A sheet intrusion, or tabular intrusion, is a planar sheet of roughly the same thickness, that forms inside a pre-existing rock.Essentials of Geology, 3rd Ed, Stephen Marshak When it cuts into another unlayered mass, or across layers, it is called a '' dike''. When it is formed between layers in a layered rock mass, it is called a '' sill''. An igneous sheet

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

is formed where a mass of molten

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

takes advantage of a pre-existing linear feature in a host rock, such as a long rupture or fault, and forces its way into these spaces. Thus the magma, intruded between existing rocks, solidifies into large thin sheets of

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

. They are among the most extensive

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

features on Earth, in the form of dikes, laccoliths, cone sheets and sills. When limestone or other precipitate forms in a fault space, it is a sedimentary sheet intrusion.

References

* Igneous rocks Volcanology {{igneous-petrology-stub he:מחדר פלוטוני#מחדר לוחי