Interior structure of the earth

Continuation of S waves shows a solid medium and velocity rise points to mantle. Exam tip: treat Moho as the crust mantle boundary.

S waves cannot pass through liquids while P waves can pass and refract. Exam tip: link liquid outer core with absence of S waves.

Partial melting in the weak upper mantle can reduce wave velocity. Exam tip: connect low velocity zone with asthenosphere.

P waves can travel through solids liquids and gases. Exam tip: remember different medium abilities of P and S waves.

At Gutenberg boundary mantle and outer core have different nature. Exam tip: remember it around 2900 kilometres.

SIAL is linked with light crustal material rich in silica and alumina. Exam tip: remember NIFE for the core.

Thick light continental crust relates to mountain roots and isostasy. Exam tip: study thickness and density together.

Dense oceanic crust can sink at plate boundaries. Exam tip: connect basaltic dense oceanic crust with subduction.

Lehmann discontinuity is related to the boundary of outer and inner core. Exam tip: treat it as a boundary within the core.

Extreme pressure prevents melting despite high temperature. Exam tip: write pressure as the reason for solid inner core.

Seismic tomography infers internal structure from changes in wave velocity. Exam tip: see it as an advanced indirect method.

Mantle convection is slow material flow linked with Earth's internal heat. Exam tip: connect convection with plate motion.

Gravity anomalies may indicate internal density distribution. Exam tip: treat gravity anomaly as indirect evidence.

Radioactive decay and formation heat contribute to internal temperature. Exam tip: keep heat source and heat flow separate.

Geothermal gradient helps understand distribution of internal heat. Exam tip: connect it with depth temperature relation.

Mining and drilling are shallow while seismic evidence indicates deeper parts. Exam tip: keep source limitation clear.

Mantle has silicate rocky materials rich in magnesium and iron. Exam tip: link ultramafic clue with mantle.

Conrad discontinuity is linked with division within continental crust. Exam tip: distinguish it from Moho.

The direct answer is option A: S waves pass through many mantle parts, and the mantle is solid but can behave plastically. A completely liquid mantle would not allow S waves to travel through it, because shear waves require a material that can resist sideways deformation. Earthquake evidence shows that the mantle is predominantly solid. Its rocks can flow extremely slowly under great heat and pressure, so “plastic behaviour” does not mean that the whole mantle is liquid. Option A is correct and gives both the seismic evidence and the correct physical description. Option B is wrong because the mantle lies inside Earth, below the crust, not in the atmosphere. Option C is wrong because it is made mainly of hot silicate rocks and minerals, not sea water. Option D is wrong because P waves do enter and travel through the mantle. The core may have liquid and solid parts, but that fact must not be wrongly applied to the entire mantle. Memory cue: S waves need rigidity; mantle is solid, hot, and slowly flowing.

Chemical order is crust mantle core and materials become heavier with depth. Exam tip: remember composition clues.

P waves travel in liquid so their shadow zone mainly forms by strong refraction. Exam tip: keep causes of P and S shadow zones separate.

S waves inability to travel in liquid directly links to liquid outer core. Exam tip: remember S wave absent equals liquid clue.

Earth average radius is about 6371 kilometres and core starts around 2900 kilometres. Exam tip: do not confuse radius and depth.

Density difference affects subduction behaviour of plates. Exam tip: remember oceanic dense and subduction together.

The direct answer is A. Pressure inside Earth is mainly caused by the weight of material lying above a particular point. As depth increases, more rocks and other matter press downward on the material below. The overlying load, or overburden, therefore becomes greater, and pressure generally increases toward the centre. High temperature also affects the state and behaviour of deep materials, but it is not the basic reason given in this question for pressure increase. Option A is correct because it identifies the increasing weight of overlying rocks and materials. Option B is wrong because sunlight does not travel directly to the core and does not create the internal pressure. Option C is wrong because ocean water cannot flow through the solid Earth to the centre. Option D is wrong because the atmosphere is only a thin outer layer; its weight is tiny compared with the enormous load of rocks above deep regions. A useful chain is: greater depth → more overlying material → greater load → greater pressure. Memory cue: “pressure comes from overburden.”