Sill (geology)
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In geology, a sill is a tabular pluton that has intruded 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. This means that the sill does not cut across preexisting rocks. Contrast this with dikes.
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 lava flows; however there are several differences between them. Intruded sills will show partial melting of 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 one side of the flow. In addition, lava flows will typically show evidence of bubbles (vesicles) where gasses were escaping 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 of weathering on their upper surface, whereas sills typically do not. An examination of the surrounding geology may be necessary to make a determination of sill or lava flow, but in most cases the attributes outlined above are sufficiently clear to allow a determination.
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.
