Intrusion
:
Intrusive redirects here, for other uses see:
Intrusive (disambiguation).:
Pluton redirects here, for the French nuclear missile see:
Pluton (missile) |
Devil's Tower, an igneous intrusion exposed when the surrounding softer rock eroded away. |
In
geology, an
intrusion is a body of
igneous rock that has
crystallized from a molten
magma below the surface of the
Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called
plutons, named for
Pluto, the
Roman god of the
underworld. Correspondingly, rocks of this kind are also referred to as
igneous plutonic rocks or
igneous intrusive rocks. This is to be contrasted with
extrusive rocks. The rock surrounding a pluton is called
country rock.
Intrusive rocks include all varieties of igneous rocks from coarse-grained, phaneritic
granites of large
batholiths to very fine grained, aphanitic,
rhyolites in
volcanic necks or feeder pipes. In composition, intrusive rocks also include the entire sequence of igneous rock types from the dense and dark ultramafic
peridotites to the very light-colored and low-density alkali granites and
syenites. A well-known example of an igneous intrusion is
Devil's Tower in
Wyoming,
USA.
Intrusive rocks also exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of
aplite. Intrusive structures are often classified according to whether or not they are parallel to the
bedding planes or
foliation of the country rock: if the intrusion is parallel, the body is
concordant, while if it cuts across the country rock, it is
discordant. Structural types include:
*
batholith: large irregular intrusions.
*
stock: smaller irregular discordant intrusions.
*
dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
*
sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but may be intruded into tilted beds or the entire package may be tilted by later deformation.
*pipe or
volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
*
laccolith: concorant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
*
lopolith: concordant body with a relatively flat to sagging top and a shallow covex base (spoon-shaped), may have a feeder dike or pipe below.
*
phacolith: a concordant lense shaped pluton that typically occupies the crest of an
anticline or the trough of a
syncline.
*
Diapir*
Mud volcano*
Ultramafic intrusion
