CDS Geography · Oceanography
Sea Floor, Salinity and Coral Reefs
The steps of the ocean floor from the shelf down to the trenches, how salty sea water is and which salts it holds, and where and why coral reefs grow.
Why this matters
Nine CDS questions from 2018 to 2025: 4 EASY, 3 MODERATE, 2 HARD. Five of them are about coral reefs, and both HARD items are single named facts: an undersea plateau and the order of three salts in sea water.
Concept 1 of 4: Relief of the ocean floor
Walk out from a beach under the sea and the floor drops in steps. First a wide, shallow shelf, then a steep slope, then a huge flat plain far below. Here and there the plain is cut by narrow trenches, the deepest places on Earth, and raised by ridges, plateaus and lone undersea mountains.
Definition
The major divisions, from the coast outwards:
- Continental shelf: the shallow, gently sloping edge of a continent, down to about 200 m. The richest fishing grounds lie here.
- Continental slope: the steep drop from about 200 m to about 3,000 m.
- Deep sea plain (abyssal plain): the flat floor, about 3,000 to 6,000 m deep, covered with fine sediment.
- Oceanic deeps or trenches: long, narrow, very deep cuts, mostly in the Pacific where plates meet. The Mariana Trench in the western Pacific holds the Challenger Deep, nearly 11,000 m, the deepest point of any ocean.
The minor features: mid-ocean ridges (undersea mountain chains, such as the Mid-Atlantic Ridge), seamounts (lone volcanic peaks), guyots (flat-topped seamounts), submarine canyons and undersea plateaus.
| Feature | What it is | Example |
|---|---|---|
| Continental shelf | Shallow edge of a continent, to about 200 m | Grand Banks off Newfoundland |
| Continental slope | Steep drop from about 200 m to 3,000 m | Edge of every continental shelf |
| Deep sea plain | Flat floor about 3,000 to 6,000 m deep | Floors of all the great oceans |
| Trench | Narrow, very deep cut where plates meet | Mariana Trench (Challenger Deep, nearly 11,000 m), western PacificQ CDS 2020 (II): the Mariana Trench is the deepest point, ahead of the Tonga, Kermadec and Philippine trenches. |
| Mid-ocean ridge | Long undersea mountain chain | Mid-Atlantic Ridge |
| Guyot | A seamount with a flat top | Common in the Pacific |
| Undersea plateau | A broad raised block of the floor | Amsterdam and St. Paul Plateau, southern Indian OceanQ CDS 2025 (II): the Amsterdam and St. Paul Plateau is in the southern Indian Ocean. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (II) 2020 — General Knowledge · Q62Easy
Example 1 · Oceanography · Sea Floor, Salinity and Coral Reefs
Which one of the following is considered as the deepest point of the oceans?
Deep is not deepest
The Tonga, Kermadec and Philippine trenches are all very deep, and all in the Pacific. The deepest point of all is the Challenger Deep in the Mariana Trench, nearly 11,000 m down.
Amsterdam and St. Paul is in the Indian Ocean
The Amsterdam and St. Paul Plateau lies in the southern Indian Ocean, carrying two small French islands. Do not place it in the Atlantic because the name sounds European.
Concept 2 of 4: Salinity and the salts in sea water
Sea water is salty because rivers have carried dissolved salts into the sea for millions of years, and evaporation takes away only the water. More evaporation makes water saltier; more rain, river water or melting ice makes it fresher. Common salt is by far the biggest share of what is dissolved.
Definition
Salinity and what changes it:
- Salinity is the weight of dissolved salt in 1,000 g of sea water, given in parts per thousand (‰). The open ocean averages about 35‰.
- Isohalines are lines on a map joining places of equal salinity (halos is Greek for salt).
- Raises salinity: strong evaporation (hot, dry, cloudless seas such as the Red Sea and the Mediterranean).
- Lowers salinity: rain, rivers and melting ice (the Baltic Sea, fed by many rivers, is very fresh).
- Of all the salt dissolved in sea water, sodium chloride is about three quarters. The rest, in falling order, is magnesium chloride, magnesium sulphate, calcium sulphate, potassium sulphate and calcium carbonate.
| Salt | Share of the dissolved salts |
|---|---|
| Sodium chloride | About 77.8% |
| Magnesium chloride | About 10.9%Q CDS 2025 (I): ascending order is calcium carbonate, calcium sulphate, magnesium chloride. |
| Magnesium sulphate | About 4.7% |
| Calcium sulphate | About 3.6% |
| Potassium sulphate | About 2.5% |
| Calcium carbonate | About 0.3% |
Sea water itself is about 3.5% salt, so these are shares of the salt, not of the water.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q7Hard
Example 2 · Oceanography · Sea Floor, Salinity and Coral Reefs
What is the correct sequence of the following salts in ascending order according to their presence in the sea water ?
