nonigneous

Nonigneous is a term used to describe rocks that are not formed by the cooling and crystallization of molten rock. In geology, it is often used as an informal umbrella category for rocks whose origin is not igneous, in contrast to igneous rocks that solidify from magma or lava. The term is less common in formal classifications, where rocks are divided into igneous, sedimentary, and metamorphic groups.

The nonigneous category primarily includes sedimentary and metamorphic rocks. Sedimentary rocks form through weathering, erosion, deposition, and lithification of fragments or from chemical or organic precipitation. Common sedimentaries include sandstone, shale, limestone, and coal. Metamorphic rocks arise when existing rocks are altered by heat, pressure, and chemically active fluids, leading to changes in mineralogy and texture; examples include slate, schist, gneiss, marble, and quartzite. Some metamorphic rocks are foliated, showing parallel mineral alignment, while others are nonfoliated and lack such textures.

In practice, nonigneous is used mainly as a descriptive contrast to igneous rocks rather than as a formal taxonomic category. It highlights origin processes—depositional and transformational histories—rather than explicit mineral composition. Fossil content is more typical of sedimentary rocks, while metamorphic rocks record past conditions of heat and pressure, which can erase or modify fossils.

Overall, nonigneous serves as a practical shorthand for rocks whose primary formation paths do not involve magma cooling, encompassing the majority of the Earth's crust outside igneous rocks. See also igneous rock, sedimentary rock, metamorphic rock, and the rock cycle.