Question

What is the specific name for a tabular igneous intrusion that runs parallel to the bedding planes of the country rock?

• A. Dyke
• B. Sill
• C. Laccolith
• D. Batholith

Hint:

Sill = \textbf{Same direction as layers} (parallel). Dyke = \textbf{Different direction} (cuts across layers).

Answer 1

The Correct Option is B

Concept: Igneous intrusions are bodies of magma that solidify beneath the Earth's surface. They are classified based on their shape and relationship with surrounding rock layers (country rock):

• Dyke: Cuts across bedding planes (discordant intrusion).
• Sill: Runs parallel to bedding planes (concordant intrusion).
• Laccolith: Dome-shaped intrusion causing uplift of overlying layers.
• Batholith: Massive, irregular deep-seated intrusion.

Step 1:Understand the meaning of "parallel to bedding planes".
If an intrusion follows the existing layers of rock without cutting across them, it is called concordant.
Step 2:Identify the correct type of concordant intrusion.
A tabular (sheet-like) intrusion that lies parallel to bedding planes is known as a sill.
Step 3:Differentiate from similar structures.

• Dyke is also tabular but cuts across layers (discordant).
• Laccolith is dome-shaped, not strictly tabular.
• Batholith is large and irregular.

Step 4:Select the correct answer.
Thus, the correct term is sill.