Sources of information

The direct answer is B: temperature rise in deep mines is a direct clue about internal heat, but only up to limited depth. A deep mine gives scientists actual observations from below the surface, so the increase in temperature with depth is direct evidence of the geothermal condition at that accessible level. However, mines do not reach the whole mantle or core, so the observation cannot be called a direct sample or direct measurement of the entire core. Option B is correct because it includes both value and limitation. Option A is wrong because a mine does not provide a sample of the whole core. Option C is wrong because the temperature increase is not merely evidence of weather; it is related to Earth’s internal heat. Option D is wrong because the observation concerns underground temperature, not atmospheric pressure. The reasoning is: deeper mine; measured higher temperature; evidence of internal heat; limited accessible depth. Memory cue: mines give direct evidence nearby, not a direct look at the core.

The direct answer is A: the medium is probably solid and more rigid. P waves can travel through solids and liquids, but S waves require a medium that can resist shearing, so they do not travel through liquids. If S waves arrive, the region is therefore consistent with solid material. An increase in P-wave velocity usually suggests that the material has become denser, more elastic, or more rigid, though velocity alone does not prove one exact property. Combining both clues gives the best conclusion: the medium is probably solid and relatively more rigid. Option A is correct because it uses both observations cautiously. Option B is wrong because S waves do not normally pass through liquids, and “definitely” is too strong. Option C is wrong because seismic waves also travel through Earth’s interior, not only the atmosphere. Option D is wrong because P and S waves are recognised seismic waves related to Earth. Exam cue: P waves pass through solids and liquids; S waves indicate a solid medium.

Shadow zones form due to changes in wave paths. Exam tip: link them with core and layer information.

The direct answer is A: a gravity anomaly gives clues about underground density and mass, not final direct proof. Gravity is the force by which Earth attracts matter. If a region has unusually dense or massive material below, the measured gravitational value may be higher; if material is less dense, it may be lower. The instrument measures a surface effect, so scientists infer what may be underground. It does not show the exact rock directly or provide a complete, certain picture. Option A is correct because it states this indirect limitation. Option B is wrong because gravity measurements do not bring out physical rock samples; drilling can do that. Option C is wrong because rainfall is measured by weather instruments, not gravity anomaly surveys. Option D is wrong because gravity is an Earth-related physical phenomenon. The evidence should be checked with seismic waves, drilling, rock studies and other information. Memory cue: anomaly means a clue or difference, not final proof.

Decrease in velocity may indicate change in state of medium. Exam tip: connect velocity change with layer properties.

Meteorites do not come directly from Earth's centre. Exam tip: treat them as indirect compositional evidence.

The Moho, or Mohorovičić discontinuity, is the boundary between the Earth’s crust and the mantle. The materials on the two sides have different physical properties, and seismic waves respond to these changes as they travel through the Earth. A noticeable change in the speed or behaviour of seismic waves therefore provides important evidence for identifying this boundary.

Option A is correct because a clear seismic-wave velocity change can mark the transition from crustal material to mantle material. Rainfall, forest growth, and river width describe surface or environmental conditions and do not reveal the internal boundary between crust and mantle. The observation is called decisive in this school-level context because seismic evidence directly helps trace a hidden subsurface discontinuity, even though the boundary is not seen in an ordinary surface sample.

The direct answer is A: drilling is useful but incomplete when it provides samples only from a limited depth. Drilling is a direct source because a drill can bring actual rock, soil or mineral material to the surface. However, even very deep drilling reaches only a small part of Earth compared with the enormous depth to the mantle and core. Thus the samples give reliable local information, but they cannot reveal the complete deep interior. Option A correctly describes this limitation. Option B is wrong because if no rock is obtained, drilling would not provide the stated direct sample evidence; the main issue in the question is limited reach. Option C is wrong because census information concerns population, not underground structure. Option D is wrong because weather forecasting is a different activity. Scientists therefore combine drilling with seismic waves, gravity, magnetic evidence and laboratory studies. Exam cue: drilling is direct, but its depth is limited.

Both clues are indirect and should be compared with other evidence. Exam tip: do not write a possible clue as final truth.

