World Geography
🔒 Log in to trackOceans, currents, straits and canals
🔒 Log in to trackOceans cover about 71% of the Earth. Warm and cold currents move heat around and shape coastal climate. Straits and canals are the chokepoints of world trade. Reefs, salinity and tides are the other facts the exam repeats.
The five oceans
Oceans cover about 71% of the Earth's surface. They 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 for shipping. Mid-Atlantic Ridge runs down the middle |
| Indian | The only ocean named after a country. The warmest |
| Southern | Surrounds Antarctica. Recognised separately in 2000 |
| Arctic | Smallest and shallowest. Partly frozen. Bering Strait joins it to the Pacific |
Salinity
Salinity is saltiness. The average is 35 parts per thousand (3.5%).
- It is higher where evaporation beats river inflow. The Red Sea is the saltiest sea, at about 41 per thousand.
- It is lower where big rivers dilute the water. The Baltic Sea is the least salty.
- The Dead Sea is the saltiest water body. It is a landlocked lake. Swimmers float in it.
Tip: Hot and dry with no rivers means salty (Red Sea). Many rivers and little evaporation means less salty (Baltic).
Ocean currents
Currents are huge loops driven by prevailing winds and the Earth's spin. Warm currents flow from the equator towards the poles. Cold currents bring polar water back.
| Type | Current | Note |
|---|---|---|
| Warm | Gulf Stream and North Atlantic Drift | Keep west European ports ice-free |
| Warm | Kuroshio, Brazilian, Agulhas | Japan, Brazil, south-east Africa |
| Cold | Labrador | Off Newfoundland |
| Cold | Canary | North-west Africa |
| Cold | Benguela | South-west Africa |
| Cold | Humboldt / Peru | Western South America |
| Cold | Oyashio | Kamchatka |
Where a warm and a cold current meet, plankton multiply and fish gather. Examples: the Grand Banks (Gulf Stream meets Labrador, off Newfoundland, known for cod and fog) and the Dogger Bank (North Sea).
Cold currents along desert coasts dry the air. They help make the Atacama (Humboldt) and the Namib and Kalahari margin (Benguela).
Sargasso Sea, El Nino and La Nina
The calm, weedy middle of the North Atlantic gyre is the Sargasso Sea. It is called the "sea without shores".
El Nino is an abnormal warming of the Peruvian coast. The Humboldt current weakens. It links with weak Indian monsoons. La Nina is the opposite. It links with strong monsoons.
Straits and canals
| Waterway | Connects | Note |
|---|---|---|
| Suez Canal (1869) | Mediterranean and Red Sea | Sea-level, no locks, about 193 km. Africa from Asia |
| Panama Canal (1914) | Atlantic and Pacific | Lock canal, about 82 km. North from South America |
| Gibraltar | Atlantic and Mediterranean | Between Spain and Morocco |
| Bosphorus and Dardanelles | Black Sea, Marmara, Aegean | Both in Turkey. Istanbul straddles the Bosphorus |
| Hormuz | Persian Gulf and Gulf of Oman | Iran and Oman/UAE. Key oil route |
| Bab-el-Mandeb | Red Sea and Gulf of Aden | Djibouti-Yemen corner |
| Malacca | Andaman Sea and South China Sea | Malaysia and Sumatra (Indonesia). Busiest Asian route |
| Palk Strait | India and Sri Lanka | Gulf of Mannar lies south of it |
| Bering Strait | Arctic and Pacific | Russia and Alaska (USA) |
| Drake Passage | South America and Antarctica | Roughest waters |
| Dover | UK and France | Narrowest part of the Channel |
Rule: Learn each strait as four boxes: two lands and two waters.
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. The Moon's pull causes most of it, helped by the Sun.
- Spring tides: at new moon and full moon the pulls add up. The range is high.
- Neap tides: at the quarter moons the pulls act at right angles. The range is low.
- The Bay of Fundy (Canada) has the world's highest tidal range.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Ocean superlatives and the five oceans
'Largest / deepest ocean', 'ocean named after a country', 'Mariana Trench lies in', ordering of oceans.
Size order: Pacific, Atlantic, Indian, Southern, Arctic.
The Pacific is also the deepest.
The Mariana Trench (Challenger Deep, about 11 km) is in the Pacific.
Indian is named after a country. Arctic is smallest and shallowest.
One size order and a few named features cover the topic.
Which is the deepest ocean?
Show solutionHide solution
The Mariana Trench is the deepest point.
It lies in the Pacific.
The Pacific Ocean
Ocean currents, their nature and effects
'Which is a warm current', 'Gulf Stream affects the climate of', 'which current makes a desert', fog, fishing grounds.
Sort the current: warm or cold.
