There are typical structural features that enable the ocean world to be divided into zones. Topographical zones, relating to the sea floor, and Bathymetric zones, relating to water depth.
Topographical zones, relating to the sea floor
The continental shelves extend on average around 60km from the land, and represent about 1/20 of the oceans. The break between the continental shelves and the continental slope occurs on average at a depth of around 200m.
The continental shelves fall away to the deep sea via the continental slopes with a gradient of around 1:40. This becomes more gradual at depths of around 3000m where it is called the continental rise with a gradient of around 1:100. In some places, however, the continental slopes are dissected by canyons with nearly vertical walls.
Finally at around 3500-4000m the slopes flatten out further to form the abyssal plain, with depths to around 6000m. The abyssal plains underlie about half of the oceans. They are interrupted by volcanic islands, seamounts, trenches and mid-oceanic ridges.
The vast mid-oceanic ridge mountain chains extend some 45 000km. They rise 2000-4000m from the ocean floor. Their highest peaks actually form islands.
Ocean ridges are the sites of tectonic activity, where new sea floor is being actively produced. They are called spreading centres. The sea floor re-descends into the earth’s crust at the continental margins and in the deep ocean trenches.
more on plate tectonics
Bathymetric zones, relating to water depth
There is a marked decrease in the density of life-forms in the sea with increasing depth. Scientists have also noted that many species tended to be restricted to specific depth ranges. Although the boundaries may be indistinct, for convenience sake, scientists have divided the ocean into vertical zones. These broad zones are found everywhere in the global oceans.
The upper-most layer is called the epipelagic, which means “upon the sea”. This layer is defined as being the light zone – where sunlight can penetrate. On average this is the top 100-150m, but light can, on clear winter days in the open ocean, penetrate down to 1000m. Many factors affect light penetration, the turbidity of the water being the most important. The epipelagic zone is the only zone that supports plant growth. In the shallow, inshore waters the epipelagic zone actually extends down to the sea-floor.
The second zone is called the mesopelagic, or middle zone. In its upper reaches it is a kind of twilight zone, because with the indistinct boundary between zones, this upper part will receive some kind of light during the day.
The third zone received no sunlight at all. It is divided up into three sub-zones; the bathypelagic (deep sea), the abyssopelagic (bottomless sea) and finally the hadapelagic (unseen). The hadapelagic waters are generally found in the ocean trenches from 6000 -10 000m. Definitions vary for the bathypelagic and abyssopelagic, with the latter beginning around 3000-4000m.
Another important ocean water zone is the benthic zone. This is defined as being the sea-floor and the waters 100m above it.
Read more about the physical challenges of the deep-sea