Sources of information

Rock samples give information about real geological material. Exam tip: treat sample based sources as direct evidence.

Temperature rise in mines gives a direct clue of internal heat. Exam tip: remember the relation between heat and depth.

Seismic waves do not give samples but give clues. Exam tip: write clue based sources as indirect.

Gravity anomaly is related to density difference. Exam tip: connect gravity with mass and density.

P wave velocity depends on the medium. Exam tip: link velocity change with layer identification.

The direct answer is option B: S waves do not pass through the outer core. Earthquake waves help scientists study the unseen interior. P waves are primary waves and can travel through solids and liquids, although their speed changes when the material changes. S waves are secondary or shear waves; they need a rigid material to transmit their sideways vibration and therefore cannot travel through a liquid. Step by step: an earthquake produces waves; instruments record them; S waves disappear at the outer-core boundary; this absence indicates that the outer core is liquid. Option A is not the best evidence because P waves arriving first only tells us that P waves travel faster than S waves; it does not by itself prove the outer core is liquid. Option B is correct because the blocked S-wave path is direct evidence of a liquid layer. Option C concerns damage caused by surface waves and does not reveal the outer core. Option D, lava colour, cannot determine the physical state deep inside Earth. Memory cue: S means shear and stops in liquid.

Seismic shadow zones are regions where direct P-waves or S-waves from an earthquake are not recorded. S-waves cannot travel through the liquid outer core, while P-waves are refracted at the core-mantle boundary. Their pattern therefore provides evidence about Earth's internal layers, especially the liquid outer core. Exam tip: link the S-wave shadow zone with the liquid state of the outer core.

Meteorites give clues about early solar system materials. Exam tip: do not treat them as direct core samples.

Drilling is direct and seismic waves are indirect. Exam tip: identify sources by type of evidence.

The geothermal gradient describes how temperature changes inside the Earth as depth increases. In general, temperature tends to rise below the surface because the Earth has internal heat. This concept helps geographers and geologists understand the thermal condition of the crust and the sources of heat within the planet. It is expressed as a temperature increase for a given increase in depth.

Option B is correct because it directly states that the gradient shows temperature increase with depth. Annual rainfall belongs to climate studies, wind speed belongs to weather and atmospheric studies, and river slope describes the fall of a river channel. These choices refer to different geographical measurements. Therefore, the correct idea is depth-related underground heating, not rainfall, wind, or river movement.

Magnetic field gives a clue linked with metallic materials. Exam tip: connect magnetic evidence with the core.

Mining is useful but does not reach great depth. Exam tip: always write the limitation of direct sources.

Artificial waves help study subsurface structure in a controlled way. Exam tip: treat it as an indirect method.

The Moho, or Mohorovičić discontinuity, is the boundary between Earth’s crust and mantle. Seismic waves travel through different materials at different speeds. When they cross from the crust into the mantle, the physical properties of the material change, and the speed and path of the waves also change. Scientists use this change in seismic-wave behaviour to locate the Moho, so option B is correct.

Earthquakes produce waves that can be recorded by instruments at different places. By comparing their arrival times and velocities, geologists identify a discontinuity beneath the crust. Colour, smell, and taste are not properties used to trace seismic waves, and seismic waves cannot be identified by those senses. Therefore, a change in wave speed is the relevant evidence and option B follows.

Gutenberg boundary is linked with mantle and outer core. Exam tip: connect major boundaries with seismic evidence.

Lava gives clues of internal material but is not a sample of the core. Exam tip: write both use and limitation of a source.

S (secondary or transverse) waves require a rigid medium for propagation and cannot travel through liquids. Therefore, the absence of S waves in a region indicates that a liquid layer may be present there. The liquid outer core is inferred partly because S waves do not pass through it. Exam tip: associate the absence of S waves with a liquid medium.

Behavior of both waves differs according to medium. Exam tip: use medium as the main basis in comparison questions.

Pressure affects behavior of materials at depth. Exam tip: treat pressure as an internal physical condition.

Mining drilling and volcanic materials give real samples or material. Exam tip: real sample identifies direct source.

These sources give clues rather than direct samples. Exam tip: treat clue based evidence as indirect.

Seismograph records waves and seismogram is the record. Exam tip: remember instrument and record separately.

Lehmann boundary is linked with outer and inner core. Exam tip: remember it as an internal boundary of the core.

Magnetic anomaly can indicate nature of internal materials. Exam tip: understand it as indirect evidence.

Decay of radioactive elements contributes to internal heat. Exam tip: keep it among causes of heat.