from the melt (by sinking, for example), the remaining magma evolves toward a more felsic composition — a process called fractional crystallization. This is why a single magma source can produce rocks of different compositions.
Recognizing igneous rocks in the field means reading both texture and composition simultaneously. A coarse-grained, light-colored rock rich in quartz and feldspar is granite (intrusive, felsic). A fine-grained, dark rock dominated by pyroxene and plagioclase is basalt (extrusive, mafic). A coarse-grained, dark rock with the same minerals as basalt is gabbro — same composition, different cooling history. This texture-composition grid is the classification system for all igneous rocks, and it connects directly to the tectonic settings where they form: basalt dominates at mid-ocean ridges and hotspots where mantle melting produces mafic magma, while granite is characteristic of continental crust where fractional crystallization and crustal melting generate felsic compositions.