Australia: The Land Where Time Began

A biography of the Australian continent 

The Oceans

According to the authors1 in descending order of size the main oceans are the Pacific Ocean (155,557,000 km2),Atlantic Ocean (76,762,000 km2) Indian Ocean (68,556,000 km2), Southern Ocean (20,327,000 km2) and Arctic Ocean (14,056,000 km2). These areas are from Link 1. Of these oceans the Southern Ocean is the only one to be dived from other oceans by ocean water characteristics and circulation rather than by land masses.

The general circulation characteristics of an ocean, as well as the processes that are involved in mixing that are essential for forcing and water properties, are determined by such things as the shape, depth and geographic location. Importation details of the circulation are often affected by features that are of smaller scales such as the locations of deep sills and fracture zones, seamounts, and the roughness of the bottom. The shapes of the 2 largest oceans differ in the Atlantic Ocean having an S-shape and the Pacific having a more oval shape. The Pacific Ocean has an east-west width that is about double that of both the Atlantic Ocean and the Indian Oceans. The ways the circulation of each ocean adjusts to forcing changes is impacted by width of the oceans. Cold, dense waters are not formed in the Indian as it has no high latitudes in the north. Subduction trenches surround the Pacific Ocean where the the oceanic plates are subducted beneath the continental plates resulting in volcanic activity and earthquakes in the overlying continental plates. Dynamic seafloor spreading is occurring in the Atlantic Ocean as basaltic lava is added to the centre of the Mid-Atlantic Ridge (MAR) that pushes the plates apart with the result that the Atlantic Ocean is growing in width by a few centimetres per year.

Marginal seas are parts of oceans that are partially cut off from an ocean but remain connected to it by channels that are described as fairly narrow1,  those being connected by very few channels being occasionally called "mediterranean seas', after the sea of the same name between Europe and Africa.

The Mediterranean Sea is an example of a sea that has a negative water balance with inflow being less than it receives from runoff or precipitation than is lost by evaporation. The Black Sea, that connects to the Mediterranean, is an example of a sea with a positive water balance, having net precipitation. The Caribbean Sea, Sea of Japan, Bering Sea, North /sea, Baltic Sea, etc. are examples where a part of the ocean is separated from the main ocean by multiple straits or island chains. The Norwegian Sea, Labrador Sea, Sargasso Sea and Tasman Sea are examples of a sea being distinguished by distinguishing local oceanographic characteristics.

Only 29 % of the Earth's surface is presently covered by land, the remainder being covered by the oceans and their marginal seas, though the latest figure for the area of the oceans is 70.96 % (Becker et al., 2009). At times in the past, in particular the Cretaceous, the land surface was even less than the present being as little as 18 % at the close of the Cretaceous, a true water world. Most of the land surface of the Earth is in the Northern Hemisphere where the ratio of water to land is 1.5:1, while in the Southern Hemisphere the ratio is 4:1.  The area of the Pacific Ocean is almost twice that of the area of the Atlantic Ocean and Indian Ocean combined, and if the section of the Southern Ocean to the south of it is included in occupies about 46 % of the total area of oceans on Earth, with the Atlantic being about 23 %, the Indian Ocean about 20 % and all the other oceans combined about 11 %.

The oceans have an average depth of 4,000 m, the marginal seas of about 1200 m or less. The oceans are much deeper below sea level than the land is high above sea level. On the surface of the land 11 % is higher than  2000 m but in the oceans 84 % of the seafloor is below 2,000 m, though when the highest point on land is compared with the maximum ocean depth the height different is much closer, on land, Mt Everest, being 8848 m while in the oceans, the Mariana Trench in the western part of the Pacific Ocean, it is 11,031 below sea level. Though the average depth of the oceans is 4 km it appears much smaller when it is compared to the width of the oceans, from 5,000-15,000 km, making them a thin skin on the surface of the Earth, though there is much detail and structure in the oceans.

 Sources & Further reading

  1. Emery, William J., Pickard, George L., Tally, Lynne D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Academic Press.


  1. World Atlas
  2. Website for source 1

Author: M. H. Monroe
Last updated 15/04/2012



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