Geomorphology — Practice Questions
UPSC Prelims-pattern practice questions on geomorphology — Earth's interior, discontinuities, plate boundaries, rocks, karst, glacial and aeolian landforms.
Questions
These are original UPSC-pattern practice questions. Attempt each one before revealing the answer.
Which one of the following discontinuities separates the Earth’s mantle from its outer core?
Solution
The Gutenberg discontinuity (about 2,900 km deep) separates the mantle from the outer core.
- Conrad — within the continental crust (upper granitic and lower basaltic layers).
- Moho — crust and mantle.
- Lehmann — outer core and inner core.
Order from top to bottom: Conrad, Moho, Repetti, Gutenberg, Lehmann.
Answer: C
Consider the following statements:
- The ocean floor is generally younger than the continental crust.
- Rocks near mid-ocean ridges are older than rocks near ocean trenches.
- Bands of alternating magnetic polarity are found symmetrically on both sides of mid-ocean ridges.
Which of the statements given above are correct?
Solution
- Statement 1 — correct. Ocean-floor rocks are nowhere older than about 200 million years; some continental rocks exceed 3,000 million years.
- Statement 2 — wrong. New crust is created at the ridges, so the youngest rocks lie at the ridge crest; age increases towards the trenches, where old crust is subducted.
- Statement 3 — correct. As new rock cools it records the Earth’s magnetic field; periodic reversals leave matching stripes on either side of the ridge — the key evidence for Hess’s sea-floor spreading.
Answer: C
Consider the following pairs:
| Plate boundary | Example |
|---|---|
| 1. Continent–continent convergence | Himalaya |
| 2. Ocean–continent convergence | Andes |
| 3. Transform boundary | Mid-Atlantic Ridge |
Which of the pairs given above is/are correctly matched?
Solution
- Pair 1 — correct. The Himalaya rose from the collision of the Indian and Eurasian plates.
- Pair 2 — correct. The oceanic Nazca plate subducts beneath the South American plate.
- Pair 3 — wrong. The Mid-Atlantic Ridge is a divergent boundary. The San Andreas Fault is the standard example of a transform boundary. (Ridges are offset by short transform faults, but the ridge itself is divergent.)
Answer: A
Which one of the following landforms is produced by glacial deposition?
Solution
An esker is a sinuous ridge of sand and gravel laid down by meltwater streams flowing under or within a glacier — a depositional landform.
A cirque (armchair-shaped hollow), an arête (knife-edge ridge between cirques) and a hanging valley (tributary valley left high above a deepened main glacial valley) are all products of glacial erosion.
Answer: D
Consider the following statements about karst topography:
- It develops mainly in areas of limestone or dolomite.
- Stalactites grow upward from the floor of limestone caves.
- Carbonation is the main chemical process responsible for it.
How many of the above statements are correct?
Solution
- Statement 1 — correct. Karst needs soluble carbonate rocks.
- Statement 2 — wrong. Stalactites hang from the ceiling (they hold tight); stalagmites rise from the ground.
- Statement 3 — correct. Carbon dioxide dissolved in rainwater forms weak carbonic acid, which dissolves limestone.
So two statements are correct.
Answer: B
Mains
“Wegener’s theory of continental drift was right about the evidence but wrong about the mechanism.” Examine this statement in the light of later developments in plate tectonics.
Solution
Introduction: In 1912 Alfred Wegener proposed that all continents once formed Pangaea and later drifted apart. His evidence was compelling, but the forces he proposed could not move continents, so the theory was widely rejected for decades.
Body:
- Evidence that survived: jigsaw fit of South America and Africa; matching rock belts across the Atlantic; tillite of the same age in India and other southern continents; fossils such as Mesosaurus and Glossopteris across now-separated continents; placer gold in Ghana sourced from Brazil.
- Mechanism that failed: pole-fleeing force and tidal force were far too weak to move continents; Wegener also pictured continents ploughing through the ocean floor, which is physically implausible.
- What filled the gap: Arthur Holmes (1930s) — mantle convection currents; Harry Hess (1961) — sea-floor spreading at ridges and destruction at trenches; palaeomagnetic striping and the young age of ocean floor as proof.
- Plate tectonics (1967–68): it is lithospheric plates — continents and ocean floor together — that move over the asthenosphere; boundaries explain earthquakes, volcanoes and fold mountains.
- Indian example: the northward drift of the Indian plate and its collision with Eurasia explain the Himalaya and the region’s seismicity.
Conclusion: Wegener correctly saw that continents move; plate tectonics explained how. The statement is broadly valid — his evidence became the foundation of a theory whose mechanism he could not supply.