Interior structure of the earth

Wave speed depends on properties of material. Exam tip: treat speed change as a clue of layer boundary.

Moho is the boundary between crust and mantle where material changes. Exam tip: connect Moho with the crust mantle boundary.

Oceanic crust has basaltic materials rich in heavier minerals. Exam tip: remember oceanic crust as basaltic and dense.

Outer core is considered liquid and inner core solid. Exam tip: remember the states of outer and inner core separately.

S waves do not travel through liquids. Exam tip: treat absence of S waves as evidence of liquid outer core.

Gutenberg discontinuity is near the mantle core boundary. Exam tip: connect 2900 kilometres with the mantle core boundary.

At depth overlying material weight and internal heat become more effective. Exam tip: connect both temperature and pressure with depth.

On chemical basis Earth is divided into crust mantle and core. Exam tip: remember chemical layering through the three main layers.

The direct answer is option A: the asthenosphere is a relatively weak, hot, and slowly flowing zone beneath the rigid lithosphere. Mechanical classification describes layers by how they behave under stress, not only by their chemical materials. The lithosphere is hard and broken into plates; the asthenosphere below it can deform slowly, allowing plates to move over it. Option A is correct because it gives both its position and mechanical nature. Option B is wrong: the outermost rigid shell is the lithosphere, not the asthenosphere. Option C is wrong: the core is mainly metallic, and the asthenosphere is not a gaseous core. Option D is wrong: it is not frozen seawater. The movement is very slow and is related to heat inside Earth. Memory cue: lithosphere is rigid; asthenosphere is weak and flowable.

Seismic waves are affected by internal materials and give clues. Exam tip: keep seismic waves as the most important indirect source.

Extreme pressure resists melting tendency. Exam tip: write pressure as the reason for solid inner core.

NIFE includes nickel and ferrum meaning iron. Exam tip: remember core composition with Ni and Fe.

Both crusts differ in thickness density and rock composition. Exam tip: remember continental as thick granitic and oceanic as thin basaltic.

P waves refract when entering a different medium. Exam tip: connect P wave shadow zone with refraction.

Shadow zones show how waves stop or bend inside Earth. Exam tip: treat shadow zone as internal evidence.

The direct answer is B, Lithosphere. The lithosphere is the hard, strong outer layer of Earth. It includes the crust and the rigid uppermost mantle. This layer is broken into large and small tectonic plates. Step by step: a rigid layer does not flow easily; Earth’s outer rigid shell is called the lithosphere; this shell is divided into plates; therefore B is correct. A, the asthenosphere, lies below the lithosphere and is hotter, weaker and capable of slow flow, so it helps plates move rather than being the plates themselves. C, the outer core, is liquid metal deep inside Earth and is not divided into surface plates. D, the inner core is solid, but it is a central metallic sphere, not a plate-bearing outer shell. Memory cue: “Litho means stone”; think of a stone-like shell broken into plates.

With depth increased pressure changes mineral behaviour. Exam tip: consider lower mantle as a high pressure layer.

SIAL relates to lighter continental material and SIMA to denser oceanic material. Exam tip: remember SIAL continents and SIMA oceans.

The direct answer is A: lava originates from internal magma and therefore can reveal information about material from inside Earth. Deep inside Earth, molten or partly molten rock is called magma. When this material reaches the surface through a volcanic opening, it is called lava. Scientists can study its minerals, chemical composition, temperature and gases to infer conditions in the interior, especially in the mantle and upper crust. It is a direct, or relatively direct, source of information compared with indirect evidence such as seismic waves, gravity or magnetism. Option A is correct because volcanic lava is surface material brought upward from Earth’s interior. Option B is wrong because lava does not fall from clouds; rain falls from clouds. Option C is wrong because sea waves may erode or move rocks, but they do not create volcanic lava. Option D is wrong because lava does much more than change soil colour; it provides rock and chemical evidence about internal materials. The safe memory cue is: magma inside, lava outside; lava carries a sample upward.

Rocks obtained from mines are useful direct evidence, but mines reach only a small depth compared with the size and radius of Earth. They can tell scientists about materials near the surface, yet they cannot provide samples from the mantle, outer core, or inner core. This limited reach is the main weakness of using mine rocks to study Earth’s interior. Therefore, option A is correct.

The other choices describe impossible or unrelated claims. Mine rocks do not come directly from the inner core or serve as samples of the outer core. They also do not stop seismic waves; seismic waves travel through Earth and are studied as indirect evidence of internal layers. Since direct access is restricted to shallow regions, scientists combine such samples with seismic data, gravity measurements, and volcanic materials to infer deeper structure. Hence option A follows.

Continental crust may be thicker beneath mountain regions. Exam tip: remember the concept of mountain roots.

Heavy materials settled below and lighter materials remained above. Exam tip: connect density differentiation with layer formation.

The direct answer is A: P waves are recorded before S waves because P waves travel faster. P waves are primary, compressional waves. They push and pull particles in the direction of travel and can pass through solids, liquids and gases. S waves are secondary, shear waves. They move particles at right angles to the direction of travel and can pass through solids but not liquids. If both waves start at the same earthquake and travel toward the same station through suitable material, the faster P wave covers the distance first. Option A is correct because higher speed produces earlier arrival. Option B is wrong because P waves are body waves and travel through Earth’s interior; they do not travel only along the surface. Option C is wrong because S waves are mechanical seismic waves, not light. Option D is wrong because S waves do not always arrive first; they generally arrive after P waves. A useful calculation idea is: for the same distance, a greater speed means a smaller travel time, using the relation travel time = distance ÷ speed. Remember the order P before S.

At a discontinuity material changes so wave behaviour changes. Exam tip: identify discontinuity by change in wave behaviour.

The direct answer is B: density generally increases from the surface toward the centre of Earth. Density means how much mass is packed into a given volume. The reasoning is: the crust is made largely of relatively lighter rocks; deeper layers contain denser materials under enormous pressure; the mantle is denser than the crust, and the core contains very heavy metallic materials, mainly iron and nickel. Therefore the general pattern is increasing density inward, although density does not change in a perfectly smooth way at every boundary. Option A is wrong because density does not generally decrease toward the centre. Option B is correct because the deepest interior is the densest major region. Option C is wrong because matter does not become zero at the centre. Option D is wrong because different layers have different compositions and pressures. Memory cue: “deeper Earth, heavier material.”