     a dike or dyke in geology is a sheet of rock that formed in a fracture in a pre-existing rock body. however, when the new rock forms within and parallel to the bedding of a layers rock, it is called a sill. it is a type of tabular or sheet intrusion, that either cuts across layers in a planar wall rock structures, or into a layer or unlayered mass of rock.essentials of geology, 3rd ed, stephen marshak dikes can be either intrusive or sedimentary in origin. for example, when molten rock intrudes into a crack then crystallizes, it is an igneous dike. when sediment fills a pre-existing crack, it is a clastic dike. {| border="0" align="left" style="float: left" |  |- |  |- | thumb|left|dikes in the black canyon of the gunnison national park, colorado, usa. |- |  |- |  |} ==magmatic dikes== an intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. thickness can vary from sub-centimetre scale to many metres, and the lateral dimensions can extend over many kilometres. a dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. dikes are usually high-angle to near-vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal. near-horizontal, or conformable intrusions, along bedding planes between [[stratum|strat
Igneous_petrology Volcanology