What is a Dike, Types of Dikes

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.

Dike (Dyke) Meaning

A dike (or dyke) is a sheet-like body of rock that forms when molten magma or fragmented rock material intrudes into a fracture in pre-existing rock, cutting across the surrounding rock layers. While typically composed of igneous rock formed from cooled and solidified magma (magmatic dikes), dikes can also be formed from cemented sediments that fill fractures (clastic dikes). Regardless of their composition, dikes are characterized by their discordant relationship with the surrounding rock and their often vertical or steeply inclined orientation. They can vary significantly in thickness and lateral extent.

Diabase dike cutting through horizontal limestone and sandstone beds near Winkelman, Arizona.

 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.

Dikes above Basalt

Magmatic Dikes

Magmatic dikes, formed by the intrusion and solidification of magma within cracks in pre-existing rock, exhibit a fascinating variety in terms of their characteristics and occurrence. 

The different types of magmatic dikes:

Dikes Types Based on Composition

Simple Dikes: Composed of magma of a single, uniform composition. These are the most common type and can range in composition from basaltic (fine-grained and dark) to granitic (coarse-grained and lighter).

• Basaltic Dikes: These are the most common type, composed mainly of basalt or diabase, and often associated with volcanic activity.
• Granitic Dikes: Formed from felsic magma with high silica content, these are less common but can be significant for understanding plutonic intrusions.
• Pegmatite Dikes: Characterized by very coarse-grained crystals and often containing rare minerals, these dikes are associated with the final stages of magmatic activity.

Composite Dikes: Formed by multiple injections of magma with different compositions, resulting in a layered or mixed appearance. Some composite dikes display stunning patterns and color variations.

Dike in La Palma, Canary Island, Spain

Dikes Types Based on Geometry

Vertical Dikes: The most common type, these dikes intrude perpendicularly to the host rock layers, often appearing as vertical walls when exposed.

Sills: Intrusions that follow the bedding planes of the host rock, forming horizontal or gently inclined sheets.

Cone Sheets: Sets of concentric, cone-shaped dikes dipping away from a central magma chamber, mimicking the shape of a volcanic cone.

Ring Dikes: Circular or elliptical intrusions, often associated with caldera collapses and volcanic centers.

Volcanic dike and sill

Dikes Types Based on Occurrence

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.
Feeder Dikes: Act as channels for magma moving from a deeper magma chamber to an eruption site or shallower intrusion.

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.

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.

Volcanic dike. Mass of igneous rock intruded into older red iron rich sedimentary rock. 1km north of Garajonay summit, La Gomera Photo Copyright: David Lyons

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.

Dike Vs. Sill

Dikes

Discordant Intrusions: Dikes exhibit a discordant relationship with the host rock, meaning they transgress pre-existing bedding planes or foliation at an angle. This discordance results from magma exploiting fractures, faults, or other planar weaknesses with orientations deviating from the layering in the surrounding rock.

Rapid cooling: Due to their smaller size and greater surface area exposure, dikes generally cool faster than sills, leading to finer-grained crystalline textures.

A dark outcrop of a Triassic sill near San Rafael in Mendoza Province, Argentina.

Sills

Concordant Intrusions: In contrast, sills display a concordant relationship with the host rock. They intercalate themselves parallel to existing bedding planes or foliation, effectively inserting a new layer amidst the pre-existing strata. This concordance often reflects the utilization of weakness planes coincident with the layering in the host rock.

Greater Lateral Extent: Compared to dikes, sills often exhibit significantly wider lateral extents, potentially covering vast areas while maintaining a relatively limited thickness (meters to tens of meters).

Slower cooling: Due to their larger size and greater insulation by surrounding rock, sills generally cool slower than dikes, leading to coarser-grained crystalline textures.

Mineralogy: Both dikes and sills can exhibit diverse compositions depending on the specific magma involved. However, dikes are often associated with basaltic magma, while sills can encompass a wider range of compositions, including felsic and mafic varieties.

Clastic dike

Clastic Dikes - Sedimentary Dikes

Clastic dikes , also known as Sedimentary dikes, are fascinating geological features that stand out like vertical stripes cutting through sedimentary rock layers. These intriguing cracks are filled with a different type of sediment than the surrounding rock, offering a glimpse into past geological events and processes.

Clastic Dikes Formation

There are two main ways sedimentary dikes form:

Fluidized injection: When a layer of coarse-grained sediment, saturated with water or other fluids, becomes overburdened by layers above, the pressure can cause the sediment to liquefy and inject itself into cracks or fissures in the underlying rock. Imagine squeezing a toothpaste tube – the soft paste shoots out where there's less pressure.

Desiccation cracks: In arid environments, clay-rich sediments can shrink and crack as they dry. These cracks can then be filled with sand or other loose sediment, creating vertical dikes. Think of mudflats drying under the sun, developing cracks that get filled with windblown sand.