intrusiviset

Intrusiviset is the Finnish term commonly used in geology to refer to intrusive igneous rocks and related intrusions that crystallize below the Earth’s surface. These rocks form when magma invades existing rock and solidifies at depth, typically cooling more slowly than surface lava and producing crystals that are large enough to be visible with the naked eye.

The texture and composition of intrusives reflect their slower cooling and chemical ingredients. The dominant textures are coarse-grained and phaneritic, where minerals such as feldspar, quartz, and dark-colored mafic minerals grow into interlocking crystals. Intrusives span a range of compositions from felsic to intermediate and some mafic varieties; common rock types include granite, granodiorite, diorite, tonalite, and gabbro. Peridotite and other ultramafic intrusions occur less commonly but are important for mantle-related processes.

Intrusive bodies are classified by shape and size. Large bodies called plutons form when magma pools and crystallizes at depth. Batholiths are very large plutons that may extend over hundreds of square kilometers. Smaller plutons can be stocks. Other intrusive features include laccoliths, which are lens-shaped and uplift overlying rock, and tabular intrusions such as dikes, which cut across existing rocks, and sills, which run parallel to bedding or layering.

Intrusives contrast with extrusive rocks, which form from erupted lava at or near the surface and typically have finer grains. Intrusive rocks are often revealed at the surface only after extensive erosion of overlying material. They play a key role in revealing crustal formation processes, dating of geological events, and, in some cases, hosting mineral deposits.

See also: intrusive rock, igneous rock, batholith, pluton, sill, dike.