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World Geography25 Essential Exam Concepts
Plutonic Igneous Intrusions GK Facts, Magma Bodies & Geology Guide
In physical geology, igneous petrology, and structural geomorphology, intrusive igneous rocks (plutonic rocks or plutons) are bodies of crystalline rock formed when molten magma originating within the Earth’s mantle or lower crust ascends into pre-existing country rock and slowly crystallizes beneath the surface. Because these subsurface magma chambers cool slowly over thousands to millions of years beneath insulating overburden rock, individual mineral crystals have ample time to grow, producing coarse-grained, phaneritic textures such as granite, granodiorite, diorite, and gabbro. Geologists classify intrusive igneous formations according to their geometric geometry, physical volume, and spatial orientation relative to the bedding planes or foliation of the surrounding host rocks into two primary groups: concordant intrusions and discordant intrusions.
Concordant intrusions are magma bodies that inject parallel to the existing stratification, bedding planes, or foliation of the host sedimentary or metamorphic country rock without disrupting their general layer orientation. A sill is a flat, tabular concordant intrusion that injects horizontally between pre-existing sedimentary strata, maintaining relatively uniform thickness over wide geographic areas. A laccolith is a dome-shaped, mushroom-like concordant intrusion formed when viscous, silica-rich acidic magma forces its way between strata, arching the overlying rock layers upward into a distinct dome while resting on a relatively flat floor. In addition, lopoliths are saucer-shaped concordant intrusions sagging downward into a basin structure, while phacoliths are lens-shaped bodies occupying the crests of anticlines or the troughs of synclines in folded mountain chains.
Discordant intrusions are magma bodies that cut across the existing bedding planes, stratification, or structural fabric of the surrounding country rock. A dyke (or dike) is a tabular, wall-like discordant intrusion that cuts vertically or steeply across the host rock bedding along pre-existing joints and fractures; when exposed by weathering and erosion, resistant dykes often stand out as prominent ridges across landscapes. The largest of all intrusive igneous bodies is a batholith: a massive, discordant plutonic rock mass composed primarily of felsic to intermediate coarse-grained granitic rocks that covers an exposed surface area exceeding one hundred square kilometers (with no visible bottom floor). Batholiths represent ancient crystallized magma chambers beneath continental volcanic arcs, such as the Ladakh Batholith in the Trans-Himalayas and the Sierra Nevada Batholith in North America, forming the crystalline foundations of continental crust.