Dike vs. Sill

What's the Difference?

Dike and sill are both geological formations that are commonly found in the Earth's crust. However, they differ in their formation and characteristics. A dike is a vertical or near-vertical intrusion of magma that cuts across the layers of pre-existing rock. It is formed when molten rock is forced into fractures and solidifies underground. On the other hand, a sill is a horizontal sheet-like intrusion of magma that is parallel to the layers of pre-existing rock. It is formed when magma is injected between the layers of rock and solidifies. While both dikes and sills are igneous intrusions, their orientations and shapes make them distinct from each other.


DefinitionA long wall or embankment built to prevent floodingA tabular sheet of igneous rock that has intruded between older layers of sedimentary rock
FormationFormed by the deposition of sediment or by the solidification of volcanic lavaFormed by the intrusion of magma into existing rock layers
ShapeTypically linear or curvedTabular or sheet-like
OrientationCan be vertical, inclined, or horizontalUsually horizontal
CompositionCan be made of various materials like soil, clay, sand, or rockComposed of igneous rock, such as basalt or granite
LocationCommonly found near rivers, coastlines, or areas prone to floodingOften found in areas with volcanic activity or where magma has intruded into the Earth's crust
FunctionPrimarily used for flood control and water managementDoes not serve a specific function, but can act as a barrier or conduit for magma

Further Detail


Dikes and sills are both types of intrusive igneous rock formations that form when magma solidifies underground. While they share some similarities, they also have distinct attributes that set them apart. In this article, we will explore the characteristics of dikes and sills, their formation processes, and their geological significance.

Formation and Composition

Dikes and sills are both formed through the intrusion of magma into existing rock formations. However, their formation processes differ. Dikes are formed when magma is injected into fractures or cracks in the surrounding rock, cutting across the existing layers. This results in a vertical or near-vertical sheet-like structure. On the other hand, sills are formed when magma is injected parallel to the existing layers of rock, creating a horizontal or near-horizontal sheet-like structure.

In terms of composition, both dikes and sills are primarily composed of igneous rock. The exact composition can vary depending on the type of magma that intrudes. Common types of igneous rocks found in dikes and sills include basalt, granite, and diorite. These rocks can exhibit a wide range of colors, textures, and mineral compositions, making each dike and sill unique.

Shape and Size

Dikes and sills can vary significantly in terms of their shape and size. Dikes typically have a narrow and elongated shape, with a width ranging from a few centimeters to several meters. They can extend for long distances, cutting across multiple layers of rock. In contrast, sills have a more tabular shape, with a relatively uniform thickness. They can range in width from a few centimeters to several meters, but their horizontal extent can be much greater than that of dikes.

The size of dikes and sills can also vary depending on the amount of magma available and the conditions under which they form. Some dikes and sills can be relatively small and localized, while others can be massive and cover extensive areas. The size of these formations can have important implications for their geological significance and potential economic value.

Geological Significance

Dikes and sills play important roles in the geological history of an area. They can provide valuable insights into the processes that have shaped the Earth's crust over millions of years. By studying the orientation and distribution of dikes and sills, geologists can gain a better understanding of the tectonic forces that have acted on the region.

Dikes, in particular, are often associated with volcanic activity and can serve as conduits for magma to reach the surface. They can be found in volcanic regions and are commonly observed in areas with extensive volcanic activity, such as the Pacific Ring of Fire. Dikes can also act as barriers, preventing the spread of magma and confining it to specific areas.

Sills, on the other hand, are often associated with the emplacement of magma at shallow depths. They can be found in areas where the crust has been stretched or thinned, such as rift zones. Sills can also have economic significance as they can host valuable mineral deposits, including copper, gold, and silver.

Impacts on the Surrounding Rocks

Both dikes and sills have significant impacts on the surrounding rocks. When magma intrudes into existing rock formations, it can cause thermal metamorphism, altering the mineral composition and structure of the surrounding rocks. This can result in the formation of new minerals and the recrystallization of existing ones.

Dikes, due to their vertical or near-vertical orientation, can cut across multiple layers of rock, causing disruption and deformation. The surrounding rocks may be fractured, folded, or faulted as a result of the intrusion. In contrast, sills, with their horizontal or near-horizontal orientation, tend to have a less disruptive impact on the surrounding rocks. They can cause some displacement and deformation but are generally less pronounced compared to dikes.


In conclusion, dikes and sills are both intrusive igneous rock formations that form when magma solidifies underground. While they share similarities in terms of their composition and impacts on the surrounding rocks, they also have distinct attributes that differentiate them. Dikes are characterized by their vertical or near-vertical orientation, narrow shape, and association with volcanic activity. Sills, on the other hand, have a horizontal or near-horizontal orientation, tabular shape, and are often associated with shallow emplacement of magma. Understanding the attributes of dikes and sills is crucial for geologists to unravel the geological history and processes that have shaped our planet.

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