CDS Geography · Free notes
Oceanography: CDS Geography
Oceanography has 20 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (I): 5 EASY, 12 MODERATE and 3 HARD. Eleven of them are about how ocean water moves, and five are about coral reefs. The older papers ask one named fact, often as 'which one is NOT', while the newer ones pack the facts into statements: of the six asked since 2024, five are statement, matching or ordering items. So learn each table as a set of true statements you can test one by one, and learn the one rule that sorts the currents: water flowing away from the equator is warm, water flowing towards it is cold.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Sea Floor, Salinity and Coral Reefs
9 PYQsThe 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.
Tides and Ocean Currents
11 PYQsWhat makes the tides and the kinds of tide, the forces that drive ocean currents, which currents are warm and which cold, and the great gyres with the Sargasso Sea inside one.
Formula & revision sheet
0 formulas · 8 reference tables · 14 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
0 formulas · 8 reference tables · 14 gotchas across all subtopics — the exam-eve cheat-sheet
Sea Floor, Salinity and Coral Reefs
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
Tides and Ocean Currents
Reference tables (4)
Tides and their kinds5 rows
| Kind of tide | Position | Effect |
|---|---|---|
| Spring tide | Sun, Moon and Earth in a line (new and full moon) | Largest range: very high high tides |
| Neap tide | Sun and Moon at right angles to the Earth (quadrature) | Smallest range |
| Perigean tide | Moon nearest the Earth | Higher than usual |
| Apogean tide | Moon farthest from the Earth | Lower than usual |
| Equinoctial spring tide | Spring tide near an equinox | Recurs about every six monthsQ CDS 2026 (I): spring 4, neap 1, apogean 2, equinoctial spring 3. |
What drives ocean currents6 rows
| Force | What it does to sea water |
|---|---|
| Heating by the Sun | Expands water near the equator, making a gentle slope |
| Wind | Drags the surface water along: the main push behind surface currents |
| Gravity of the Earth | Pulls heaped-up water down the slope |
| Coriolis force | Turns currents right in the north, left in the south |
| Differences in density | Make cold, salty water sink and drive the deep circulation |
| Pull of the Sun and Moon | Raises tides; not a cause of currentsQ CDS 2022 (II): the gravitational pull of the Sun and Moon is the one that does not influence currents. |
Warm and cold currents, and El Niño12 rows
| Current | Where it flows | Warm or cold |
|---|---|---|
| Gulf Stream | East coast of the USA, North Atlantic | Warm |
| North Atlantic Drift | Towards north-west Europe | WarmQ CDS 2020 (I): the North Atlantic Drift and the Brazil current are the warm pair. |
| Brazil Current | East coast of South America | Warm |
| Kuroshio | East coast of Japan, North Pacific | Warm |
| Agulhas Current | South-east coast of Africa (South Africa) | WarmQ CDS 2016 (II): Agulhas South Africa, Humboldt Peru, Labrador Canada, Falkland Argentina. |
| California Current | West coast of North America | ColdQ CDS 2021 (I): the California current is the cold one. |
| Humboldt (Peru) Current | West coast of South America (Peru) | Cold |
| Canary Current | North-west coast of Africa | Cold |
| Benguela Current | South-west coast of Africa | Cold |
| Labrador Current | East coast of Canada | Cold |
| Falkland Current | East coast of Argentina | Cold |
| El Niño | Off Peru and Ecuador, replacing the Humboldt | WarmQ CDS 2022 (I): the south-east trades blowing towards the Indian Ocean is the normal state, not a result of El Niño. |
Gyres and the Sargasso Sea4 rows
| Side of the Sargasso Sea | Current that bounds it |
|---|---|
| West | Gulf StreamQ CDS 2019 (I): the sea lies east of the Gulf Stream and west of the Canary current, not the other way round. |
| North | North Atlantic Drift |
| East | Canary Current |
| South | North Equatorial CurrentQ CDS 2026 (I): it is the one sea without a land boundary; it is salty, not the least salty part of the Atlantic. |
Watch out for (7)
- Spring tides are not about the season→ Tides and their kinds
- The direction of the Earth's spin does not explain tides→ Tides and their kinds
- Gravity is on the list, the Moon's pull is not→ What drives ocean currents
- Pairs in the options mix one warm with one cold→ Warm and cold currents, and El Niño
- El Niño weakens the trades, it does not cause them→ Warm and cold currents, and El Niño
- Gulf Stream west, Canary east→ Gyres and the Sargasso Sea
- No land boundary does not mean fresh→ Gyres and the Sargasso Sea
PYQ weightage by concept
8 concepts · 20 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
8 concepts · 20 PYQs — where the marks actually sit, so you know what to drill first
Sea Floor, Salinity and Coral Reefs9 PYQs · 45%
| Concept | PYQs | Share |
|---|---|---|
| Where coral reefs are | 4 | 20% |
| Relief of the ocean floor | 2 | 10% |
| Salinity and the salts in sea water | 2 | 10% |
| How coral reefs grow | 1 | 5% |
Tides and Ocean Currents11 PYQs · 55%
| Concept | PYQs | Share |
|---|---|---|
| Warm and cold currents, and El Niño | 4 | 20% |
| Gyres and the Sargasso Sea | 3 | 15% |
| Tides and their kinds | 2 | 10% |
| What drives ocean currents | 2 | 10% |
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.