DEFINITION OF BATHOLITH

Large igneous intrusion with a surface area

greater than 60 square miles (100 km2). Several types of

igneous intrusions are produced by magmas (generated from

melting rocks in the Earth) and intrude the crust, taking one

of several forms. A pluton is a general name for a large

cooled igneous intrusive body in the Earth. The specific type

of pluton is based on its geometry, size, and relations to the

older rocks surrounding the pluton, known as country rock.

Concordant plutons have boundaries parallel to layering in

the country rock, whereas discordant plutons have boundaries

that cut across layering in the country rock. Dikes are

tabular but discordant intrusions, and sills are tabular and

concordant intrusives. Volcanic necks are conduits connecting

a volcano with its underlying magma chamber. A famous

example of a volcanic neck is Devils Tower in Wyoming.

Some plutons are so large that they have special names.

Batholiths and plutons have different characteristics and

relationships to surrounding country rocks, based on the

depth at which they intruded and crystallized. Epizonal plutons

are shallow and typically have crosscutting relationships

with surrounding rocks and tectonic foliations. They may

have a metamorphic aureole surrounding them, where the

country rocks have been heated by the intrusion and grew

new metamorphic minerals in response to the heat and fluids

escaping from the batholith. Rings of hard contact metamorphic

rocks in the metamorphic aureole surrounding

batholiths are known as hornfels rocks. Mesozonal rocks

intrude the country rocks at slightly deeper levels than the

epizonal plutons, but not as deep as catazonal plutons and

batholiths. Catazonal plutons and batholiths tend to have

contacts parallel with layering and tectonic foliations in the

surrounding country rocks, and they do not show such a

large temperature gradient with the country rocks as those

from shallower crustal levels. This is because all the rocks are

at relatively high temperatures. Catazonal plutons tend to be

foliated, especially around their margins and contacts with

the country rocks.

Batholiths are derived from deep crustal or deeper melting

processes and may be linked to surface volcanic rocks.

Batholiths form large parts of the continental crust, are associated

with some metallic mineral deposits, and are used for

building stones.

See also IGNEOUS ROCKS; PLUTON.

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