MPSC 2023 Prelims Paper-I · Question 29 of 100

Correct — option (1). A trench is not simply a deep place in the ocean; it is a long, narrow, arc-shaped depression, thousands of kilometres in length and several kilometres deeper than the ocean floor beside it, and it is the surface expression of one specific process. At a convergent plate boundary where oceanic lithosphere meets another plate, the denser oceanic slab bends downward and descends into the mantle. The line along which it bends and begins its descent is the trench. Everything else about a trench follows from that: it is deep because the sea floor is being pulled down there; it is narrow because the bend is sharp; and it is curved in plan because a rigid shell descending into a sphere folds along an arc. The western Pacific is the most heavily trenched margin on the Earth because it is where oceanic lithosphere is being consumed most extensively — the Pacific plate and the Philippine Sea plate converge along this side against the plates carrying Asia and Australasia, and the whole margin is part of the circum-Pacific belt of subduction commonly called the Ring of Fire. That is why the trenches there come as a chain rather than as isolated features: the Aleutian, Kuril, Japan, Izu-Bonin, Mariana, Philippine and Tonga-Kermadec systems all mark the same process repeated along one boundary, and the Mariana Trench contains the Challenger Deep, the deepest point known in any ocean. The subduction also accounts for the company the trenches keep, since each is paralleled by a belt of earthquakes whose foci grow deeper away from the trench, tracing the descending slab, and by a line of volcanic islands built from magma generated above it. Option (1) names that process, and no other option on the list names a process at all.

• (2)Along the western border of the Pacific Ocean parallel mountain ranges are aligned — The western Pacific margin genuinely is lined with chains of islands and submarine ridges running parallel to the trenches, so this option describes something real. What it gets wrong is the direction of causation: the arcs are a product of subduction, not a cause of it. As the descending slab sinks, it releases water into the hot mantle above it, and the added water lowers the melting point of that mantle enough to generate magma, which rises and builds the volcanic islands. The chain therefore cannot pre-date the trench, since it is manufactured by the same descent that makes the trench. Offering a companion feature as the explanation of the feature asked about is one of the commission's standard constructions, and the test is chronological: ask which of the two could exist without the other. A trench can form before an arc has been built; an arc of this kind cannot form without a subducting slab beneath it.
• (3)The continental slope of the western part of the Pacific Ocean is steep — This offers a description of the sea floor's shape in place of an explanation of how it came to have that shape, and the description is not even accurate to a trench. A continental slope is the descent from the edge of the continental shelf to the deep ocean floor, and it exists on every kind of ocean margin, including entirely quiet ones. The Atlantic is the decisive comparison: its margins have continental slopes as pronounced as any in the Pacific, yet the Atlantic contains almost no trenches, because its edges are passive margins where continent and ocean floor belong to the same plate and nothing is being consumed. Steepness, in other words, does not produce a trench; subduction does. A trench is also a different order of feature altogether, a linear furrow far deeper than the abyssal plain it adjoins and traceable for thousands of kilometres.
• (4)The western part of the Pacific Ocean is characterised by a large number of trenches — This option restates the fact in the stem and offers it as the reason for itself. The stem observes that trenches are found mainly along the western Pacific; the option explains that observation by saying the western Pacific has a large number of trenches. Nothing has been added, and a candidate who accepts it has answered the question with its own premise. Circular options of this shape are worth learning to recognise on sight, because they can be eliminated without any knowledge of the subject at all — a genuine explanation must introduce something the stem did not already contain, whether a process, a structure or a cause. Under time pressure this recognition is valuable: striking out the circular option immediately reduces a four-way choice to a three-way one on grounds of logic alone.

Plate tectonics accounts for the major relief of the ocean floor through three kinds of boundary. At divergent boundaries, plates move apart and new oceanic crust is created; the mid-oceanic ridges are the result, and sea-floor spreading is the process. At transform boundaries, plates slide past one another and crust is neither made nor destroyed. At convergent boundaries, plates move towards each other and something must give way, and what gives way is whichever lithosphere is denser. Oceanic lithosphere is denser than continental, so where ocean meets continent the ocean floor descends beneath the continent, producing a trench offshore and a chain of fold mountains with volcanoes onshore — the Peru-Chile trench and the Andes are the standard example. Where ocean meets ocean, the older and colder of the two descends, producing a trench and, on the overriding plate, an arc of volcanic islands. Where continent meets continent neither will sink, so the crust crumples upward instead and there is no trench at all, which is why the Himalaya has none. The descending slab reveals itself in a belt of earthquakes whose foci deepen away from the trench, the Benioff zone, and the deepest earthquakes recorded anywhere occur along such zones. The Pacific is almost entirely ringed by convergent boundaries of these kinds, which is why the world's trenches, its most active volcanoes and the great majority of its earthquakes are concentrated around it.

