Dikes

Geological Dike

A dike is a sheet of rock that formed in a crack in a pre-existing rock body. However, when the crack is between the layers in a layered rock, it is called a sill, not a dike. 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.

Dikes can be either magmatic or sedimentary in origin. A magmatic dike is formed when magma (lava) flows into a crack and solidifies as a sheet or tubular intrusion. It can cut through layers of rocks or the contagious rock mass. Clastic or sedimentary dikes form when sediment fills a pre-existing crack in the rock.

Dikes and sills

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 strata are called intrusive sills.

Dike Vs. Sill

Dike and sills describe the same thing; intrusions or mass of rock that has forcibly entered, penetrated, and embedded into another rock or landform. However, there are some differences as well. Dikes and sills are often associated with volcanoes but are not exclusive to that particular landform.

Dikes and sills occur naturally and are considered foreign rocks in relation to the rock in which they form. The rock in which a dike or sill occurs is known as local or original rock.

Dikes are characterized as steep, vertical, and nearly vertical structures. Sills have a different direction than dikes. Sills are uniform in orientation because the surrounding rock often gives consistency to their form.  Sills can be non-uniform in appearance as far as color and direction are concerned.

Both dikes and sills can be magmatic or sedimentary in nature. Dikes are easier to identify because the intrusion is evident between bedding planes and rocks. Sills can be difficult to identify because they mostly exist parallel to planes and rocks. It requires proper testing and discoloration to identify and differentiate a sill from the original or local rock.

Types Of Dikes

There are two types of dikes; magmatic and sedimentary.

MAGMATIC DIKES

Multiple and Composite Dikes: Multiple magmatic dikes occur when multiple injections of similar composition occur. However, this rarely happens. When the injections have different compositions, the dikes formed are known as composite dikes. The compositing of a composite dike can vary from diabase to granite. These variances are common in composite dikes of Scotland and Northern Ireland.

Dike Swarms: Dikes can also appear in swarms. A typical dike swarm can have several to 100s of dikes that form during a single intrusive event. Dike swarms are usually made of diabase and are associated with flood basalts of huge igneous rock collections. There are various examples of dike swarms. The most common are Jurassic dike swarms in England, dikes in Iceland, and dike swarms in the eroded rift zones of Hawaiian volcanoes.

Dikes can also form as radial swarms from a certain volcano or intrusion. The radial swarms appear to originate in the central intrusion; the dikes can have different ages and compositions from the intrusion. Such radial swarms may have been created over the intrusion and were later altered by a rising body of magma or regional tension.

Sheeted Dike Complexes: A sheeted dike complex is a series of sub-parallel intrusions of igneous rocks. It forms a dike layer within the oceanic crust. The sheeted dikes often show a chilled margin on only one side that indicates each dike was cut in half by magma eruption after dike formation.

Ring Dikes & Cone Sheets: These are special dikes associated with caldera volcanism. These dikes are usually distributed around shallow magma chambers.

Feeder Dike: It is a dike that acts as a channel for magma moving from a magma chamber to an intrusion.

Sole Injection: It is a dike that is injected along a thrust fault plane. A thrust fault plane is a break in the earth’s crust where older rocks are pushed above the younger rocks.

   

A magmatic dike cross-cutting horizontal layers of sedimentary rock, in Makhtesh Ramon, Israel Photo: Andrew Shiva

SEDIMENTARY DIKES (CLASTIC DIKES)

These are vertical bodies of sedimentary rocks that cut through other local or original rocks. These dikes are formed from sedimentary rocks.  Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) or passively by water, wind, and gravity (sediment swept into open cracks). Diagenesis may play a role in the formation of some dikes.
 

 

Volcanic dyke. Mass of igneous rock intruded into older red iron rich sedimentary rock. 1km north of Garajonay summit, La Gomera Copyright: DAVID LYONS