Sources of information

A direct source gives real samples of Earth's material. Exam tip: link the word sample with direct source.

Temperature and pressure are very high at depth. Exam tip: write this as the reason for indirect sources.

Mining provides actual rock and mineral samples from the upper layers of the Earth, so it is a direct source of information about Earth’s interior. A school map represents surface features, while rainfall records and census tables are unrelated to the Earth’s internal structure. Exam tip: Remember that samples from mining and deep drilling are direct sources.

Volcanic material comes from inside Earth to the surface. Exam tip: remember lava as a direct clue.

Wave behavior gives clues about internal materials. Exam tip: connect speed and direction change with internal structure.

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

Magnetic field is considered linked with metallic materials inside Earth. Exam tip: connect magnetism with the core.

Meteorites are linked with early solar system materials. Exam tip: treat them as indirect compositional evidence.

P waves pass through many media and change speed. Exam tip: also remember P wave as primary wave.

S waves do not travel in liquids so their absence indicates a liquid region. Exam tip: connect this fact with the core.

The direct answer is B: a seismograph records seismic waves. A seismograph is an instrument designed to detect and record ground movement caused by earthquakes. Its record, called a seismogram, can show the arrival and movement of different waves. Scientists use these records to study earthquake strength, timing, location, and Earth’s interior. Option B is correct because recording seismic motion is the instrument’s main function. Option A is wrong because soil fertility is tested by soil analysis, not a seismograph. Option C is wrong because counting raindrops is not its purpose. Option D is wrong because wind direction is measured with instruments such as a wind vane. The instrument must be matched with its function. Exam cue: seismic means earthquake-related; seismograph records earthquake waves.

A seismogram is the written record of seismic waves. Exam tip: remember the difference between seismograph and seismogram.

Time differences of waves help infer internal media. Exam tip: connect arrival time with layer identification.

The direct answer is option A: the Moho boundary separates the crust from the mantle. Moho is short for Mohorovičić discontinuity, named after the scientist who identified it from earthquake-wave behavior. Earth has major layers. The crust is the thin outer rocky layer, while the mantle lies below it and is generally denser. At the boundary, seismic-wave speed changes because the material and its physical properties change. The reasoning is: identify the outer solid layer as crust; identify the thick layer below it as mantle; the dividing surface between them is the Moho. Option A is correct. Option B is wrong because the mantle–outer-core boundary is a deeper boundary, not the Moho. Option C is wrong because the outer-core–inner-core boundary is still deeper and is a different discontinuity. Option D is wrong because the lithosphere–atmosphere boundary concerns the solid Earth and air, not the seismic crust–mantle boundary. The Moho is not a visible empty gap; it is a change in material properties detected mainly through seismic waves. Memory cue: “Moho” means crust below? More accurately, think “Moho marks the move from crust to mantle.”

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

The Lehmann boundary is the discontinuity within Earth’s core that separates the liquid outer core from the solid inner core. Therefore, option D is correct. The boundary between the crust and mantle is the Mohorovičić (Moho) discontinuity, while the mantle–outer core boundary is the Gutenberg discontinuity. Exam tip: remember Moho–crust/mantle, Gutenberg–mantle/outer core, and Lehmann–outer/inner core.

The direct answer is A: change in the medium or layer. In Earth science, waves travel through materials such as rocks. When a wave enters material with different density or composition, its speed changes. Because its speed changes, its path may bend; this bending is called refraction. Step by step: the wave travels through one layer, reaches a boundary, enters another layer, changes speed, and bends. This helps scientists infer boundaries between Earth’s internal layers, even when those layers cannot be seen directly. Option A is correct because it states the geological meaning of refraction. Option B is wrong because air humidity is an atmospheric condition and does not explain seismic refraction inside Earth. Option C is wrong because rainy seasons do not produce this evidence. Option D is wrong because soil colour is not the cause or interpretation of wave refraction. Memory cue: refraction means a changed path, so think of a changed material or layer.

Reflection indicates waves returning from layer boundaries. Exam tip: study reflection and boundaries together.

Shadow zones form due to bending and blocking of seismic waves. Exam tip: connect this with core information.

Mining does not reach deep internal parts. Exam tip: remember both usefulness and limitation of direct sources.

Drilling provides real samples of rocks below. Exam tip: consider drilling as direct but limited source.

The direct answer is A: a borehole gives a direct clue about temperature conditions below the surface. A borehole is a narrow hole drilled into the ground. Instruments can measure temperature at different depths inside it. Reasoning step by step: the hole reaches below the surface, a thermometer records temperature at a chosen depth, readings from several depths are compared, and the pattern gives evidence about underground heat. This is direct thermal information because the temperature is measured in the subsurface rather than guessed only from surface signs. Option A is correct. Option B is wrong because boreholes do not form clouds; clouds are atmospheric water-vapour formations. Option C is wrong because a borehole does not control or change river direction. Option D is wrong because population counting uses demographic methods, not underground temperature measurements. Borehole observations are useful in studying the geothermal gradient and Earth’s internal heat. Memory cue: borehole means “hole below,” so it helps measure conditions below.

The direct answer is A: increase of temperature with depth. The geothermal gradient describes how temperature changes inside Earth as depth increases. Earth has internal heat, so, in the usual meaning of this school-level term, deeper rocks generally become hotter. It is expressed as the temperature increase for a certain increase in depth. Option A is correct because it directly defines this relationship. Option B, increase of rainfall with depth, is unrelated to underground temperature and rainfall is an atmospheric process. Option C, increase of population with depth, concerns human geography, not Earth science. Option D, increase of air with depth, is not the definition; air pressure may change underground, but geothermal gradient specifically concerns temperature. Memory cue: geo means Earth, thermal means heat, and gradient means change with distance or depth.

Decay of radioactive elements releases heat. Exam tip: remember it among causes of internal temperature.

The direct answer is A, the weight of overlying layers. Pressure inside Earth increases mainly because deeper material has to bear the weight of all the rocks and layers above it. As depth increases, the column of overlying material becomes larger, so the downward force and pressure generally increase. In simple terms, deeper rocks are squeezed by the enormous mass above them. Option A is correct because it directly explains this pressure. Option B, moonlight, provides illumination but does not create the great pressure inside Earth. Option C, raindrops, affect the surface only in a very limited way and cannot explain pressure deep underground. Option D, the height of trees, has no meaningful connection with the pressure in Earth’s interior. The reasoning is: greater depth means more overlying material, more weight, and therefore greater pressure. Memory cue: depth increases the load above, so pressure rises; do not confuse pressure with temperature, although both generally increase downward.