Marble is a metamorphic rock formed from which of the following parent rocks?

The correct answer is B, the Mohorovicic discontinuity. Named after the Croatian seismologist who noticed that earthquake waves suddenly speed up at a certain depth, it marks the base of the crust and the top of the mantle, and is commonly shortened to the Moho.

Option A, the Conrad discontinuity, lies inside the crust itself, between its lighter upper part and its denser lower part. Option C, the Gutenberg discontinuity, lies much deeper, at about 2,900 kilometres, and separates the mantle from the outer core; it is the usual distractor in this question. Option D, the Lehmann discontinuity, separates the liquid outer core from the solid inner core and is named after the Danish seismologist who identified the inner core. Revise the four names in order of depth, from Conrad to Lehmann, and the matching questions become easy marks.

The correct answer is B, secondary or S waves. They are transverse waves, in which particles vibrate at right angles to the direction of travel, and such motion cannot be sustained in a fluid, so S waves die out at the outer core. Their absence beyond about 105 degrees from an epicentre is the main evidence that the outer core is liquid.

Option A, P waves, are longitudinal, the fastest of all and the first to reach a seismograph, and they travel through solids, liquids and gases alike. Option C, L waves, are surface waves that arrive last but shake buildings the most and are responsible for the greatest damage in an earthquake. Option D is wrong because only one of the three is stopped by liquid. The property, the order of arrival and the destructive power of each wave are the three points papers combine here.

The correct answer is B, basalt. The Deccan Trap was formed by vast fissure eruptions in which lava spread over the surface and cooled quickly, giving a fine-grained extrusive igneous rock, basalt. Its weathering produces the black regur or cotton soil of Maharashtra, Madhya Pradesh and Gujarat.

Option A, granite, is also igneous but intrusive: it cools slowly deep inside the crust and therefore has large visible crystals. Option C, limestone, is a sedimentary rock of marine origin used in cement manufacture, not a lava rock. Option D, gneiss, is a metamorphic rock of the old peninsular shield, banded in appearance and derived from granite. The examiner is testing two ideas at once here, that basalt is extrusive and that the Deccan lava flows are the parent of black soil.

The correct answer is B, sedimentary rocks. They are built up layer by layer as sediment settles over the remains of living things and hardens gently, so shells, bones and plant matter survive as fossils. The same layered basins hold coal and petroleum, which makes this family the most valuable one economically.

Option A, igneous rocks, form from molten magma or lava at temperatures that destroy every trace of organic material. Option C, metamorphic rocks, may begin as fossil-bearing sedimentary rock, but the heat and pressure of metamorphism deform and erase the fossils. Option D, plutonic rocks, is only another name for intrusive igneous rocks such as granite, so it fails for the same reason as option A. Remember that layering, or stratification, and fossils always point to the sedimentary family.

The correct answer is C, oxygen. Almost half the weight of the crust, about 46.6 per cent, is oxygen, because it is locked into the silicate minerals of which nearly all crustal rocks are made rather than existing as free gas.

Option A, silicon, comes second at about 27.7 per cent, and together with oxygen it forms the silica that gives the crust its character. Option B, aluminium, is third at about 8.1 per cent, and it is the most abundant metal in the crust, a fact often asked as a separate question. Option D, iron, follows at about 5 per cent in the crust, though it is the dominant element of the core, which is called Nife for nickel and iron. Keep two answers ready: oxygen for the most abundant element and aluminium for the most abundant metal.