Water (Oceans)

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 12.

What does the other seventy per cent of the Earth look like?

Drain every ocean and you would not find a flat bowl. You would find mountain ranges longer than any on land, canyons deeper than the Grand Canyon, flat plains smoother than any farmland, and trenches that could swallow Mount Everest. The ocean surface hides a landscape as varied as the continents. This chapter has four jobs: follow water around its cycle, map the ocean floor, explain why ocean water is warm at the top and cold at the bottom, and explain why some seas are saltier than others.

Water is a cyclic resource

Earth is called the Blue Planet because it has an abundant supply of water on its surface. Water is a cyclic resource: it can be used and reused. The hydrological cycle is the circulation of water within the earth's hydrosphere in liquid, solid and gaseous forms. It is the continuous exchange of water between the oceans, the atmosphere, the land surface, the subsurface and organisms.

About 91 per cent of the planetary water is found in the oceans. The rest is freshwater, held in glaciers and ice caps, groundwater, lakes, soil moisture, the atmosphere, streams and living things. Nearly 59 per cent of the water that falls on land returns to the atmosphere through evaporation from oceans and other places. The remainder runs off over the surface, soaks into the ground, or becomes part of a glacier.

Renewable water is constant, but demand is growing fast, and river pollution makes things worse. This leads to a water crisis in space and in time, because some places and some seasons have plenty and others very little.

Beyond the textbook. For the exam: about 91 per cent of the planetary water is in the oceans. In reality: most scientific estimates put the oceans at roughly 97 per cent of all water on Earth, with only about 3 per cent as freshwater, most of it locked in ice. Why it matters: both figures point to the same conclusion, that almost all of Earth's water is salty and the usable freshwater is a tiny fraction.

The five oceans

Geographers divide the ocean into five: the Pacific, Atlantic, Indian, Southern and Arctic oceans. The oceans merge so naturally into one another that they are hard to separate. The various seas, bays and gulfs are parts of these oceans. The Arctic is the smallest of the five.

Relief of the ocean floor

Oceans fill the great depressions in the earth's outer layer. A major part of the ocean floor lies 3 to 6 km below sea level. The ocean floor is as varied as land, with the world's largest mountain ranges, deepest trenches and largest plains. They form in the same way as continental features: by tectonic, volcanic and depositional processes.

The floor has four major divisions, which you meet in order as you go from the coast to the open ocean.

Continental shelf in detail. Shelf width varies a lot. It is almost absent or very narrow along some margins, such as the coast of Chile and the west coast of Sumatra. The Siberian shelf in the Arctic Ocean is the largest in the world, stretching to 1,500 km. The thick sediments laid down over a long time become the source of fossil fuels, which is why shelves matter for oil and gas.

Oceanic deeps in detail. Trenches occur at the bases of continental slopes and along island arcs, and are linked to active volcanoes and strong earthquakes. That makes them important for the study of plate movements. Of the 57 deeps explored so far, 32 are in the Pacific, 19 in the Atlantic and 6 in the Indian Ocean.

Pause and predict: the Pacific has the most trenches. Without looking back, what do trenches tell you about the Pacific's edges?

Check your answer: they are zones of plate movement. Trenches sit where one plate dives beneath another, near active volcanoes and earthquakes, so the Pacific rim is a busy zone of both.

Beyond the textbook. For the exam: trenches are the deepest parts of the oceans and their count by ocean (32, 19, 6) is the figure to remember. In reality: the deepest trench of each ocean is usually given as the Mariana Trench in the Pacific, the Puerto Rico Trench in the Atlantic and the Java (Sunda) Trench in the Indian Ocean. The Challenger Deep in the Mariana Trench is about 11 km below sea level, deeper than Mount Everest is high. Why it matters: it answers the common question about the deepest trench of each ocean.

Minor relief features

Apart from the four major divisions, some smaller but important features dominate parts of the oceans.

An oceanic deep is a major feature, not a minor one. The guyot's flat top is the clue to its history: it was once a seamount that sank, and its summit was levelled before it submerged.

Temperature of ocean waters

Ocean water is heated by solar energy just as land is, but it heats up and cools down more slowly than land.

