Page 177 - Sedimentology and Stratigraphy

164 The Marine Realm: Morphology and Processes

Fig. 11.1 A cross-section from the

continental shelf through the continental
slope and rise down to the abyssal plain.

parts of the bathymetric profile from the deep ocean the shelf edge break. Large areas of continental crust
to the shelf. The angle of the continental slope is that are covered by seawater, which are mainly bor-
relatively steep, usually between about 2˚ and 7˚, dered by land masses and connected by straits to the
while the continental rise is a lower angle slope oceans, are called epicontinental seas (sometimes
down to the edge of the abyssal plain. called epeiric seas). The areas of epicontinental seas
The continental shelf itself is underlain by conti- are greatest when relative sea levels are at the highest
nental crust, and the junction between the shelf and worldwide. A nomenclature for the division of the
the slope usually occurs at about 200 m below sea marine realm based on these depth zones is shown
level at present-day margins (the shelf edge break). in Fig. 11.2. The shelf area, down to 200 m water
Continental shelves are very gently sloping with gra- depth, is called the neritic zone, the bathyal zone
dients ranging from steep shelves of 1 in 40 to more corresponds to the continental slope and extends from
typical gradients of 1 in 1000. They may extend for 200 m to 2000 m water depth, while the abyssal
tens to hundreds of kilometres from the coastline to zone is the ocean floor below 2000 m. A depth limit

Fig. 11.2 Depth-related divisions of the marine realm: (a) broad divisions are defined by water depth; (b) the shelf is described
in terms of the depth to which different processes interact with the sea floor, and the actual depths vary according to the
characteristics of the shelf.