Depth Zones of Sea
The sea has variance in depth from coastal floor up to the deep sea floor. The profile of the sea floor has been divided into four zones: (i) littoral zone, (ii) continental shelf, (iii) continental slope, and (iv) deep sea zone
(1). Littoral Zone: This is the initial zone of a sea, which is also called the ”shore zone”. This zone includes the area between the levels of high tide and low tide. The littoral zone separates the coastal landmass from the sea. This zone contains the sediment derived from the land (terrigenous sediment).
(2). Continental Shelf: It starts just after the Littoral zone. It is a gently sloping submerged platform that extends from the edge of the continent to a depth of about 200 meters. The outer edge of the continental shelf is marked by rapid steepening of the gradient. Since it is underlain by continental type crust, it is clearly a flooded extension of the continental shelves that vary in width, the average width being about 65 km. The sediment on it is mostly terrigenous. Coral islands are commonly formed in this zone.
(3). Continental Slope: The continental slope lies between the continental shelf and the deep-sea zone. The depth of water varies from 200 to 900 meters. Beyond the shelf, the downward slope increases abruptly to form a continental slope. The continental slope marks the boundary between the continental crust and the oceanic crust. The sediments here are very fine, which include black mud, blue mud, green mud, coral, and volcanic mud. These sediments are derived mainly from continental erosion.
Along some mountainous coasts, the continental slope merges into deep oceanic trenches. In such cases, the shelf is very narrow or does not exist at all. Such a situation occurs along the west coast of South America. Here the vertical distance from the high peaks of the Andes Mountains to the floor of the Peru-Chile trench exceeds 12,200 meters.
(4). Deep-Sea Zones: Deep seafloor is included in this zone. The depth of the seafloor is about 4000 meters. The deep-sea zones contain very fine sediments of inorganic as well as organic nature, which are called ”ooze”. Some of the chief oozes of the deep-sea zones are:
- Radiolarian ooze: A deep-sea ooze in which at least 30 percent of the sediment consists of the siliceous radiolarian tests.
- Foraminiferal ooze: It is composed of the calcareous sediments of the shells of dead foraminifera.
- Diatom ooze: It is composed of microscopic unicellular algea, which have cells walls consisting of silica or resembles silica.
- Red clay: It is also known as Pelagic Red Clay or simply Pelagic Clay or abbysal red clay. This types of clay is found in the floor of deep sea. It is found in the form of lithogenic fine grained material, the composition of which is clay, silt, and little amount of sand. It is found in the sea floor of generally more than 4000 meters depth.