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Define Sill

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In Geology, Sill, also known as a sheet, is a flat intrusion of igneous rock that forms between the preliminary layers of rocks and has a large horizontal extent when compared with its thickness. To define sill, let us understand that it is a concordant intrusive sheet. This implies that it does not cut across preliminary rock beds. Sills can often resemble volcanic flows that were interspersed with the sedimentary unit. 

Sill Rock Meaning

Before learning what is a sill, let us understand the meaning of sill rock. A sill is a flat sheet-like igneous rock mass that is formed when magma intrudes into between the older layers of rocks and crystallizes. Sills can form from magma with a range of silica contents. 

A renowned example of the sill is the tabular mass of quartz trachyte found near the summit of Engineer Mountain near Silverton, Colorado.

What is a Sill?

A sill is a tabular sheet intrusion that is formed between the older layers of sedimentary rocks, beds of volcanic lava or tuff, along the direction of foliation in metamorphic rocks. Sills occur in parallel to the bedding of other rocks that surround them, and though they may have vertical to horizontal orientation, horizontal sills are most commonly found. Sills may weigh a fraction of an inch to hundreds of feet thick and up to hundreds of miles long. 

The rock compositions of different types are found in sills. The famous basic sills get much attention for the knowledge they provide related to the crystallization behavior of molten magmas.

Sills are split based on the number of intrusions that are formed and the variability of the types of rocks that are involved. A simple sill is formed by a single intrusion whereas multiple sills are formed by two or more intrusions. A composite sill is composed of more than one type of rock found between the older layers of rocks during more than one intrusive period.

Sill V/S Dykes

Both dykes and sills are geological formations that are made up of igneous rocks. These rocks are formed when the hot magma from the hot core or mantle of the Earth gets released upward through cracks, fissures, or joints. This magma does not reach the surface of the Earth in the case of the Sill and dyke as in the case with Lava that exploded from the opening of a volcano. Hence, sill and dyke are the results of the cooled down magma before it reaches the surface of the Earth. Understanding the difference between dyke and sill is important for students of Volcanology.


What is the Difference Between Dykes and Sills?

  • Both dykes and sills are rock formations resulting from volcanic activities and are younger than their surrounding rocks.

  • The different colors of both a dyke or a sill from the surrounding rocks are exposure to volcanic activity.

  • A sill is generally formed when the dyke cannot move upward further and starts to move horizontally. Hence, a sill is fed by a dike.

  • When magma intrusion is found along the preexisting rocks, then the resulting formation is known as sill whereas when magma flows across the rocks, the dyke is formed.

  • Both dykes and sills are subterranean geological formations that remain hidden from our eyes until they are visible due to the continuous weathering and person of the top surface of the Earth. 

Did You Know?

  • Sills are considered to be a horizontal intrusion.

  • Sills are formed underground and hence composed of plutonic igneous rocks.

  • The texture of a sill ranges from aphanitic to phaneritic.  

  • All the types of rock formations are found in sills.

  • The measures of a sill range from a fraction of an inch to hundreds of feet thick and up to hundreds of miles long.

FAQ (Frequently Asked Questions)

1. What are Igneous Rocks?

Ans: The igneous rock, also known as the magmatic rock is one of the three types of rocks, the other being a sedimentary and metamorphic rock. Igneous rock is formed through the solidification and cooling of magma or lava. Igneous rock can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic). Igneous rock can occur in a broad range of geological settings including platforms, orogens, basins, extended crust, and oceanic crust).

2. What is a Sheet Intrusion?

Ans: A sheet intrusion or tabular intrusion is a plain sheet of approximately the same thickness, that forms inside a pre-existing rock. When sheet intrusion cuts into another unlayered mass, or across layers, it is known as ‘dike’ whereas when sheet intrusion is formed between layers in a layered rock mass is known as a ‘sill’.

3. What are Transgressive Sills?

Ans: Despite the concordant nature of sills, many large sills change stratigraphic levels within the intruded sequence, with each concordant part of intrusion joined by relatively short dye-like segments known as transgressive sills. Whin sills and sills within the Karoo basin are examples of transgressive sills.