Plutons

Plutons are bodies of intrusive igneous rock that crystallized from magma cooling underground within the crust. They form when magma pools and solidifies beneath the surface, either from a single large intrusion or from repeated injections of magma. Over geological time, uplift and erosion may remove surrounding rock, exposing these intrusions at the surface or as large bedrock outcrops.

Most plutonic rocks are coarse-grained because cooling occurs slowly underground. They are commonly felsic to intermediate in composition, including granite, granodiorite, tonalite, diorite, and monzonite. Some plutons exhibit a porphyritic texture, with large phenocrysts surrounded by a finer-grained groundmass, reflecting changes in cooling rate within the intrusion. Mineralogy typically features quartz, plagioclase and alkali feldspar, along with micas, amphibole, and other accessories.

Geologists classify plutons by size and shape. Batholiths are large bodies covering more than about 100 square kilometers; smaller stocks are less than this threshold. In addition, tabular intrusions include sills, which run parallel to surrounding layers, and dikes, which cut across them. Laccoliths are lens-shaped intrusions that dome the overlying strata.

Notable examples of plutons include the Sierra Nevada batholith in western North America, the Coast Plutonic Complex in western Canada, and the exposed granitic rocks of the Adirondack Mountains in New York. Plutons record episodes of crustal magmatism and are important for understanding crustal differentiation, tectonics, and the creation of ore deposits.