Which one of the following is not an igneous rock ?
- (a)Granite
- (b)Slate
- (c)Basalt
- (d)Gabbro
Correct — B, Slate. Igneous rocks form from the cooling and solidification of molten magma or lava. Granite (coarse-grained, cooled slowly at depth), basalt (fine-grained, from lava at the surface) and gabbro (the coarse-grained deep equivalent of basalt) are all igneous. Slate is the odd one out: it is a metamorphic rock, formed when the sedimentary rock shale is compressed and heated, so it is not igneous.
- (a)Granite — Granite is a classic coarse-grained igneous rock, formed when magma cools slowly deep underground — so it belongs to the igneous group and is not the answer.
- (c)Basalt — Basalt is a fine-grained igneous rock formed from lava cooling quickly at the surface (it makes up the Deccan Traps) — igneous, so not the exception.
- (d)Gabbro — Gabbro is an igneous rock — the coarse-grained, deep-seated equivalent of basalt — so it too is igneous, not the odd one out.
Rocks are grouped by how they form. Igneous rocks solidify from molten magma or lava; sedimentary rocks form by the deposition and compaction of sediments; metamorphic rocks form when existing rocks are transformed by heat and pressure. Slate is metamorphic, produced from shale, while granite, basalt and gabbro are all igneous.
Scan the list for the rock that is not born of cooling magma. Granite, basalt and gabbro are textbook igneous rocks; slate is the give-away metamorphic rock (shale → slate). Remembering the shale-to-slate transformation quickly isolates the answer.
- Igneous rocks solidify from magma (intrusive, e.g. granite, gabbro) or lava (extrusive, e.g. basalt).
- Slate is a metamorphic rock formed from shale under heat and pressure.
- Gabbro is the coarse, deep-seated equivalent of fine-grained basalt — both are igneous.
- Basalt forms the Deccan Traps of peninsular India.
- Confusing gabbro with a non-igneous rock — it is simply coarse-grained basalt.
- Forgetting that slate comes from shale (metamorphic), not from cooling magma.
NDA and UPSC give a list of rocks and ask which one is not igneous / not metamorphic / not sedimentary — know the common members of each class and the standard metamorphic pairs.
Quartzite is metamorphosed from
- (a) Limestone
- (b) Obsidian
- (c) Sandstone
- (d) Shale
Answer(c) Sandstone
Tests the metamorphic pairs directly — sandstone→quartzite, and (in its explanation) shale→slate, the very transformation that makes slate metamorphic rather than igneous.
Consider the following statements made about the sedimentary rocks: I. Sedimentary rocks are formed at Earth's surface by the hydrological system. II. The formation of sedimentary rocks involves the weathering of pre-existing rocks. III. Sedimentary rocks contain fossils. IV. Sedimentary rocks typically occur in layers. Which of these statements are correct?
- (a) I and II
- (b) I and IV
- (c) II, III and IV
- (d) I, II, III and IV
Answer(d) I, II, III and IV
Tests the defining features of the sedimentary class — the same rock-classification theme, here for the sedimentary parent (shale) whose metamorphism produces slate.
Which one of the following is an example of both an igneous as well as a metamorphic rock?
- (a) Gabbro
- (b) Geyserite
- (c) Gneissoid
- (d) Granite
Answer(d) Granite
Another rock-classification item turning on igneous versus metamorphic — the same distinction that marks slate (metamorphic) as the non-igneous rock here.
- practice — not a real PYQ
Which one of the following is a metamorphic rock?
- (a)Basalt
- (b)Marble
- (c)Granite
- (d)Sandstone
Answer(b) Marble — it is metamorphosed limestone; basalt and granite are igneous and sandstone is sedimentary.
- practice — not a real PYQ
Basalt is an example of which type of rock?
- (a)Sedimentary
- (b)Metamorphic
- (c)Igneous (extrusive)
- (d)Igneous (intrusive)
Answer(c) Igneous (extrusive) — basalt forms from lava cooling quickly at the Earth's surface.