Interior structure of the earth

At the Moho material and elastic properties change so wave velocity changes. Exam tip: identify Moho as the crust mantle boundary.

S waves do not travel in liquids so their absence proves a liquid outer core. Exam tip: remember S waves and liquid together.

The direct answer is B: a sudden difference in the density and elasticity of the medium. P waves are seismic body waves. They travel through solid, liquid, and gas because they push and pull particles in the direction of travel. When a P wave reaches the boundary between the mantle and the outer core, the material changes greatly. Its density and elastic properties are different, so the wave speed changes. A wave changes direction when its speed changes across a boundary; this bending is called refraction. Step by step: first, the P wave travels through the mantle; second, it reaches the outer-core boundary; third, its speed changes because the material changes; fourth, its path bends. Option A is wrong because rainfall does not stop or bend deep earthquake waves. Option B is correct because density and elasticity control seismic-wave speed and refraction. Option C is wrong because surface wind affects air, not waves deep inside Earth. Option D is wrong because the Moon’s gravity does not suddenly end and is not the main cause of this seismic bending. The shadow-zone pattern is related to refraction and changes in wave speed. Memory cue: a change of material causes a change of speed, and a change of speed causes bending.

Pressure in the inner core is so high that melting is suppressed. Exam tip: write pressure as the cause of solid inner core.

Crust mantle core are based on composition while lithosphere asthenosphere are based on behaviour. Exam tip: keep the basis separate.

Weak and partly molten behaviour of asthenosphere can reduce velocity. Exam tip: connect low velocity zone with upper mantle.

Oceanic crust is basaltic and denser so it can sink downward. Exam tip: connect density with subduction.

Gutenberg discontinuity lies between mantle and outer core. Exam tip: connect it with about 2900 kilometres depth.

Shadow zones form due to bending and stopping of waves. Exam tip: treat shadow zones as evidence of core nature.

SIAL is linked with silica alumina and SIMA with silica magnesium. Exam tip: remember them with crust types.

Gravity values may be affected by dense or light materials inside. Exam tip: treat gravity study as indirect evidence.

Motion of liquid metals in the outer core is linked with geomagnetism. Exam tip: connect outer core with magnetic field.

Direct sources are useful but do not represent Earth's full depth. Exam tip: remember the limitation of direct sources.

Meteorites can give clues about materials that formed Earth. Exam tip: place them under indirect sources.

P waves can pass through liquid but S waves cannot. Exam tip: think of outer core in such questions.

Mantle is mostly solid but can flow slowly under high temperature and pressure. Exam tip: understand mantle as solid with plastic behaviour.

Lehmann discontinuity marks a state change boundary within the core. Exam tip: connect it with inner core boundary.

Heat flow is linked with heat coming from Earth's interior to surface. Exam tip: treat heat flow as an internal heat clue.

Geothermal gradient indicates Earth's internal heat. Exam tip: connect it with temperature and depth.

Continental crust is generally granitic and lighter. Exam tip: remember granite as a continent clue.

Oceanic crust is mainly basaltic and is denser than continental, granitic crust. Therefore, a basaltic and relatively denser rocky layer is the best indicator of oceanic crust. A granitic, lighter layer is characteristic of continental crust, while iron and nickel are associated with Earth’s core. Exam tip: remember ‘basalt—higher density’ for oceanic crust and ‘granite—lower density’ for continental crust.

High mountains may have deep roots of lighter crust below them. Exam tip: study mountain roots with isostasy.

Continental and oceanic crust differ in thickness density and composition. Exam tip: remember the comparison together.

The direct answer is D: Gutenberg–crust and mantle is the incorrect match. A discontinuity is a boundary where seismic-wave behaviour changes because the material or composition changes. The Moho lies between the crust and mantle, so A is correct. The Gutenberg discontinuity lies between the mantle and the outer core, so B is correct. The Lehmann discontinuity is associated with the boundary between the outer and inner core, so C is accepted as correct at this level. Gutenberg is not the crust–mantle boundary; that boundary is Moho. Therefore D is the incorrect pairing. Do not confuse the names: Moho means crust–mantle, Gutenberg means mantle–outer core, and Lehmann means outer–inner core.

The direct answer is A. Earth became internally differentiated when it was much hotter and partly molten. Dense materials, especially iron and nickel, moved downward under gravity and became concentrated in the core, while lighter rocky materials remained above. This process is called density differentiation or gravitational differentiation. Therefore density generally rises from the crust toward the centre. Option A is correct because it gives the actual cause: heavy elements concentrated at depth. Option B is wrong because sunlight does not reach Earth’s centre and does not arrange the layers. Option C is wrong because rainfall cannot travel through the crust to the centre, and water is not the cause of the metallic core. Option D is wrong because the atmosphere is a very thin outer envelope and does not simply settle into the core. The core is denser than the mantle, and the mantle is generally denser than the crust. Memory cue: “heavy sinks, light stays above.”