UPPSC 2022 Prelims GS Paper-I · Question 21 of 150

Correct — B, A-3, B-4, C-1, D-2. Every pair in this code is the recognised deepest sounding of its ocean. A–3: the Pacific's floor drops to the Challenger Deep in the Mariana Trench, at roughly 10,925 m the deepest known point in any ocean. B–4: the Arctic Ocean's deepest point is the Molloy Deep, a small hole about 5,550 m down in the Fram Strait between Greenland and Svalbard — modest by trench standards, because the Arctic has no subduction zone of its own. C–1: the Indian Ocean bottoms out in the Sunda (Java) Trench, where the Java Deep lies about 7,200 m down along the arc where the Indo-Australian plate dives beneath the Sunda plate. D–2: the Atlantic's deepest point is the Milwaukee Deep in the Puerto Rico Trench, roughly 8,376 m, on the boundary between the North American and Caribbean plates. Read the four together and the code A-3, B-4, C-1, D-2 falls out.

• (a)A-3, B-2, C-1, D-4 — This is the near-miss, and the only one worth real attention: it gets the Pacific and the Indian Ocean right but swaps the Arctic and the Atlantic. It puts the Puerto Rico Trench in the Arctic and the Molloy Deep in the Atlantic — the reverse of the truth. Puerto Rico is in the Caribbean, at about 18° N; the Molloy Deep is at about 79° N in the Fram Strait.
• (c)A-4, B-3, C-2, D-1 — Every pair is wrong. It sends the Mariana Trench to the Arctic — a trench that lies just east of the Mariana Islands in the western Pacific — and the shallow Molloy Deep to the Pacific, which would make the world's deepest ocean the one with the shallowest deepest point.
• (d)A-1, B-2, C-3, D-4 — Also wrong throughout: it moves the Sunda Trench (an Indian Ocean feature off Sumatra and Java) into the Pacific and the Mariana Trench into the Indian Ocean. It is the code you get by pairing A-1, B-2, C-3, D-4 in sequence without reading List-II at all — a pattern UPPSC plants often, and it is almost never right.

Ocean trenches are the deepest features of the sea floor and they are not scattered at random: they mark subduction zones, where one lithospheric plate bends and descends beneath another. That is why the Pacific — ringed almost entirely by subduction zones, the 'Ring of Fire' — holds the deepest trenches in the world, while the Arctic, which has a spreading ridge (the Gakkel Ridge) but no subduction of its own, has a deepest point about two-thirds as deep as the Atlantic's. Each ocean's 'maximum deepest point' is therefore a named deep inside a named trench, and the two names are usually different: Challenger Deep sits inside the Mariana Trench, Milwaukee Deep inside the Puerto Rico Trench, Java Deep inside the Sunda Trench.

The efficient way in is to fix the two you certainly know — Pacific = Mariana — and then use the odd name out. 'Molloy deep' is the only entry in List-II that is not called a trench, and it is the only one that belongs to the Arctic; once B-4 is fixed, the code is decided, because options (a) and (b) differ in nothing else. The trap is geographic instinct: 'Puerto Rico' sounds tropical and 'Molloy' sounds Nordic, which is exactly right — yet option (a) inverts them, and candidates who match on the code pattern rather than on the names walk into it.

• Pacific Ocean — Challenger Deep, Mariana Trench, about 10,925 m: the deepest known point on Earth's sea floor
• Atlantic Ocean — Milwaukee Deep, Puerto Rico Trench, about 8,376 m, at the North American–Caribbean plate boundary
• Indian Ocean — Java Deep, in the Sunda (Java) Trench, about 7,200 m, off Sumatra and Java
• Arctic Ocean — Molloy Deep, about 5,550 m, in the Fram Strait between Greenland and Svalbard
• Trenches form at subduction zones; the Pacific's near-continuous ring of them explains why it holds the deepest water

• Confusing the trench with the deep inside it: the Mariana Trench is the trench, the Challenger Deep is the point
• Assuming the Arctic must have a deep trench because it is an ocean — it has no subduction zone and its Molloy Deep is the shallowest of the four
• Answering match-list questions from the code pattern (A-1, B-2, C-3, D-4) instead of reading List-II

UPPSC asks ocean-floor relief as a four-way match: ocean to trench, or trench to location. UPSC has asked the same content the other way round — naming a single trench and asking which ocean it sits in, or matching several trenches to North Pacific / South Pacific / Indian / South Atlantic. Learn the trench-plus-deep pairs and both formats are covered.

• practice — not a real PYQ

  The Challenger Deep, the deepest known point of the world's oceans, lies in which trench?

  • (a)Puerto Rico Trench
  • (b)Mariana Trench
  • (c)Tonga Trench
  • (d)Sunda Trench

  Answer(b) Mariana Trench — the Challenger Deep is a small slot at its southern end, about 10,925 m below sea level, in the western Pacific.

• practice — not a real PYQ

  Which one of the following statements about ocean trenches is correct?

  • (a)Trenches are formed along mid-oceanic ridges where plates move apart
  • (b)Trenches are formed at subduction zones where one plate descends beneath another
  • (c)Trenches are the drowned valleys of rivers that flowed during the Ice Age
  • (d)Trenches occur only in the Atlantic Ocean

  Answer(b) Trenches are formed at subduction zones where one plate descends beneath another — which is why the Pacific, ringed by subduction zones, holds the deepest of them.