Interior structure of the earth

Seismology reveals internal structure using waves. Exam tip: write it as the most important indirect source.

Low velocity may indicate weak or partly molten material. Exam tip: connect low velocity zone with asthenosphere.

Mines go to very small depths compared with Earth radius. Exam tip: consider mines as limited direct sources.

The lithosphere is Earth’s rigid outer layer, consisting of the crust and the uppermost rigid part of the mantle. This rigid layer breaks into tectonic plates. The asthenosphere below it is relatively soft and plastic, allowing plates to move over it; it does not form the rigid plates themselves. Exam tip: remember that tectonic plates are lithospheric plates.

The direct answer is A: the core contains very dense metallic materials. Earth’s mass depends mainly on how much matter it has and how densely that matter is packed. The core is made chiefly of iron and nickel, which are dense metals. Thus, although the core occupies a smaller volume than the mantle, its material contributes greatly to Earth’s total mass. B is wrong because being the thinnest layer would not itself explain a large mass contribution. C is wrong because the core is not gas; it has a solid inner part and a liquid outer part. D is wrong because ocean water is not the material of the core. The reasoning is: dense iron-nickel material → high density → considerable mass. Exam cue: mass is linked with density, not merely thickness.

Waves refract when velocity changes in different media. Exam tip: connect refraction with identification of layers.

Earth internal structure is understood through combined evidence from many sources. Exam tip: mention both direct and indirect evidence.

At depth rocks may remain solid yet behave ductile over long time. Exam tip: do not write mantle as fully liquid.

Mechanical division mainly depends on material behaviour. Exam tip: write chemical and mechanical bases separately.

Arrival time helps estimate wave path and speed. Exam tip: connect it with indirect evidence of internal structure.

Pressure and heavy materials increase with depth. Exam tip: link density increase with metallic nature of the core.

Shadow zone forms due to absence or bending of waves. Exam tip: remember it as evidence of the core.

The direct answer is C: the layer is liquid or behaves like a liquid. S-waves are shear waves. They move by making particles vibrate sideways, so the material must resist shearing. Solids can do this, but liquids cannot transmit S-waves because their particles flow past one another. Thus, if S-waves stop at a boundary, the next layer is inferred to be liquid or liquid-like. A is wrong because stopping S-waves does not prove the layer is gas. B is wrong because there is no basis for saying it is completely ice. C is correct and helps explain the liquid outer core. D is irrelevant: the observation concerns Earth’s interior, not the Sun. Memory cue: P-waves pass through solids and liquids; S-waves pass through solids only.

The correct answer is D: wave velocity changes mainly because the materials of the crust and mantle have different properties. Seismic-wave speed depends on factors such as composition, density, rigidity, and pressure. At the Moho, waves enter mantle material, so their speed changes noticeably. Option D is correct because a change in material properties produces the discontinuity and the velocity change. Option A is wrong because atmospheric pressure has little relevance to this deep internal boundary. Option B is wrong because river deposits occur at the surface and do not create the Moho. Option C is wrong because faster sea waves are unrelated to the speed of seismic waves inside Earth. The key chain is: crust material changes to mantle material, properties change, and seismic velocity changes. Memory cue: Moho is a material boundary, not a weather or water boundary.

The direct answer is A: crust, mantle, core. Scientists divide Earth chemically according to composition. Starting at the surface, the thin rocky crust comes first. Beneath it lies the much thicker mantle, made of dense silicate rocks. At the centre is the metallic core, mainly associated with iron and nickel. Therefore the sequence from outside to inside is crust → mantle → core. B reverses the sequence and begins at the centre. C places the mantle before the crust and is therefore incorrect. D wrongly places the core between the crust and mantle. Do not confuse this chemical division with mechanical divisions such as lithosphere and asthenosphere; those describe physical behaviour, not the three main compositional layers. Memory cue: surface to centre means C-M-C: crust, mantle, core.

The asthenosphere is a relatively weak and ductile (plastic) zone of the upper mantle below the lithosphere. Its ductile behaviour enables the movement of lithospheric plates above it. Option A describes the rigid and brittle nature of the lithosphere, not the asthenosphere. Exam tip: link the asthenosphere with plate tectonics and plate movement.

Continental crust is generally thicker and less dense than oceanic crust, so it stands relatively higher on the mantle. Oceanic crust is comparatively thinner and denser because it is mainly basaltic. Exam tip: remember continental crust as granitic, thicker, and less dense.

Dense oceanic crust can sink at convergent margins. Exam tip: write the relation between density and subduction.

Absence of S-waves shows liquid nature of outer core. Exam tip: write Gutenberg as mantle-outer core boundary.

The direct answer is C: volcanic material is a direct source for studying Earth’s interior. A direct source gives material or evidence that comes from Earth itself, such as rocks obtained by mining, drilling, and volcanic eruptions. The reasoning is: magma rises from depth; when it erupts, lava, ash, and rock fragments reach the surface; scientists can examine their composition and minerals; this gives direct, though limited, evidence about material inside Earth. Option C is correct. Option A is wrong because meteorology studies the atmosphere and weather, not the origin of volcanic rock as a source of interior evidence. Option B is wrong because volcanic material is geological, not only biological. Option D is wrong because an artificial satellite is a human-made observing instrument and does not provide a physical sample from inside Earth. Volcanic material may not come from the deepest core, so it is a limited direct source, but among the given options “direct source” is clearly correct. Memory cue: eruption brings underground material to the surface.

Meteorites give indirect clues about original planetary composition. Exam tip: write them as indirect evidence.

P-waves are compressional and longitudinal in nature. Exam tip: remember them among the fastest waves.

The direct answer is B: S-waves do not propagate through liquids. S-waves are secondary or shear waves. In a shear wave, particles must move sideways relative to the direction of travel, so the material must resist shearing. Solids can do this, but liquids cannot maintain a shear shape; therefore S-waves stop at a liquid boundary. Step by step: an earthquake produces waves; S-waves travel through solid rock; when they reach a liquid layer, they cannot continue; the missing S-wave signal helps scientists infer a liquid layer, especially the outer core. Option B is correct. Option A is wrong because S-waves do not become faster in liquid; they cannot pass through it. Option C is wrong because earthquakes occur beneath land and oceans, and S-waves do not originate only in oceans. Option D is wrong because S-waves are seismic waves, not instruments for measuring air temperature. Memory cue: “S means shear and solid”; no shear strength in liquid means no S-wave transmission.

Low velocity zone is linked with the weak part of upper mantle. Exam tip: connect it with asthenosphere.

Enormous pressure keeps inner core material solid. Exam tip: mention pressure along with temperature.