Topic: Properties of the Ocean
Theme: Systems and Structures

Key Concepts

Water and Seawater

  • Earth is the only planet in our solar system with liquid water on its surface.
  • All known life forms on Earth depend on water.
  • Water is unique among substances.
  • The high surface tension of water, which results in water "piling up" on a flat surface, is just one of the unusual properties of water. Water's characteristics are important in determining how water interacts with other substances.
  • Water can dissolve more substances and greater quantities of these substances than any other liquid.
  • Ocean water contains salts and minerals that make it different from fresh water.
  • Most of Earth's water (a little over 97%) is in the oceans. Freshwater may be the most precious and scarce natural resource supporting humanity.

Density and Layers in the Ocean

  • Salts dissolved in water increase the density of water.
  • The concentration (quantity) of salts in seawater is expressed as salinity.
  • Very small changes in salinity can significantly alter seawater density.
  • Objects or substances that have a higher density than water sink and ones that have a lower density rise and float.
  • Liquid water can also rise or sink if its density is different from that of the surrounding water.
  • Most substances have higher density in the solid phase than in the liquid phase. Water is an exception to this rule. Ice floats because it is less dense than water.
  • The world's oceans may be thought of as having three layers where water conditions such as temperature, salinity, and density are essentially the same:
    • Surface (mixed) layer - This layer is 100 to 500 meters thick and waters are well mixed by winds and waves. The salinity and temperature of this layer changes seasonally. This zone contains 2% of the total ocean volume. It has the warmest and least dense water in the ocean.
    • Intermediate layer - Water density changes markedly in this layer. Its depth is dependent on temperature and salinity and whether it's in the open ocean or near a coastal area.
    • Deep (bottom) layer - About 80% of the ocean's volume is found in this layer. Low temperature, high salinity, and high density mark it.
  • Many water movements in the ocean are driven by differences in density.


  • Water density is controlled by changes in temperature, and salinity (concentration of dissolved salts and minerals), and (to a lesser degree) pressure.
  • Temperature is a measure of the amount of heat gained or lost by an object or substance.
  • Decreases in seawater temperature cause density to increase uniformly due to the salinity of the water.
  • Water is resistant to compression, therefore, only very slight increases in density occur with an increase in pressure.

Bathymetry (submarine topography)

  • The sea bottom was thought at one time to be flat and featureless. Parts of the ocean bottom, however, are as uneven and rugged as the familiar mountain topography of land.
  • The part of the ocean nearest the continentents is divided into three parts:
    • The continental shelf,
    • The continental slope ( it's here that we find submarine canyons),
    • The continental rise.
  • Beyond the continental margins lie deep ocean basins. The topography here is very varied. The following bathymetric features can be found:
    • Abyssal plains
    • Abyssal hills
    • Seamounts
    • Deep-sea trenches
    • Mid-ocean ridge