World Geography
🔒 Log in to trackOceans, currents, straits and canals
🔒 Log in to trackOceans
| Ocean | Key facts |
|---|---|
| Pacific | Largest and deepest; Mariana Trench (Challenger Deep, about 11 km) |
| Atlantic | Second largest, S-shaped; busiest shipping; Mid-Atlantic Ridge |
| Indian | Only ocean named after a country |
| Southern | Surrounds Antarctica |
| Arctic | Smallest and shallowest |
Ocean currents
| Warm currents | Cold currents |
|---|---|
| Gulf Stream (N Atlantic, warms NW Europe) | Labrador (off Canada) |
| Kuroshio (Japan) | Oyashio (Japan) |
| Brazil, Agulhas (SE Africa), East Australian | Humboldt / Peru (W South America) |
| North Atlantic Drift | Benguela (SW Africa), Canary, California, West Australian, Falkland |
- Cold currents off western coasts of continents in the subtropics create deserts: Humboldt → Atacama, Benguela → Namib, California → Sonoran/Mojave, West Australian → Great Australian desert.
- Meeting of warm and cold currents creates rich fishing grounds: Grand Banks (Newfoundland) where the Gulf Stream meets the Labrador current; Kuroshio–Oyashio off Japan.
- El Niño is the periodic warming of the eastern Pacific off Peru; it tends to weaken the Indian monsoon.
Straits and canals
| Strait | Connects / separates |
|---|---|
| Palk | India – Sri Lanka |
| Gibraltar | Spain – Morocco; Mediterranean ↔ Atlantic |
| Bering | Russia (Asia) – Alaska (N America); near the Date Line |
| Malacca | Malay Peninsula – Sumatra; Andaman Sea ↔ South China Sea |
| Hormuz | Iran – Oman; Persian Gulf ↔ Gulf of Oman (oil route) |
| Bab-el-Mandeb | Yemen – Djibouti; Red Sea ↔ Gulf of Aden |
| Bosphorus | Istanbul; Black Sea ↔ Sea of Marmara |
| Dover | England – France |
| Magellan | South American mainland – Tierra del Fuego |
| Sunda | Java – Sumatra |
| Cook | North and South islands of New Zealand |
| Bass | Australia – Tasmania |
Canals: Suez (1869) — Mediterranean ↔ Red Sea (Egypt); Panama (1914) — Atlantic ↔ Pacific; Kiel — North Sea ↔ Baltic (Germany).
Detailed notes
The five oceans
Oceans cover about 71% of the Earth's surface and hold about 97% of its water.
| Ocean | Remember for |
|---|---|
| Pacific | Largest and deepest; Mariana Trench (Challenger Deep, about 11 km) - the deepest point on Earth; ringed by the Ring of Fire |
| Atlantic | Second largest, S-shaped; busiest shipping ocean; Mid-Atlantic Ridge runs down its middle |
| Indian | The only ocean named after a country; the warmest |
| Southern | Surrounds Antarctica (recognised separately in 2000) |
| Arctic | Smallest and shallowest; partly frozen; the Bering Strait joins it to the Pacific |
Salinity (saltiness) averages 35 parts per thousand (3.5%). It is higher where evaporation beats river inflow - the Red Sea is the saltiest sea (about 41 per thousand) - and lower where big rivers dilute the water, as in the Baltic Sea, the least salty. The saltiest water body on Earth is the landlocked Dead Sea lake, so dense that swimmers float.
Ocean currents - rivers in the sea
Currents are huge loops driven by prevailing winds and the Earth's rotation. Warm currents flow from the equator towards the poles; cold currents return polar water.
- Warm: Gulf Stream + North Atlantic Drift (keep the ports of western Europe ice-free), Kuroshio (Japan), Brazilian, Agulhas (south-east Africa).
- Cold: Labrador (Newfoundland), Canary (north-west Africa), Benguela (south-west Africa), Humboldt/Peru (western South America), Oyashio (Kamchatka). Where a warm and a cold current meet, plankton multiply and fish gather: the Grand Banks (Gulf Stream x Labrador, off Newfoundland, famous for cod and fog) and the Dogger Bank (North Sea). Cold currents running along desert coasts dry the air - they help create the Atacama (Humboldt) and Namib/Kalahari margin (Benguela) deserts. The calm, weed-choked middle of the North Atlantic gyre is the Sargasso Sea, the 'sea without shores'. An El Nino is an abnormal warming of the Peruvian coast (the Humboldt current weakens), linked with weak Indian monsoons; La Nina is its opposite, linked with strong monsoons.
