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The ocean floor is divided, in accordance to its different features, intocontinental margin, deep-ocean basins, and mid-ocean ridges. It is further divided in accordance to the distance from the coast and the depth of the water into continental shelf, continental slope, and the abyss.
A cross-section of the oceans shows, that continental margins may be classified into two basic types: active and passive continental margins. An active continental margin, withvolcanic activity and frequent earthquakes, is found in the subduction zones. The coast of South America is an example of this type. A passive continental margin extends alongside the coast of North America. It is quite removed from the plate boundaries. There are no volcanoes in this region, and earthquakes occur very rarely.
An important region, called shelf, is situated between the abyss and the coast. Continental shelf surrounds all continents. This region, where the average depth of water is 150 metres, was originally a part of the continental mass. On the passive continental margins, the shelf is relatively flat and wide. On the coast of Siberia, the shelf extends up to 1600 kilometres into the sea. On the active continental margins, the shelf is generally more narrow and morphologically more fragmented. The cause of the slightly irregular relief of these regions, withvalleys and hills, is their glaciation during the Pleistocene. At that time, all shelf regions, which today are 100 metres below the surface of the water, were above the sea level. During that time they were shaped into what they are today.
Deep trenches, hills, and ridges formed as a result of theaction of waves, erosion, and glacial sediments. Powerful volcanoes, extensive deltas, and coral reefs created a surface rich in contrasts. Deep canyons incise the continental shelf. An example is the Congo Canyon (Africa), which is 10 kilometres long and 3 kilometres deep.
The continental shelf zones are very important regions for the economy. They are the most productivefishing grounds. These areas still receive sufficient sunlight, which means they support a wide variety of plant and animal life. Oil and natural gas deposits, which exist in the continental shelf areas, are also very important. In addition, various raw materials are exploited by means of drilling. For example, the entire North Sea is a continental shelf.
Large areas of the continental slope, which is the continuation of the shelf, are covered with blue mud. It is a mixture of fine-grained sand, soil, organic substances, and particles of iron sulphide that, combined, create this blue-grey colour. Waves and thetide are not a factor in the deep-water motion. Soil and sand sediments shift from the shelf to the slope. Oceanic shift processes are at times produced by storms or earthquakes.
They form so-called turbidity currents that run down the continental slope. The sediment is disturbed and creates a layer of turbid water. This muddy water, the density of which is higher than the density of clear water, runs relatively quickly (80 km/h) across the calm ocean floor and carries material which settles down again when the current subsides..
This material also contains remnants of plants and animals, which can live only in the shallow waters of the continental shelf, because the temperatures are not as low as in the deep ocean waters. This explains their occurrence in thesediment samples taken from the ocean floor. Coarse grains and rock particles are the first to settle. Gradually, the lighter particles also settle down. Turbidity currents generally travel up to 100 kilometres, however, some of those leaving the North American continent will reach the Mid-Atlantic Ridge.
The steep continental slope gradually transitions into not so steep continental rise in the depth of approximately 3500 metres into the abyss. Exploration of these regions is very difficult. The abyss was formed by the volcanic activity in the Mid-Ocean Ridge, as well as by sediments. In these regions, there are guyots, plains,volcanic islands, and deep ocean ridges, creating a diversified landscape. Some of the volcanoes rise above the sea level. Similar to deep ocean mountain ridges, volcanoes also occur in groups as well as individually.
They could form in two places. One was on the edge of the Mid-Ocean Ridge, and the other in the places where a continental plate moved over the so-called hot spot, which rose from the earth's mantle.
The term hot spot describes the melt zone in the earth's mantle underneath thelithosphere. They originated due to increased temperature and are more than 10 million years old. On the surface, they are visible as volcanoes. Since hot spots are locally stable, the lithospheric plates slide over them, which creates new eruptive points in the plate. Volcanic crests form in this way.
The deep ocean sediments consist of a variety of components. In the polar regions, there are larger fragments of rocks and sands on the ocean floor. They were transported by melting glaciers (moraine deposits).
The so-called pelagic sediments consist of red and brown clay (which may be used to make pottery) of the deep ocean, containing first of all solidified particles of smallorganisms living in more shallow waters. Fine-grain particles fall at a very slow speed from the upper layers to the deep ocean floor.
Certain part of this dust is carried by thewind over the continents to the sea. Chemical reactions of the sea water with these sediments produce manganese concretions. These are nodules forming around some sort of a nucleus (for example, a grain of sand or a particle of a bone). These formations grow very slowly, they need approximately 1 million years to produce less than one centimetre of concretion. In addition to 30 percent of manganese, they contain compounds of iron, nickel, tin, and copper. Large areas of the abyss are covered by these nodules, which are the size of a potato. Twenty to thirty percent of the Pacific Ocean floor is covered by these formations.
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