Interior structure of the earth

Correct answer: A. It is a source of internal heat

Radioactive decay contributes to maintaining Earth's internal heat. Exam tip: remember it among heat sources.

Correct answer: B. Due to very high pressure and concentration of heavy materials

Inner core has extreme pressure and concentrated heavy metallic material. Exam tip: remember maximum density near the centre.

Correct answer: C. Mechanical classification

Rigidity and flow are mechanical properties. Exam tip: place lithosphere and asthenosphere in mechanical classification.

Correct answer: D. Asthenosphere

The direct answer is D, Asthenosphere. Chemical classification divides Earth according to what materials the layers are made of. By this system, the broad layers are crust, mantle and core. The crust is the outer rocky layer, the mantle is the thick layer below it, and the core is mainly metallic, especially iron and nickel. Asthenosphere is different: it is identified mainly by its physical or mechanical behaviour. It is a weak, hot, partly plastic zone in the upper mantle on which tectonic plates can move. Option A, Crust, is correct as a chemical layer. Option B, Mantle, is also a chemical layer. Option C, Core, is also a chemical layer. Option D, Asthenosphere, is not a chemical division; it belongs to the mechanical or physical classification of Earth’s interior. Do not confuse the material-based names crust, mantle and core with behaviour-based names such as lithosphere, asthenosphere, mesosphere, outer core and inner core. Memory cue: “chemical means composition; mechanical means behaviour.”

Correct answer: A. It provides a soft, semi-fluid base for the movement of lithospheric plates

The asthenosphere is a relatively weak, hot and semi-fluid zone in the upper mantle below the rigid lithosphere. Its plastic nature helps lithospheric plates move slowly, so option A is correct. Option B describes the inner core, whereas the asthenosphere is not located at Earth’s centre. Exam tip: remember the asthenosphere as the weak layer beneath the plates.

Correct answer: A. Seismic wave behaviour changes around 2900 kilometres depth

Around 2900 kilometres Gutenberg discontinuity marks mantle core boundary. Exam tip: remember this depth.

Correct answer: C. It is boundary of outer core and inner core

Lehmann discontinuity is linked with boundary between outer and inner core. Exam tip: treat it as boundary within core.

Correct answer: B. Mantle

Mantle is the most extensive major layer of Earth. Exam tip: remember mantle as thickest and largest volume layer.

Correct answer: B. SIAL continental light and SIMA oceanic dense

SIAL is linked with lighter continental material and SIMA with denser oceanic material. Exam tip: remember both with crust types.

Correct answer: B. Lithosphere

Lithosphere includes crust and the rigid part of upper mantle. Exam tip: understand it as rigid outer shell.

Correct answer: C. Mesosphere

Lower mantle is linked with mesosphere in mechanical classification. Exam tip: do not confuse it with atmospheric mesosphere.

Correct answer: A. Silica and magnesium are associated with SIMA, whereas the core is mainly nickel and iron (NIFE)

Silica and magnesium are associated with SIMA, a term used for the Earth’s upper rocky layers. In contrast, the core is composed mainly of iron and nickel, called NIFE. Option D is also incorrect because the inner core is largely solid and the core contains nickel as well. Exam tip: Remember SIAL, SIMA and NIFE as composition-based terms for the Earth’s layers.

Correct answer: A. Because their speed and type depend on elastic properties of medium

Wave behaviour is affected by rigidity and elasticity of medium. Exam tip: treat seismic waves as clues of material properties.

Correct answer: B. Studying direct samples with indirect geophysical clues

Direct sources are limited so study with indirect clues is more reliable. Exam tip: remember combined evidence method.

Correct answer: C. Both increase with depth

With depth internal heat and weight of overlying rocks increase. Exam tip: connect both temperature and pressure with depth.

Correct answer: A. Density increases from crust to core

Heavy materials are more concentrated near centre so density is highest in core. Exam tip: study density order from surface to centre.

Correct answer: A. Change in properties of medium

P wave speed depends on properties like density and elasticity. Exam tip: link speed decrease with medium change.

Correct answer: B. Oceanic basaltic and continental granitic

Oceanic crust is basaltic and continental crust is granitic. Exam tip: remember basalt ocean and granite continent.

Correct answer: A. Continental crustal roots may be deeper there

High mountains may have roots of thick light crust below them. Exam tip: study mountain roots with isostasy.

Correct answer: B. Liquid and metallic

The outer core is the liquid part of Earth’s core and is composed mainly of metals such as iron and nickel. Therefore, “liquid and metallic” is correct. The solid metallic part is the inner core, while silica-aluminium-rich material is associated with the crustal SIAL layer. Exam tip: remember that the motion of liquid metals in the outer core helps generate Earth’s magnetic field.

Correct answer: A. Solid and under extreme pressure

Inner core is considered solid due to extreme pressure. Exam tip: write pressure as reason for solidity despite high temperature.

Correct answer: A. Its thickness is much less than mantle and core

Crust is the outer layer but very thin in thickness. Exam tip: connect both outermost and thinnest with crust.

Correct answer: B. Incorrect because mantle is mostly solid but may show slow flow

The mantle is the thick layer between Earth’s crust and core. It is made mainly of very hot, solid rock, not water or gas and not a completely liquid ocean. Because of the enormous heat and pressure, some mantle rock can deform and move extremely slowly over long periods. In certain regions, limited melting can occur, but this does not mean that the entire mantle is molten. Its slow movement helps explain processes such as convection and the movement of tectonic plates, while the mantle as a whole remains predominantly solid.

Option B is correct because it states both essential facts: the mantle is mostly solid, and it can show slow flow-like movement. Option A uses a false explanation and wrongly treats the mantle as water; option C confuses rock with gas; and option D incorrectly places the mantle outside Earth. Therefore, calling the mantle a completely molten ocean is an inaccurate statement. The careful evaluation is to reject the claim while remembering that solid rock can slowly deform under great heat, pressure and geological time.

Correct answer: C. From about 2900 kilometres depth

Core begins around 2900 kilometres depth where Gutenberg boundary is considered. Exam tip: remember core start with Gutenberg.

Correct answer: A. Combined analysis of seismic waves with gravity magnetic and direct sources

One source can be limited so combined study of many evidences is better. Exam tip: keep seismic evidence most central for internal structure.