Sill

Definition

A sill is a tabular sheet of igneous rock that forms when magma intrudes into existing layers of rock and solidifies horizontally between them. This formation typically occurs at shallow depths within the Earth's crust, and sills can be found in various geological settings, providing insights into the history and movement of magma beneath the surface.

5 Must Know Facts For Your Next Test

1. Sills can vary in thickness from a few centimeters to hundreds of meters, depending on the conditions of their formation.
2. The texture of a sill can range from fine-grained to coarse-grained, reflecting the cooling rate of the magma as it solidifies between existing rock layers.
3. Sills can often be traced laterally over great distances, providing valuable information about the extent and movement of magma within the crust.
4. They may be associated with volcanic activity but do not erupt at the surface; instead, they remain hidden beneath layers of older rock.
5. Studying sills helps geologists understand the processes of magma intrusion and the evolution of igneous rock formations over geological time.

Review Questions

• How do sills differ from dikes in terms of their formation and orientation?
  • Sills and dikes are both types of intrusive igneous rock formations but differ mainly in their orientation. Sills form horizontally between layers of pre-existing rock, while dikes intrude vertically or at steep angles across those layers. This difference in orientation affects how they interact with surrounding rock and their overall geological significance.
• Discuss the role of sills in understanding the geological history and processes occurring within the Earth's crust.
  • Sills play a crucial role in piecing together the geological history by providing evidence of past magmatic activity and conditions within the Earth's crust. Their presence indicates where magma was able to intrude into existing rock formations, offering insights into the temperature and pressure conditions during formation. Additionally, studying sills helps geologists track magma movement patterns over time, enhancing our understanding of tectonic processes.
• Evaluate the significance of sills in relation to other intrusive structures like batholiths and how they contribute to our understanding of igneous processes.
  • Evaluating sills alongside other intrusive structures like batholiths reveals essential aspects of igneous processes. Sills are often smaller and less massive than batholiths but provide critical information about shallow magmatic activity. By studying both sills and batholiths, geologists can better comprehend how varying depths of magma intrusion affect mineral composition, texture, and overall geological formations. This comparative approach enhances our understanding of the dynamics within the Earth's crust and the evolution of its features.

Related terms

Dike: A dike is a vertical or steeply inclined sheet of igneous rock formed when magma intrudes into pre-existing rock layers and solidifies, typically cutting across the layers.

Batholith:

A batholith is a large mass of intrusive igneous rock that forms from the slow cooling of magma beneath the Earth's surface, often creating a significant geological feature.

Pluton:

A pluton is a general term for any body of intrusive igneous rock, including sills and dikes, that has formed from cooled magma at depth.