Interior structure of the earth

The direct answer is A: mineral phase changes can change seismic-wave velocity. A phase change means the same mineral material adopts a different crystal structure under changed pressure and temperature. That structural change can alter density and elasticity; seismic velocity depends on these physical properties, so the speed or path of waves may change. This is why phase changes help explain seismic discontinuities and the Earth’s transition zone. B is wrong because a deep mineral phase change does not immediately increase rainfall. C is unrelated to underground crystal structure. D is also unrelated; it cannot by itself end atmospheric oxygen. A is correct because it follows the physical chain: phase change → altered structure and properties → altered seismic velocity. Memory cue: a mineral phase boundary can produce a seismic jump.

Temperature and pressure are very high at depth. Exam tip: write limitation of direct evidence and importance of indirect evidence.

In planetary differentiation heavy materials moved inward and lighter materials upward. Exam tip: connect it with layer formation.

There is major change in density and state at core-mantle boundary. Exam tip: link it with Gutenberg discontinuity.

Both may contain metallic materials but pressure is higher on inner core. Exam tip: distinguish composition and state.

Magnetic measurement indicates core nature but gives no sample. Exam tip: write it as indirect source.

One source does not give full information so many evidences are combined. Exam tip: mention both direct and indirect sources.

Layered structure of Earth is established by different kinds of evidence. Exam tip: do not depend on only one evidence.

Medium changes at discontinuities so wave behaviour changes. Exam tip: connect discontinuities with layer boundaries.

The direct answer is B: entry into more rigid and denser material. P waves are compressional seismic waves. Their speed depends on the material’s elasticity, rigidity, density and state. At the Mohorovičić discontinuity, or Moho, seismic properties change sharply as waves pass from crust into mantle. Mantle rock is generally more rigid and denser than crustal material, so P-wave velocity increases. A is wrong because increasing air would usually reduce rigidity and would not explain the mantle boundary. C is unrelated: ocean-water expansion does not cause this seismic discontinuity. D concerns atmospheric pressure, whereas P waves travel through the solid Earth. The reasoning is: crust-to-mantle boundary → changed, more rigid dense rock → faster P waves. Exam cue: a sudden seismic-speed change marks a discontinuity such as the Moho.

S-waves do not travel through liquid so they stop in outer core. Exam tip: connect S shadow zone with outer core.

Mines and drilling are very shallow compared with Earth radius. Exam tip: write indirect evidence for deeper parts.

Materials separated by density in early Earth. Exam tip: connect planetary differentiation with core formation.

Mineral phase change can alter density and elasticity. Exam tip: link transition zone with mineral changes.

Motion of metallic liquid in outer core generates magnetic field. Exam tip: connect geodynamo with outer core.

Lehmann discontinuity shows change of state and properties inside core. Exam tip: link it with outer-inner core boundary.

Oceanic crust is composed mainly of basaltic and gabbroic rocks, so its average density is greater than that of continental crust. Continental crust is relatively richer in lower-density granitic rocks. Because of its greater density, an oceanic plate commonly subducts into the mantle at convergent boundaries. Exam tip: remember oceanic crust as thinner but denser, and continental crust as thicker but less dense.

Rigidity and thickness of lithosphere vary with heat and plate setting. Exam tip: remember oceanic and continental differences.

Asthenosphere is not fully liquid but a weak ductile zone. Exam tip: understand it as the base region for plates.

Arrival time reveals speed and path of different waves. Exam tip: remember importance of travel time curves.

The mantle is not a completely uniform layer. As depth increases, pressure and temperature rise, and minerals can change their crystal structure or physical state. These changes affect the density, strength, and movement of mantle material. The boundary between the upper and lower mantle is therefore identified mainly through changes in seismic-wave behaviour and mineral structure caused by increasing pressure.

Option A is correct because it gives a genuine scientific basis for distinguishing the two regions. Rainfall, river-valley shape, and forest height describe surface geography or ecology, not the internal structure of Earth. The key idea is that the upper and lower mantle are parts of one broad mantle but have different physical and mineralogical conditions at different depths.

Metallic nature of core is supported by many indirect evidences. Exam tip: write density magnetism and meteorites together.

Gravity value is affected by density of hidden materials below. Exam tip: write gravity anomaly as indirect source.

Mountains may have isostatic roots below them. Exam tip: write relation between mountains and thick continental crust.

Velocity changes when medium changes and waves refract. Exam tip: link refraction with layer identification.