Dike vs Sill
Dike (Dyke in British English) and sill are geological formations that are made of igneous rocks. These rocks get formed when hot magma from the hot core or the mantle of the earth gets released upwards through cracks, fissures, or joints. This magma does not reach the surface of the earth in the case of the sill and dike as is the case with lava that erupts from the opening of a volcano. Thus, sill and dike are the result of cooled down magma before it reaches the surface of the earth. Though not very important for us, the difference between these two geological formations is important for students of vulcanology.
Dike
Magma from the mantle always moves upwards cutting through rocks, trying to reach the surface of the earth. Magma gets added, and the pressure from below makes it move up through fissures, cracks, and joints. The wall of the magma chamber gives way in many instances and the hot magma, instead of shooting up through the opening, begins it journey through these cracks that can go to hundreds of kilometers. A dike is formed when the magma moves vertically through the fissures, cutting through various rock layers. The important thing to remember is that the magma cools down and hardens inside the rocks instead of reaching the surface of the earth. It is only because of constant weathering and erosion of top layers of rocks for thousands of years that we are able to see a dike as a geological formation. A dike is seen as an igneous rock that is at a very steep angle or almost vertical to the existing rock structure.
Sill
Magma, when it moves in a horizontal fashion along a bed of older rocks through fissures and cracks, is referred to as a sill. A sill does not form in thin air, and content or the magma is fed to it from a dike. The dike does not find any way to continue its upward journey and instead begins its lateral journey and later cools down into igneous rock that is referred to as a sill. The width of a sill is never more than a couple of meters, but it can continue up to hundreds of kilometers.
What is the difference between Dike and Sill?
• Both dikes and sills are subterranean geological formations that remain hidden from our eyes until they are visible because of continuous weathering and erosion of the top surface of the earth.
• When magma intrusion is along the preexisting rocks, the resulting formation is called a sill whereas when magma flows across the rocks, dike is formed.
• Mostly a sill is formed when a dike cannot go up further and starts to move horizontally. Thus, a sill is fed by a dike.
• Dikes and sills are rock formations resulting from volcanic activities and are always younger than their surrounding rocks.
• The different color of a dike or a sill from the surrounding rocks is a giveaway to volcanic activity.