CDS (II) 2020 General Knowledge · Question 62 of 120

Correct — B, Mariana Trench. Its southern end holds the Challenger Deep, the deepest measured point on the ocean floor at roughly 10,900 to 11,000 metres below sea level — deep enough to swallow Mount Everest with about two kilometres of water to spare. The trench lies in the western Pacific, east of the Mariana Islands, and is formed where the Pacific plate is forced under the smaller Mariana plate. Depth figures here should always be quoted loosely: modern dives corrected for salinity and temperature are published with an uncertainty of around fifteen metres, and the Challenger Deep has been given as anything from 10,911 m to 10,994 m depending on the survey. The point being tested is the ranking, and on that there is no argument.

• (a)Tonga Trench — Deep, but not the deepest. The Tonga Trench in the south-west Pacific reaches about 10,800 m at Horizon Deep, which puts it second in the world and first in the southern hemisphere.
• (c)Philippine Trench — The Philippine Trench, east of the Philippine islands, bottoms out near 10,540 m at the Galathea Depth. Close, and a favourite distractor precisely because it is in the same corner of the Pacific.
• (d)Kermadec Trench — The Kermadec Trench runs north-east of New Zealand and reaches about 10,000 m. It is the continuation of the Tonga Trench system, which is why the two are so often offered together.

Ocean trenches are long, narrow, arc-shaped troughs that form at convergent boundaries, where one plate is subducted beneath another. They are the deepest parts of the ocean floor and are seats of deep-focus earthquakes; the volcanic island arcs that parallel them are built from magma generated above the descending slab. NCERT groups them with continental margins in its three-fold division of ocean-floor relief, alongside deep-sea basins and mid-oceanic ridges.

This item is pure recall, but it becomes secure once you attach one deep to each ocean rather than memorising a list of trenches in isolation. The Pacific holds the Mariana Trench and its Challenger Deep; the Atlantic's deepest point is the Milwaukee Deep in the Puerto Rico Trench at about 8,376 m; the Indian Ocean's is the Java or Sunda Trench at about 7,192 m; and the Arctic's is the Molloy Deep, about 5,550 m, in the Fram Strait west of Svalbard. Three of the four options here are Pacific trenches, which is the real difficulty of the question — it asks you to rank within one ocean, not across four.

• The Challenger Deep in the Mariana Trench is the deepest known point of the world ocean, about 10,900-11,000 m; published depths range from 10,911 m to 10,994 m and are best quoted as approximate.
• The Mariana Trench lies in the western Pacific and marks the subduction of the Pacific plate beneath the Mariana plate.
• The Tonga Trench, at roughly 10,800 m, is the second deepest and the deepest in the southern hemisphere.
• Deepest point by ocean: Pacific — Mariana Trench; Atlantic — Puerto Rico Trench (Milwaukee Deep, about 8,376 m); Indian — Sunda/Java Trench (about 7,192 m); Arctic — Molloy Deep (about 5,550 m).
• Trenches occur at convergent boundaries and are the source of the deepest-focus earthquakes recorded.

• Confusing the trench with the deep inside it — Challenger Deep is a point within the Mariana Trench, not a separate feature.
• Assuming the Indian Ocean's deepest point is the Diamantina Trench; the Sunda (Java) Trench is deeper, and the Diamantina comes second.
• Quoting the Challenger Deep to the metre. Surveys disagree by tens of metres.

As a straight 'which is deepest' item, or as a match-the-trench-to-its-ocean list.

• practice — not a real PYQ

  The Challenger Deep, the deepest known point of the world ocean, lies in which one of the following trenches?

  • (a)Puerto Rico Trench
  • (b)Sunda Trench
  • (c)Mariana Trench
  • (d)Peru-Chile Trench

  Answer(c) Mariana Trench — the Challenger Deep lies at its southern end, in the western Pacific.

• practice — not a real PYQ

  Ocean trenches are typically formed at which one of the following?

  • (a)Divergent plate boundaries
  • (b)Convergent plate boundaries
  • (c)Transform faults
  • (d)Hot spots

  Answer(b) Convergent plate boundaries — a trench marks the line where one plate bends down and is subducted beneath another.