A sheeted dyke complex, or sheeted dike complex, is a series of sub-parallel intrusions of

igneous rock 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 l ...

, forming a layer within the

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafi ...

. At

mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...

s, dykes are formed when magma beneath areas of

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large t ...

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 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 natura ...

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 Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial' ...

, forming an ophiolite.

Geometry

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 margin A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine-grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid crystalliz ...

s, 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.

Dyke formation

Sheeted dyke complexes are most commonly found at divergent plate boundaries marked by the presence of

s. 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 The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is ...

is subject to upwelling, pushing hot magma up towards the seafloor. The magma that reaches the surface is subject to fast cooling and creates

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

ic 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. Dykes are formed when the rising magma that does not reach the surface cools into upright columns of igneous rock beneath areas of divergence.

Ophiolites

Dykes are perpetually formed as long as magma continues to flow through the plate boundary, creating a distinct,

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostr ...

-like sequences of rocky columns within the seafloor. Ophiolites are formed when these sections of oceanic crust are revealed above sea level and embedded within coastal crust. Older dykes formed near divergence zones are pushed away as new seafloor is created, a phenomenon known as seafloor spreading, and over time, the oldest dykes are pushed far enough from convergence zones to be exposed above sea level.

Seafloor spreading and continental drift

The creation of sheeted dykes is a perpetual and continuous process that promotes the phenomenon known as seafloor spreading. Seafloor spreading is the creation of new oceanic crust by volcanic activity at mid-ocean ridges, and as magma continues to rise and solidify at mid-ocean ridges, the existing older dykes are pushed out of the way to make room for newer

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

. The rate at which new oceanic crust is created is referred to as ''spreading rate'', and variations in spreading rate determine the geometry of the mid-ocean ridge being created at plate boundaries.

Fast-spreading ridges

Mid-ocean ridges with a spreading rate greater than or equal to 90 mm/year are considered to be fast-spreading ridges. Due to the large amounts magma being expelled from the asthenosphere in a relatively short period of time, these formations typically protrude much higher from the seafloor.

Slow-spreading ridges

Mid-ocean ridges with a spreading rate less than or equal to 40 mm/year are considered to be slow-spreading ridges. These formations are typically characterized by a large depression in the seafloor, known as

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear de ...

s, and are formed due to the lack of magma present to solidify.

Examples

* Troodos Ophiolite, Cyprus * Maydan

Syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

, Oman, part of the Semail Ophiolite - A sheeted dyke complex on the coast of Oman has been discovered to have been formed during a single sea-floor spreading episode. * Hole 504b, Costa Rica - Hole 504b is a scientific ocean drilling program that burrowed 1562.3 m below the seafloor directly through layers of sediment exposing sheeted dykes and pillow lava.

References

{{reflist Igneous petrology Magmatic dikes