Intrusives

Intrusives, or intrusive igneous rocks, are rocks that crystallize below the Earth's surface from magma that intrudes into surrounding rock. Cooling underground is slow, producing coarse-grained, easily visible crystals and a phaneritic texture, in contrast to the fine-grained or glassy textures typical of most volcanic rocks.

Intrusions vary in size and form. They include dikes, which cut across existing rock; sills, which run parallel to layering; plutons, irregular bodies of solidified magma; stocks, smaller plutons; laccoliths, lens-shaped intrusions that lift overlying strata; and batholiths, large, often multi-stage plutonic complexes. Most granitic and many dioritic and gabbroic rocks crystallize as plutons or related bodies.

Common rock types are classified by silica content and mineralogy. Felsic rocks such as granite and granodiorite are rich in quartz and feldspar; intermediate rocks include diorite and quartz diorite; mafic rocks include gabbro and diabase; ultramafic rocks such as peridotite are less common at the surface. Pegmatites are extremely coarse-grained intrusions that may host rare minerals and gemstones.

Formation and significance: Intrusive rocks form when magma is trapped underground and cools slowly, allowing large crystals to develop. They are exposed at the surface through uplift and erosion, providing important records of crustal processes and tectonic settings. Economically, intrusions can host mineral deposits, including ore veins near contacts and pegmatite-rich zones with gemstones and rare-earth elements.