Interior structure of the earth

Volcanic material gives a direct sample from depth. Exam tip: separate direct and indirect sources.

Meteorites can indicate original composition of Earth-like bodies. Exam tip: write them as indirect sources.

Gravity anomaly relates to density differences below. Exam tip: treat it as indirect evidence of internal structure.

Geomagnetism indicates metallic and moving parts of the core. Exam tip: connect it with core study.

Mantle transition zone is linked with mineral changes. Exam tip: remember 410 and 660 kilometres as key depths.

Material properties change with depth so wave speed changes. Exam tip: link velocity change with layer identification.

Temperature and pressure become very high with depth. Exam tip: link drilling limit with lack of direct sources.

The direct answer is A: a sudden increase in seismic-wave velocity between the crust and mantle indicates a change in chemical and physical properties. Seismic waves travel differently through different materials. At a boundary, a sudden change in speed shows that the material below is not exactly the same as the material above. The crust–mantle boundary is called the Mohorovičić discontinuity, or Moho. Thus the evidence is a discontinuity and a change in composition, density and physical condition. Option A is correct because it explains why wave velocity changes. Option B concerns ocean-surface waves, not the interior Earth boundary. Option C concerns atmospheric temperature and cannot explain seismic velocity. Option D is a surface river feature and is unrelated to Earth’s internal layers. Exam cue: sudden seismic change means a boundary or discontinuity, not a weather or surface-landform event.

Sial relates to lighter continental and sima to denser oceanic nature. Exam tip: place them under crustal composition.

Basaltic oceanic crust, especially when it becomes old and cool, is denser than continental crust. At a convergent plate boundary, it can subduct beneath a less dense plate and descend into the mantle, where its material may take part in melting and magma-forming processes. This is why it is associated with crustal recycling in plate tectonics. Option B is incorrect because oceanic crust is generally denser, not lighter, than continental crust. Exam tip: link greater density and a convergent boundary with subduction.

Waves from focus behave differently in different media. Exam tip: mention the importance of wave paths from focus.

Surface samples show only a small part of depth. Exam tip: write the importance of indirect evidence for deep Earth.

Mantle convection is linked with internal heat and material movement. Exam tip: include it among causes of plate movement.

Radioactive decay is a major source of Earth internal heat. Exam tip: remember it along with primordial heat.

High density and magnetism of the core indicate metallic composition. Exam tip: connect iron-nickel with the core.

Discontinuity is identified by change in wave velocity and material properties. Exam tip: remember boundaries of major discontinuities.

On chemical basis Earth is divided into crust mantle and core. Exam tip: keep chemical and mechanical divisions separate.

The rigid upper part on mechanical basis is called lithosphere. Exam tip: distinguish it from chemical crust.

The direct answer is A: mineral phase changes are more clearly identified through changes in seismic velocity. The mantle is deep and cannot usually be sampled directly. Earth scientists therefore study earthquake waves travelling through it. When minerals change crystal structure under different pressure and temperature conditions, the density and elastic properties of the material can change. Seismic waves then travel at different speeds or show discontinuities, especially near transition zones. The reasoning is: a mineral phase change alters physical properties; altered properties affect wave movement; instruments record changes in seismic velocity; scientists use those changes as evidence. Option A is correct because seismic velocity is directly related to the internal physical structure of Earth. Option B, leaf colour, is a surface biological observation and gives no evidence about deep mantle minerals. Option C, the number of marine fish, concerns ecology and cannot reveal a mineral transition deep underground. Option D, raindrop size, concerns weather and has no direct relation to mantle structure. Seismic evidence is evidence, not a photograph of the mantle, so scientists interpret it with other data and experiments. Exam cue: mantle transition zone plus phase change means discontinuity or change in seismic-wave velocity.

Crustal thickness varies by location. Exam tip: remember the difference between oceanic and continental thickness.

The direct answer is A: pressure generally increases the melting point of materials at depth. Melting point is the temperature at which a solid changes into a liquid. In many Earth materials, greater pressure holds atoms more tightly together and makes melting more difficult, so a higher temperature is needed. The reasoning is: depth increases pressure; increased pressure often stabilises the solid structure; the material therefore needs more heat to melt; its melting point generally rises. This principle helps explain why the inner core can remain solid even though it is extremely hot: the pressure there is enormous. Option A is correct because it states the usual general effect asked in the question. Option B is wrong because pressure does not make the melting point always become zero. Option C is wrong because melting behaviour depends on the material and its conditions; pressure does not make the material irrelevant. Option D is wrong because melting is not controlled only by the Sun; pressure, temperature, composition and sometimes water or other substances matter. The word 'generally' is important because Earth materials can behave differently under special conditions. Exam cue: high pressure usually raises the melting point and can keep deep material solid.

The direct answer is A: the D double-prime layer lies near the base of the lower mantle, close to the mantle–core boundary. In studies of Earth’s interior, D″ is a deep zone just above the outer core. It is not a separate surface layer like the crust, ocean or atmosphere. The reasoning is: Earth has the crust, mantle and core; the lower mantle reaches down to the core; D″ refers to the unusual region at the bottom of that lower mantle; therefore it is located near the mantle–core boundary. Option A is correct. Option B is wrong because the upper crust is shallow, while D″ is extremely deep. Option C is wrong because the ocean surface is external. Option D is wrong because the atmosphere is far above the solid Earth. Memory cue: the double-prime symbol points to the deep bottom of the mantle, just above the core.

P (primary) waves are longitudinal waves that can travel through solids, liquids, and gases. Their velocity is greater than that of S (secondary) waves, so they reach and are recorded by a seismograph first. S-waves are slower and cannot travel through liquids. Exam tip: the first seismic arrival on a seismogram is generally a P-wave.

Inability of S-waves to travel through liquid reveals outer core state. Exam tip: link this property with shadow zone.

The geothermal gradient means the rate at which temperature rises as depth increases. This rate is not identical everywhere or at every depth. It is influenced by the distribution of internal heat, the type and conductivity of rocks, pressure, physical state, and local geological conditions. Near the surface, measured increases may differ from those deeper inside, so a simple straight-line assumption is unsuitable for the whole Earth.

Option A is the best answer because it recognises that heat sources, conductivity, and pressure vary with depth and location. The other choices are scientifically false: Earth is not completely cold inside, sunlight cannot reach the centre, and ocean water cannot cool the deep interior. Thus, the temperature increase is real but its rate is not uniform throughout Earth.