Oceans cover about 71% of the Earth's surface and contain about 97% of the water on the planet. The oceans are not merely large water bodies; they are a dynamic system that absorbs solar radiation, stores heat, moderates climate and supports a rich diversity of life. The study of the oceans, their physical and chemical properties and the processes within them is called oceanography. This chapter focuses on the relief features of the ocean floor, the temperature and salinity of ocean water, and the factors that control them.
The oceans play a crucial role in the Earth's heat balance. Water has a high specific heat, so the oceans absorb large amounts of solar energy in the tropics and release it slowly, moderating coastal climates and transferring heat towards the poles through currents. The vertical and horizontal distribution of temperature and salinity in the oceans determines density, which drives oceanic circulation. Understanding the properties of ocean water is essential for the study of ocean currents, tides and waves in the next chapter.
The ocean floor is not a flat plain; it has a varied relief with mountains, plains and deep trenches. The major relief features of the ocean floor are:
The continental shelf is the gently sloping, submerged extension of the continent that extends from the shore to an average depth of about 200 metres. It is the shallowest part of the ocean and is rich in marine life and mineral resources. Continental shelves are wider on the east coast of North America and narrower along the west coast of South America. The width varies from almost nothing to 1,500 kilometres (as in the Arctic Ocean).
The continental slope connects the continental shelf with the deep ocean floor. It is a steep descent, with an average slope of about 2 to 3 degrees, extending to depths of about 3,000 to 5,000 metres. Submarine canyons are found on many continental slopes.
The abyssal plains are the flat, extensive regions of the deep ocean floor at depths of about 3,000 to 6,000 metres, covered with fine sediments. These are the most extensive relief features on Earth.
Ocean trenches are the deepest parts of the oceans, formed by subduction at convergent plate boundaries. The deepest trench is the Mariana Trench in the Pacific Ocean, which extends to about 11,000 metres (its deepest point, Challenger Deep).
Mid-oceanic ridges are long, elevated volcanic mountain chains running through the oceans, where new crust is formed by sea floor spreading. The Mid-Atlantic Ridge is a prominent example. The ridges rise to about 2,000 to 3,000 metres above the ocean floor.
The temperature of ocean water is influenced by the amount of insolation received, which depends on latitude; the loss of heat by evaporation; and the distribution of land and sea. Ocean currents, winds and the depth of water also affect temperature. In general, ocean temperature decreases from the equator towards the poles.
The average surface temperature of the ocean is about 26-27 degrees Celsius in the equatorial and tropical regions. In the mid-latitudes it is about 20 degrees Celsius, and near the poles it falls below freezing. The highest surface temperatures occur in the tropics, and warm currents raise the temperature of the coastal water they pass.
Temperature decreases with depth, but the rate is not uniform. The thermocline is the zone of rapid temperature decrease found between about 300 and 1,000 metres depth in the mid-latitudes. Below the thermocline, temperature decreases very slowly with depth. At the bottom, temperature is close to 0 to 2 degrees Celsius in most of the deep ocean. In the polar regions, the water is nearly isothermal (uniform temperature) from surface to bottom.
Salinity is the total amount of dissolved salts in water, expressed in grams per kilogram of sea water, or parts per thousand (per mille). The average salinity of ocean water is about 35 per mille (35 g per kg). The major salts are sodium chloride (about 85% of the salts), magnesium chloride, calcium carbonate and magnesium sulphate.
Salinity is controlled by the balance between evaporation and precipitation: - High salinity occurs in regions of high evaporation and low precipitation, such as the subtropical latitudes. - Low salinity occurs in regions of high precipitation, runoff and melting ice, such as the equatorial belt and polar regions. - Freshwater inflow from rivers, freezing and melting of ice, and ocean currents also affect salinity.
The average salinity of the oceans is about 35 per mille, but it varies. The highest salinity is found in the Red Sea (about 40-41 per mille) and the Persian Gulf, and the lowest is found near the mouths of large rivers and in polar regions where ice melts. The maximum salinity is generally found in the subtropical regions, while the equatorial region has slightly lower salinity due to heavy rainfall.
The density of sea water depends on temperature, salinity and pressure. Cold, saline water is denser than warm, fresh water. Density increases with depth, and the densest water is found in the deep ocean and in polar regions where cold, saline water sinks. These density differences drive the thermohaline circulation of the oceans, a global conveyor belt of ocean currents.
| Feature | Depth / Character | Example |
|---|---|---|
| Continental shelf | 0-200 m, gently sloping | East coast of North America |
| Continental slope | To about 3,000-5,000 m, steep | --- |
| Abyssal plains | 3,000-6,000 m, flat | Most extensive features |
| Ocean trenches | Deepest parts, over 11,000 m | Mariana Trench |
| Mid-oceanic ridges | Elevated chains | Mid-Atlantic Ridge |
| Factor | Effect on Temperature | Effect on Salinity |
|---|---|---|
| Latitude | Decreases towards poles | Subtropics highest, equator lower |
| Evaporation | Lowers temperature | Increases salinity |
| Precipitation | --- | Decreases salinity |
| Ocean currents | Warm/cold modify temperature | Warm currents higher salinity |
| Ice melt | Cools water | Decreases salinity |
The oceans are vast, dynamic systems that cover nearly three-quarters of the Earth and hold most of its water. The ocean floor has a varied relief of continental shelves, slopes, abyssal plains, deep trenches and mid-oceanic ridges, reflecting the same plate tectonic processes that shape the continents. Ocean water has characteristic temperature and salinity distributions: temperature decreases with latitude and depth, with a sharp thermocline in mid-latitudes, while salinity, averaging about 35 per mille, is highest in subtropical regions and lowest near the equator and poles. Temperature and salinity together control the density of sea water, which drives the circulation of the oceans. These properties are the basis for understanding the movements of ocean water, which we study in the next chapter.
Keywords: Oceanography, continental shelf, continental slope, abyssal plain, Mariana Trench, mid-oceanic ridge, thermocline, salinity, thermohaline circulation.