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Geology is the Way

Sill

A sill is a sheet intrusion emplaced parallel to the structures present within its host rocks. Its primary feature is to be concordant and never crosscut, indeed, the foliation or bedding planes in the host, which magma exploits as planes of weakness to move underground. Sills are typically tabular (i.e. table-like), developing mainly along 2 directions and showing limited thickness. A dike is also a tabular intrusion but, contrarily to sills, is discordant with the structures of the host. Sills may laterally evolve to dikes or vice-versa and split into several dikes and sills. Sills are usually fed by diked (feeder dikes) that are linked to other magmatic intrusions or to the magma source at depth. Large concentrations of sills in the same area that share common characteristics like age, composition, and orientation are sill swarms or sill complexes.

At outcrop scale, sills are parallel with layers or foliations above and below and may be confused with lava flows interlayered within sedimentary beds or volcanic rocks. However, they are different from lava flows as they show intrusive contacts on both sides. They may contain xenoliths originated from the overlying and underlying rocks. Contact metamorphism occurring again above and below the sill is another evidence for intrusive emplacement. For the same reason, a sill may show evidence of fast cooling (e.g. chilled margin) at the contact with all the surrounding country rocks in the same way. On the other hand, a lava flow tend to show evidence of very rapid cooling and exposure to atmospheric agents only on the upward side.
Care is needed in metamorphic sequences, since metamorphism may destroy these evidences, making it difficult to distinguish a metamorphosed sill from a metamorphosed lava flow.

Dolerite (diabase) sill of Mid-Carboniferous age emplaced parallel to shales and sandstones of the Lower Carboniferous Horton Group, Horton Bluff, near Cheverie, Nova Scotia, Canada. The sill is easily recognizable as it is more resistant to erosion. Photo by Mike Norton.

Sills can sometimes be hard to identify. Can you recognize the sill in this sequence of sandstone and shales exposed on the banks of the Nahal Ramon river (Israel)? Photo by Shayshal2.

The dark band is a sill of dolerite which intruded into the Ards Quartzite. Dunfanaghy, Ireland. The sill appears folded, since it follows the structures in the host rocks, or because it was folded together with them by later deformation. Photo by Anne Burgess at geograph.uk.

Diorite sill and bleached reaction zone of contact metamorphism in the adjacent country rock. Purcell Sill intruding Proterozoic Helena Fm, Glacier National Park, Montana, USA. Sill is about 2 meters thick. Photo by Marli Miller at geologypics.com.

Swarm of pegmatite sills in Proterozoic basement rocks exposed next to a small glacier on northeastern Baffin Island (Canada). Sills coexist with and laterally pass to small dikes. Photo by Mike Beauregard.

 

References
Cartwright, J., & Møller Hansen, D. (2006). Magma transport through the crust via interconnected sill complexes. Geology34(11), 929-932.
Maccaferri, F., Bonafede, M., & Rivalta, E. (2011). A quantitative study of the mechanisms governing dike propagation, dike arrest and sill formation. Journal of Volcanology and Geothermal Research208(1-2), 39-50.
Muirhead, J. D., Airoldi, G., White, J. D., & Rowland, J. V. (2014). Cracking the lid: Sill-fed dikes are the likely feeders of flood basalt eruptions. Earth and Planetary Science Letters406, 187-197.
Valentine, G. A., & Krogh, K. E. (2006). Emplacement of shallow dikes and sills beneath a small basaltic volcanic center–The role of pre-existing structure (Paiute Ridge, southern Nevada, USA). Earth and Planetary Science Letters246(3-4), 217-230.
Walker, R. J. (2016). Controls on transgressive sill growth. Geology44(2), 99-102.

        

See also
Geologypics.com

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Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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