Which of the following statements regarding Earth's internal structure is/are correct? 1. The oceanic crust is heavier than the continental crust. 2. Most of the Earth's internal heat is contained within the mantle. 3. Large convective cells in the crust circulate heat and drive plate-tectonic processes. Select the answer using the code given below:
- (a)3 only
- (b)1 and 2 only
- (c)2 and 3 only
- (d)1, 2 and 3
Correct — B, statements 1 and 2 only. Oceanic crust is made of dense basaltic rock (about 3.0 g/cm3), which is heavier than the lighter granitic continental crust (about 2.7 g/cm3), so statement 1 holds — this density contrast is why oceanic plates subduct beneath continental ones. The mantle occupies roughly 84 percent of Earth's volume and about two-thirds of its mass, so it holds most of the planet's internal heat, making statement 2 correct. Statement 3 is wrong on location: the great convection cells that circulate heat and drag the plates flow within the hot, plastic mantle, not within the thin, rigid crust. With statement 3 removed, the correct combination is 1 and 2 only.
- (a)3 only — Statement 3 is the one false statement — convection cells operate in the mantle, not the crust — and this option also drops the two true statements about crustal density and mantle heat.
- (c)2 and 3 only — Keeps the false statement 3 (convection is a mantle, not a crustal, process) and wrongly discards the true statement 1 about oceanic crust being denser than continental crust.
- (d)1, 2 and 3 — Wrongly includes statement 3; the convection currents that drive plate tectonics circulate in the mantle, so the crust cannot be their seat.
Earth is layered into a thin outer crust, a thick mantle, and a liquid outer core surrounding a solid inner core. The crust comes in two kinds — lighter granitic continental crust and denser basaltic oceanic crust. Because the mantle is by far the largest layer, it stores most of Earth's internal heat, and slow convection within it is the engine of plate tectonics.
The trap is statement 3's phrase 'in the crust'. Heat does move outward through the crust, but the organised convective cells that overturn material and drag the lithospheric plates live in the hot, ductile mantle (asthenosphere and below). The crust is too thin and rigid to convect. Fixing that one word settles the question.
- Oceanic crust is basaltic and denser (about 3.0 g/cm3); continental crust is granitic and lighter (about 2.7 g/cm3), which is why oceanic crust subducts.
- The mantle makes up roughly 84 percent of Earth's volume and about two-thirds of its mass, storing most of the planet's internal heat.
- Convection currents that drive plate movement occur in the mantle, not the crust.
- Earth's internal heat comes from leftover accretional heat and the radioactive decay of elements such as uranium, thorium and potassium.
Statements 1 and 2 hold; the convection cells sit in the mantle, so statement 3 fails — answer 1 and 2 only.
- Placing the convection cells in the crust instead of the mantle.
- Assuming continental crust is denser because it is thicker — it is actually lighter (granitic) than oceanic (basaltic) crust.
A statement-code question on Earth's interior — confirm the crust densities and mantle heat, and remember convection is a mantle process, not a crustal one.
In the structure of planet Earth, below the mantle, the core is mainly made up of which one of the following?
- (a) Aluminium
- (b) Chromium
- (c) Iron
- (d) Silicon
Answer(c) Iron — the core (liquid outer and solid inner) is predominantly an iron-nickel alloy, while aluminium, chromium and silicon are concentrated in the crust and mantle.
Tests the same layered model of Earth's interior — crust, mantle and core — that this NDA question builds on, here pinning the composition of the core.
- practice — not a real PYQ
Convection currents that drive the movement of lithospheric plates occur mainly in which layer of the Earth?
- (a)Crust
- (b)Mantle
- (c)Outer core
- (d)Inner core
Answer(b) Mantle — slow convection in the hot, plastic mantle drags the overlying plates.
- practice — not a real PYQ
Which type of Earth's crust is denser?
- (a)Continental crust
- (b)Oceanic crust
- (c)Both are equally dense
- (d)Neither has a definite density
Answer(b) Oceanic crust — basaltic (~3.0 g/cm3) and denser than granitic continental crust (~2.7 g/cm3).