NDA (I) 2023 GAT Q135: Which one of the following is the lowermost/innermost

Q: What is the answer to NDA (I) 2023 GAT Q135?

B. Correct — B, Batholith. A batholith is a very large, deep-seated (plutonic) mass of intrusive igneous rock that forms the lowermost/innermost intrusive body, cooling slowly far below the surface. The other forms (laccolith, lopolith, phacolith) are shallower intrusions.

NDA (I) 2023 General Ability Test · Question 135 of 149

Question text verified against the official NDA question paper.

Which one of the following is the lowermost/innermost intrusive igneous rock?

(a)Laccolith
(b)Batholith
(c)Lopolith
(d)Phacolith

Answer

Why

Correct — B, Batholith. A batholith is a very large, deep-seated (plutonic) mass of intrusive igneous rock that forms the lowermost/innermost intrusive body, cooling slowly far below the surface. The other forms (laccolith, lopolith, phacolith) are shallower intrusions.

Why the others are wrong

(a)Laccolith — A laccolith is a shallower, dome-shaped intrusion that pushes the overlying strata upward; it sits above a batholith, not at the deepest level.
(c)Lopolith — A lopolith is a large saucer-shaped (basin-like) intrusion; it is not the deepest, innermost body.
(d)Phacolith — A phacolith is a small lens-shaped intrusion occupying the crest or trough of a fold — a shallow, minor form.

Concept

Intrusive (plutonic) igneous rocks form when magma cools within the crust. They take characteristic shapes: batholiths (huge, deep discordant masses), laccoliths (dome-shaped, concordant), lopoliths (saucer-shaped), phacoliths (lens-shaped in folds), plus sheet-like sills (concordant) and dykes (discordant). The batholith is the largest and deepest-seated of these bodies.

'Lowermost/innermost intrusive' points to the deep-seated batholith, the foundation from which shallower intrusions branch. The other options are progressively shallower and smaller forms.

Key facts

A batholith is a very large, deep-seated intrusive igneous body.
It cools slowly at depth, forming coarse-grained rock like granite.
Laccoliths, lopoliths and phacoliths are shallower intrusions.
Sills (concordant) and dykes (discordant) are sheet-like intrusions.

Intrusive igneous forms (deep -> shallow)

Batholith→very large, deepest-seated

Laccolith→dome-shaped, uplifts strata

Lopolith→saucer/basin-shaped

Phacolith→lens in a fold crest/trough

Study next

Intrusive vs extrusive igneous rocksBatholith, laccolith, lopolith, phacolithSills and dykesIgneous rock textures and coolingLandforms from intrusions

Common traps

Confusing the dome-shaped laccolith with the deep batholith.
Assuming a small lens (phacolith) could be the innermost body.

Asked as identifying the deepest/largest intrusion, or matching intrusive forms to shapes.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
A dome-shaped intrusion that arches up the overlying strata is a
- (a)batholith
- (b)laccolith
- (c)sill
- (d)dyke
Answer(b) laccolith.
practice — not a real PYQ
A concordant, sheet-like intrusion parallel to rock layers is a
- (a)dyke
- (b)sill
- (c)batholith
- (d)phacolith
Answer(b) sill.