Define Sill
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.
FAQs on Sill
1. What is a sill in geography?
A sill is a type of igneous intrusion where magma pushes its way between existing layers of sedimentary rock, volcanic lava, or tuff. Because it forms parallel to the pre-existing rock layers, it is known as a concordant tabular intrusion. Sills are typically horizontal or gently dipping, following the orientation of the surrounding rock strata.
2. How does a sill form within existing rock layers?
A sill forms when magma from beneath the Earth's surface rises but does not reach the surface to erupt. Instead, it finds a path of least resistance, which is often along the horizontal weaknesses between layers of rock, known as bedding planes. The pressure of the magma forces these layers apart, and the magma then spreads out laterally, cooling and solidifying to form the sill.
3. What is the main difference between a sill and a dyke?
The main difference lies in their orientation relative to the surrounding rock layers. A sill is a concordant intrusion, meaning it runs parallel to the bedding planes of the host rock. In contrast, a dyke is a discordant intrusion, meaning it cuts across the existing rock layers, typically in a vertical or near-vertical sheet.
4. What are the different types of sills found in nature?
Sills can be classified based on their structure and composition. The main types include:
- Simple Sills: Formed from a single injection of magma.
- Multiple Sills: Consist of two or more injections of the same type of magma, separated by thin screens of country rock.
- Composite Sills: Formed from multiple injections of different types of magma, leading to varied compositions within the same sill structure.
- Differentiated Sills: Large sills where slow cooling allows different minerals to crystallise and settle at different levels, creating layers within the sill itself.
5. Can you give a real-world example of a major sill?
A famous example of a major sill is the Whin Sill in northern England. This quartz-dolerite sill is extensive and forms several prominent landscape features, including parts of Hadrian's Wall and High Force waterfall. Another classic example is the Palisades Sill in the USA, which is visible along the Hudson River.
6. Why are sills considered important for geological studies?
Sills are geologically important for several reasons. They provide valuable information about the composition of magma deep within the Earth. The heat from a sill can also alter the surrounding rock, creating metamorphic rocks in a process called contact metamorphism. Furthermore, the resistance of sill rock to erosion can lead to the formation of distinct landforms like mesas, escarpments, and waterfalls.
7. What makes a sill 'transgressive' if they are supposed to be parallel to rock layers?
While sills are fundamentally concordant, a transgressive sill is one that locally cuts across stratigraphic levels. This happens when the magma, while moving along a bedding plane, encounters a blockage or a change in rock structure. It then breaks across the layers in a short, steep, dyke-like segment before continuing along a different, parallel bedding plane. This creates a step-like appearance in the intrusion's path.
8. In geology, is it spelled 'sill' or 'cill'?
In geological and geographical contexts, the correct and standard spelling for this type of igneous intrusion is 'sill'. The spelling 'cill' is an archaic variant and is now considered incorrect in scientific literature. The term 'cill' (often spelled with a 'c') is more commonly associated with the base of a window or door in construction.