1. Calcium Carbonate
2. Calcium Sulphate
3. Magnesium Chloride
Select the answer using the code given below :
Iso-lines by their root
An isohaline joins equal salinity, an isotherm equal temperature, an isobar equal pressure and an isohyet equal rainfall. The root halos means salt.
Shares of the salt, not of the water
Sodium chloride is about 78% of the dissolved salt, not 78% of sea water. Sea water as a whole is only about 3.5% salt.
Concept 3 of 4: How coral reefs grow
A coral reef is built by tiny animals called polyps, which make limestone skeletons. Tiny algae live inside them and feed them by photosynthesis, so a reef needs sunlight. That one fact explains the rest: the water must be shallow, clear, warm and salty, and a coral dies in fresh or muddy water.
Definition
What a reef needs:
- Warm water: best at about 23 to 25 °C; no reefs where the water falls below about 20 °C.
- Shallow water: not deeper than about 50 m, so sunlight reaches the algae.
- Salt water that is clear and free of mud: fresh water and silt kill the polyps.
The three kinds of reef:
- Fringing reef: close to the shore of an island or continent.
- Barrier reef: parallel to the coast but separated from it by a wide lagoon. The largest is the Great Barrier Reef.
- Atoll: a ring of coral around a lagoon with no island in the middle. The Lakshadweep islands are atolls.
A coral island is built by reefs; an island made by volcanoes or cut off from a continent is not a coral island.
| Condition | What corals need | Why |
|---|---|---|
| Temperature | Warm, about 23 to 25 °C | Polyps die in cold water |
| Depth | Shallow, not deeper than about 50 m | Sunlight must reach the algae |
| Salinity | Salt water only | Fresh water kills the polypsQ CDS 2018 (II): 'coral can grow abundantly in freshwater' is the wrong statement. |
| Clarity | Clear water, free of silt | Mud blocks light and smothers the polyps |
| Sunlight | Plenty of it | The algae inside the coral photosynthesise |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (II) 2018 — General Knowledge · Q21Easy
Example 3 · Oceanography · Sea Floor, Salinity and Coral Reefs
Which one of the following statements with regard to growth of coral reefs is not correct?
A fringing reef touches the shore
A fringing reef grows right at the shore. A barrier reef runs parallel to it across a wide lagoon. An atoll is a ring with no island inside. Keep the three apart by how far they sit from land.
Concept 4 of 4: Where coral reefs are
India has four reef areas: two gulfs on the mainland coast and two island groups. One more gulf on the west coast, Khambhat, has none, because its water is muddy. Outside India, the giant is Australia's Great Barrier Reef.
Definition
Reefs in India:
- Gulf of Kachchh (Gujarat) and Gulf of Mannar (Tamil Nadu), on the mainland coast.
- Lakshadweep: coral atolls in the Arabian Sea.
- Andaman and Nicobar Islands: fringing reefs, the largest reef area in India. Swaraj Dweep (once Havelock Island) has coral reefs.
- No reefs in the Gulf of Khambhat (Cambay): a very large tidal range and heavy silt from rivers keep its water muddy.
Abroad:
- Great Barrier Reef: the world's largest barrier reef system, over 2,000 km long in the Coral Sea, off Queensland on Australia's north-eastern (east) coast.
- Rainbow Reef, Fiji: a well-known soft-coral reef.
| Reef area | Where | Note |
|---|---|---|
| Andaman and Nicobar Islands | Bay of Bengal | Largest reef area in India; fringing reefsQ CDS 2022 (II): largest area under coral reefs among the states and union territories. |
| Lakshadweep | Arabian Sea | Coral atolls |
| Gulf of Mannar | Tamil Nadu | Reefs around small islands near the coast |
| Gulf of Kachchh | Gujarat | Reefs in the Marine National Park near Jamnagar |
| Gulf of Khambhat (Cambay) | Gujarat | No reefs: muddy water and a huge tidal rangeQ CDS 2019 (II): the Gulf of Cambay is the region without coral reefs. |
| Great Barrier Reef | Coral Sea, off Queensland, east coast of Australia | World's largest barrier reef systemQ CDS 2020 (II): it lies off the east coast of Australia. |
| Kyushu | Japan | Not a coral island: one of Japan's four main islands, made by volcanoesQ CDS 2020 (II): Kyushu is the odd one out against the Great Barrier Reef, Rainbow Reef and Swaraj Island. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (II) 2019 — General Knowledge · Q67Moderate
Example 4 · Oceanography · Sea Floor, Salinity and Coral Reefs
Coral reefs are not found in which one of the following regions ?
