Sources of information

Core samples provide direct geological material so their information is strong. Exam tip: write core sample as a direct source.

The direct answer is B: Indirect source. A direct source provides actual physical material, such as a rock sample collected from a mine or a drilling core. A wave observation does not give scientists a piece of the interior; it gives information from which they infer the nature, depth or boundaries of hidden layers. The reasoning is: waves are observed; their speed, direction or reflection is studied; these behaviours provide clues; no solid sample is obtained; therefore the source is indirect. Option B is correct. Option A is wrong because a direct source requires actual material or a directly observed exposed feature, not only wave behaviour. Option C is wrong because biological sources concern living organisms or biological evidence, which is not the subject here. Option D is wrong because climatic sources concern climate records or atmospheric conditions, not seismic evidence. Thus the important distinction is evidence versus sample. Memory cue: seismic waves describe what is hidden indirectly; mines, volcanoes and drilling can provide material directly.

Ash and gases are linked with volcanic magma. Exam tip: remember them as parts of volcanic source.

Velocity change indicates different medium or layer. Exam tip: connect velocity change with internal boundaries.

P waves pass through many media and their speed indicates the medium. Exam tip: remember the medium capacity of P waves.

Absence of S waves indicates that the outer core is liquid. Exam tip: this evidence is very important.

The direct answer is B: blocking and bending of waves in internal layers. Seismic shadow zones are areas on Earth’s surface where certain seismic waves from an earthquake are not detected directly. P waves can travel through solids and liquids, but they are refracted, or bent, when they pass between materials with different properties. S waves cannot travel through liquids, so they disappear beyond the region blocked by the liquid outer core. These facts help scientists infer that Earth has distinct internal layers and that the outer core is liquid. Option A, crop rotation, concerns agriculture and has no connection with seismic-wave paths. Option B is correct because shadow zones are produced by the stopping, absence, or bending of waves as they meet internal layers, especially the core. Option C, population growth, is a demographic topic and does not explain seismic observations. Option D, rainfall distribution, concerns climate and weather, not earthquake-wave transmission. The reasoning chain is: earthquake produces waves; waves meet different layers; their speed, direction, or ability to pass changes; missing-wave regions reveal internal structure. Memory cue: seismic shadow zones are clues about the core, not about crops, people, or rainfall.

At Moho wave velocity changes between crust and mantle. Exam tip: remember Moho through seismic evidence.

Gutenberg boundary is considered between mantle and outer core. Exam tip: connect it with wave change.

Lehmann boundary is an important boundary within the core. Exam tip: connect it with the inner core.

Gravity values are affected by density and mass. Exam tip: treat gravity anomaly as an indirect source.

Magnetic study can indicate metallic materials inside. Exam tip: connect it with nature of the core.

Meteorites provide comparative indirect evidence. Exam tip: do not write them as direct core samples.

The geothermal gradient refers to the rate at which temperature changes as depth below the Earth’s surface increases. The interior of the Earth cannot be observed completely by ordinary surface inspection, so temperature patterns provide useful evidence about internal heat and conditions. The concept is therefore linked with depth and temperature, not with language, political boundaries, or forests.

As we move downward, temperature generally rises, although the rate of increase is not identical everywhere. Studying this change helps geographers and geologists understand the heat of the Earth’s interior and make informed interpretations about deep layers. Hence, option B gives the main use of the geothermal gradient. The other options concern human or surface features and have no direct connection with this measurement.

Decay of radioactive elements can release heat. Exam tip: remember it among sources of internal heat.

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

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

Direct sources give real material while indirect sources help in inference. Exam tip: remember this basic difference.

The direct answer is B: drilling core and rock composition. A drilling core is a cylindrical sample removed from below Earth’s surface. Scientists can examine this actual material to determine its minerals, texture, age, and chemical composition, although drilling reaches only a limited depth compared with Earth’s full radius. Therefore the correct match is between drilling core and rock composition. Option A is incorrect because a rain gauge measures rainfall near the surface and cannot reveal the outer core. Option B is correct because a core sample is direct physical evidence about the rock from the depth reached by drilling. Option C is incorrect because a census counts and describes population; it does not locate or analyse the Mohorovičić discontinuity, or Moho boundary. Option D is incorrect because a crop report gives agricultural information and cannot directly describe the mantle. The reasoning method is to identify what each source actually records: instrument or sample first, claimed information second, then check whether the relationship is logical. Memory cue: a core sample is like a small piece of the underground book—it gives real rock information, while census, crop reports and rain gauges describe people, farming and weather.

These sources give geophysical clues rather than samples. Exam tip: keep them under indirect sources.

These provide real rock or volcanic material. Exam tip: understand material based sources as direct.

The direct answer is A: a liquid region may exist in the path. P waves are compressional waves. They can travel through solids, liquids, and gases, although their speed changes in different materials. S waves are shear waves and require a rigid medium; they cannot pass through a liquid. Therefore, if a P wave arrives at a station but an expected S wave does not, the most suitable inference is that the wave path includes a liquid region or that the S wave was blocked by such a region. This is one important reason scientists infer that Earth’s outer core is liquid. Option A is correct because it matches the physical behaviour of P and S waves. Option B is wrong because a forest at the surface does not normally stop an earthquake’s deep seismic S-wave path in the relevant way. Option C is wrong because seismic waves do not create weather; weather involves the atmosphere. Option D is wrong because the observation actually supports the existence of different layers, not the absence of layers. The reasoning chain is: P arrives; S does not; P crosses liquid; S cannot; therefore liquid material may lie along the route. Memory cue: P passes through liquid; S stops in liquid—think “P passes, S stops.”

Arrival interval indicates wave speed and internal medium. Exam tip: consider seismogram as a record.

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

Seismic-wave refraction occurs when a wave enters a layer or medium with different physical properties and its speed or direction changes. Earth is made of layers whose composition, density, pressure, and other properties are not identical. When seismic waves cross a boundary between such layers, their paths can bend. By recording these changes, scientists can infer the presence of boundaries and learn about the structure of Earth’s interior. Thus refraction is evidence of a change of medium or layer.

The process does not directly indicate rainfall, forest growth, or population change. Those subjects require different observations and measurements. The important link is between a changed wave path and a boundary between materials with different properties. Therefore option A is correct, because it expresses what seismic refraction reveals about Earth’s layered interior.