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.