Igneous Rock
Igneous rock forms when magma or lava cools and hardens, producing intrusive rocks like granite or extrusive rocks like basalt.
Definition
Igneous rock formed when melted rock cooled and turned solid: intrusive if it cooled underground, extrusive if it cooled on the surface. Intrusive (plutonic) rocks cool slowly beneath the surface and are coarse-grained, while extrusive (volcanic) rocks cool rapidly and are fine-grained or glassy. Composition ranges from light-colored felsic rocks rich in silica to dark mafic rocks rich in iron and magnesium. Geologists classify igneous rocks by texture (phaneritic, aphanitic, porphyritic, glassy, vesicular) and by silica content: felsic above about $63\%$ $SiO_2$, intermediate, mafic at about $45$ to $52\%$, and ultramafic. Minerals crystallize in a predictable order described by Bowen's reaction series, with olivine and calcium plagioclase first at roughly $1{,}200^\circ C$ and quartz last near $700^\circ C$. Fractional crystallization removes early minerals and drives the remaining melt toward higher silica.
Example
Granite cooled slowly underground and has big visible crystals, while basalt, the dark rock that makes up Hawaii, cooled quickly from lava and has tiny crystals. Granite (felsic, intrusive) and basalt (mafic, extrusive) are the two most common examples; pumice is extrusive rock so full of gas bubbles that it floats, and Yellowstone's rhyolite flows are felsic lava that cooled at the surface. A porphyritic andesite from the Cascades contains large plagioclase phenocrysts in a fine groundmass, showing two-stage cooling: slow growth in a magma chamber, then rapid quenching on eruption. Oceanic crust is almost entirely basalt and gabbro formed at mid-ocean ridges.
Key Insight
The word "igneous" comes from the Latin word for fire, and every igneous rock started as something hot enough to glow. Grain size tells cooling history and composition tells source: a coarse-grained dark rock (gabbro) and a fine-grained dark rock (basalt) have the same chemistry but different cooling rates. Bowen's series also explains weathering. Minerals that crystallize first at the highest temperatures, like olivine, are furthest from surface conditions and weather fastest, while quartz, which forms last, survives to become beach sand.