Interior structure of the earth

Conrad discontinuity is linked with upper and lower parts of continental crust. Exam tip: connect Conrad with continental crust.

Lithospheric plates move over the soft asthenosphere. Exam tip: connect plate tectonics with the asthenosphere.

Movement of metallic materials in the liquid outer core is linked with the magnetic field. Exam tip: connect magnetic field with the outer core.

Continental crust generally has granitic composition. Exam tip: connect granite with continents.

Oceanic crust is mainly associated with basaltic rocks. Basalt is a dark, dense volcanic rock formed when molten material cools, often at or near the ocean floor. New oceanic crust is produced at mid-ocean ridges, where magma rises, cools, and becomes basalt. This composition also helps explain why oceanic crust is generally denser and thinner than continental crust, which is more commonly linked with granitic rocks.

Option B is therefore correct. Granite is chiefly associated with continental crust, while limestone and sandstone are sedimentary rocks that may occur in layers but do not form the dominant basic material of oceanic crust. The most useful school-level association is “basalt with ocean floor” and “granite with continents.” Although oceanic crust can contain sediment and other minor materials, its major rock type is basaltic. Hence the correct choice is basaltic, not granitic, limestone, or sandstone.

Earth is arranged in several broad layers, and the layer nearest its exact centre is the inner core. It lies inside the outer core and forms the deepest part of the planet. The inner core is mainly composed of iron and nickel and remains solid because the pressure at this depth is extremely high, even though the temperature is also very great. Thus, the correct choice is C, Inner core.

To identify the answer, compare each option with its position from the surface inward. The crust is the thin outer layer, while the lithosphere includes the crust and the rigid uppermost mantle. The mantle lies below the crust and surrounds the core, so neither is closest to the centre. The core has an outer and an inner part; the inner core is the innermost layer of all. Therefore, option C follows directly. This question asks about location, not which layer is thickest or hottest.

The crust is Earth’s outermost solid layer, so it is encountered first below the surface. The mantle lies beneath the crust, while the outer core and inner core are much deeper inside Earth. Exam tip: remember the order—crust, mantle, outer core, inner core.

The core lies directly below the mantle and is composed mainly of iron and nickel. The crust is the outermost solid layer, so it lies above the mantle, not below it. Exam tip: remember the main internal layers from outside to inside as crust–mantle–core.

The highest temperature is near the centre in the inner core. Exam tip: connect highest temperature with the inner core.

The highest pressure is near the centre in the inner core. Exam tip: connect maximum pressure with the core.

The direct answer is A: S waves are slower than P waves. Earthquakes produce different seismic waves. P means primary because these waves usually reach a recording station first, while S means secondary because they usually arrive later. The main reason is speed: P waves are compressional waves and can travel through solids, liquids and gases; S waves are shear waves and travel only through solids. Therefore, when both move through the same suitable material, P waves generally have a greater speed than S waves. Option A is correct because slower means that S waves take more time to cover the same distance. Option B is wrong because S waves do not normally move faster than P waves. Option C is wrong because their speeds are not equal; the difference in arrival time is useful to scientists. Option D is wrong because S-wave speed is not zero in solid rocks, although S waves cannot pass through liquid layers. The accurate memory cue is: P comes before S, so P is faster and S is slower.

A shadow zone is an area where some seismic waves do not reach. Exam tip: connect shadow zone with evidence of Earth's interior.

On chemical basis Earth is divided into crust mantle and core. Exam tip: connect chemical layering with crust-mantle-core.

On mechanical basis the lithosphere is the rigid outer layer. Exam tip: place lithosphere first in mechanical layers.

Asthenosphere is the weak and soft part of the upper mantle. Exam tip: connect asthenosphere with upper mantle.

The lower mantle is called mesosphere in many classifications. Exam tip: connect mesosphere with lower mantle.

Volcanic materials come from inside Earth so they give direct information. Exam tip: remember volcanic materials as direct source.

Seismic waves are indirect sources because they indicate internal structure. Exam tip: seismic waves are the most important indirect source.

Mines and drilling reach only shallow depths so they inform about the upper crust. Exam tip: remember the limit of drilling.

Heavier materials moved downward and lighter materials stayed above causing layers. Exam tip: connect density difference with layering.

The crust has lighter silicate materials so it is the lightest major layer. Exam tip: remember crust as the least dense layer.

The behaviour of seismic waves reveals the position and nature of layers. Exam tip: treat wave behaviour as evidence of internal structure.

Oceanic crust is generally thinner and denser than continental crust. It is mainly associated with basaltic rocks, which contain relatively heavy minerals, whereas continental crust is generally thicker and has a lower average density. Because oceanic crust is denser, it commonly lies lower and can be pushed beneath less-dense continental crust at a subduction zone. Thus, option B correctly gives both comparisons.

The question asks for the general relationship, not an exception or a special local case. Oceanic crust is usually only a few kilometres thick compared with the much thicker continental crust, and its average density is higher. Option A reverses both properties, option C ignores the difference, and option D is scientifically false because crust is made of rocks, not only ice. Therefore, “thinner and denser” is the correct description.

The direct answer is C: the order from outermost to innermost is crust, mantle, core. Start at Earth’s surface. The crust is the thin, solid outer layer on which we live. Beneath it lies the mantle, a much thicker layer of hot rock that can move very slowly. At the centre is the core, mainly made of iron and nickel; its outer part is liquid and its inner part is solid because of enormous pressure. Option C is correct because it follows the required outside-to-inside direction: crust first, mantle second, core last. Option A is the reverse inward arrangement and begins with the core, so it is wrong. Option B places the mantle before the crust, so it does not begin with the outermost layer. Option D puts the core between the crust and mantle, which is structurally incorrect. Memory trick: “C-M-C” means crust, mantle, core when moving inward.

The direct answer is A: AL represents alumina, commonly written as aluminium oxide. The word SIAL is formed from SI for silica and AL for alumina. In school geography, SIAL is used for the lighter, silica-and-alumina-rich upper continental crust, although it is a traditional compositional term rather than a complete description of every rock. Option A is correct because AL in SIAL refers to alumina, the aluminium-rich component. Option B is wrong because iron is represented by Fe and is associated more strongly with the heavier sima or nife terms, not AL. Option C is wrong because nickel is represented by Ni; nickel is commonly remembered with iron in NIFE, a term for the core. Option D is wrong because magnesium is represented by Mg and is not what AL means. The reasoning is therefore: split SIAL into Si + Al, identify Si as silica and Al as alumina, and select A. A helpful memory cue is “SIAL = silica plus alumina,” while “NIFE = nickel plus ferrous iron.”