Because sills generally form at shallow depths up to many kilometers below the surface, 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, if still covered by country rock, typically do not.
Figure 5. Certain 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 igneous complex of the United States.
These intrusions often contain concentrations of gold, platinum, chromium and other rare elements. Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by relatively short dike-like segments. Such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin. Improve this page Learn More.
Skip to main content. Module 9: Volcanoes. Search for:. Reading: Dikes and Sills Dikes Figure 1. Figure 2. Figure 4. Illustration showing the difference between a dike and a sill. Did you have an idea for improving this content? Roest; Removing varying directional trends in aeromagnetic data, Geophysics, vol. See also, Polteau, S. Cones are thin intrusive sheets that expand upwards and outwards in cones. Individual sheets are generally a few metres thick, but arranged so that the outer cones dip at lower angles to the inner ones so that they all converge towards a common source at depth.
These have a much thicker top that plunges down to a very narrow neck. A classic example is the Skaergaard intrusion of Greenland. Funnel dykes are elongated in outcrop like a dyke but with a V-shaped cross-section that narrows downwards. All known examples are very large over km in length. Lopoliths are the largest known intrusions of dense magma and form a thick saucer shape within the surrounding country rocks. A famous example is the Bushveld Complex of South Africa, which is over km across and up to 8 km in thickness.
Lopoliths contain many important economic deposits of nickel, copper, platinum, palladium and chromium. The Sudbury intrusion of Ontario, Canada formed in an oval-shaped depression probably caused by a large meteorite impact. Dykes reach their highest numbers in dyke swarms, which may form lines or radial patterns. Radial dykes usually converge on volcanic centres or igneous intrusions. Linear dyke swarms are more extensive than radial dyke swarms but can also be concentrated around large intrusions or volcanic centres.
In south-eastern Iceland, the Cenozoic lava succession is cut by thousands of aligned near-vertical basaltic dykes averaging less than a metre thick. Diatremes are steep pipe-like bodies filled with fragments of both igneous rocks and wallrocks. They form by explosion which results from the release of contained carbon dioxide gas CO2 and water vapour H2O near the surface. Some very silica-poor magma types are economically important as the host rocks for diamonds. The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.
Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer An igneous intrusion cut by a pegmatite dyke, which in turn is cut by a dolerite dyke.
The oldest igneous rocks in this photo show features caused by magma mingling or magma mixing. What are intrusions? Types of intrusions Dykes Dykes are sheet-like bodies of igneous rock that cut across sedimentary bedding or foliations in rocks.
Silica-rich intrusions Stoped stocks Stoping happens when a rising magma breaks off jointed blocks from the overlying country rock. Ring dykes and bell-jar plutons Ring dykes form a cylinder around a subsided block of country rock, fill ing the ring-shaped fracture with magma. The principle of inclusions and components states that, with sedimentary rocks, if inclusions or clasts are found in a formation, then the inclusions must be older than the formation that contains them.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them. The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past. The principle of Original Horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal.
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