Sill (geology)

In geology, a sill is a tabular, often horizontal mass of igneous rock that has been intruded laterally between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet.

Certain mafic and ultramafic layered intrusions are a variety of sill that often contain important ore deposits. Precambrian examples include the Bushveld, Insizwa, and the Great Dyke, complexes of southern Africa, the Duluth intrusive complex of the Superior District, and the Stillwater intrusive of the United States. Phanerozoic examples are usually smaller and include the Rum peridotite complex of Scotland and the Skaergaard intrusive of east Greenland. These intrusives often contain concentrations of gold, platinum, chromium, and other rare elements.

See also

• Dike
• Laccolith