NDA (II) 2017 General Ability Test · Question 57 of 150

Correct — B, Western Pacific Ocean. The Mariana Trench is a crescent-shaped gash in the floor of the western Pacific, running for well over two thousand kilometres to the east of the Mariana Islands and not far from Guam, at roughly 11 degrees north and 142 degrees east. It holds the Challenger Deep, the deepest known point in any ocean at about eleven kilometres below the surface, deeper than Mount Everest is high. The trench exists because the old, cold and heavy Pacific plate is being forced down beneath the smaller Mariana plate; the volcanic arc of the Mariana Islands is the surface expression of that same subduction.

• (a)Southern Atlantic Ocean — The South Atlantic's trench is the South Sandwich Trench, near the island arc of the same name. The Mariana Trench is on the other side of the world from it.
• (c)Eastern Pacific Ocean — The right ocean but the wrong side of it, which makes this the closest and most tempting wrong answer. The eastern Pacific's trenches lie along the Americas — the Peru-Chile or Atacama Trench and the Middle America Trench — while the Mariana Trench is close to Guam in the far west.
• (d)Northern Atlantic Ocean — The North Atlantic's deepest feature is the Puerto Rico Trench with its Milwaukee Deep, north of the Greater Antilles. The Atlantic as a whole is dominated by a spreading ridge rather than by long subduction trenches.

Ocean trenches form where one lithospheric plate slides beneath another. They are therefore concentrated around the rim of the Pacific, the belt of subduction zones and volcanic arcs known as the Ring of Fire, and are comparatively rare in the Atlantic, whose floor is mainly being pulled apart at the Mid-Atlantic Ridge. Each trench is paired with an island arc or a mountain chain on the overriding plate.

Two of the four options say Pacific, so the marks turn on east against west. Fix the trench to the Mariana Islands and Guam, which are in the far western Pacific near the Philippine Sea, and the answer follows. A short list worth memorising: Mariana Trench in the western Pacific holds the Challenger Deep, the deepest point in the world ocean; the Puerto Rico Trench is deepest in the Atlantic; the Sunda or Java Trench is deepest in the Indian Ocean. Depths are quoted with real uncertainty, so say 'about eleven kilometres' rather than a figure to the metre.

• The Mariana Trench lies in the western Pacific, east of the Mariana Islands and near Guam.
• Its Challenger Deep, at about eleven kilometres, is the deepest known point in any ocean.
• The trench is a subduction zone where the Pacific plate descends beneath the Mariana plate.
• The Puerto Rico Trench is the deepest part of the Atlantic and the Sunda or Java Trench the deepest of the Indian Ocean.
• Most of the world's deep trenches ring the Pacific, along the Ring of Fire.

• Settling for 'Pacific' without deciding between the eastern and western sides.
• Quoting the depth of the Challenger Deep to the metre; published figures differ by tens of metres.
• Expecting long trenches in the Atlantic, whose floor is dominated by a spreading ridge.

NDA and UPSC both ask trench-to-ocean matching, so learn a handful of trenches with the ocean and the plate boundary that made each one.

• practice — not a real PYQ

  The deepest known point of the world ocean, the Challenger Deep, lies in

  • (a)the Puerto Rico Trench
  • (b)the Mariana Trench
  • (c)the Sunda Trench
  • (d)the South Sandwich Trench

  Answer(b) the Mariana Trench — the Challenger Deep sits at its southern end in the western Pacific.

• practice — not a real PYQ

  Deep ocean trenches are formed mainly at

  • (a)divergent plate boundaries
  • (b)convergent plate boundaries where one plate subducts
  • (c)transform faults
  • (d)hotspots in the middle of a plate

  Answer(b) convergent plate boundaries where one plate subducts — the descending slab drags the sea floor down into a trench.