Which of the following igneous intrusions is horizontally inclined?

Analyze the question: It asks which igneous intrusion lies horizontally, meaning it runs parallel to the layers of the surrounding rock.

Explain the concept: Intrusive igneous features form when magma cools underground without reaching the surface. Their shapes differ:

• Sill: A sheet of magma that squeezes between rock layers (bedding planes) and hardens. It is horizontal or concordant, meaning it lies parallel to the bedding.
• Dyke: A sheet of magma that cuts across the rock layers. It is vertical or steeply inclined (discordant).
• Laccolith: Magma forces its way between layers and pushes the overlying rock up into a dome. It has a flat base and a domed or mushroom-like top, so it is not simply horizontal.
• Batholith: A huge, irregular mass of intrusive rock (like granite) that forms deep underground. It is a large body, not a sheet, and it has no particular horizontal inclination.

Solution:

1. Dyke: vertical or cross-cutting, so it is eliminated.
2. Batholith: a massive, irregular body, so it is eliminated.
3. Laccolith: dome-shaped, so it is eliminated. It does have a horizontal base, but its defining feature is the bulge.
4. Sill: a flat, horizontal sheet parallel to the rock layers, so it fits.

Answer: A. Sill

Exam tip: Remember that a sill is sleeping flat (horizontal), while a dyke is like a divider wall standing upright and cutting across the layers.