batholiths

A batholith is a very large mass of intrusive igneous rock that crystallized from magma beneath the surface. Batholiths are exposed at the surface only after extensive erosion of the overlying rock. By convention, a batholith covers at least about 100 square kilometers and typically contains volumes of 1000 cubic kilometers or more.

Most batholiths are granitic in composition, especially granodiorite to granite, though dioritic to granitic varieties occur. They have coarse-grained, phaneritic textures formed by slow cooling at depth. Batholiths commonly consist of many individual intrusions (plutons) that coalesce into a single, much larger body.

Formation and tectonics: They form as magma crystallizes at considerable depth in the crust, often sourced by partial melting of existing crust or subducted lithosphere. The magma intrudes through the crust along large fault systems or along tectonic plate margins, and can be emplaced during multiple intrusion events over tens of millions of years. Erosion later exposes the batholith.

Geologic significance and examples: Batholiths provide records of crustal growth and magmatic differentiation. They are often associated with ore deposits and metamorphism in surrounding rocks. Notable batholiths include the Sierra Nevada Batholith in California, the Coast Plutonic Complex of western North America, and the numerous Andean batholiths along South America’s west coast.

See also: They are distinct from smaller plutons, stocks, and dikes; the terms reflect scale rather than process.