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
Dyke (UK) or dike (US) may refer to:
General uses
* Dyke (slang), a slang word meaning "lesbian"
* Dike (geology), a subvertical sheet-like intrusion of magma or sediment
* Dike (mythology), ''Dikē'', the Greek goddess of moral justice
* Dikes, ...
". When it is formed between layers in a layered rock mass, it is called a "
sill
Sill may refer to:
* Sill (dock), a weir at the low water mark retaining water within a dock
* Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma
* Sill (geostatistics)
* Sill (river), a river in Austria
* Sill plate, a ...
".
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 is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
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 (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
. They are among the most extensive
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
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
{{Volcanology-stub
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