Straits and canals - the chokepoints
| Waterway | Connects / separates | Note |
|---|---|---|
| Suez Canal (1869) | Mediterranean - Red Sea | Sea-level canal (no locks), about 193 km; separates Africa from Asia |
| Panama Canal (1914) | Atlantic - Pacific | Lock canal, about 82 km; separates North from South America |
| Strait of Gibraltar | Atlantic - Mediterranean | Between Spain and Morocco |
| Bosphorus + Dardanelles | Black Sea - Sea of Marmara - Aegean | Both in Turkey; Istanbul straddles the Bosphorus |
| Strait of Hormuz | Persian Gulf - Gulf of Oman | Between Iran and Oman/UAE; the world's key oil route |
| Bab-el-Mandeb | Red Sea - Gulf of Aden | Djibouti-Yemen corner; the Suez route's gate |
| Strait of Malacca | Indian Ocean (Andaman Sea) - South China Sea | Between Malaysia and Sumatra (Indonesia); busiest Asian route |
| Palk Strait | India - Sri Lanka | With the Gulf of Mannar to its south |
| Bering Strait | Arctic - Pacific; Asia - North America | Between Russia and Alaska (USA); the two Diomede islands face each other |
| Drake Passage | South America - Antarctica | The roughest waters |
| Strait of Dover | UK - France | Narrowest part of the Channel |
Coral reefs and tides
Coral polyps are tiny animals; their limestone skeletons build reefs. They need warm (over about 20 degrees C), clean, shallow salt water, so reefs grow in tropical seas and die near muddy river mouths. The Great Barrier Reef (north-east Australia, Coral Sea) is the largest living structure on Earth. Reef shapes: fringing, barrier and atoll (a ring around a sunken volcano). Warming seas cause coral bleaching. Tides are the twice-daily rise and fall of the sea, caused mainly by the Moon's pull (assisted by the Sun). At new moon and full moon (Sun, Moon, Earth in line) the pulls add up - high-range spring tides; at the quarter moons the pulls work at right angles - low-range neap tides. The Bay of Fundy (Canada) records the world's highest tidal range.
Quick revision
- Pacific largest + Mariana Trench deepest; Atlantic S-shaped + busiest; Indian = country's name; Arctic smallest.
- Average salinity 35 per thousand; Red Sea saltiest sea, Baltic least, Dead Sea saltiest lake.
- Warm: Gulf Stream, Kuroshio, Agulhas; Cold: Labrador, Canary, Benguela, Humboldt (El Nino).
- Fishing grounds form where warm meets cold: Grand Banks, Dogger Bank; Sargasso Sea = 'sea without shores'.
- Suez 1869 (Africa-Asia, no locks), Panama 1914 (locks, N-S America); Gibraltar = Spain-Morocco; Hormuz = oil gate.
- Spring tide at new/full moon, neap at quarters; Bay of Fundy = highest range.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Ocean superlatives and the five oceansvery common4 practice Q
'Largest/deepest ocean', 'which ocean is named after a country', 'Mariana Trench lies in', ordering of the five oceans.
- Size order: Pacific > Atlantic > Indian > Southern > Arctic; Pacific is also the deepest and holds most of the world's water.
- Deepest point: Mariana Trench (Challenger Deep, about 11 km) in the Pacific.
- The Indian Ocean is the only one named after a country; the Southern Ocean (2000) encircles Antarctica; the Arctic is the smallest and shallowest.
Example: Which is the deepest ocean of the world?
The Pacific Ocean - it holds the Mariana Trench, whose Challenger Deep (about 11 km) is the deepest known point on Earth.
Type 2: Ocean currents ↔ nature ↔ effectsvery common4 practice Q
'Which is a warm current', 'Gulf Stream affects the climate of', 'which current makes a desert', fog and fishing grounds.
- Warm: Gulf Stream + North Atlantic Drift (keeps west Europe warm), Kuroshio, Brazilian, Agulhas, Eskimo-free East Australian.