Kachchh has reefs, Khambhat does not
Both gulfs are in Gujarat. The Gulf of Kachchh has coral reefs; the Gulf of Khambhat (Cambay) has none, because its water is muddy.
Largest reef area is the Andamans, not Lakshadweep
Lakshadweep is the island group made of coral, but the largest area under reefs is around the Andaman and Nicobar Islands.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Relief of the ocean floor7 rows
| Feature | What it is | Example |
|---|---|---|
| Continental shelf | Shallow edge of a continent, to about 200 m | Grand Banks off Newfoundland |
| Continental slope | Steep drop from about 200 m to 3,000 m | Edge of every continental shelf |
| Deep sea plain | Flat floor about 3,000 to 6,000 m deep | Floors of all the great oceans |
| Trench | Narrow, very deep cut where plates meet | Mariana Trench (Challenger Deep, nearly 11,000 m), western PacificQ CDS 2020 (II): the Mariana Trench is the deepest point, ahead of the Tonga, Kermadec and Philippine trenches. |
| Mid-ocean ridge | Long undersea mountain chain | Mid-Atlantic Ridge |
| Guyot | A seamount with a flat top | Common in the Pacific |
| Undersea plateau | A broad raised block of the floor | Amsterdam and St. Paul Plateau, southern Indian OceanQ CDS 2025 (II): the Amsterdam and St. Paul Plateau is in the southern Indian Ocean. |
Salinity and the salts in sea water6 rows
| Salt | Share of the dissolved salts |
|---|---|
| Sodium chloride | About 77.8% |
| Magnesium chloride | About 10.9%Q CDS 2025 (I): ascending order is calcium carbonate, calcium sulphate, magnesium chloride. |
| Magnesium sulphate | About 4.7% |
| Calcium sulphate | About 3.6% |
| Potassium sulphate | About 2.5% |
| Calcium carbonate | About 0.3% |
Sea water itself is about 3.5% salt, so these are shares of the salt, not of the water.
How coral reefs grow5 rows
| Condition | What corals need | Why |
|---|---|---|
| Temperature | Warm, about 23 to 25 °C | Polyps die in cold water |
| Depth | Shallow, not deeper than about 50 m | Sunlight must reach the algae |
| Salinity | Salt water only | Fresh water kills the polypsQ CDS 2018 (II): 'coral can grow abundantly in freshwater' is the wrong statement. |
| Clarity | Clear water, free of silt | Mud blocks light and smothers the polyps |
| Sunlight | Plenty of it | The algae inside the coral photosynthesise |
Where coral reefs are7 rows
| Reef area | Where | Note |
|---|---|---|
| Andaman and Nicobar Islands | Bay of Bengal | Largest reef area in India; fringing reefsQ CDS 2022 (II): largest area under coral reefs among the states and union territories. |
| Lakshadweep | Arabian Sea | Coral atolls |
| Gulf of Mannar | Tamil Nadu | Reefs around small islands near the coast |
| Gulf of Kachchh | Gujarat | Reefs in the Marine National Park near Jamnagar |
| Gulf of Khambhat (Cambay) | Gujarat | No reefs: muddy water and a huge tidal rangeQ CDS 2019 (II): the Gulf of Cambay is the region without coral reefs. |
| Great Barrier Reef | Coral Sea, off Queensland, east coast of Australia | World's largest barrier reef systemQ CDS 2020 (II): it lies off the east coast of Australia. |
| Kyushu | Japan | Not a coral island: one of Japan's four main islands, made by volcanoesQ CDS 2020 (II): Kyushu is the odd one out against the Great Barrier Reef, Rainbow Reef and Swaraj Island. |
Watch out for (7)
- Deep is not deepest→ Relief of the ocean floor
- Amsterdam and St. Paul is in the Indian Ocean→ Relief of the ocean floor
- Iso-lines by their root→ Salinity and the salts in sea water
- Shares of the salt, not of the water→ Salinity and the salts in sea water
- A fringing reef touches the shore→ How coral reefs grow
- Kachchh has reefs, Khambhat does not→ Where coral reefs are
- Largest reef area is the Andamans, not Lakshadweep→ Where coral reefs are
Test yourself on Oceanography
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.