Key Points

Our planet is called the Blue Planet because approximately 71 percent of its surface is covered with water, primarily in the oceans. Water is an essential component for all life forms.

The hydrological cycle describes the continuous movement of water on, in, and above the earth. It is a cyclic resource that can be used and re-used, moving from the ocean to land and back.

The ocean floors can be divided into four major divisions: the Continental Shelf, the Continental Slope, the Deep Sea Plain, and the Oceanic Deeps or Trenches.

The continental shelf is the shallow, extended margin of each continent. Massive sedimentary deposits on the shelves can become sources of fossil fuels.

The continental slope connects the shelf to the ocean basin and is characterized by a steep gradient. Its depth varies between 200 and 3,000 meters.

Deep sea plains are the flattest and smoothest regions of the world, located in the deep ocean basins. They are covered with fine-grained sediments like clay and silt.

These are the deepest parts of the oceans, forming relatively steep-sided, narrow basins. They are associated with active volcanoes and strong earthquakes.

Minor but significant ocean floor features include Mid-Oceanic Ridges (underwater mountains), Seamounts (volcanic mountains not reaching the surface), Guyots (flat-topped seamounts), and Atolls (coral islands).

The distribution of temperature in ocean water is affected by latitude, the unequal distribution of land and water, prevailing winds, and ocean currents.

Ocean temperature generally decreases with depth. This creates a three-layer system in low to middle latitudes: a warm top layer, a thermocline layer, and a cold deep layer.

The thermocline is a boundary region in the ocean where there is a rapid decrease in temperature with increasing depth. It separates the warm surface water from the cold deeper water.

The average surface temperature of oceans is about 27 C, and it decreases from the equator towards the poles. Oceans in the northern hemisphere are relatively warmer than in the southern hemisphere.

Salinity is the total content of dissolved salts in seawater. It is calculated as the amount of salt in grams dissolved in 1,000 grams of seawater and expressed as parts per thousand (%oo).

Surface salinity is mainly affected by evaporation and precipitation. It is also influenced by freshwater flow from rivers, the freezing and thawing of ice, winds, and ocean currents.

The halocline is a distinct zone in the ocean where salinity increases sharply with depth. High salinity water is denser and generally sinks below lower salinity water.