Sill (geology)
- Sill (geology)
In
geology , a sill is a tabularpluton that has intruded between older layers ofsedimentary rock , beds ofvolcanic lava ortuff , or even along the direction of foliation inmetamorphic rock . The term "sill" is synonymous with "concordant intrusive sheet". This means that the sill does not cut across preexisting rocks, in contrast to dikes, which do cut across older rocks.Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations. They can be confused with solidified lava flows; however there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both the "upper" and "lower" contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the
atmosphere . Because sills generally form at depth (up to many kilometers), the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence ofweathering on their upper surface, whereas sills, if still covered by country rock, typically do not.Certain
ultramafic to mafic layered intrusions are a variety of sill that often contain importantore deposits.Precambrian examples include the Bushveld,Insizwa , and theGreat Dyke complexes of southernAfrica , the Duluth intrusive complex of the Superior District, and theStillwater igneous complex of theUnited States .Phanerozoic examples are usually smaller and include theRùm peridotite complex ofScotland and the Skaergaard igneous complex of eastGreenland . Theseintrusion s often contain concentrations ofgold ,platinum ,chromium , and other rare elements.See also
*
Batholith
* Dike
*Laccolith
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