- Cold: Labrador (icebergs, fog off Newfoundland), Canary (Canary Islands), Benguela (Namib), Humboldt/Peru (Atacama, rich anchovies, El Nino connection), Oyashio.
- Where a warm and cold current meet (Grand Banks, Dogger Bank) plankton blooms - hence the world's best fishing grounds; cold currents running along coasts make the Atacama and Namib deserts.
Example: The warm North Atlantic Drift keeps the climate of which region unusually mild for its latitude?
Western Europe (British Isles, Norway's coast) - it is the eastward continuation of the Gulf Stream; the Labrador cold current meets the Gulf Stream near Newfoundland, making the Grand Banks foggy and rich in fish.
Type 3: Straits: two water bodies and two landsvery common3 practice Q
'Which strait separates X from Y', 'which strait connects the Red Sea and the Gulf of Aden', match-the-column on straits.
- Palk Strait - India-Sri Lanka (Gulf of Mannar south of it); Bering Strait - Russia-Alaska (Arctic-Pacific); Drake Passage - South America-Antarctica (roughest).
- Hormuz - Persian Gulf (oil route); Bab-el-Mandeb - Red Sea-Gulf of Aden; Malacca - Andaman Sea-China Sea (longest, busiest); Bosphorus + Dardanelles - Black Sea-Mediterranean through Turkey.
- Gibraltar - Spain (Europe)-Morocco (Africa), Atlantic-Mediterranean. Learn each strait as four boxes: two lands, two waters.
Example: The Strait of Hormuz connects the Persian Gulf with the:
Gulf of Oman (Arabian Sea) - the world's most important oil-shipping bottleneck between Iran and Oman/UAE.
Type 4: Canals and man-made sea linkscommon3 practice Q
'Which canal joins the Mediterranean and the Red Sea', 'Suez/Panama canal connects', locks, year of opening.
- Suez Canal (1869) - Mediterranean-Red Sea; in Egypt; sea-level, no locks, about 193 km; Europe-Asia trade shortcut.
- Panama Canal (1914) - Atlantic-Pacific; in Panama; a lock canal about 82 km; joins Colon to Panama City.
- 'Kiel Canal' joins the North Sea and the Baltic (Germany) - a favourite third option.
Example: The Suez Canal, opened in 1869, lies in which country and connects which two seas?
It lies in Egypt and joins the Mediterranean Sea with the Red Sea, cutting the Europe-Asia voyage by about 7,000 km; being sea-level, it has no locks.
Type 5: Salinity, coral reefs and tidescommon4 practice Q
'Highest salinity is in', 'coral reefs need', 'Great Barrier Reef lies off', spring vs neap tides, Bay of Fundy.
- Average ocean salinity 35 per mille; highest in the Red Sea (about 41) - hot, dry, no fresh rivers; lowest in the Baltic (many rivers, low evaporation). Dead Sea = saltiest lake, bodies float.
- Corals need clean, shallow, warm (over 20 C), salt water; types: fringing, barrier, atoll (ring); Great Barrier Reef off Queensland (Australia) is the largest living structure; warm-water stress causes bleaching.
- Tides: spring tides - new/full moon (highest); neap tides - first/third quarter (lowest); Bay of Fundy (Canada) = highest tidal range.
Example: Which water body has the highest average salinity among the seas?
The Red Sea (about 41 per mille) - high evaporation, low rainfall and hardly any fresh-water rivers. The Baltic has the lowest.
Shortcut tricks
⚡ Cold current → desert pairs
Humboldt–Atacama, Benguela–Namib, California–Sonoran. West coasts + cold current = dry air = desert.
Example: Which cold current is responsible for the Namib desert?
The Benguela current.
⚡ Strait → two countries
Gibraltar = Spain|Morocco; Hormuz = Iran|Oman; Bering = Russia|USA; Malacca = Malaysia|Indonesia; Palk = India|Sri Lanka; Dover = UK|France.
Where students lose marks
Calling the Gulf Stream a cold current — it is warm.
Saying the Atlantic is the deepest ocean — the Pacific (Mariana Trench) is.
Linking the Suez Canal to the Atlantic — it joins the Mediterranean and the Red Sea.
Practice sets — 20 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.