The direct answer is B: S waves do not pass through the outer core. Seismic waves travel through Earth after an earthquake. P waves can travel through solids and liquids, although their speed changes when the material changes. S waves are shear waves: they require a material that can resist sideways distortion, so they travel through solids but not liquids. The reasoning is: seismographs record earthquake waves; scientists notice an S-wave shadow zone; S waves disappear where the outer core lies; therefore the outer core behaves as liquid. Option A is wrong because P-wave speed is not the same in every medium. Option B is correct and is the strongest listed seismic evidence. Option C is an effect of surface waves, not proof of the outer core’s state. Option D merely describes recording equipment and gives no evidence about core material. Memory cue: S means shear, and shear cannot travel through liquid.

The direct answer is B: P- and S-wave behaviour changes when waves pass from the mantle toward the outer core, helping identify the Gutenberg boundary. P waves are compressional waves and can travel through solids and liquids, though their speed changes. S waves are shear waves and cannot travel through liquids. The outer core is liquid, so S waves are not transmitted through it, while P waves are strongly refracted and slowed. This change in wave paths and arrival times reveals the boundary between the solid or largely solid mantle and liquid outer core. Option B is correct. A is wrong because these waves are not weather indicators. C is wrong because body P and S waves travel through Earth’s interior, not only along the surface. D is wrong because waves do not extract rock samples. Remember: Gutenberg boundary means mantle–outer core; missing S waves are especially important evidence for a liquid outer core.

P waves can pass through liquid but S waves cannot. Exam tip: this comparison is important in core study.

Mining is a direct source but does not reach great depth. Exam tip: remember the limit of direct sources.

Lava comes from inside but has limits. Exam tip: write both use and limitation of volcanic sources.

Higher mean density indicates denser material inside. Exam tip: connect density with internal structure.

Artificial waves help infer subsurface structure. Exam tip: write it as an indirect geophysical method.

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

Returning from a boundary is reflection and indicates internal boundary. Exam tip: connect reflection with boundary identification.

Refraction means that a wave changes its direction when it passes from one material or layer into another. Seismic waves travelling through Earth may bend because different internal layers have different composition, density, temperature, and physical state. Therefore, their bending gives scientists evidence that Earth is not uniform throughout and contains boundaries between layers. This is why refraction is useful in studying the interior, although it is interpreted together with other seismic observations.

When a seismic wave reaches a boundary, its speed may change and its path may bend. By recording the changed path and travel time at different places, scientists infer the position and nature of hidden layers. Option A correctly connects a change in direction with a change in medium. The other choices concern moisture, clouds, or population and have no relation to seismic evidence. Thus the supplied answer is scientifically and conceptually correct.

Bending of wave path indicates medium change. Exam tip: connect path change with internal structure.

These sources provide clues rather than real samples. Exam tip: keep them in indirect source group.

Data from many stations improves inference of internal structure. Exam tip: connect network data with reliability.

The direct answer is A, understanding the structure of the deep mantle and core. Direct sources give information obtained by reaching or observing material directly, while indirect sources provide clues about places that cannot be reached easily. The deep mantle and core lie far below the surface, so scientists combine evidence such as seismic waves, rock studies, gravity and magnetic observations. Option A is correct because this difficult, inaccessible question requires comparison and interpretation of both kinds of evidence. Option B, telling today's weather, mainly uses current atmospheric observations and forecasts, not evidence about Earth's deep interior. Option C, deciding a crop harvesting date, depends on crop maturity, local weather, soil and farming conditions, not mantle-core evidence. Option D, writing the name of a river, is a simple identification task and does not need comparison of geological sources. The important reasoning is: inaccessible interior equals limited direct observation, so indirect evidence must be checked along with available direct evidence. Memory cue: the deeper and less reachable the Earth part, the more important combined evidence becomes.

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

The direct answer is A: They are not direct but indirect sources giving clues. Seismic waves are vibrations produced by earthquakes or other disturbances. Scientists record how these waves travel through Earth, including changes in speed, direction and reflection or refraction at boundaries. This information helps them infer the internal structure, but the waves do not bring an actual piece of deep rock to the surface. The reasoning is: a wave is observed; its behaviour is measured; the measurement suggests hidden layers; no physical sample is collected; therefore it is indirect evidence. Option A is correct. Option B is wrong because seismic waves do not themselves provide rock samples. Option C is wrong because they are not weather sources; weather concerns the atmosphere. Option D is wrong because population sources concern demographic information, not Earth’s interior. The difference between a clue and a sample is the key. Memory cue: seismic waves tell scientists about hidden material by inference, while drilling or mining can provide direct samples.