Question

What is the difference between a concordant tabular intrusion and a discordant tabular intrusion?

Short answer

A concordant tabular intrusion runs parallel to rock layers; a discordant tabular intrusion cuts across them.

Step-by-step solution

Understand Tabular Intrusions

Tabular intrusions are sheet-like bodies of igneous rock that have intruded into pre-existing rock formations.

Define Concordant Tabular Intrusion

A concordant tabular intrusion, also known as a sill, is a sheet-like body of igneous rock that intrudes parallel to the existing rock layers. It does not cut across the stratification of the surrounding rock.

Define Discordant Tabular Intrusion

A discordant tabular intrusion, or a dike, is a sheet-like body of igneous rock that cuts across the existing rock layers. It intrudes at an angle to the bedding planes of the surrounding rock.

Summarize the Difference

The key difference is that a concordant tabular intrusion (sill) runs parallel to the existing rock layers, while a discordant tabular intrusion (dike) cuts across them.

Key concepts

Concordant Tabular Intrusion

A concordant tabular intrusion, also known as a sill, is an igneous rock formation that aligns itself with pre-existing layers of rock. Imagine a molten rock pushing itself into the space between sedimentary layers deep within Earth's crust. It solidifies in this parallel alignment without disturbing the original stratification of the rock layers above and below it.

Key features of concordant tabular intrusions include:

• Alignment parallel to existing rock layers.
• Formation typically seen within sedimentary layers.
• Not cutting or breaking through other rock formations.

This creates a distinct separation line, but harmony or concordance with the surrounding geological structures remains intact. Knowing this helps in identifying sills within the larger context of how Earth's crust is structured and formed.

Discordant Tabular Intrusion

In contrast, a discordant tabular intrusion, which is often referred to as a dike, behaves differently. Instead of settling parallel to pre-existing rock layers, this igneous rock forcefully cuts through the layers. Visualize molten rock making a vertical or angled cut across a stack of layered rock.

Here are some defining characteristics of discordant tabular intrusions:

• Orientation that is perpendicular or at an angle to existing rock layers.
• Capability to cut across multiple layers of different compositions.
• Formation resulting from the rock forcefully breaking through the surrounding rock.

This intrusion disrupts and creates noticeable geological features, making dikes a significant point of interest in the study of geological formations. Geologists can trace the path of these intrusions to understand the forces that shaped a particular geological region.

Igneous Rock Formations

Igneous rock formations are formed from the cooling and solidification of magma or lava. These rocks can form either below the Earth's surface as intrusive rocks or on the surface as extrusive rocks. Tabular intrusions like sills and dikes represent just a small part of the diverse ways that igneous rocks can form.

Major types of igneous rock formations include:

• Intrusive (Plutonic) Rocks: Formed deep within the Earth. Examples include granite and diorite.
• Extrusive (Volcanic) Rocks: Formed on the Earth's surface. Examples include basalt and pumice.

Each type of rock gives clues about the geological processes and environments that existed at the time of its formation. For example:

• Sills are often associated with low-viscosity magma that can spread easily.
• Dikes can signal rapid movement and cooling of magma.
• Surface formations like lava flows can indicate volcanic activity.

Understanding these rock types provides insights into the history of Earth’s crust and allows geologists to reconstruct past geological events and environments.