Dike Geology: Definition, Formation and Types

Dikes refer the fractures that divide the bedding planes in different eroded parts vertically.

Dike is very easy to define. In plain words, we can say that dike is a fracture that cuts the bedding planes of any rock vertically. Dikes cannot be viewed in vertical form only. They can be seen in horizontal, ringed and oval forms.

Dike VS Sill

The function of dike and sill are the same. The only difference is their direction. Dike intrudes the cuts vertically whereas; sill intrudes the cuts on the bedding planes of a rock vertically. When the dikes and sills occur on a titled rock, they will also be titled. The classification of dikes and sills on regular and titled rocks shows their formation actually. It is not according to their appearance but on their formation.

Dikes’ Formation

Dikes or Dykes are formed due to the overflow of magma from below to the higher level of rocks. Since magma is a mixture of molten crystals; it cuts the currently existing strata. It follows the same way created by the previous faults and cracks. If the flow of magma is more powerful, it may create some new paths upward. So, it crystallizes the pre-existing path and forms dikes. Resultantly, the plutonic rocks come into existence.

When the erosion occurs, it allows us to peep through the erosions and to view the dikes. It may be impossible for magma in a dike to access the surface of the earth. Once the magma is poured out on the surface, the rock will be turned into extrusive one. You can see the Spanish Peaks to examine the dikes in rocks. It is famous for having dikes. All of them are different on the basis of age, composition, and structure. So far, the geologists have been mapped more than 500 dikes.

Dikes on Sediment Rocks

Types of Dikes

There are basically three types of dikes: Sedimentary, Ring and Igneous. I am going to elaborate on each of them in plain words to make you understand them very well.

Sedimentary Dikes

These are also known as sandstone dikes or clastic dikes. These dikes occur when the sediment and minerals play a vital role to build up the rock and then convert it into a rock fracture. The sedimentary dikes are usually formed in the sedimentary rock as the name suggests, but they can be formed in an igneous rock too.

Clastic/Sedimentary dikes can occur in numerous ways. Following are these.

  • When the earthquakes occur, the liquefaction or fractures appear in the rocks. These dikes usually serve as paleoseismic indicators.
  • The passive movement of sediment rock into already existing fissures can also form the sedimentary dikes. Just imagine a mudslide or glacier that moves over a part of fractured rock. While moving over it, the glacier or mudslide adds clastic material in the fractured area of the rock and cause sedimentary dikes.
  • The hydrocarbons and gases revolve in the form of a thick sand bed covered by mud. The air exerts pressure on the sand bed and transfers the hydrocarbons and gases from the thick sand bed to the above layer. This process forms sandstone dikes. This kind of dikes can be observed by examining the fossils of cold seep clique.

Igneous Dikes

Igneous dikes are formed on the igneous/metamorphic rocks. When the magma from a volcano starts flowing through the rock, the vertical fractures of the rock form igneous dikes. Once the igneous dikes cool down, the fractures appear wider. The sheets are thick from a few millimeters to numerous meters.

Igneous dikes are longer and not very thick. If you measure the length, it can go up to thousands of meters high. In the same way, the height can go up to many kilometers. One dike sheet consists of hundreds of dikes. In this context, we can take an example of Mackenzie dike swarm on the Canadian Shield rock. It is 1100 miles wide and 1300 miles long. But it is not the longest and tallest dike. Igneous dikes can be longer than this too.

Ring Dikes

Ring Dikes are formed into circular, oval and arcuate shapes. These are not formed vertically or horizontally on the rocks. The ring dikes are formed on igneous rocks due to caldera collapsing. Sometimes, shallow magma releases the remaining content with somehow lower pressure. Consequently, the roof of the rock collapses. So, the roof collapsing constitutes vertical or steep dip-slip fractures. These fractures can give way to the shallow magma. In this context, I will give an example of the Pilanesberg Mountains of South Africa. The harder minerals of the dike caused erosions in the adjacent rocks. The Ossipee Mountains, New Hampshire can also be examined for ring dikes.


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