Sources of information

Real material is direct and clues are indirect evidence. Exam tip: classify sources on this basis.

Ocean floor rocks show the nature of the crust. Exam tip: remember them as direct geological material related to crust.

The direct answer is option A: refraction of seismic waves shows that the medium changes in Earth’s layers. Refraction means a wave changes direction and speed when it enters a region where the material has different physical properties. Earthquakes produce seismic waves that travel through the planet. If their paths bend or their speeds change, scientists infer that the waves have entered material with different density, composition, temperature, or state. The reasoning is: an earthquake sends waves; the waves meet a boundary; their speed and direction change; this change indicates a different medium or layer. Option A is correct because it expresses the geological conclusion. Option B is wrong because the freshness of river water is unrelated to seismic refraction. Option C is wrong because rainfall is a weather process and is not proved by seismic-wave paths. Option D is wrong because population distribution has no connection with the physical passage of earthquake waves. Refraction alone may not reveal every detail, but it is valuable evidence for boundaries and differences inside Earth. Memory cue: seismic waves bend when the material changes.

Reflection indicates waves returning from boundaries. Exam tip: connect reflection with internal boundaries.

Magnetic field gives a clue linked with metallic core. Exam tip: study magnetic and seismic evidence together for the core.

Travel time of waves indicates internal media and distances. Exam tip: connect time difference with layer identification.

Wave speed and gravity values give clues linked with density. Exam tip: remember combined indirect evidence for density.

Seismic gravity and magnetic clues are needed for deep parts. Exam tip: write indirect evidence for deep study.

The direct answer is D: gravity anomaly and direct sample source is the incorrect match. A source is direct when scientists obtain material or make an observation directly from the underground object, as in mining or a drilling sample. An indirect source provides evidence or a clue without bringing up a physical sample. Step by step: mining gives material directly, so A is correct; seismic waves travel through Earth and are interpreted to infer internal structures, so B is correct; a drilling sample is physically obtained, so C is correct; a gravity anomaly is a measured difference in gravitational effect used to infer hidden mass or structure, so it is indirect and not a direct sample source. Therefore D is the wrong match. Option A is not the answer because mining is direct. B is not the answer because seismic evidence is indirect. C is not the answer because a drilling sample is direct. D works because it incorrectly calls a gravity signal a direct sample. Memory cue: sample in hand means direct; signal or anomaly means indirect.

Volcanic ash gives clues about material linked with internal magma. Exam tip: keep ash lava and gases under volcanic sources.

P waves can pass through liquid but S waves cannot. Exam tip: remember medium capacity of both waves together.

Data from many stations make wave paths clearer. Exam tip: consider multi-station records reliable evidence.

Direct and indirect evidences are combined for correct conclusions. Exam tip: write the importance of combined evidence.

Rock samples can be obtained directly from the crust. Exam tip: remember sample study for crustal composition.

The direct answer is Option B: direct thermal information. A borehole is a deep, narrow hole drilled into the ground. If instruments measure temperature at different depths inside it, the readings directly record heat at those reached locations. This is thermal information because it concerns temperature, and it is direct because the instrument measures the temperature itself rather than inferring it from an unrelated effect. Option B is correct, with an important limitation: boreholes reach only limited depths, so they cannot directly measure the whole interior of Earth. Option A, biological information, concerns living organisms and is not what a temperature instrument measures. Option C, political information, concerns governments and institutions, not underground heat. Option D, population information, concerns the number and distribution of people and cannot be obtained from borehole temperature readings. Exam cue: identify both the measured thing and the method—temperature measured in the borehole means direct thermal information.

More than one evidence is compared for the core. Exam tip: draw conclusions from combined evidence.

The direct answer is A: a source that gives only clues instead of a real sample should be treated cautiously before being called direct. In studying Earth’s interior, a direct source gives material actually obtained from inside Earth, such as a rock brought up by mining or drilling. A clue, measurement, or indirect effect may tell us something about the interior without giving the material itself. Option A is correct because clues alone do not establish that the source is direct. Option B is generally direct because a real rock sample can be examined physically, although its depth and representativeness must still be considered. Option C is generally direct because drilling brings underground material to the surface. Option D is also generally direct because mining exposes or removes actual subsurface material. The important exam distinction is not simply “useful information” versus “useless information”; it is actual material versus evidence or inference. Memory cue: real sample means direct evidence; clue or effect usually means indirect evidence.

Decrease in wave speed may indicate different material or state. Exam tip: connect velocity change with internal boundary.

Seismic wave does not give a direct sample. Exam tip: check wave based options carefully in direct source questions.

Both are materials linked with volcanic activity. Exam tip: consider volcanic materials as direct clues from inside Earth.

Gravity study gives clues about internal mass and density. Exam tip: check subject relation for correct matching.

Behavior of seismic waves gives information about layers. Exam tip: treat them as the main indirect source.

Scientific conclusions are made by matching several evidences. Exam tip: consider comparison of sources as scientific approach.

Volcanic material is useful but not a direct sample of every layer. Exam tip: answer after understanding limits of evidence.

Both do not give direct samples of Earth's interior but give clues. Exam tip: keep clue based sources in indirect group.