What is Intrusion?
[[File:A Yool DevilsTower 04Sep03 exif.jpg|thumb|[[Devils Tower National Monument]], [[Wyoming]]. This is an igneous intrusion exposed when the surrounding softer rock eroded away.]] An '''intrusion''' is [[magma]] (melted rock) which cools and becomes solid under the [[Earth]]'s surface. It occurs when there are lines of weakness such as [[fault]]s, [[joint]]s, or [[strata|bedding plane]]s in the crust. Then magma enters these lines of weakness. By contrast, an [[extrusion]] is magma which cools into rock above the surface of the [[Earth's crust|crust]]. Both intrusive and extrusive rocks are classed as [[igneous rock]]s. The process of forming intrusive rocks may take millions of years. As the rock slowly cools into a solid, the different parts of the magma [[Crystallization|crystallize]] into [[minerals]]. The crystals are usually larger than in extrusive rocks. Many mountain ranges, such as the [[Sierra Nevada (US)|Sierra Nevada]] in [[California]], are formed mostly by intrusive rock. Often they are huge granite (or related rock) [[Formation (geology)|formations]]. Intrusions vary widely, from [[mountain range]] sized [[batholith]]s to thin [[vein]]-like fracture fillings. When exposed by [[erosion]], these cores called batholiths may occupy huge areas of [[Earth]]'s surface. Large bodies of magma that solidify underground before they reach the surface of the crust are called [[pluton]]s. Intrusive structures may go between [[strata|bedding planes]] of earlier 'country' rock. A [[Sill (geology)|sill]] is an intrusion which makes a table along bedding planes. A [[Dyke (geology)|dyke]] is an intrusion which moves up, crossing older [[strata]]. == Related pages == *[[Volcanic plug]] [[Category:Geology]] [[Category:Igneous rocks]]