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.
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