Sources of information

Earth's interior is understood by combining many sources. Exam tip: remember sources in two groups.

Rocks show materials and minerals of Earth's layers. Exam tip: consider rock samples as direct evidence.

Temperature increases with depth so mines indicate internal heat. Exam tip: connect heat increase with depth.

Seismology studies earthquakes and their waves. Exam tip: remember this term with seismic sources.

The direct answer is C, the nature of the medium. A wave is a disturbance that travels through a material medium or, for some waves, through space. Its speed depends on the physical properties of the material, such as density, elasticity and state. Inside Earth, seismic waves travel differently through solid, liquid and partly molten materials. This difference helps scientists infer the structure of Earth’s interior. Option A is wrong because the time of day does not determine the internal speed of seismic waves. Option B is wrong because rainfall at the surface does not control wave speed deep inside Earth. Option C is correct because the medium’s nature determines how the disturbance travels. Option D is wrong because trees have no role in the speed of waves within Earth. A small scientific caution is useful: the exact speed can also depend on factors such as pressure, temperature and wave type, but “nature of the medium” is the correct school-level answer here. Memory cue: change the medium, and wave speed can change.

Reflection of waves indicates layer boundaries. Exam tip: connect reflection with internal boundaries.

The Moho boundary, or Mohorovičić discontinuity, was identified through the study of seismic waves. Seismic-wave velocity changes suddenly at this boundary, indicating a change in the properties of materials between the Earth’s crust and mantle. A soil survey examines only near-surface soil and cannot identify such a deep internal boundary. Exam tip: remember Moho as the crust–mantle boundary detected using seismic waves.

Arrival time differences give information about internal media. Exam tip: link arrival time with structure.

Gravity measurement is affected by mass and density. Exam tip: remember gravity clues for heavy materials.

Magnetic field is considered linked with the metallic core. Exam tip: connect magnetism with core information.

Geothermal energy comes from Earth's internal heat. Exam tip: remember it as evidence of heat source.

The direct answer is A. Radioactive elements inside Earth release energy through radioactive decay. This decay produces heat and is an important source of Earth’s internal heat, along with the heat left from Earth’s formation and other sources. Internal heat drives processes such as mantle convection, plate movement and volcanic activity, so radioactive elements help explain why Earth remains geologically active. Option A is correct because radioactive decay produces thermal energy inside Earth. Option B is wrong: clouds form mainly through condensation of water vapour in the atmosphere, not because radioactive elements create them. Option C is wrong: rivers are formed by flowing water, rainfall, melting snow and erosion, not directly by radioactive elements. Option D is wrong: radioactive elements do not colour the air; the colour of air and sky is connected mainly with the scattering of sunlight and atmospheric particles. The wording “produce internal heat” is the relevant textbook fact, even though radioactive decay is not the only heat source. Memory cue: radioactive decay means heat from inside Earth.

Higher gravity is generally linked with higher density or mass. Exam tip: connect gravity anomaly with density.

Drilling brings out cylindrical rock pieces called core samples from below the Earth's surface. Studying them provides direct evidence about the composition, structure, and layering of subsurface rocks. Population growth and weather change cannot be determined from such rock samples. Exam tip: remember drilling and mining as direct sources of information about Earth's interior.

The direct answer is Option D: gravity anomaly. An indirect source does not physically reveal the deep interior; instead, it provides measurable evidence from which scientists infer hidden structures. A gravity anomaly is a difference between the measured gravitational effect at a place and the value expected for that place. Such differences can indicate changes in density below the surface, so they help scientists study Earth’s interior indirectly. Option D is therefore correct. Option A, volcanic lava, comes from inside Earth and can be examined directly as a material sample, so it is treated as a direct source, although it does not come from the deepest mantle. Option B, mine rock, is obtained by directly excavating and observing rock. Option C, a drilling sample, is also physically collected from below the surface and is a direct source for the material reached by drilling. All direct sources have depth limitations, but that does not turn them into indirect sources. Memory cue: gravity, seismic waves and magnetic evidence usually suggest what is below; lava, mine material and drilling samples can be directly examined.

Volcanic gases are linked with internal magma. Exam tip: consider both gas and lava as sources.

Earthquake waves do not all travel at the same speed. Primary waves, or P waves, are the fastest body waves and can pass through solids and fluids. Secondary waves, or S waves, arrive later and cannot travel through liquids, while surface waves generally move more slowly but may cause strong damage near the surface. Seismographs record these waves in order of arrival.

Thus, option B, P wave, is correct because it reaches distant recording stations first in normal earthquake-wave comparisons. The word “primary” is a useful memory aid for both its name and its first arrival. S waves and surface waves arrive later, and a sea wave is not an earthquake body wave used for this comparison.

P and S waves pass through Earth's interior. Exam tip: consider them important for internal study.

Shadow zones form due to refraction and obstruction of waves. Exam tip: connect them with core information.

P (primary) waves are longitudinal seismic waves. Their particles vibrate parallel to the direction of travel, so P waves can pass through solids, liquids and gases. In contrast, S waves cannot travel through liquids. Exam tip: remember that P waves are the fastest seismic waves and travel through all three states of matter.

Inability of S waves to pass through liquid is important evidence. Exam tip: remember this clue for outer core.

S waves do not travel in liquids so the outer core is considered liquid. Exam tip: remember the relation between waves and state.

When a wave travels from one material into another, its speed may change. Because of this change, the wave can change its direction at the boundary. This bending is called refraction. Seismic waves passing through different internal layers of the Earth can be refracted, so their paths and arrival patterns help scientists infer boundaries between layers and study the planet’s interior.

Option C, refraction, is therefore correct. Condensation means a gas changing into a liquid, weathering is the breakdown of rocks at or near the surface, and deposition is the laying down of transported material. None of these terms describes a wave changing direction at a boundary. The supplied explanation and answer correctly connect refraction with internal-layer boundaries.

Gutenberg boundary lies between mantle and outer core. Exam tip: remember major boundaries with layers.

The Lehmann boundary is the boundary between Earth’s liquid outer core and solid inner core. Scientists inferred it from the way seismic waves behave after travelling through the core. Changes in wave paths and arrivals showed that the core is not a single uniform layer, but has an outer part and an inner part with different physical properties. This boundary is also called the inner-core boundary.

Therefore, option C is correct. Clouds, soil and rivers are surface features and have no connection with the Lehmann boundary. The phrase “within the core” is important: it distinguishes this boundary from the core–mantle boundary, which lies between the mantle and the outer core.