Ocean-floor relief and plate tectonics are standing topics in the MPSC geography section, and they are asked in a way that rewards understanding rather than lists, because every feature in the topic — ridge, trench, island arc, rift valley, fold mountain — is the product of one of three plate movements, and knowing which movement produces which feature answers most of the questions. This item is a good example of a question that can be reasoned to the answer without recalling a single trench by name. Of the four options only one names a process; the others name a companion feature, a shape and the question's own premise. That pattern recurs constantly in MPSC's 'because' questions, and it gives a useful first pass through the options: eliminate anything that merely describes, anything that restates the stem, and anything that offers an effect as a cause. A second habit worth carrying is to test any proposed explanation against a place where the explanation would apply but the feature is absent — the Atlantic's continental slopes without trenches settle this question by themselves.

• An oceanic trench is a long, narrow and very deep depression of the sea floor that marks a convergent plate boundary, formed where the denser oceanic lithosphere bends downward and descends into the mantle at a subduction zone.
• Where an oceanic plate subducts beneath another oceanic plate the result is a trench paired with a volcanic island arc, as in the Aleutian, Kuril, Japan, Izu-Bonin, Mariana, Philippine and Tonga-Kermadec systems of the western Pacific.
• The Mariana Trench in the western Pacific contains the Challenger Deep, the deepest point known in any ocean.
• A subduction zone is marked by a belt of earthquakes whose foci grow progressively deeper away from the trench, called the Benioff zone, and by volcanoes fed by magma generated where water released from the descending slab melts the mantle above it.
• The Pacific is nearly encircled by subduction zones, the belt known as the Ring of Fire, while the Atlantic margins are largely passive and carry almost no trenches despite having well-developed continental slopes.

• Accepting a companion feature such as an island arc as the cause of a trench, when both are products of the same subduction and the arc cannot exist without it
• Taking a description of shape, such as a steep slope, for an explanation of origin; the Atlantic has steep continental slopes and almost no trenches
• Failing to spot a circular option that repeats the stem's own observation as its explanation
• Assuming trenches occur only in the western Pacific; they ring the whole ocean, but the western margin has the greatest number and the deepest of them

MPSC asks plate tectonics both as recall and as reasoning. The recall questions name a trench, a ridge or a mountain system and ask which plate movement produced it, or ask which is the deepest trench or the longest ridge. The reasoning questions, of which this is one, state an observed distribution and ask why it exists, and their option sets are typically built from one genuine process and three near misses — an associated feature, a descriptive property and a restatement of the premise. Expect the Pacific to appear more often than the other oceans, because its ring of subduction zones supplies trenches, island arcs, volcanoes and earthquake belts in a single package, and expect the Himalaya to be used as the contrasting case of continental collision where no trench forms.

No directly related past PYQ was found.

• practice — not a real PYQ

  The Challenger Deep, the deepest point so far measured in any ocean, is situated in which of the following trenches ?

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

  Answer(a) The Mariana Trench — it lies in the western Pacific east of the Mariana islands, where the Pacific plate is descending beneath the Philippine Sea plate, and the Challenger Deep at its southern end is the deepest sounding recorded anywhere. The Puerto Rico Trench is the deepest point of the Atlantic, the Sunda Trench lies off Indonesia in the Indian Ocean, and the Peru-Chile Trench marks the subduction of the Nazca plate beneath South America.

• practice — not a real PYQ

  Which of the following pairs of features is formed where an oceanic plate is subducted beneath a continental plate ?

  • (a)A mid-oceanic ridge and a rift valley
  • (b)A deep trench offshore and a chain of fold mountains with volcanoes onshore
  • (c)A transform fault and a submarine canyon
  • (d)A continental shelf and an abyssal plain

  Answer(b) A deep trench offshore and a chain of fold mountains with volcanoes onshore — the descending oceanic slab creates the trench at the line of its bend, while the magma it generates rises through the overriding continental plate to build volcanoes along a mountain belt. The Peru-Chile Trench together with the Andes is the classic example. Ridges and rift valleys belong to divergent boundaries, and shelves and abyssal plains are ordinary features of the sea floor unrelated to subduction.