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Difference Between Dyke and Sill

Dyke vs Sill

“Dyke” and “sill” are geological terms used to describe an intrusion, usually a mass of igneous or volcanic rocks that forcibly entered, penetrated, and embedded into layers of another rock or land form. Dykes and sills are often associated with volcanoes though they are not exclusive to that particular land form.

As intrusions, both dykes and sills are igneous rocks that were left or as a result of crystallization of molten magma flow that exists beneath the Earth’s surfaces. They are naturally occurring in nature and considered as “foreign” rock in relation to their surrounding rock environment or form which is “local” or “original” rock. They can be injected into existing cracks in the bedding plates or erupt as a pressure or force from a particular point of origin.

Dykes and sills “intrude” due to the pressure, stress, and deformation from the surrounding rocks or beneath the Earth’s surface. It is often in a molten or unstable state when it “intrudes” into another formation and hardens as it cools down over the passage of time. The main forms of dykes and sills are magmatic and sedimentary.

Compared to the rock that they intrude or cut, dykes and sills are often slimmer or less thick in comparison. They are also considered wider compared to their surrounding rocks and younger in age from their surrounding rocks. A dyke (also spelled as dike) is a mass of rocks that intrudes or cuts across two layers of another rock or land form. A large number of dykes is called dyke swarms. Dyke swarms are a common occurrence in the creation of dykes and usually consist of hundreds in numbers.
Dykes can be classified as vertical, nearly vertical or steep in nature since its creation is dependent on the consistency of pressure that originates from below.
The usual classification of dykes can be diabase, basaltic to granitic or rhyolitic but other forms like pegmatite dikes and aplite dikes.
Dykes are classified as discordant intrusions since they are not parallel but across the pre-existing land or rock form.
On the other hand, sills are formed in the same way as dykes but their direction is different. Sills are created in between and parallel to its surrounding bedding plates
Meanwhile, sills are often uniform in orientation since the surrounding rocks can give consistency to its form. However, it can also be not uniform in appearance in terms of color and direction.
Sills are usually composed of medium-grained rock and often contain have certain rare types of ore deposits in its formation.
As an intrusion, a sill as a body of rock is concordant because it is parallel to the original rock or land from.
Summary:

1.Dykes (or dikes) are igneous rocks that intrude vertically (or across) while sills are the same type of rocks that cuts horizontally (or along) in another land or rock form.
2.Dykes are discordant intrusion while sills are concordant intrusions.
3.Dykes and sills from due to pressure, force and stress from one point of oriogn. Dykes form when the point of origin is beneath the forming dyke and sill si formed when the starting point is either on the left or right side.
4.Both dykes and sills can be magmatic or sedimentary in nature. They are also slimmer, younger and wider than their surrounding rocks or plates. Dykes and sills as intrusion often have a different color from their surrounding rocks.
5.Dykes are easier to distinguish because the intrusion is evident between bedding planes and rock while sills can be harder to identify since it exists in parallel with the planes and the rocks. Only proper testing and discoloration between rocks can state whether a plane is a sill or part of the original rock formation.


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