In igneous geology, an intrusion of magma that spreads and solidifies parallel to the existing bedding plane of sedimentary rocks is called a what?

Introduction / Context:
Intrusive igneous bodies are classified according to their size, shape, and relationship with surrounding rock layers. Knowing the difference between features like dykes, sills, batholiths, and laccoliths is an important part of structural and physical geography. This question focuses specifically on the intrusive form that follows or parallels the bedding planes of pre existing sedimentary rocks.

Given Data / Assumptions:

• Magma intrudes into existing layered sedimentary rocks.
• The intrusion runs parallel to the bedding planes rather than cutting across them.
• Options include dyke, sill, batholith, and laccolith.
• We assume standard definitions used in geomorphology and igneous geology.

Concept / Approach:
A sill is an intrusive sheet of igneous rock that is emplaced parallel to the bedding or layering of the host rocks. In contrast, a dyke cuts across bedding planes, a batholith is a very large, irregular deep seated intrusive body, and a laccolith is a dome shaped intrusion that pushes overlying layers upward. Because the question describes magma intruding along a bedding plane, the correct term for this concordant intrusion is sill.

Step-by-Step Solution:
Step 1: Identify the key phrase in the question: the magma intrusion follows or runs along the bedding plane of existing rocks. Step 2: Recall that concordant intrusions that lie parallel to bedding are called sills. Step 3: Dyke is the name given to discordant intrusions that cut across bedding and joints, so it does not fit the description of following the bedding plane. Step 4: Batholith refers to a very large, deep seated intrusive body with no simple sheet like relationship to bedding planes. Step 5: Laccolith describes an intrusion that has a lens or dome shape, lifting overlying layers but not simply lying flat along them as a thin sheet. Step 6: The only intrusive body that correctly matches a sheet of magma intruding parallel to bedding is a sill.

Verification / Alternative check:
Geology textbooks and geomorphology diagrams typically show sills as horizontal or gently inclined sheets running along sedimentary bedding, while dykes cut vertically or steeply through the layers. Famous examples include the Great Whin Sill in northern England. The definitions of batholith and laccolith always emphasise their large size or doming effect rather than simple concordant sheet like geometry, confirming that sill is the correct technical term here.

Why Other Options Are Wrong:

• Dyke: A discordant intrusion cutting across bedding planes, not parallel to them.
• Batholith: A massive, irregular intrusive body, usually deep seated and not limited to a single bedding plane.
• Laccolith: A dome shaped concordant intrusion that lifts overlying strata, different in form from a thin sheet like sill.

Common Pitfalls:
Students sometimes confuse sills and dykes because both are tabular intrusions. The key difference is orientation with respect to bedding: sills are parallel, and dykes cut across. Others may choose laccolith simply because it also lies roughly along layers, overlooking the specific dome shape required for that term. To avoid errors, remember the simple rule: sheet parallel to bedding is a sill, sheet cutting across bedding is a dyke.

Final Answer:
An intrusion of magma along a bedding plane is called a sill.