The World Ocean is the continuous body of water surrounding the Earth and the main part of the hydrosphere. It is the climate’s "heat store": it absorbs heat in summer, releases it slowly in winter and carries it from the equator to the poles in currents. It teems with microscopic algae that supply an important share of atmospheric oxygen, and it provides fish, oil and gas, and shipping routes. Exams ask about the four oceans, the relief of the ocean floor, salinity and currents; often you must identify an ocean from a description.
§1. The World Ocean and its parts
Area ≈ 361 million km² — ≈ 71% of the Earth’s surface; mean depth about 3.7 km
The continents divide the World Ocean into four oceans: the Pacific, Atlantic, Indian and Arctic. Some classifications also treat the waters around Antarctica as a separate Southern Ocean. Parts of the ocean include:
seas — inland (the Mediterranean, the Black Sea), marginal (the Barents Sea, the Sea of Japan) and inter-island;
gulfs and bays — parts of an ocean or sea reaching into the land (the Bay of Bengal, the Gulf of Mexico, the Persian Gulf);
straits — narrow waterways linking two bodies of water (Gibraltar, Bering, Magellan, Drake Passage).
The Northern Hemisphere has much land, while ocean dominates the Southern.
Worked example
The Earth’s total surface is about 510 million km² and the World Ocean covers 361 million km². What share of the surface is ocean?
Formula: share = ocean area / total area · 100%.
361 / 510 ≈ 0.708.
0.708 · 100% ≈ 71%. Answer: about 71%; land is about 29%.
§2. Properties of sea water
Salinity is the amount of dissolved salts in 1 kg (about 1 litre) of sea water, measured in per mille (‰).
Mean salinity ≈ 35‰ (about 35 g of salt per litre)
Salinity depends on the balance between evaporation and the input of fresh water (rain, rivers, melting ice):
in the tropics evaporation is high and rain scarce, so salinity is highest;
at the equator heavy rain makes it slightly lower;
where big rivers enter and ice melts it is low.
The saltiest sea is the Red Sea (over 40‰): it is hot and dry and no rivers flow into it. The Baltic, by contrast, is very fresh. Temperature: surface water cools from the equator to the poles; it also falls with depth and stays at about +2 °C at great depths. Polar waters freeze over. Water movements: wind waves; tsunamis — huge waves caused by undersea earthquakes and eruptions; tides — mainly caused by the Moon’s gravity; and currents.
Worked example
5 litres of sea water with a salinity of 35‰ are evaporated. How much salt remains?
35‰ means about 35 g of salt in every litre.
In 5 litres: 5 · 35 = 175 g.
Answer: about 175 g of salt.
§3. Relief of the ocean floor
The continental shelf is the flat sea floor near the coast, down to 200 m; it has plenty of light and food, so it is rich in fish, and it holds many oil and gas fields.
The continental slope is the steep drop beyond the shelf, down to 2–3 km.
The ocean floor (abyssal plain) consists of wide plains and hills, mostly at 4–6 km.
Mid-ocean ridges are underwater mountain chains where plates move apart, with rift cracks along their crests; in places they rise above the sea as islands (Iceland).
Deep-sea trenches form where an oceanic plate sinks beneath a continental one; the deepest is the Mariana Trench in the Pacific (about 11 km).
§4. The four oceans
The Pacific is the largest (about 178 million km², bigger than all the land), the deepest and the warmest at the surface. Around its edges runs a chain of volcanoes and earthquakes — the "Ring of Fire" — with many trenches and islands. Magellan, the first European to cross it, named it "Pacific" (peaceful).
The Atlantic is the second largest (about 92 million km²). The S-shaped Mid-Atlantic Ridge runs down its middle. Its shelves are wide, and it has many straits and inland seas. It is the saltiest ocean and contains the warm Gulf Stream. The busiest shipping lanes are here.
The Indian Ocean is the third (about 76 million km²), mostly in the Southern Hemisphere. In its northern part the monsoons reverse the currents with the seasons. The Red Sea and the Persian Gulf belong to it; the Persian Gulf region is extremely rich in oil.
The Arctic Ocean is the smallest and the shallowest; shelf covers most of its area. Much of it is ice-covered all year. Its floor has the Lomonosov, Mendeleev and Gakkel ridges. As the ice shrinks, the Northern Sea Route is gaining importance.
Worked example
Description: "Several parallel ridges cross the ocean floor with deep basins between them; shelves cover a very large area; most of the surface is covered with ice." Which ocean is it?
A very large shelf area marks a shallow ocean.
Ice cover marks a polar ocean.
Parallel ridges (Lomonosov, Mendeleev, Gakkel) are also typical of it.
Answer: the Arctic Ocean.
§5. Ocean currents
Currents are steady movements of large masses of ocean water. They are driven mainly by prevailing winds (trade winds, westerlies); the Earth’s rotation and the shape of the continents affect their direction.
Warm: Gulf Stream, North Atlantic Drift, Kuroshio, Brazil, Guinea. Cold: Labrador, Canary, California, Peru (Humboldt), Benguela, West Wind Drift
Warm currents usually flow from the equator towards the poles, cold currents from the poles towards the equator.
A warm current brings warmth and moisture to a coast: thanks to the North Atlantic Drift, ports on the Norwegian coast and the port of Murmansk on the Barents Sea stay ice-free in winter, although they lie north of the Arctic Circle.
A cold current chills the air from below; the air does not rise and rain is scarce, so coastal deserts form (the Atacama — Peru Current; the Namib — Benguela Current).
The West Wind Drift is the most powerful current, circling Antarctica.
Where warm and cold currents meet, waters mix and nutrients rise, so fish are plentiful (around Newfoundland).
§6. Ocean resources and their protection
Biological: fish, seafood and seaweed; the most productive areas are shelves and places where currents meet.
Mineral: oil and gas on the shelf, iron-manganese nodules on the floor, and table salt from sea water.
Energy: tidal, wave and wind power.
The water itself: desalination in dry countries (the Persian Gulf states).
Transport: most international freight goes by sea.
Problems include oil spills, plastic waste, overfishing and warming of ocean water. Protection relies on international agreements, fishing quotas and marine reserves.
Check yourself
Which ocean is the saltiest?
What is the continental shelf, and how deep is it?
Which current helped form the Namib Desert?
Which ocean is surrounded by the "Ring of Fire"?
What kind of current is the Kuroshio?
In which ocean do monsoons reverse the currents?
How much salt is in 2 litres of water with a salinity of 35‰?
Which is the saltiest sea?
Common mistakes
Putting the Gulf Stream in the Pacific — it is in the Atlantic.
Calling the Peru and Canary currents warm — they are cold.
Confusing the saltiest ocean with the saltiest sea (the Atlantic and the Red Sea).
Reading per mille as per cent: 35‰ is 35 g per litre, not 35%.
Thinking the deepest point is in the Atlantic — the Mariana Trench is in the Pacific.