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What Is a Sheet Intrusion?

Andrew Kirmayer
Andrew Kirmayer

A sheet intrusion is a rock formation generally created by the rising of molten rock, or magma, in between the top layers of the Earth’s crust. The top layers can expand as a result, and the underlying material is sometimes exposed because of surface erosion. Rock sheets are usually somewhat flat in shape, but can take on an undulating or dome-like structure; geological processes can alter a relatively flat sheet intrusion into a more vertical position. The intrusion also usually appears different from material that originated as flowing lava, by incorporating components of the surrounding rock into the top and bottom layers.

Rising magma typically causes heated material to crystallize as it cools, forming igneous rock. It can form dikes, or sheet-like formations that cut across layers; a sheet intrusion, however, is generally oriented across, over, or under the landscape. Both variations are often seen in volcanically active areas, as well as places known to have been so in the past.

A rock formation that is created by the rising of magma in between the top layers of the Earth's crust is referred to as a sheet intrusion.
A rock formation that is created by the rising of magma in between the top layers of the Earth's crust is referred to as a sheet intrusion.

One type of sheet intrusion is called a laccolith. The molten material is typically pushed up between layers of overlying sedimentary rock, which in turn rises because of the force. A laccolith’s base is usually horizontal and the rock normally cools slowly. By the time these are discovered, the overlying layers have usually eroded away and it can be difficult to determine the original shape of the intrusion. Some of these structures or dome-shaped or they can be vertical in nature; wind and water often remove many layers of rock over time.

A sill is generally a kind of sheet intrusion that sits flat in between two other layers of rock, or even solidified lava. The material that is part of this structure usually forms deep underground, so pressure typically prevents gases from escaping while it hardens. Bubble-like formations that are common in lava are generally not seen in sills, which are protected for much of their life by the layers of rock surrounding them.

Computerized seismic mapping is often used to see where a sheet intrusion is. Sills and laccoliths can be mapped fully and the flow pattern of an underground sheet intrusion can be determined as well. The pressure during formation of the structure in the host rock, and how much space there was in the sediment when it formed, can be determined as well. Molten rock movement and erosion have led to the formation of many different features of landscapes throughout the world.

Frequently Asked Questions

What is a sheet intrusion in geological terms?

A sheet intrusion, also known as a sill or dyke, is a tabular body of igneous rock that forms when magma intrudes into a crack then solidifies between older layers of rock. Sills are parallel to the existing rock layers, while dykes cut across them. These formations are significant as they can provide insights into the history of Earth's geological processes.

How does a sheet intrusion differ from other types of intrusions?

Sheet intrusions are distinct in their thin, planar nature, contrasting with other types such as plutons or batholiths, which are massive and irregularly shaped. While sheet intrusions are typically horizontal (sills) or vertical (dykes), plutons and batholiths form large, deep-seated blobs within the Earth's crust, often exposed over time through erosion.

What types of rocks are commonly found in sheet intrusions?

Sheet intrusions are primarily composed of igneous rocks, such as basalt and gabbro when formed from mafic magma, or granite and diorite in the case of felsic magma. The specific rock type depends on the chemical composition of the magma and the rate at which it cooled, which affects the size of the crystals within the rock.

Can sheet intrusions be seen on the Earth's surface?

Yes, sheet intrusions can be exposed on the Earth's surface through the process of erosion, which removes the overlying material. When visible, they often appear as prominent, linear features in the landscape. The famous Palisades Sill along the Hudson River in New York is a well-known example of a sill visible at the surface.

What role do sheet intrusions play in the Earth's crust?

Sheet intrusions play a crucial role in reinforcing the Earth's crust, adding new material and altering its composition. They can act as conduits for magma movement, influence rock deformation, and contribute to the formation of mineral deposits. Their study helps geologists understand tectonic processes and the evolution of the Earth's lithosphere.

How are sheet intrusions important for economic geology?

Sheet intrusions are significant for economic geology as they often contain valuable mineral deposits, including metals like copper, platinum, and nickel. For instance, the Bushveld Igneous Complex in South Africa, a large sheet intrusion, is one of the richest sources of platinum-group metals in the world, vital for various industries.

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    • A rock formation that is created by the rising of magma in between the top layers of the Earth's crust is referred to as a sheet intrusion.
      By: kmiragaya
      A rock formation that is created by the rising of magma in between the top layers of the Earth's crust is referred to as a sheet intrusion.