Phacolith

A phacolith is a pluton of igneous rock parallel to the bedding plane or foliation 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 migration and emplacement.

Cross-sectional diagram of phacoliths (red) in older folded rocks

The term was coined and initially defined by Alfred Harker in his The Natural History of Igneous Rocks in 1909.

Examples

• in the Franklin and Hamburg areas of Sussex County, New Jersey^[1]
• the Omey pluton in Ireland^[2]
• near Bayalan, Ajmer district, Rajasthan in India^[3]
• Corndon Hill, in Shropshire, England^[4][5]

See also

• Laccolith
• Lopolith
• Batholith