THE SHAPE OF THE SEA FLOOR

The topography of the floor of the deep ocean (Fig. 2.1) is a balance between the parameters of seafloor spreading and sedimentation of inorganic and organic particles. Around the periphery of the ocean basins lies a continental shelf of varying width ending at the shelf break, usually at a depth of c. 200 m, below which plant life is supposed to be absent. In the Antarctic, owing to the weight of the ice cap, the shelf edge is at c. 500 m. If we accept topographic criteria, the deep sea may be said to begin at the shelf break. This is a safer distinction than one based on photosynthetic depth since attached seaweeds have been found as deep as 268 m off the Bahamas (Littler et al., 1985).

Seaward of the shelf edge there is a marked increase in the downward gradient of the seabed indicating the continental slope (Fig. 2.2). The slope may be a simple structure where the isobaths are parallel to each other and evenly spaced or it may contain a series of irregularities to give a very uneven gradient. The slope marks the underlying boundary between oceanic and continental crust. The theory of plate tectonics tells us that these areas of crust consist of a dynamic system of plates where crust is both being formed at mid-ocean ridge spreading centres and consumed by subduction at ‘active’, or seismic margins typical in the Pacific (Fig. 2.3).