UKPSC Prelims 2024 Q2: The shape of Atoll

Q: What is the answer to UKPSC Prelims 2024 Q2?

C. Correct — C, Horse Shoe/Ring Shaped. An atoll is a coral reef that grows around a central body of shallow water — the lagoon — so its outline closes on itself. That is why the key reads "**Horse Shoe/Ring Shaped**": the reef rim runs right around the lagoon, and where the rim is broken by tidal passages or where one arm has not kept pace, the closed ring opens out into a horseshoe. The two words in the option are not alternatives you have to choose between; they are the same form described tightly or loosely. **The idea to carry away is that the shape of a reef follows what the reef is growing around.** A fringing reef is growing against a shoreline, so it hugs the coast. A barrier reef is growing offshore from a coast or island, so it runs long and roughly parallel to it. An atoll has no land left inside it to hug or parallel — only water — so the reef closes into a ring around that water.

UKPSC Prelims 2024 General Studies Paper-I · Question 2 of 147

Verified against the official question booklet. Answer as per UKPSC's provisional answer key.

Answers from UKPSC's provisional answer key. The Commission publishes a provisional key for UKPSC Prelims 2024 and invites objections before issuing a final key. Every answer shown here is the Commission's own provisional answer (key PDF), not our derivation. If the final key changes an answer, this page is updated to match.

The shape of Atoll is:

(a)Rectangular
(b)Rod-shaped
(c)Horse Shoe/Ring Shaped
(d)Triangular

Answer

Why

Correct — C, Horse Shoe/Ring Shaped.

An atoll is a coral reef that grows around a central body of shallow water — the lagoon — so its outline closes on itself.

That is why the key reads "Horse Shoe/Ring Shaped": the reef rim runs right around the lagoon, and where the rim is broken by tidal passages or where one arm has not kept pace, the closed ring opens out into a horseshoe.

The two words in the option are not alternatives you have to choose between; they are the same form described tightly or loosely. The idea to carry away is that the shape of a reef follows what the reef is growing around.

A fringing reef is growing against a shoreline, so it hugs the coast. A barrier reef is growing offshore from a coast or island, so it runs long and roughly parallel to it.

An atoll has no land left inside it to hug or parallel — only water — so the reef closes into a ring around that water.

Why the others are wrong

(a)
Rectangular — Coral grows outward toward light, food and open ocean water, so reef outlines are curved and lobed, not built of straight sides meeting at right angles.
Rectangular is a real descriptor elsewhere in geomorphology — a rectangular drainage pattern, where tributaries join the trunk stream at sharp right angles because the streams are exploiting two sets of joints or faults that intersect at right angles in the underlying rock.
That pattern belongs to structural control of rivers, not to reef building.
(b)
Rod-shaped — This option fits a barrier reef, not an atoll. A barrier reef is genuinely long and narrow, running roughly parallel to the coast it fronts, and "rod-shaped" is a loose fit for that.
But an atoll is a reef closed around a lagoon, and that definitional point holds whichever origin theory you follow; on Darwin's subsidence sequence it is the stage past the barrier reef, after the central land has gone under. Outside geography, rod-shaped is the standard descriptor for bacillus bacteria.
(d)
Triangular — Triangular belongs to a different family of landforms. An arcuate delta is described as triangular — the Nile delta is the reason the Greek letter delta names the feature at all — and triangular facets are the blunt triangles left on a faulted mountain front by spur truncation.
Neither has anything to do with coral. As a form, the atoll is a closed or near-closed ring or horseshoe around a lagoon; individual atolls take irregular outlines, and some, such as Tarawa in Kiribati, are roughly triangular in plan. "Triangular" therefore names an accident of one atoll's outline, not the form.

Concept

Reef-building corals are colonial animals that live with symbiotic algae in their tissues, so they need warm, shallow, clear, well-oxygenated saltwater with enough light reaching them.

Because they can only build upward and outward within that narrow depth window, the reef they leave behind records the shape of whatever platform they were growing on.

