Consider the following statements about rock structure: 1. Conglomerates are clastic sedimentary rocks. 2. Slate is a non-clastic sedimentary rock. 3. Granite is a plutonic igneous rock. 4. Basalt is an extrusive igneous rock. Which of the statements given above are correct?

Correct — D, 1, 3 and 4 only. Conglomerate is made of rounded pebbles cemented in a finer matrix, and because those pebbles are broken fragments of older rock it is a clastic, mechanically formed sedimentary rock — statement 1 holds. Granite cools slowly at depth, which is why its crystals are large enough to see, and that makes it plutonic or intrusive igneous — statement 3 holds. Basalt is the same magma cooled fast at the surface, giving a fine-grained extrusive igneous rock, and it is what the Deccan Traps are built of — statement 4 holds. Statement 2 is the false one, and it is false twice over: slate is not sedimentary at all but metamorphic, formed when shale is compressed under low-grade regional metamorphism, and the term non-clastic belongs to sedimentary rocks formed chemically or organically, such as limestone, gypsum, rock salt and coal.

• (a)1, 2 and 3 only — Carries statement 2 and drops basalt, although basalt is the standard textbook example of an extrusive igneous rock.
• (b)3 and 4 only — Names only the two igneous statements, discarding the accurate description of conglomerate as a clastic sedimentary rock.
• (c)1 and 2 only — Pairs a true statement with the false one, and both of the igneous statements are left out.

Rocks are classified by how they form. Igneous rocks solidify from magma, and split into intrusive or plutonic types that cool slowly at depth and are coarse-grained, and extrusive or volcanic types that cool quickly at the surface and are fine-grained. Sedimentary rocks are laid down in layers, and divide into clastic or mechanically formed ones built of transported fragments, and non-clastic ones precipitated chemically or built from organic remains. Metamorphic rocks are pre-existing rocks altered in place by heat, pressure or chemically active fluids, and slate, marble, quartzite and gneiss are the standard examples.

The false statement here is an unusually efficient one, because it misplaces slate on two different axes at once — wrong family and wrong sub-type. A candidate who knows the parent-and-product pairs of metamorphism will never fall for it: shale becomes slate, limestone becomes marble, sandstone becomes quartzite, granite becomes gneiss, and coal becomes graphite. Those five pairs are worth memorising as a block, because they answer a large share of the rock questions that appear in these papers, and they immediately expose any statement that files a metamorphic rock under sediments.

• Conglomerate is a clastic sedimentary rock of rounded pebbles; breccia is the same thing with angular fragments.
• Granite is intrusive or plutonic igneous, coarse-grained because it cooled slowly at depth; basalt is extrusive, fine-grained because it cooled fast at the surface.
• Slate is metamorphic, derived from shale by low-grade regional metamorphism, and splits along a cleavage that has nothing to do with the original bedding.
• Non-clastic sedimentary rocks are formed chemically or organically — limestone, gypsum, rock salt, geyserite and coal.
• The standard parent-and-product pairs are shale to slate, limestone to marble, sandstone to quartzite, granite to gneiss and coal to graphite.

• Filing slate under sedimentary rocks because it is layered; the layering is metamorphic cleavage, not bedding.
• Confusing non-clastic with metamorphic; non-clastic is a sub-type of sedimentary.
• Assuming granite and basalt differ in composition rather than in where and how fast they cooled.

As a four-statement classification item where one statement misfiles a rock into the wrong family, which is the single most common way this topic is set.

• practice — not a real PYQ

  Slate is formed by the metamorphism of

  • (a)limestone
  • (b)shale
  • (c)sandstone
  • (d)granite

  Answer(b) shale — limestone gives marble, sandstone gives quartzite and granite gives gneiss.

• practice — not a real PYQ

  Which one of the following is a non-clastic sedimentary rock?

  • (a)Sandstone
  • (b)Conglomerate
  • (c)Limestone
  • (d)Breccia

  Answer(c) Limestone — chemically or organically formed, while the other three are built of transported fragments.