Sill (geology)

A sill is a sheet-like body of igneous rock intruded between pre-existing rock layers or along another planar structure, usually parallel to them.

Magma: Molten or partially molten rock beneath or at Earth’s surface, containing dissolved gases and crystals. Magma supplies the molten material that intrudes between rock layers to form a sill.
Dike (geology): A sheet-like igneous intrusion that cuts across pre-existing rock layers or structures. A dike is discordant to surrounding strata, unlike the typically layer-parallel sill.
Igneous intrusion: A body of igneous rock formed when magma enters and solidifies within pre-existing rock. A sill is one tabular, concordant form of igneous intrusion.
Palisades Sill: A Jurassic diabase intrusion exposed along the western Hudson River in New York and New Jersey. Its layered minerals and extensive exposures provide a classic example for studying sill structure.
Contact aureole: A zone of thermally altered rock surrounding an igneous intrusion. Sills can produce paired aureoles in the strata immediately above and below them.
Fracture propagation: The extension of a crack through a material as stress drives its tip forward. Magma can force existing fractures open or extend new ones as it advances along a layer.
Lopolith: A large, basin-shaped igneous intrusion with layers that dip inward toward its center. Some lopoliths contain extensive sill-like sheets, but their overall geometry is basin-shaped.
Concordant intrusion: An igneous intrusion whose boundaries run broadly parallel to structures in its host rock. Sills are classified as concordant because their contacts follow surrounding layers or structures.
Whin Sill: A large Carboniferous dolerite intrusion exposed across northern England. Its prominent outcrops show how a sill can shape topography and local building stone.
Hornfels: A hard, fine-grained metamorphic rock formed by heating near an igneous intrusion. Host strata heated by a sill may recrystallize into hornfels.

Linked from 7 pages