Darwin's subsidence theory ties the three reef forms into one sequence: coral first builds a fringing reef right against the shore of a volcanic island;

as the island slowly sinks, the coral keeps growing upward while the shoreline retreats inward, so a lagoon opens between the island and the reef and the reef becomes a barrier reef;

when the island finally disappears below the surface, only the ring of coral is left, enclosing the lagoon that now sits where the island used to be. That ring is the atoll.

The shape is not a piece of trivia — it is the visible evidence for the subsidence theory. If atolls were just reefs that happened to grow on shallow banks, there would be no reason for them to be rings enclosing deep central water rather than solid caps.

The ring form, and the fact that the lagoon floor sits well below the depth at which reef corals can live, is what points to a sunken island underneath.

Deep drilling at Enewetak Atoll in the Pacific found volcanic rock beneath more than a kilometre of accumulated reef limestone, which is the physical confirmation of exactly that picture.

For an Indian candidate the same idea also explains a map fact worth holding: the Lakshadweep group in the Arabian Sea is built of coral atolls, reefs and submerged banks, whereas the Andaman and Nicobar Islands are not coral islands of this kind.

Key facts

An atoll is a coral reef enclosing a lagoon, with no substantial central island remaining above water.
Real atoll rings are frequently incomplete — broken by tidal channels and passages, and often oval, elongated or horseshoe-shaped rather than circular. A neat closed circle is the textbook diagram, not the general rule.
Charles Darwin set out the subsidence explanation of reef forms in The Structure and Distribution of Coral Reefs (1842).
The developmental sequence in that theory runs fringing reef, then barrier reef, then atoll, as the volcanic island subsides.
Two rival explanations also appear in standard texts: John Murray's submarine-platform theory, and Reginald Daly's glacial-control theory, which invokes falling sea level and wave planation during glacial periods.
Reef-building corals are restricted to warm, shallow, clear, saline water — heavy sediment, freshwater dilution and depths beyond the sunlit zone all suppress them.
Drilling at Enewetak Atoll reached volcanic basement beneath over a kilometre of reef carbonate, consistent with long-continued subsidence.
The word atoll comes from the Dhivehi language of the Maldives, a country made up of such reef groups.
The Lakshadweep islands are coral formations of the Arabian Sea; the Great Barrier Reef off Queensland is a barrier reef, not an atoll.

Reef form follows what the coral is growing around

Fringing reef→Attached to the shore itself; no lagoon, or only a narrow shallow one

Barrier reef→Long and narrow, running roughly parallel to the coast, separated from it by a wide, deeper lagoon

Atoll→Ring or horseshoe outline enclosing a lagoon, with no substantial land left inside

Darwin's sequence→Fringing to barrier to atoll, as the volcanic island subsides and the coral keeps growing upward

The ring in practice→Often broken by passages and often oval or elongated, which is why the option pairs horseshoe with ring

The three forms are one story at three stages, not three unrelated types — which is why the atoll is the only one whose outline closes.

Study next

Darwin's subsidence theory and the evidence for it, set against Murray's submarine-platform theory and Daly's glacial-control theoryConditions required for reef-building coral: temperature, depth and light, salinity, sediment load, wave actionCoral bleaching and reef degradation, and the mechanism by which the symbiotic algae are expelledIndia's coral areas — Lakshadweep, Gulf of Mannar, Gulf of Kachchh, Andaman and Nicobar — and which reef type each carriesCoastal lagoons of a completely different origin, such as Chilika and Pulicat, which are enclosed by sand spits and bars rather than by coral

Common traps

Reaching for a linear or rod-like answer because you retrieved the barrier reef, which really is long and narrow, instead of the atoll.
Rejecting the correct option because it offers two words. Horseshoe and ring are not competing claims here; the option is worded that way because atoll rims are commonly incomplete.
Assuming an atoll must be a clean circle and then doubting the option because it says horseshoe.
Assuming the lagoon of an atoll surrounds an island. In an atoll the lagoon is what sits in the middle; the island that was once there has gone under.
Reading the question as being about cross-section or depth profile; it asks for the outline in plan view.

