The structure of the ocean basins has only really been thoroughly investigated in the last 50 years. Until the discovery of plate tectonics in the 1960's, the structure of the ocean floor was thought of as a rather boring and static environment. However, we now know that parts of the seafloor are very dynamic environments affected by massive landslides and sediment collapses. At the other end of the spectrum, the abyssal plains are areas of cold calm, only occasionally disturbed by dramatic events.

The structure of the ocean basins

If you removed all the seawater and looked at the edge of a typical ocean basin, this is what it would look like in profile:

The continental shelf

Nearest to the land, there is an area of the continental landmass which is covered by the sea - this is called the continental shelf. Continental shelves occupy apporximately 7% of the seafloor. The shelf slopes gently away from the land at an angle of about 1 - 3º and is covered by shallow seas, usually less than 200 metres deep (average 60m deep).

Some landmasses, such as the UK, are surrounded by a wide continental shelf, whereas other areas like the coast off Portugal have a narrow continental shelf. The width of the shelf is mostly controlled by the underlying geological structure, and can vary from a few kilometres to over 1000km wide. Active continental margins (eg, the Pacific rim), where the oceanic plate is being subducted beneath the continental plate, tend to have narrow continental shelves. Passive margins (eg, the Atlantic coast which has no tectonic plate boundary) have much wider, shallower shelf areas. The edge of the continental shelf is believed to represent the sea level during the last glaciation, about 18,000 years ago. The sea level was much lower at this time because so much water was locked up in the ice caps and glaciers.

The shelf environment is rich in biology because there is good light penetration, which is essential for the food chain. It is also an area of active currents which transport sediment along the coastlines and funnel sediment across the shelf towards the deep sea.

The continental slope

At the edge of the continental shelf, the seafloor drops away suddenly to form the continental slope. The ocean's continental slopes account for around 9% of the seafloor. The slope here is much steeper than on the shelf (usually around 30º), and is the site of submarine landlsides, turbidity currents and sediment slumps. The continental slope is often cut by massive underwater canyons and gorges, created by currents carrying sediment from the continental slope down to the deep sea. The canyons are usually (but not always) linked to major river mouths across the continental shelf, and provide a chute for the sediment spewed out by rivers. Water depth on the continental slope can reach thousands of metres within a few kilometres of the shelf break.

Left: Bathymetric image showing submarine canyons cutting through the seafloor offshore of the USA. Image source: NGDC.

The continental rise

At the base of the continental slope, the gradient becomes shallower. The area between the continental slope and the abyssal plain (the deepest part of an ocean basin) is called the continental rise. Here, sediment which has moved down from the continental shelf piles up at the base of the slope, and is gently sculpted by slow-moving currents which travel along the contours of the slope break. On passive continental margins, the continental rise tends to mark the transition between continental cruse and oceanic crust.

The abyssal plain

The abyssal plains are the deepest regions of the ocean basins (with the exception of subduction trenches), and they form vast expanses of flat, cold, dark terrain. Here, the water can reach depths of over 5000 metres, so little or no light penetrates into the murky depths. If the ocean basin is split by a mid-ocean spreading ridge (for example, the Mid Atlantic Ridge in the Atlantic Ocean), the seafloor will start to gently rise upwards again as you approach the spreading ridge. This is because of the thermal buoyancy of the ocean crust around the hot spreading centre. As you move away from the ridge back out onto the abyssal plain, the ocean crust is cooler and less buoyant, so the ocean is deeper at this point.

Only very fine sediment reaches the depths of the abyssal plain. The mud on the floor of the abyssal plain is mostly composed of extremely fine particles of clay and microscopic marine organisms. However, large turbidity currents can carry sediment from the continental slope down onto the abyssal plain, where it spreads out and forms thin layers on the seafloor. These layers are very distinctive in the stratigraphic sequence (the sequence of mud and sand layers making up the sediment on the ocean floor), and they can be used to date large geological events, such as earthquakes and large landslides (for an example, see the Great Lisbon Quake of 1755).

Life in the abyss?

For centuries, it was assumed that no life could exist in the deepest part of the oceans because of the inhospitable coniditions - after all, would you like to live in total darkness in temperatures of around 2ºC?! Amazingly, life thrives in places in the deep ocean, mainly around hydrothermal vents associated with spreading ridges. As you can see from the picture of the fangtooth fish to the left, the creatures of the deep are weird and wonderful - click here to find out more!