Sill (geology)
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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.
Sills are always parallel to beds (layers) of country rock. They can often be confused with lava flows when looking at them from a geological point of view, however there is one main difference between them. When lava flows come into the rock, they always come on top, therefore only heating one side of country rock and baking it (a baked margin). However sills intrude into the rock, therefore creating a baked margin on both sides.
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 Rùm peridotite complex of Scotland and the Skaergaard intrusive of east Greenland. These intrusives often contain concentrations of gold, platinum, chromium, and other rare elements.
