batholit

A batholith is a very large igneous intrusion, a pluton that forms when substantial volumes of magma crystallize below the surface. By convention, batholiths cover at least about 100 square kilometers (roughly tens to hundreds of miles across) and extend for several kilometers in thickness. They are exposed at the surface mainly through long-term erosion of the overlying rock.

Most batholiths consist predominantly of felsic to intermediate rocks, such as granite or granodiorite, though they may include quartz diorite and other lithologies. They originate at depths of several to tens of kilometers within continental crust, often in association with subduction zones or continental collision. They crystallize slowly, over millions of years, producing coarse-grained textures and a compositional zoning from felsic cores to more intermediate margins.

Batholiths are the dominant structural features in many ancient and active mountain belts. They commonly show contact metamorphism at their margins where surrounding rocks were heated by the intruding magma. Erosion eventually removes much of the overlying rock, leaving the batholith as a prominent rocky exposure, sometimes forming the cores of mountain ranges.

Notable examples include the Sierra Nevada Batholith in western North America and the Coast Range Batholith of California, both among the largest batholiths. The Adirondack Batholith in New York is another well-studied example. In geology, batholiths are a subset of plutons, defined by large areal extent; smaller intrusions are called stocks or dikes. Note: the standard spelling is batholith; batholite is a less common variant.