igneux

Igneous rocks are rocks formed by the cooling and solidification of molten material. The melt may originate below the surface as magma or erupt at the surface as lava. As the melt cools, crystals form and interlock, creating a texture that reflects the rate of cooling.

Igneous rocks are commonly classified by chemical composition into felsic, intermediate, mafic, and ultramafic categories. Felsic rocks are rich in silica and light-colored minerals such as quartz and feldspar and include granite and rhyolite. Intermediate rocks, like diorite and andesite, have intermediate silica content. Mafic rocks contain more magnesium and iron and include basalt and gabbro. Ultramafic rocks are very high in magnesium and iron, with examples such as peridotite. This compositional scheme often correlates with color, density, and mineralogy.

Texture in igneous rocks depends on the cooling history. Intrusive or plutonic rocks crystallize underground, cooling slowly to form coarse-grained textures (phaneritic). Extrusive or volcanic rocks erupt onto the surface and cool rapidly, producing fine-grained textures (aphanitic) and, in some cases, glassy textures (obsidian) if cooling is extremely fast. Some rocks are porphyritic, containing large crystals (phenocrysts) embedded in a finer-grained matrix that records different cooling rates.

Formation settings include divergent plate boundaries, subduction zones, and hotspots, where mantle-derived magmas crystallize to form a wide range of rock types. Igneous rocks preserve evidence of planetary differentiation and crust formation and remain essential to petrology and geologic history.