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Introduction to Dike

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Dike or dyke as also known as geology is a type of tabular or sheet like igneous rock that is vertically or steeply inclined to the bedding of the pre-existing intruded rocks. The dike igneous rock is a kind of similar body that is oriented parallel to the bedding of the enclosing rocks which are known as sills. Hence, an igneous dike is made up of several parallel dikes and when the number of dike igneous rocks is larger than the term used is dike swarm. The igneous dike rocks have a very wide range of rock compositions. The most common type of porphyritic texture i.e. the larger crystals within the finer-grained groundedness, indicating the two periods of crystallization.

Magnetic Igneous Dike

An intrusive dike is one that has a very high aspect ratio i.e. the thickness of the dike igneous rock is smaller than the other two parameters of measurements. The thickness of the dike igneous rock can range in-between sub-centimeter scale to many meters and the lateral measurements can be over many kilometers in size. A dike is an intruding formation into the opening of a cross-cutting fissure. It is shouldering aside from the other pre-existing rocks. This means that a dike will always be yonder than the rocks that form a part of the dike. The dike igneous rocks are normally high-angle approaching a near-vertical angle in orientation. The following tectonic deformation can rotate the sequence of the strata so much so that the dike propagates making the dike horizontal. The almost near-horizontal intrusions along with the bedding of the planes in-between the strata are known as intrusive sills. Therefore, the dikes and sills are generally also known as sheets in relevant contexts. 

Sometimes many of the dikes appear together as a couple which is known as a dike swarm. The swarms of dikes consist of several hundreds of the dikes put into position at a single time during a single intrusive event. An example of a dike swarm is the Mackenzie dike swarm present in the Northwest Territories in Canada. It is also the world’s largest dike swarm.

The dikes often form shapes that are either radial or concentric when they are arranged in a swarm around the events of plutonic intrusive or volcanic necks or feeder vents found in the volcanic cones. The latter forms are also known as ring dikes.

The dikes can differ in the form of textures and their composition. They can range from the diabase of the basaltic to granitic or rhyolitic with a global perspective that the basaltic composition is prevalent showing ascent of very vast volumes of the mantle-derived magmas through the fractured lithosphere throughout the history of the Earth. The pegmatite dikes usually comprise the extremely coarsely formed crystalline granite rocks and often they are associated with the late stages of granite intrusion or the metamorphic segregations. The aplite dikes are the ones that have fine-grained or sugary-texture intrusive in the granite composition. 

There is another term known as ‘feeder dike’ which is used for the dike that acts as a conduit for the magma. This feature is formed when the magma flows along and then out of the dike. Contrary to the magmatic dikes, a sill is the magmatic sheet of intrusive igneous rocks that forms within and is parallel to the bedding of the layered rock. 

Sedimentary Dikes

The sedimentary dikes which are also known as the clastic dikes are the vertical bodies of sedimentary rocks that cut off the layers of other rocks. They are usually formed in two ways:

  • When shallow unconsolidated sediment is composed of any of the sediment formed from alternating coarse-grained and impermeable layers of clay, the pressure of the fluid inside the coarser layers reach any critical value because of the lithostatic overburden. The sediment breaks through the layers, forming the dike when it is driven by the fluid pressure.

  • A soil formed under the permafrost conditions leads to a totally frozen pore water. When any cracks are formed in any such rocks, they can get filled up with the sediments that may fall in from the above. This results in a vertical body of the sediment being cut through the horizontal layers and forming a dike.


In geology, a dike or a dyke is a sheet formed by the fracture of the pre-existing rock. The dikes can either be magmatic or can be sedimentary in origin. The magmatic dikes are formed when magma flows through a crack solidifying as intrusion as a sheet while cutting across the layers of the rock or through the contiguous mass of rocks. The clastic dikes are the ones that are formed when sediment is filled in pre-existing rock. 

Although the dikes can range in size from a few centimeters to widths greater than 10 meters. The average width of the dikes is between 0.3 and 6-metre width. The length of the dike usually depends on how far it can trace across the surface with some dikes that can go up to hundreds of miles long. The longest dike in the world is the Great Dike of Zimbabwe as it stretches for more than 550 km southwest to northeast across the center of the country.

FAQ (Frequently Asked Questions)

1. How are Dikes and Sills Formed?

Ans: A sheet of rock formed by the fracture of a previous pre-existing body is known as dikes. They can either be magmatic or sedimentary in origin. The magmatic dikes are formed when the magma intrudes into the crack which then crystallizes as a sheet or sheet intrusion by either cutting across the layers of the rock or through an unlayered mass of the given rock.

2. What is an Example of a Dike?

Ans: The Ossipee Mountains of the New Hampshire and the Pilanesberg Mountains of South Africa are the two examples amongst others which are examples of the ring dikes. In both these examples, the minerals of the dike are harder than the rock that they are intruded into. 

3. What is a Volcano Dike?

Ans: Dikes are usually the tabular or sheet-like bodies of magma that usually cuts through and across the layering of the adjacent rocks. When the magma rises into the existing fractures that create the new crack by the force of the way through the existing rock and then solidifies. 

4. Are Dikes Man-Made?

Ans: Dikes are usually used to hold the water back and such dikes are usually made of earth. So, yes the dikes are man-made. Sometimes, the dikes can be natural as well. More often people usually create or construct dikes to prevent flooding. When they are constructed along the river banks, they are used to avoid floods and control the flow of water.

5. How Are Igneous Rocks Formed?

Ans: The igneous rocks are formed when any of the hot and molten rock crystallizes which later solidifies. The melt usually originates deep within the Earth and near the active plate boundaries of hot spots which then rises towards the surface.