Which of the following statements is/are correct? 1. The Earth's crust is brittle in nature. 2. The mean thickness of the oceanic crust is 15 km, whereas that of the continental crust is around 30 km. Select the correct answer using the code given below.
- (a)1 only
- (b)2 only
- (c)Both 1 and 2
- (d)Neither 1 nor 2
Correct — A, 1 only. Statement 1 is right and statement 2 fails on one number. The crust is the outermost solid shell of the Earth and it is brittle — it responds to stress by fracturing rather than flowing, which is exactly why faults form and why shallow earthquakes are crustal events. Statement 2 gets the continental figure right, around 30 km, but overstates the oceanic crust more than twofold. Oceanic crust is thin and remarkably uniform, about 5 km in the standard school figure and roughly 7 km in geophysical measurements, and it is essentially nowhere as thick as 15 km. Since one number in a compound statement is wrong, the whole statement is wrong, and only statement 1 survives.
- (b)2 only — This rejects the brittleness of the crust, which is one of its defining mechanical properties, and accepts a thickness figure that is wrong. Both halves of the choice fail.
- (c)Both 1 and 2 — Tempting because the 30 km continental figure in statement 2 is correct and reads familiar. But the oceanic figure of 15 km is roughly two to three times the real value, and a compound statement is only as true as its weakest number.
- (d)Neither 1 nor 2 — This denies statement 1 as well. The crust's brittleness is not in doubt — it is what distinguishes the rigid crust and uppermost mantle from the ductile asthenosphere below.
The Earth is layered into crust, mantle and core, and the crust is the thin outermost skin, separated from the mantle by the Mohorovicic discontinuity. The crust is not uniform. Oceanic crust is made of dense basaltic rock, is constantly created at mid-ocean ridges and destroyed at subduction zones, and is therefore both thin and young. Continental crust is lighter granitic rock, is not recycled the same way, and is far thicker — thickest of all beneath young fold mountains, where it can exceed 60 km under the Himalaya.
Compound statements that bundle two numbers are among the most common ways an examiner plants an error, and the fix is to check each number separately rather than judging the sentence as a whole. Here the continental number is right and the oceanic number is wrong, and a reader who recognises only the familiar 30 km will accept the whole line. Fix the contrast instead — oceanic crust is thin and dense, continental crust is thick and light. Textbook and research figures differ slightly for the oceanic crust, with NCERT giving 5 km and geophysical surveys about 7 km, but nothing supports 15 km, so the statement fails on either source.
- The crust is the outermost solid layer of the Earth and behaves brittly, fracturing under stress instead of flowing.
- Oceanic crust is thin and uniform, about 5 km in the standard textbook figure and roughly 7 km in geophysical measurement, and generally less than 10 km.
- Continental crust averages about 30 km and thickens greatly beneath young fold mountains such as the Himalaya.
- Oceanic crust is denser, about 3.0 g/cm³ against roughly 2.7 g/cm³ for continental crust, and the Mohorovicic discontinuity marks the base of the crust.
The continental figure in statement 2 is right and the oceanic figure is wrong, which is why only statement 1 stands.
- Accepting a two-number statement because one of the numbers is familiar and correct.
- Confusing crust with lithosphere; the lithosphere is the crust plus the rigid uppermost mantle.
- Assuming the crust is thickest under the oceans because ocean basins are deep — it is thinnest there.
As a statement pair like this, as a discontinuity-naming item, or as a question on where the crust is thickest and thinnest.
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 next layer down in the same cross-section. Both items reward one memorised diagram of the Earth's interior — crust thickness and behaviour at the top, mantle in the middle, iron-nickel core at the bottom.
Which one of the following best describes the Lithosphere?
- (a) Upper and lower mantle
- (b) Crust and core
- (c) Crust and upper mantle
- (d) Lower mantle and core
Answer(c) Crust and upper mantle
The companion definition. This NDA item fixes what the lithosphere includes, while the 2022 item tests how thick and how brittle its crustal part is — and the brittleness in statement 1 is exactly what makes the lithosphere behave as rigid plates.
- practice — not a real PYQ
The boundary that separates the Earth's crust from the mantle is called the
- (a)Conrad discontinuity
- (b)Mohorovicic discontinuity
- (c)Gutenberg discontinuity
- (d)Lehmann discontinuity
Answer(b) Mohorovicic discontinuity — the Gutenberg discontinuity lies between mantle and core.
- practice — not a real PYQ
Compared with continental crust, oceanic crust is
- (a)thicker and lighter
- (b)thinner and denser
- (c)thicker and denser
- (d)thinner and lighter
Answer(b) thinner and denser — a few kilometres of basaltic rock against about 30 km of lighter granitic rock.