Oceans, Ocean Currents, Trenches and Major Rivers and Lakes
The oceans cover about 71 per cent of the Earth's surface and contain more than 97 per cent of all the water on the planet. They regulate climate, carry currents that move heat from the equator towards the poles, and are home to the deepest places on the Earth. Inland, rivers and lakes provide fresh water and have shaped the settlement of the world's population. For competitive examinations, this topic brings together the names, sizes and records of the oceans, the warm and cold currents, the deep trenches, and the world's major rivers and lakes. Examiners ask this area mostly through record facts and matching questions, so careful memorisation is essential.
The five oceans
There are five oceans. The Pacific Ocean is the largest and the deepest ocean, covering about one third of the Earth's surface. The Atlantic Ocean is the second largest, and the Indian Ocean is the third. The Southern (Antarctic) Ocean surrounds Antarctica, and the Arctic Ocean is the smallest and the shallowest. The bottom of the ocean is not flat; from the shore outwards we find the continental shelf (the shallow flat area richest in marine life), the continental slope, the continental rise, the abyssal plain (the vast deep floor) and the deepest features called trenches. The Mariana Trench in the Pacific Ocean is the deepest trench in the world, and its deepest point is the Challenger Deep, about 11,034 metres below sea level.
Ocean currents
An ocean current is a continuous, directed movement of seawater, driven mainly by prevailing winds, differences in temperature and salinity, and the rotation of the Earth. Currents are broadly of two types. Warm currents flow from near the equator towards the poles and raise the temperature of the coasts they touch; the Gulf Stream and the Kuroshio are warm currents. Cold currents flow from higher latitudes towards the equator and lower the coastal temperature; the Labrador, Canary and Benguela currents are cold. Currents also bring nutrients and plankton, which is why the meeting zones of warm and cold currents are the richest fishing grounds of the world. Fog is common where a warm and a cold current meet.
Trenches, ridges and the deepest places
A trench is a narrow, very deep depression in the ocean floor, usually formed where an oceanic plate subducts beneath another plate. The Mariana Trench (Pacific) is followed by the Puerto Rico Trench (Atlantic) and the Java or Sunda Trench (Indian Ocean). In contrast, a mid-oceanic ridge is a long underwater mountain system built at divergent boundaries, such as the Mid-Atlantic Ridge, and the ridge system is the longest mountain range on Earth. Trenches mark the deepest, most unstable parts of the oceans and are closely associated with earthquakes, tsunamis and volcanic activity.
Major rivers of the world
The Nile in Africa is generally regarded as the longest river in the world, flowing about 6,650 kilometres through eleven countries into the Mediterranean Sea. The Amazon in South America is the largest river by volume of water (discharge) and has the largest drainage basin in the world, carrying about one fifth of all the fresh water entering the oceans. Other important rivers include the Yangtze (longest river of Asia and China), the Mississippi-Missouri (United States), the Congo (Africa), the Volga (longest river of Europe) and, in India, the Ganga, the Brahmaputra and the Indus. The Ganga-Brahmaputra delta is the largest delta in the world.
Major lakes of the world
A lake is a large body of water surrounded by land. The Caspian Sea is the largest lake in the world, although it is called a sea because of its size and salinity. Lake Baikal in Russia is the deepest lake and the largest freshwater lake by volume. Lake Superior in North America is the largest freshwater lake by surface area. The Dead Sea, between Israel and Jordan, is the lowest point on land and one of the saltiest water bodies. Lake Titicaca in the Andes is the highest navigable lake in the world, and Lake Victoria, Tanganyika and Victoria are among the great lakes of Africa. The Great Lakes of North America are a group of five large freshwater lakes.
Quick revision
Remember that the Pacific is the largest and deepest ocean and the Arctic the smallest and shallowest, with the Mariana Trench (Challenger Deep) the deepest point. Warm currents such as the Gulf Stream and Kuroshio flow polewards, while cold currents such as the Labrador, Canary and Benguela flow towards the equator. The Nile is the longest river and the Amazon is the largest by discharge with the biggest basin. The Caspian is the largest lake, Baikal the deepest and largest freshwater lake by volume, Superior the largest freshwater lake by area and the Dead Sea the lowest land point.
How to prepare this topic
- Learn the five oceans in order of size and depth.
- Fix the extreme facts: Mariana Trench deepest, Challenger Deep, Nile longest river, Amazon largest discharge, Caspian largest lake, Baikal deepest lake.
- Classify currents into warm and cold with two examples each.
- Remember the ocean floor features from shelf to trench.
- Keep the rivers and lakes of each continent ready for matching questions.
Common exam traps
- The Nile is the longest river, but the Amazon carries the largest volume of water.
- The Caspian Sea is actually a lake, and the Dead Sea is a lake too.
- The Arctic Ocean is the smallest and shallowest, not the Southern Ocean.
- Lake Baikal is the deepest lake, while Lake Superior is the largest freshwater lake by area.
- Warm currents raise coastal temperature, cold currents lower it - do not interchange them.
Frequently Asked Questions
Which is the largest, deepest, smallest and shallowest ocean?
The Pacific Ocean is the largest and deepest ocean, covering about one third of the Earth's surface, and it contains the Mariana Trench, whose deepest point is the Challenger Deep at about 11,034 metres. The Atlantic is the second largest and the Indian is the third. The Arctic Ocean is the smallest and shallowest, while the Southern Ocean surrounds Antarctica.
What are warm and cold ocean currents with examples?
Warm currents flow from near the equator towards the poles and raise the temperature of the coasts they touch; the Gulf Stream and the Kuroshio are examples. Cold currents flow from higher latitudes towards the equator and lower coastal temperatures; the Labrador, Canary and Benguela are examples. The meeting zones of warm and cold currents are the richest fishing grounds in the world, and fog commonly forms there.
Which is the longest river and the largest river in the world?
The Nile in Africa is generally regarded as the longest river in the world at about 6,650 kilometres. The Amazon in South America is the largest river by volume of water, or discharge, and has the largest drainage basin, carrying about one fifth of all fresh water entering the oceans. The Ganga-Brahmaputra delta is the largest delta in the world.
What is the difference between a trench and a ridge?
A trench is a narrow, very deep depression in the ocean floor, usually formed where an oceanic plate subducts beneath another plate, and the Mariana Trench is the deepest. A mid-oceanic ridge is a long underwater mountain system built at divergent boundaries where new crust forms, such as the Mid-Atlantic Ridge. The ridge system is the longest mountain range on Earth.
Which are the largest, deepest and lowest lakes in the world?
The Caspian Sea is the largest lake in the world, though it is called a sea because of its size and salinity. Lake Baikal in Russia is the deepest lake and the largest freshwater lake by volume, while Lake Superior is the largest freshwater lake by surface area. The Dead Sea is the lowest point on land and one of the saltiest water bodies, and Lake Titicaca in the Andes is the highest navigable lake.