A phacolith is a
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
of
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...
parallel to the bedding plane or
foliation
In mathematics ( differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition ...
of folded
country rock
Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...
. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an
anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
or the trough of a
syncline
In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...
. In rare cases the body may extend as a
sill
Sill may refer to:
* Sill (dock), a weir at the low water mark retaining water within a dock
* Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma
* Sill (geostatistics)
* Sill (river), a river in Austria
* Sill plate, a ...
from the crest of an anticline through the trough of an adjacent syncline, such that in
cross section it has an ''S'' shape. In intensely folded terrain the hinge of folds would be areas of reduced pressure and thus potential sites for
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...
migration and emplacement.
The term was coined and initially defined by
Alfred Harker
Alfred Harker FRS (19 February 1859 – 28 July 1939) was an English geologist who specialised in petrology and interpretive petrography. He was Lecturer in Petrology at the University of Cambridge for many years, and carried out field mapping ...
in his ''The Natural History of Igneous Rocks'' in 1909.
Examples
* in the Franklin and Hamburg areas of
* the Omey pluton in
Ireland
Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
* near Bayalan,
Ajmer district, Rajasthan in India
*
Corndon Hill
Corndon Hill ( cy, Cornatyn) is a hill in Powys, Mid Wales, whose isolated summit rises to 1,683 ft above sea level.
It is surrounded on three sides by the English county of Shropshire and forms a prominent landmark in the Wales-England b ...
, in
Shropshire
Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to ...
, England
See also
*
Laccolith
A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...
*
Lopolith
*
Batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
References
*Davis A. Young (2003) ''Mind Over Magma: The Story of Igneous Petrology'', page 335, Princeton University Press.
*American Geological Institute. ''Dictionary of Geological Terms''. New York: Dolphin Books, 1962.
Igneous rocks
Igneous petrology
{{Petrology-stub