A phacolith is a
pluton of
igneous rock parallel to the bedding plane or
foliation
In mathematics (differential geometry), a foliation is an equivalence relation on an topological manifold, ''n''-manifold, the equivalence classes being connected, injective function, injectively immersed submanifolds, all of the same dimension ...
of folded
country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an
anticline or the trough of a
syncline. In rare cases the body may extend as a
sill 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 (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
migration and emplacement.
The term was coined and initially defined by
Alfred Harker
Alfred Harker Fellow of the Royal Society, 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, a ...
in his ''The Natural History of Igneous Rocks'' in 1909.
Examples
* in the Franklin and Hamburg areas of
Sussex County, New Jersey
* the Omey pluton in
Ireland
Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...
* near Bayalan,
Ajmer district, Rajasthan in India
*
Corndon Hill, in
Shropshire
Shropshire (; abbreviated SalopAlso used officially as the name of the county from 1974–1980. The demonym for inhabitants of the county "Salopian" derives from this name.) is a Ceremonial counties of England, ceremonial county in the West M ...
, England
See also
*
Laccolith
*
Lopolith
*
Batholith
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