Sources of information

Mines reach limited depth while waves give clues of deeper parts. Exam tip: write seismic evidence for deep study.

Borehole is useful but does not reach the deep core. Exam tip: always write limitation of direct sources.

Velocity decrease and weak S wave may indicate change in medium state. Exam tip: write it as a possible conclusion.

Gravity anomaly is an indirect clue. Exam tip: use sample analysis or combined evidence for chemical composition.

Magnetic field gives metallic clue but seismic evidence is needed for liquid or solid state. Exam tip: separate roles of evidence.

The direct answer is D: meteorites are indirect evidence used for comparison with early materials. Scientists cannot normally obtain a sample directly from Earth’s core. Meteorites formed from early solar-system material, so their composition can be compared with ideas about the materials from which Earth developed. This does not make them direct pieces of the core; it makes them useful comparative evidence about planetary composition. Option A is wrong because meteorites do not come directly from Earth’s core. Option B is wrong because meteorites are not sources of rainfall; rainfall is part of the water cycle. Option C is wrong because mined rocks are samples collected from accessible parts of Earth, whereas meteorites come from space. Option D is correct because they provide indirect clues through comparison with primitive material. Memory cue: meteorites are space samples and comparative clues, not core samples.

The Mohorovičić discontinuity (Moho) is a deep boundary between Earth’s crust and mantle. Direct rock samples from this depth are generally unavailable, so it is identified mainly from the abrupt increase and refraction of seismic-wave velocities. Therefore, option A is correct. Rainfall, population density and river beds are unrelated to the Moho. Exam tip: remember the Moho as the crust–mantle boundary marked by a change in P-wave velocity.

The direct answer is B: the shadow zone forms because seismic waves bend, change direction, or are blocked when they meet different materials inside Earth. Earthquakes produce P-waves and S-waves. Their speed and path change at boundaries, and S-waves cannot pass through liquid. The outer core is liquid, while the inner core is solid; this difference creates regions on Earth’s surface where certain waves are not detected. Scientists use the position and size of these zones to infer the core’s boundaries and physical state. Option A is wrong because sunlight has nothing to do with seismic shadow zones. Option B is correct because wave refraction, reflection, and blocking create the zones. Option C is wrong because rainfall is a weather measurement, not a seismic result. Option D is wrong because a shadow zone gives no physical mineral sample; it gives indirect information from wave behaviour. Memory cue: no wave arrival means look for a hidden boundary or changing material.

S waves do not pass through the liquid outer core. Exam tip: connect Gutenberg boundary with mantle and outer core.

S waves do not travel in liquids and pass through solids. Exam tip: link S wave presence with solid property.

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

Higher mean density indicates dense internal materials. Exam tip: connect this with core composition.

Lava comes from inside but its source region may be limited. Exam tip: write limitation of volcanic source.

Both are indirect geophysical clues. Exam tip: strengthen possible conclusion with combined evidence.

Reflection shows return from boundary and refraction shows direction change with medium. Exam tip: understand both as different but related processes.

A crustal sample does not represent the core. Exam tip: identify scope and limitation of source.

Artificial waves give clues but not samples. Exam tip: write it as indirect geophysical method.

Difference in arrival time indicates internal path and medium. Exam tip: connect seismogram data with analysis.

Rock analysis is sample based and seismic analysis is clue based. Exam tip: separate sample and clue.

Bending of P wave may indicate refraction and medium change. Exam tip: connect path change with internal boundary.

Mining and drilling have limited reach to deep layers. Exam tip: write seismic gravity and magnetic evidence for deep parts.

Ocean floor rocks give direct material related to crust. Exam tip: do not infer core directly from them.

The direct answer is A: radioactive decay can be a heat source, while mine temperature is limited direct observation. To understand Earth’s internal heat, first separate cause from evidence. Radioactive elements inside Earth release energy as they decay, so radioactive decay is one possible source of heat. Mines and deep boreholes allow scientists to measure temperature below the surface, but only through a small and relatively shallow part of Earth. Option A is correct because it makes this distinction. Option B is wrong because neither item is a population source. Option C is wrong because neither refers to rainfall or rivers. Option D is wrong because neither mine measurements nor radioactive decay is a core sample; core samples are physical pieces, and direct core sampling is not available in the ordinary sense. The reasoning is source versus observation: decay helps explain why heat exists, while mine temperature helps show how temperature changes with depth. Exam cue: cause is not the same as evidence.

The direct answer is A: the absence of S waves can indicate a liquid region, not empty space. Seismic waves travel through the Earth after an earthquake. S waves are shear waves: particles move at right angles to the direction of travel. A liquid cannot support this kind of sideways shear motion, so S waves do not pass through liquids. Scientists therefore use an S-wave shadow zone as evidence that part of the outer core is liquid. Option A is correct because it gives the physical reason and the correct inference. Option B is wrong because S waves do not pass through every material; their failure in liquids is important evidence. Option C is wrong because S waves are produced by earthquakes, not rainfall. Option D is wrong because seismic waves provide information about internal layers, not mineral samples. The memory cue is: P waves pass through solids and liquids, but S waves stop at liquid layers.

A source is called direct when it provides actual material from the object or region being studied, while an indirect source gives information through measurements, comparisons, or effects. A drilling core is a physical cylinder of rock brought up from below the surface, so it provides direct material evidence from the drilled depth. A gravity anomaly is an indirect geophysical clue: variations in gravitational measurements can suggest differences in underground density, but they do not themselves provide a rock sample.

Option C is correct because it matches drilling core with direct material evidence. Option B pairs a gravity anomaly with a direct rock sample, but a gravity anomaly is not itself direct material evidence, so that pairing is not correct as a source-nature match. Lava may offer material from the interior but is not population evidence, and a rain gauge cannot provide core evidence. Therefore C is the precise answer.