Beyond the textbook. For the exam: heating and cooling of ocean water is slower than land. In reality: water needs much more heat to change temperature than rock or soil does, and currents and mixing carry heat through the water. So the sea moderates the climate of nearby land: coasts have a smaller temperature range than continental interiors. Why it matters: it links this chapter to the marine climates of the previous one.

Factors that affect ocean temperature. Four factors are tested.

The Gulf Stream (warm) raises temperature near the eastern coast of North America and the west coast of Europe. The Labrador Current (cold) lowers temperature near the north east coast of North America. Enclosed seas in low latitudes are warmer than open seas, while enclosed seas in high latitudes are cooler than open seas.

Horizontal pattern. The average surface temperature of the oceans is about 27°C, and it decreases from the equator to the poles at roughly 0.5°C per degree of latitude.

The northern hemisphere oceans are warmer than those of the southern hemisphere, with annual averages of about 19°C and 16°C. The highest temperature is not recorded at the equator but slightly north of it, a result of the unequal distribution of land and water.

Vertical pattern. The warmest water is always at the surface, because the sun heats it directly, and heat then moves down by convection. Temperature falls very rapidly to a depth of about 200 m, and more slowly after that. The boundary region where temperature drops rapidly is the thermocline. It usually begins at 100 to 400 m below the surface and extends several hundred metres downward. About 90 per cent of the ocean's volume lies below the thermocline, where temperatures approach 0°C.

In middle and low latitudes the ocean behaves like a three layer system.

At the Arctic and Antarctic circles, the surface water is already close to 0°C, so there is only one layer of cold water from the surface to the floor and the temperature changes very little with depth.

Pause and predict: at 70°N, would you expect a sharp thermocline to separate warm surface water from cold deep water? Why?

Check your answer: no. The surface water is already near 0°C, so there is no warm layer to contrast with the deep water. Polar oceans have one cold layer all the way down.

Salinity of ocean waters

All natural water, whether rain or sea, holds dissolved salts. Salinity is the total content of dissolved salts in seawater. It is calculated as the grams of salt dissolved in 1,000 grams (1 kg) of seawater and usually expressed in parts per thousand (ppt). Salinity of 24.7 ppt is the upper limit for brackish water.

What controls it. Salinity changes mainly because water is added or taken away.

Salinity of selected water bodies. The textbook lists these as the most saline.

Beyond the textbook. For the exam: the textbook ranks Lake Van, the Dead Sea and the Great Salt Lake in that order of salinity. In reality: direct measurements put the Dead Sea at roughly 340 ppt, among the saltiest bodies of water in the world, while Lake Van is a soda lake with a salinity of only about 22 ppt. Why it matters: if a question lists the Dead Sea without Lake Van, the Dead Sea is the saltiest choice.

Horizontal distribution. The salinity of the normal open ocean ranges from 33 to 37 ppt. In hot, dry regions with high evaporation it can reach 70 ppt. In estuaries and the Arctic it varies seasonally from 0 to 35 ppt. The land locked Red Sea is as high as 41 ppt.

The equator does not have the highest salinity, even though it is the hottest belt, because heavy rainfall and cloud dilute the surface water there. Salinity peaks in the drier subtropical belts where evaporation exceeds rainfall.

Vertical distribution. Surface salinity rises with evaporation or the freezing of water into ice, and falls with the input of fresh water, such as river inflow. At depth, salinity is almost fixed, because no water is lost and no salt added. Salinity generally increases with depth, and the zone where it increases sharply is the halocline. Higher salinity makes water denser, so salty water tends to sink below less salty water. This causes stratification by salinity: lighter, less salty water rests above denser, saltier water.

Pause and predict: the Baltic Sea lies in the high latitudes, yet its salinity is very low. Which factor from the table above explains it?

Check your answer: the influx of river water. A large flow of fresh water dilutes the sea. Cold, slow evaporation at high latitude adds to the effect.

Exam traps

60-second revision

Where this chapter connects

• Previous: World Climate and Climate Change shows how ocean temperature and currents affect coastal climates.
• Next: Movements of Ocean Water takes the temperature and salinity differences here and shows how they drive waves, tides and currents.
• Related: Distribution of Oceans and Continents explains how plate movements create trenches and mid oceanic ridges, and Water in the Atmosphere covers the evaporation and precipitation that control surface salinity.