Sources of information

Refraction occurs due to different materials or layers. Exam tip: link change in direction with internal differences.

A seismograph records the ground vibrations produced by seismic waves during an earthquake. A rain gauge measures rainfall, while a compass shows direction, so they are not correct. Exam tip: learn each geographical instrument along with its main function—seismograph for earthquakes.

Rock samples obtained through mining and deep drilling provide direct evidence mainly about the Earth’s upper crust, because human access is limited to shallow depths. The outer and inner core lie far deeper and are studied mainly through indirect evidence such as seismic waves. Exam tip: direct samples indicate the upper crust, while deeper layers are inferred indirectly.

Volcanic eruption brings material from inside Earth. Exam tip: connect natural source with volcanoes.

Passing or blocking of waves shows the state of material. Exam tip: remember the example of S waves.

Rock samples reveal minerals and chemical elements. Exam tip: sample-based information is direct.

Temperature and pressure are very high at depth. Exam tip: write difficult access as the reason for indirect study.

Seismic waves travel through materials inside Earth, and their behaviour depends on the properties of those materials. When a wave reaches a boundary between layers with different density, composition or physical state, its speed may change. Its path can also bend through refraction, and some waves may be reflected or stopped. Instruments record these changes at different places on Earth.

By comparing arrival times, paths and missing wave zones, scientists infer the positions of internal boundaries. Thus, option A is correct: seismic-wave speed and direction change at boundaries. Rainfall, soil formation and plant nutrition are unrelated to the use of seismic waves for mapping Earth’s internal layers.

Waves indicate the state of core and magnetism indicates metallic nature. Exam tip: remember both clues for the core.

The direct answer is Option A: mining and seismic waves. A direct source gives information by actually reaching or sampling Earth materials, while an indirect source gives clues about the inside without directly seeing or collecting the deeper material. Mining exposes rocks and minerals from below the surface, so it is a direct source, although mining cannot reach the deepest layers. Seismic waves produced by earthquakes travel through Earth, and their speed, direction and behavior reveal boundaries and materials inside Earth; therefore they are an indirect source. Option A is correct because it contains one direct and one indirect source. Option B, rainfall and clouds, concerns weather and does not provide the standard pair of sources for Earth’s interior. Option C, forest and grass, concerns vegetation, not interior structure. Option D, river and lake, concerns surface water and also does not represent direct and indirect geological investigation. Remember: mining or drilling touches material directly; seismic waves, gravity and magnetism help scientists infer what lies below.

The direct answer is A: distribution of heat inside Earth. Drilling allows scientists to go below the surface and record temperature at different depths and places. Step by step, measurements are taken near the surface and deeper underground; the values are compared; the change of temperature with depth and location gives evidence about the geothermal condition and the internal heat of Earth. Such data cannot reveal every detail of the interior, but they provide direct information from accessible underground points. Option A is correct because it matches the scientific use of drilling temperature observations. Option B, bird migration, is studied through animal movement, seasons, and habitats, not underground temperature drilling. Option C, city size, is a human-geography topic based on population and urban area. Option D, length of the rainy season, concerns atmospheric and climatic observations. Memory cue: drilling goes down, so its temperature data tell us about Earth’s inside heat.

The correct answer is A: deep drilling is a human-made direct attempt to learn about Earth’s interior. Direct sources involve actually obtaining material or observations from inside Earth, although drilling reaches only a very small depth compared with Earth’s radius. Step by step: people make a borehole using drilling equipment; rock samples and measurements are collected from below the surface; these samples give direct information about composition and temperature at that limited depth; scientists combine this evidence with indirect evidence such as seismic waves. Option A is correct because it is deliberately carried out by humans and physically reaches underground material. Option B is wrong because a lunar eclipse is a natural astronomical event and does not directly sample Earth’s interior. Option C is wrong because ocean tides are a natural result of gravitational forces and are not a drilling or sampling operation. Option D is wrong because wind direction is an atmospheric observation. Deep drilling is limited by heat, pressure, cost, and technology, so it cannot directly reach the core. Exam cue: drilling is direct but shallow; seismic evidence is indirect and reaches much deeper.

Density differences indicate different materials and layers. Exam tip: connect density with internal structure.

Volcanic material comes from inside Earth. Exam tip: it is an important example of direct information.

A school attendance register is not related to Earth's interior. Exam tip: eliminate unrelated options first.

Behavior of P and S waves shows state and structure of layers. Exam tip: remember the comparison of both waves.

A seismogram contains the record of seismic waves. Exam tip: remember the link between seismograph and seismogram.

Meteorites are linked with early solar system materials. Exam tip: study them as indirect clues of composition.

Radioactive materials can generate heat inside Earth. Exam tip: remember it among causes of internal heat.

Mining is a direct source of information about Earth’s interior because actual rock and mineral samples are obtained from mines. However, mines extend to only a limited depth compared with Earth’s radius, so they cannot reveal conditions up to the centre. Seismic waves are an indirect source, not mining. Exam tip: remember mining and drilling as direct sources, and seismic waves as indirect sources.

The direct answer is option A: gravity studies help infer the distribution of density and mass inside Earth. Gravity is the attractive force associated with mass. If rocks or underground regions have different densities, they do not pull equally strongly. Careful measurement of small gravity differences, called gravity anomalies, can therefore suggest where denser or lighter materials occur. The reasoning is: gravity depends on mass; mass is related to density and the amount of material; uneven underground density produces measurable gravity differences; scientists use those differences as indirect evidence of internal structure. Option A is correct because it describes the information gravity surveys can provide. Option B is wrong because air humidity is mainly studied with atmospheric instruments, not inferred as Earth's internal structure from gravity. Option C is wrong because wave height belongs to oceanographic observations; gravity may affect oceans broadly, but that is not the intended internal information. Option D is wrong because raindrop size is a weather or cloud-physics matter. Gravity studies do not provide a direct rock sample; they provide clues, so this is an indirect source. Memory cue: gravity variations point to variations in underground mass and density.

A direct sample from the deep core is not available. Exam tip: core information mainly comes from indirect sources.

Arrival time of waves gives information about internal media. Exam tip: connect time difference with layer identification.

Mining drilling volcanoes and seismic waves are all useful sources. Exam tip: identify direct and indirect sources together.

Both types of sources are necessary in internal study. Exam tip: remember conclusions are drawn from combined evidence.