Interior of the Earth – NDA Geography PYQs

NDA 2024-I

Q. 1. 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

Answer: (b) 1 and 2 only

Explanation: Statement 1 (Oceanic crust heavier than continental): Correct. Oceanic crust is composed mainly of basalt (density ~3.0 g/cm³). Continental crust is composed mainly of granite (density ~2.7 g/cm³). Oceanic crust is denser and heavier: this is why continental plates override oceanic plates at subduction zones. Statement 2 (Most heat in the mantle): Correct. The mantle, being the largest layer by volume, contains the majority of Earth’s internal heat. Heat is generated by radioactive decay in the mantle and core, but the mantle’s volume gives it the largest total heat content. Statement 3 (Convective cells in the crust): Incorrect. Convective cells (convection currents) occur in the MANTLE, not the crust. The crust is too rigid and thin for convection. Mantle convection drives plate tectonics by causing tectonic plates (lithosphere) to move. Concept Tested: Earth structure: oceanic crust denser; most heat in mantle; convective cells in mantle (NOT crust)

NDA 2023-I

Q. 2. Which one of the following is found in the innermost part of the Earth?

(a) Conrad discontinuity

(b) Moho discontinuity

(c) Guttenberg discontinuity

(d) Lehman discontinuity

Answer: (d) Lehman discontinuity

Explanation: Discontinuities mark boundaries between Earth’s layers, from outer to inner: Conrad discontinuity: within the crust (boundary between upper and lower continental crust). Mohorovičić (Moho) discontinuity: boundary between crust and mantle. Gutenberg (Wiechert-Gutenberg) discontinuity: boundary between mantle and outer core. Lehmann discontinuity: boundary between outer core and inner core; the innermost of all the listed discontinuities. Since the Lehmann discontinuity marks the boundary between the liquid outer core and the solid inner core, it is found deepest: in the innermost part of the Earth. Concept Tested: Discontinuities from surface to core: Conrad (crust), Moho (crust-mantle), Gutenberg (mantle-outer core), Lehmann (outer-inner core)

★ JOVIK Exam Insight Earth’s internal boundaries (discontinuities) for NDA: from outermost to innermost: 1. Conrad discontinuity: within continental crust (~20 km depth). 2. Mohorovičić (Moho) discontinuity: crust-mantle boundary (~30-70 km). 3. Repetti discontinuity: upper-lower mantle boundary (~650 km). 4. Gutenberg (Wiechert-Gutenberg) discontinuity: mantle-core boundary (~2,900 km). 5. Lehmann discontinuity: outer core–inner core boundary (~5,100 km). Lehmann = innermost = deepest boundary.

NDA 2022-II

Q. 3. 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

Answer: (a) 1 only

Explanation: Statement 1 (Earth’s crust is brittle): Correct. The Earth’s crust is a brittle, rigid layer. When stress exceeds its strength, it fractures rather than flows. This is why earthquakes occur: sudden brittle fracture along faults. Statement 2 (Oceanic crust = 15 km, continental = 30 km): Incorrect. The mean thickness of the oceanic crust is approximately 5–10 km (NOT 15 km). Continental crust averages about 35–40 km (not 30 km exactly). The oceanic crust figure of 15 km significantly overstates its actual thickness. Concept Tested: Earth’s crust: brittle in nature; oceanic crust is ~5-10 km (not 15 km); continental ~35-40 km (not 30 km)

NDA 2021-II

Q. 4. 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

Explanation: This is the third NDA question on Lithosphere. The consistent answer across NDA 2015-II, 2019-II, and 2021-II is: Lithosphere = Crust + Upper Mantle. NDA has tested this concept three times because it is fundamental and frequently confused. The lithosphere does NOT include the core or the lower mantle. Concept Tested: Lithosphere = Crust + Upper Mantle: tested 3 times in NDA (2015-II, 2019-II, 2021-II)

NDA 2021-I

Q. 5. The maximum depth of Lithosphere is found in the:

(a) Pacific Ocean

(b) Siberian Plain

(c) Patagonian Desert

(d) Himalayan Mountains

Answer: (d) Himalayan Mountains

Explanation: The lithosphere is thickest (deepest) under mountain ranges, particularly ancient and geologically active ones. Under the Himalayan Mountains, the continental crust has deep roots extending downward: the crust-mantle boundary (Moho) can be at depths of 60–70+ km. This gives the Himalayan lithosphere the maximum depth among the options. Under ocean floors, the lithosphere is thinner. Siberian Plain and Patagonian Desert have moderate lithospheric thickness. Concept Tested: Thickest (deepest) lithosphere under the Himalayan Mountains: mountain roots extend the crust to ~70 km

NDA 2019-II

Q. 6. Lithosphere consists of:

(a) Upper and lower mantle

(b) Crust and core

(c) Crust and uppermost solid mantle

(d) Mantle and core

Answer: (c) Crust and uppermost solid mantle

Explanation: This is the same concept as Q. 2 (NDA 2015-II), phrased slightly differently. The Lithosphere consists of the crust and the uppermost solid portion of the mantle. The key distinction: the lithosphere includes only the solid (rigid) part of the mantle: not the semi-molten asthenosphere below it. Option (c) correctly captures this by specifying “uppermost solid mantle”. Concept Tested: Lithosphere = crust + uppermost solid mantle; the rigid layer above the semi-fluid asthenosphere

NDA 2015-II

Q. 7. Which one of the following describes the Lithosphere?

(a) Upper and lower mantle

(b) Crust and upper mantle

(c) Crust and core

(d) Mantle and core

Answer: (b) Crust and upper mantle

Explanation: The Lithosphere is the outermost rigid layer of the Earth. It consists of the crust (both oceanic and continental) and the uppermost part of the mantle. Together, these form the solid, rigid outer shell that is broken into tectonic plates. The Lithosphere is distinct from the Asthenosphere: the semi-fluid layer of the upper mantle below the lithosphere, on which the tectonic plates float and move. The term “Lithosphere” comes from the Greek “lithos” (rock) + “sphaira” (sphere). Concept Tested: Lithosphere = Earth’s crust + upper mantle (rigid outer shell; divided into tectonic plates)

★ JOVIK Exam Insight The Lithosphere question has appeared in NDA 2015-II, 2019-II, and 2021-II: always asking the same concept. Remember: Lithosphere = Crust + Upper Mantle (rigid part). Asthenosphere = semi-fluid upper mantle below the lithosphere. Mesosphere = lower mantle. Outer Core = liquid iron-nickel (causes magnetic field). Inner Core = solid iron-nickel (hottest, densest).

NDA 2009-II

Q. 8. The Earth’s crust is the thinnest:

(a) under the mountain ranges

(b) under continental masses

(c) at ocean bottoms

(d) at mid-oceanic ridges

Answer: (c) at ocean bottoms

Explanation: The oceanic crust (beneath the ocean floors) is the thinnest part of Earth’s crust: approximately 5–10 km thick on average. The continental crust (under land masses) is much thicker: approximately 30–50 km on average, and even thicker (up to 70 km) under mountain ranges where the crust has deep roots. Mid-oceanic ridges are where new oceanic crust is being created: the crust is thinnest here among oceanic areas, but the ocean bottom overall is the correct answer among the given options. Concept Tested: Oceanic crust is thinnest (~5-10 km); continental crust is thicker (~30-70 km under mountains)

---