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Translations of Encyclopedia about Geology

 

Oceans, Lakes, and Rivers

Approximately 70 percent of the earth's surface is covered by oceans. Oceans contain 96.5 of the earth's water. Only 3.5 percent of water is freshwater. The largest supplies of fresh water are found in glaciers, snow, and underground water, only then followed by rivers and lakes

Oceans are divided into different regions, based on their distance from the coast and their depth. The regions linked to the coasts are called the continental shelf. Continental shelf changes into continental slope, which descends steeply to a depth of two thousand metres, ending at the continental foot or continental rise. Beyond is the deepest part of the ocean, mostly flat, called abyss or abyssal plain.

On the ocean floor there are certain distinctive formations, such as, for example, deep ocean trenches or mid-ocean ridges. Earthquakes and volcanic phenomena occur in these tectonically active regions. Deep ocean trenches are places where the oceanic crust collapses into the depth; new ocean floor forms on the mid-ocean ridges where magma rises from the earth's interior. The relief of the sea landscape is also shaped by the motion of the waves, the fluctuating sea level, and scouring.

The average content of salt in the sea water is 3.5 percent. However, when that water mixes with fresher water from the surrounding seas (Mediterranean, Baltic), this figure may be lower. In contrast, when higher rate of evaporation is present (Dead Sea, Red Sea), this figure will be higher. The salt main components are kitchen salt (sodium chloride), magnesium chloride, magnesium sulphate, calcium, and potassium.

Ocean currents are an integral part of the motion of the water in the oceans and cover considerable distances. Near the water surface, they are generally given by prevailing winds and partially deflected by the earth's rotation. This occurs at right angle to the current, to the right in the northern hemisphere and to the left in the southern hemisphere. In addition to the surface currents there are also slow-flowing deep-ocean currents. The circulating masses of water originate here. This is linked to the exchange of energy, nutrients, and sediments. Cold or warm currents may play a role in climatic conditions of certain regions. The global climate also depends on the evaporation of seawater, because that is the most important prerequisite for the forming of precipitation.

Lakes also collect water. They are not connected with the oceans, however, and are delineated by the earth's surface. Based on their size, they may be classified as pond, pool, lake, and so on. The depression needed for the formation of a lake may be created by a moraine, separation of a river bend, movements of the earths crust, or a crater of an extinct volcano.

The Dead Sea is also a lake, and it is an example of salt-water lakes. There is no water outlet. Gradual evaporation results in a high concentration of salt, which in this case reaches 28 percent. That means that salt concentration here is nine times higher than it is in the oceans.

Rivers represent one of the most important geological forces shaping the surface of the earth. They shape the landscape, because the rivers flow through extensive portions of the earth. Rivers carry sediments to the ocean. On their journey to the sea they create steep gorges and banks, valleys, and fertile flood basins. In places, where the hard rock softens, we find white waters and waterfalls. Deltas form in the coastal regions due to the accumulation of a large amount of sediments.

Rivers also have a considerable economic value. As waterways, they connect individual countries and their industrial centres. A system of a river and its confluents is an important supplier of water to the population as well as the industry.

In addition, rivers are a vital component in the hydrological cycle of precipitation and evaporation, between the atmosphere and the oceans. Rivers carry to the sea most of the precipitation the land receives.

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