Interior structure of the earth

Both clues relate to medium properties and density. Exam tip: do not write a possible conclusion as final fact.

The Moho, or Mohorovičić discontinuity, is the boundary between Earth’s crust and mantle. Seismic waves do not travel through every material at the same speed. When they cross from crustal rocks into mantle material, their speed changes because the composition, density, and elastic properties of the material change. This measurable change helps scientists identify the boundary.

The observation does not indicate a rainfall, population, or river-bed boundary. It is evidence of a physical change inside Earth. The Moho may vary in depth beneath different regions, but its essential meaning remains the crust–mantle boundary. Therefore, option B is correct: a change in wave velocity at the Moho indicates that the properties of the crust and mantle are different.

The Lehmann boundary is a seismic boundary inside the Earth. It separates the liquid outer core from the solid inner core. When seismic waves reach this boundary, their speed and path can change because they are entering material with different physical properties. Scientists used such changes in wave behaviour to infer that the Earth’s core is divided into an outer and an inner part.

Option D is correct because it identifies the boundary between the outer core and the inner core. The other choices concern the atmosphere, crops, or rivers and cannot explain a deep seismic boundary. It is also useful to distinguish Lehmann discontinuity from the Moho discontinuity: Moho separates the crust and mantle, whereas Lehmann concerns the inner structure of the core. Thus the changed wave behaviour points to the outer-core–inner-core boundary.

The geothermal gradient is the rate at which temperature increases with depth inside Earth. It reflects Earth’s internal heat, whereas surface weather concerns short-term atmospheric conditions such as temperature, rainfall and wind near the surface. Therefore, option B is correct; option A refers to rainfall, not the temperature change with depth. Exam tip: associate the geothermal gradient with the phrase “temperature change with depth.”

Mean density higher than surface samples indicates dense material inside. Exam tip: understand surface and depth separately.

Magnetism shows metallic nature and absence of S waves shows liquid state. Exam tip: remember combined evidence.

A sample from the upper crust provides direct evidence mainly about the composition and properties of that crustal region. It cannot safely represent the entire mantle or core because pressure, temperature and material composition change with depth. Exam tip: keep the scope of the conclusion within the known sampling region.

Moho and Lehmann are names for different seismic boundaries, so treating them as identical mixes up separate parts of Earth’s structure. The Moho is the discontinuity between the crust and the mantle. It is relatively shallow compared with boundaries inside the core. The Lehmann discontinuity is generally used for the boundary between the liquid outer core and the solid inner core, within the core itself.

Seismic waves reveal these boundaries because their speed, direction, or ability to pass through a material changes at each discontinuity. Thus, the two names must be linked with their correct locations and layers. They are not rainfall, river, or population boundaries. Option A is correct because it preserves the distinction between the crust–mantle boundary and a boundary within the core.

S waves propagate only through solids; therefore, the liquid outer core does not allow them to pass through, producing an S-wave shadow zone in seismic records. Option C is wrong because P waves can travel through liquids, although their velocity changes at a boundary. The Gutenberg boundary separates the mantle from the core, not the crust from the mantle. Exam tip: the absence of S waves beyond certain regions is key evidence that the outer core is liquid.

The high pressure and temperature at depth can alter the structure, density, state and other physical properties of minerals. Therefore, samples collected under surface conditions may not fully represent how the same materials behave inside Earth. Option B is wrong because igneous, metamorphic and sedimentary rocks can all occur at the surface, while option D is wrong because samples reveal composition and mineral characteristics, not merely colour. Exam tip: always consider pressure and temperature effects when interpreting evidence about Earth’s interior.

Seismic wave speed and bending reveal internal layers. Exam tip: link wave behaviour with Earth structure.

The correct answer is A: the Moho discontinuity is the boundary between the crust and the mantle. Earth has layers with different compositions and physical properties. Seismic waves travel at different speeds when they enter a new material, so a marked change in wave velocity helps scientists identify the Moho. Option A is correct because the Moho separates the thin outer crust from the denser mantle below it. Option B is wrong because the boundary between the outer and inner core is a different internal boundary, not the Moho. Option C is wrong because lithosphere and hydrosphere are not the two layers separated by the Moho; the hydrosphere means Earth’s water. Option D is wrong because the troposphere and stratosphere are atmospheric layers. Exam cue: Moho means crust–mantle boundary.

Continental crust is lighter and thicker. Exam tip: remember thickness and density differences together.

Oceanic crust is mainly basaltic and dense. Exam tip: distinguish it from granitic continental crust.

The lithosphere is Earth’s rigid outer layer, made of the crust and the uppermost strong part of the mantle. It is broken into tectonic plates. The outer core is a liquid metallic layer, not the lithosphere, and the mantle is not completely liquid. Exam tip: remember that the lithosphere includes more than just the crust.

Asthenosphere ductility helps plate movement. Exam tip: remember it as a weak upper mantle zone.

The Lehmann discontinuity is the boundary between Earth’s liquid outer core and solid inner core. It shows a major change in material and physical state inside the core. Seismic waves behave differently when they pass across this boundary, which helps scientists identify it. Therefore, the transition from the outer core to the inner core, given in option A, is correct. The crust–mantle boundary is called the Mohorovičić or Moho discontinuity, so option B describes a different boundary. The remaining choices concern unrelated parts of Earth or its atmosphere.

Earth is commonly described as having the crust, mantle, outer core and inner core. The Lehmann discontinuity occurs deep within the core, not near the surface. The outer core is mainly liquid, whereas the inner core is solid because of the enormous pressure at the centre. Thus, when the question asks what change the Lehmann discontinuity represents, the correct reasoning is to identify the core boundary. Hence, option A is the appropriate answer.

S-waves cannot travel through liquids. Exam tip: connect S-wave blockage with the liquid outer core.

P-wave refraction reveals internal boundaries. Exam tip: link velocity change with discontinuities.

Shadow zone is the area where waves do not arrive. Exam tip: write it as evidence for core state.

Pressure and heavy elements increase with depth. Exam tip: link density increase with core composition.

The outer core is mainly a liquid layer of iron and nickel, whereas the inner core remains solid despite very high temperatures because of immense pressure. Option C reverses their states, so it is incorrect. Exam tip: remember the pair—liquid outer core and solid inner core.

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

Low velocity zone is linked with weak asthenosphere. Exam tip: remember it in relation to plate movement.

Crust is thin but supports landforms and human activity. Exam tip: consider functional importance along with thickness.