Batholiths

A batholith (from Greek bathos, depth + lithos, rock) is a large mass of igneous rock intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite (see also granite dome).

Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or plutons, bodies of igneous rock of irregular dimensions (typically at least several kilometers) that can be distinguished from adjacent igneous rock by some combination of criteria including age, composition, texture, or mappable structures. Individual plutons are crystallized from magma that traveled toward the surface from a zone of partial melting near the base of the Earth's crust.

Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma in large masses called plutonic diapirs. Because the diapirs are liquified and very hot, they tend to rise through the surrounding native "country" rock, pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcanoes, but instead slow down, cool, and usually solidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; in reference to the Roman god of the underworld Pluto). An alternate view is that plutons commonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation of smaller volumes of magma that ascended as dikes.

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