Sheeted dyke complex

A sheeted dyke complex, or sheeted dike complex, is a series of sub-parallel intrusions of igneous rock, forming a layer within the oceanic crust. At mid-ocean ridges, dykes are formed when magma beneath areas of tectonic plate divergence travels through a fracture in the earlier formed oceanic crust, feeding the lavas above and cooling below the seafloor forming upright columns of igneous rock. Magma continues to cool, as the existing seafloor moves away from the area of divergence, and additional magma is intruded and cools. In some tectonic settings slices of the oceanic crust are obducted (emplaced) upon continental crust, forming an ophiolite. The individual dykes typically range in thickness from a few centimetres to a few metres. Most of the dykes show evidence of one-sided chilled margins, consistent with most dykes having been split by later dykes. It is also common for the chilled margins to be consistently on one side, suggesting that most dykes in any one exposure were gradually moved away from the spreading centre by further stages of intrusion in a constant location. The layer of sheeted dykes that makes up the lower part of Layer 2 of the oceanic crust is typically between one and two kilometres thick. At the top, the dykes become increasingly separated by screens of lava, while at the base they become separated by screens of gabbro. Sheeted dyke complexes are most commonly found at divergent plate boundaries marked by the presence of mid-ocean ridges. These subaqueous mountain ranges are made up of newly created oceanic crust due to tectonic plates moving away from each other. In response to the separation of plates, magma from the asthenosphere is subject to upwelling, pushing hot magma up towards the seafloor. The magma that reaches the surface is subject to fast cooling and creates basaltic formations such as pillow lava, a common extrusive rock created near areas of volcanic activity on the seafloor. Although some magma is able to reach the surface of oceanic crust, a considerable amount of magma solidifies within the crust.