Exercise 2: The topography of the ocean floor.

Let us return to the system of ridges. The oceanographic importance of the ridges derives from their influence on water movement at great depth.

Below about 4000 m depth, all water in the ocean is of polar origin. This is because water at the surface of the polar regions gets so cold that it becomes the densest water in the ocean. It sinks to the ocean floor and spreads towards the tropics. Its path is guided, and occasionally obstructed, by the oceanic ridges.

Ridges determine whether a deep ocean basin is easily accessible by currents or isolated from the rest of the world ocean. In other words, ridges play an important role for the renewal of water, or ventilation, in deep basins. Water movement by currents, or advection, is a very effective way of ventilation.

In general, the earth's rotation produces a concentration of water movement on the western side of the ocean basins. The existence of ridges that reach much above the 4000 m level can block the flow of water along the most preferred route. Some deep ocean basins are therefore ventilated from the east rather than from the west.

The following screens will explore the pathways of bottom water in the major ocean basins.