Warm: Gulf Stream, Kuroshio, Brazilian, Agulhas. Cold: Labrador, Canary, Benguela, Humboldt, Oyashio.
Warm meets cold gives fishing grounds and fog.
Cold current on a west coast gives a desert.
Every current has a fixed nature and a fixed effect.
The warm North Atlantic Drift keeps which region mild for its latitude?
Show solutionHide solution
It continues the Gulf Stream eastwards.
It reaches western Europe.
Western Europe
Straits: two waters and two lands
'Which strait separates X from Y', 'strait joining the Red Sea and Gulf of Aden', match-the-column on straits.
Write the two lands and the two waters.
Palk: India and Sri Lanka. Bering: Russia and Alaska. Drake: South America and Antarctica.
Hormuz: Persian Gulf, oil route. Bab-el-Mandeb: Red Sea and Gulf of Aden. Malacca: busiest Asian route.
Gibraltar: Spain and Morocco.
Straits are asked as four-box facts.
The Strait of Hormuz connects the Persian Gulf with the:
Show solutionHide solution
Hormuz lies between Iran and Oman/UAE.
It is the key oil route.
Gulf of Oman
Canals and man-made sea links
'Which canal joins the Mediterranean and the Red Sea', 'Suez / Panama connects', locks, year of opening.
Suez: 1869, Egypt, Mediterranean and Red Sea, no locks, about 193 km.
Panama: 1914, Atlantic and Pacific, locks, about 82 km.
Kiel Canal joins the North Sea and Baltic (Germany) and is a common third option.
Canals differ by seas joined, year and locks.
The Suez Canal opened in 1869. Which two seas does it connect?
Show solutionHide solution
It lies in Egypt.
It links the Mediterranean to the Red Sea.
The Mediterranean Sea and the Red Sea
Salinity, coral reefs and tides
'Highest salinity is in', 'coral reefs need', 'Great Barrier Reef lies off', spring vs neap tides, Bay of Fundy.
Average salinity is 35 per thousand. Red Sea is highest. Baltic is lowest.
Corals need warm (over about 20 degrees C), clean, shallow salt water.
Spring tides at new and full moon. Neap tides at the quarters.
Bay of Fundy has the highest range.
Each fact ties one cause to one place.
Which sea has the highest salinity?
Show solutionHide solution
Heat, low rain and no big rivers raise salt.
That fits the Red Sea, at about 41 per thousand.
The Red Sea
Sargasso Sea, El Nino and La Nina
'Sea without shores', 'El Nino is', 'La Nina is linked with', Peruvian coast warming.
Sargasso Sea is the weedy middle of the North Atlantic gyre.
El Nino is warming of the Peruvian coast with a weak Humboldt current.
El Nino links with weak Indian monsoons. La Nina with strong ones.
El Nino and La Nina are opposites, so learn one and flip it.
El Nino is an abnormal warming off which coast?
Show solutionHide solution
The Humboldt current weakens.
The Humboldt runs along the coast of Peru.
The Peruvian coast (western South America)
Shortcuts that save time
Humboldt gives Atacama. Benguela gives Namib. West coasts with a cold current have dry air, so deserts form.
Which cold current helps make the Namib desert?
Show solutionHide solution
The Namib is on the south-west coast of Africa.
The cold current there is Benguela.
The Benguela current
Gibraltar: Spain and Morocco. Hormuz: Iran and Oman. Bering: Russia and USA. Malacca: Malaysia and Indonesia. Palk: India and Sri Lanka. Dover: UK and France.
Mistakes to avoid
Where most students lose marks on this subtopic.
Calling the Gulf Stream a cold current.
It is a warm current.
Saying the Atlantic is the deepest ocean.
The Pacific is, with the Mariana Trench.
Linking the Suez Canal with the Atlantic.
It joins the Mediterranean and the Red Sea.
Calling the Red Sea the least salty sea.
It is the saltiest sea. The Baltic is the least salty.
Placing spring tides at the quarter moons.
Spring tides come at new and full moon. Neap tides come at the quarters.
Quick revision
Read this the night before the exam.
Pacific largest and deepest (Mariana Trench). Atlantic S-shaped. Indian named after a country. Arctic smallest.
Average salinity is 35 per thousand. Red Sea saltiest sea. Baltic least. Dead Sea saltiest lake.
Warm: Gulf Stream, Kuroshio, Agulhas. Cold: Labrador, Canary, Benguela, Humboldt.
Fishing grounds: Grand Banks, Dogger Bank. Sargasso Sea is the sea without shores.
Suez 1869: Africa and Asia, no locks. Panama 1914: locks, joins the two Americas. Hormuz is the oil gate.
Spring tide at new and full moon. Neap at quarters. Bay of Fundy has the highest range.
Practice: 20 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.