Structure of the Earth and Rocks: Layers, Waves and Types

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 C, limestone. Limestone is a sedimentary rock made largely of calcium carbonate, and when heat and pressure recrystallise it without melting it, the result is marble, the stone used in the Taj Mahal and in the Dilwara temples.

Option A, sandstone, turns into quartzite under metamorphism, a rock so hard that it resists weathering and forms ridges. Option B, shale or clay, turns into slate, which splits into thin sheets and was long used for roofing and writing boards. Option D, granite, an intrusive igneous rock, turns into gneiss, recognised by its banded appearance. Since one or another of these pairs appears in nearly every paper, revise them as a single list: limestone to marble, sandstone to quartzite, shale to slate, granite to gneiss and coal to graphite.

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.

The correct answer is B, the Lehmann discontinuity. It marks the change from the molten outer core to the solid inner core, and is named after Inge Lehmann, who deduced from the behaviour of earthquake waves that the innermost part of the planet is solid.

Option A, the Repetti discontinuity, lies far above it, between the upper and the lower mantle. Option C, the Moho or Mohorovicic discontinuity, is the crust and mantle boundary and is the shallowest of the named surfaces after the Conrad. Option D, the Conrad discontinuity, lies within the crust itself, dividing its upper and lower parts. Candidates lose this mark by learning only two names, so list all five from the surface downward: Conrad, Moho, Repetti, Gutenberg and Lehmann, and attach one depth to the list, the 2,900 kilometres of the Gutenberg surface, since that figure anchors every other layer in the diagram.

The correct answer is A, an intrusive igneous rock. Granite forms when magma cools slowly deep below the surface, which gives the crystals time to grow, so the rock is coarse and speckled. Such rocks are also called plutonic, and granite is the classic example used in textbooks.

Option B, extrusive igneous rock, describes lava that cools quickly on the surface, giving fine-grained rocks such as basalt and the frothy pumice that floats on water. Option C, chemically formed sedimentary rock, covers rock salt and gypsum, deposited when mineral-rich water evaporates. Option D, foliated metamorphic rock, covers banded rocks such as gneiss and schist; gneiss is in fact what granite becomes after metamorphism. So one rock name can move between families, and the process named in the question decides the answer.

The correct answer is B, the shadow zone. The core bends P waves sharply and stops S waves altogether, so a belt of the Earth surface receives neither type of wave from a given earthquake. Beyond about 105 degrees no S waves arrive at all, and the belt between roughly 105 and 145 degrees receives no P waves either.

Option A, the focus or hypocentre, is the point inside the Earth where the rupture begins, with the epicentre lying vertically above it on the surface. Option C, the Benioff zone, is the inclined belt of earthquakes along a subducting plate, a term from plate tectonics. Option D, a fault zone, is the fractured belt along which rocks slip, the setting of an earthquake rather than a record of one. This question is asked almost every year, so the two angle figures are worth memorising exactly.

The correct answer is B, the asthenosphere. It lies in the upper mantle just below the rigid outer shell, is weak and partly molten, and is the main source of the magma that reaches the surface through volcanoes; the plates slide over it.

Option A, the lithosphere, is that rigid outer shell itself, made of the crust plus the solid uppermost mantle, and it is what the plates are made of rather than what they move on. Option C, the barysphere, is an older term for the heavy central part of the Earth, that is the core. Option D, the biosphere, is the zone of life where land, water and air meet, and has nothing to do with the interior. The pair lithosphere and asthenosphere is the trap here, so fix the idea that the rigid one rides on the weak one.

The correct answer is A, graphite. Coal is a sedimentary rock formed from buried plant matter, and prolonged heat and pressure drive off its volatile content and rearrange the carbon, first into harder anthracite and finally into graphite, a metamorphic rock of nearly pure carbon.

Option B, slate, is the metamorphic form of shale or clay, not of coal. Option C, quartzite, comes from sandstone and is among the hardest common rocks. Option D, gneiss, comes from granite and shows alternate light and dark bands. Note also that graphite and diamond are both carbon, which is why this question sometimes appears in general science as an example of allotropes. The list of parent and product pairs is short, so learning it once settles several questions across subjects.

The correct answer is D, the core. Nife is formed from the chemical symbols of nickel and iron, the two metals believed to make up the central part of the planet, and the flow of that metal in the liquid outer core is what generates the Earth magnetic field.

Option A, the continental crust, is called Sial, from silica and aluminium, because those are its dominant constituents and its density is low. Option B, the oceanic crust, is called Sima, from silica and magnesium, and is thinner but denser than the continental crust. Option C, the mantle, is the largest layer by volume and is rich in silicates of iron and magnesium, but the Nife label is not used for it. Learn the three short names together, Sial, Sima and Nife, from the surface downward.