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Glossary - Igneous

Igneous

An igneous rock is formed by the cooling and crystallization of molten rock. The term igneous is derived from ignius, the Latin word for fire. Scientists have divided igneous rocks into two broad categories based on where the molten rock solidified.

Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava (flows and fragmented debris called pyroclasts).

Plutonic rocks (also called intrusive igneous rocks) are those that have solidified below ground; plutonic comes from Pluto, the Greek god of the underworld.

The initial distinction between volcanic and plutonic rocks is made on the basis of texture (fine-grained volcanic vs. coarse-grained plutonic). Volcanic and plutonic rocks are divided further on the basis of chemistry and mineral composition. The classification scheme below is based on chemistry, and is perhaps the simplest method; there are many other classification methods for igneous rocks. These rock types all have different characteristics, including temperature when fluid, viscosity (resistance to flow), composition, explosiveness, and types, amounts, and sizes of minerals.

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