This idea surfaces in a few recognisable shapes. The plainest, as here, is a one-line definition item asking for the outline of an atoll.

A second form gives the three reef types and asks you to place them in Darwin's developmental order, or to match each with its relation to the shoreline and lagoon.

A third works through a named example — asking which reef type the Great Barrier Reef is, or what the Lakshadweep islands are built of — where the shape knowledge is what lets you classify the example.

A fourth comes at it from ecology, listing conditions and asking which set favours reef-building coral. Holding the subsidence sequence rather than three isolated definitions answers all four, because the sequence explains why each form looks the way it does.

Related PYQs

No eligible match among this question's citable set.

Practice

practice — not a real PYQ
Which one of the following coral reef forms encloses a lagoon with no substantial island remaining inside it?
- (a)Fringing reef
- (b)Barrier reef
- (c)Atoll
- (d)Patch reef
Answerc — An atoll is by definition a reef that has closed around a lagoon after the central land subsided beneath the surface. (a) fails because a fringing reef is attached to the shore and leaves no lagoon, or at most a narrow shallow channel.
(b) fails because a barrier reef does have a lagoon, but that lagoon lies between the reef and a coast or island that is still above water — land remains inside.
(d) fails because a patch reef is an isolated mound of coral growing on the floor of a lagoon or shelf; it does not enclose water at all.
practice — not a real PYQ
According to Charles Darwin's subsidence theory, the correct developmental sequence of coral reefs around a slowly sinking volcanic island is:
- (a)Atoll, then barrier reef, then fringing reef
- (b)Fringing reef, then barrier reef, then atoll
- (c)Barrier reef, then fringing reef, then atoll
- (d)Fringing reef, then atoll, then barrier reef
Answerb — Coral first builds against the island's own shore as a fringing reef; as the island subsides the reef grows upward while the shoreline retreats, opening a lagoon between the two, which makes it a barrier reef; when the island finally sinks out of sight only the ring remains, which is the atoll.
(a) reverses the whole sequence, which would require the island to rise rather than sink. (c) is impossible because the lagoon of a barrier reef opens only after a fringing reef already exists to be separated from the shore.
(d) puts the atoll before the barrier reef, but the atoll stage cannot precede the stage in which the island is still above water.
practice — not a real PYQ
Which one of the following statements about the Lakshadweep islands is correct?
- (a)They lie in the Bay of Bengal and are the emerged crests of a submerged mountain range
- (b)They are coral formations — atolls, reefs and submerged banks — lying in the Arabian Sea
- (c)They are alluvial islands built by sediment brought down by west-flowing peninsular rivers
- (d)They contain India's only active volcano
Answerb — Lakshadweep is a coral archipelago of the Arabian Sea, made up of atolls, reefs and submerged banks.
(a) fails on both counts: the group lies in the Arabian Sea, not the Bay of Bengal, and they are coral build-ups, not the emerged crests of a submerged mountain range — that is the Andaman and Nicobar group.
(c) fails because the islands are built of coral limestone, not river alluvium. (d) fails because India's only active volcano is Barren Island, in the Andaman Islands, in the Andaman Sea.
practice — not a real PYQ
Reef-building corals grow best under which one of the following sets of conditions?
- (a)Cold, deep water with a heavy load of suspended sediment
- (b)Warm, shallow, clear and saline water within the sunlit zone
- (c)Brackish estuarine water at the mouths of large rivers
- (d)Dark abyssal water on the deep ocean floor
Answerb — Reef-building corals depend on symbiotic algae living in their tissues, so they need light, which restricts them to shallow, clear, warm and normally saline tropical water. (a) fails on every count: cold and deep denies them warmth and light, and suspended sediment smothers the polyps.
(c) fails because river mouths deliver both freshwater dilution and sediment, which is why reef-building coral is generally poor or missing off the mouths of large rivers. (d) fails because no light reaches abyssal depths, so reef-building corals cannot live there.