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Translations of Encyclopedia about Geology

 

Mid-Ocean Ridge

Mid-Ocean Ridge is a system of mountains in the oceans. In the region of the ridge, the plates of the earth's crust drift away from each other.

A fascinating discovery was made less then 100 years ago, when the telegraph cable between England and North America was laid down. In parallel to the coast, the Atlantic Ocean is furrowed by highlands and lowlands, similar to the surface of the land. Twenty years later, something very similar was discovered also in the Pacific Ocean. Thus it was discovered, that all ocean floors have almost identical features. We know now, that a chain of ocean ridges circle the entire earth. Some of these mountains have steep summits that rise above the surface of the water.

The Mid-Atlantic Ridge includes the Azores Archipelago, St. Paul, San Diego Alvarez. Iceland is also situated on the door-step of the Atlantic. The width of this ridge reaches almost 1000 kilometres, its height (depth) is almost 5000 metres.

Mid-ocean ridges are the centres of the most active volcanic and tectonic activities. Typically, their centre is always a central trench 20 to 50 kilometres wide. This trench always runs alongside an island chain or the continental shelf. There is always a steep wall adhered to the continent. This wall descends in the shape of a V-form into the trench. On the opposite side, there is a gradual slope into the open sea.

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Islands

Islands are pieces of land (with the exception of continents), which are completely surrounded by water. The water may be an ocean, a river, or a lake.

In general, islands situated near a continent are remnants of an originally one landmass. Reasons for their separation might have been sea erosion (for example, Halligen islands in the German Bay), a fragment of the neighbouring land (Sunda), or sinking of land.

The geological structure of islands is always very similar to that of the land. Fauna and flora are often almost identical. Sedimentary islands, for example some West Frisian islands, are constantly exposed to the tide and coastal currents, which means that there shape changes quite often.

Some islands also formed from fragments of continental crust, which separated from a larger land, for example Madagascar or Japan. A special form of islands is formed by coral animals when they colonise an extinct volcano that originated on the ocean floor. A major part of the ocean islands are of volcanic origin. In part, they are volcanoes that rose above the sea level.

A large concentration of volcanic islands formed in the so-called seafloor-spreading zones. This is where mid-ocean ridges, which in places rise above the sea level as islands, are formed. Iceland and Azores belong to this group.

Volcanic island arcs are the result of the subduction processes. An example of these are the islands of the Aleutian island arc. All of them formed within the same period of time. Among the islands that formed above a hot spot are Hawaii and Canary Islands. There, the magma rises from the earth's mantle and forms a volcanic island. If the plate moves further on, another volcanic island will form over the constant hot spot, forming a volcanic arc. The rule here is, that the further away are the individual volcanoes from the hot spot, the older they are. As the distance from the hot spot increases, the volcanic activity diminishes.

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Atolls and Coral Reefs

A special form of marine mounds are the groups of coral reefs or atolls. They are found not only in shallow waters, but in the deep oceans as well.

During the last two hundred years, exploring scientists were always fascinated by these marvellous "structures." Charles Darwin, during his ocean journeys between 1831 and 1836, was the first of the scientists to geologically study coral reefs. The theory he proposed then is still valid today.

Corals are marine organisms that can live only in water containing salt. These organisms, which form reefs, require relative high water temperature, between 15 and 20 degrees Celsius, to be able to construct branching calcite formations. They flourish mainly in tropical waters reaching a maximum of 50 metres. Calcareous algae, gastropods, crustaceans, fungi, and similar organisms also belong to the living community of corals.

Corals often settle on the summit of submarine protuberances, mainly on volcanoes. They create ring-shaped reefs on the edges of extinct or dormant volcanoes. When the ocean floor descends as a consequence of tectonic processes, or when due to postglacial processes the sea level rises, the volcanic island sinks.

Layers of corals form an atoll. The atoll contains the captured sea, a lagoon. The lagoon is surrounded by reefs. Lagoons may reach a size of 30 square kilometres.

The long island chain of atolls in the Pacific Ocean could form only when the ocean floor sank. A fringing reef changed into a barrier reef, and when the island, on which the reef was built, sank, it formed an atoll.

Other types of reefs formed in the immediate vicinity of tropical coasts. They are the fringing or the barrier reefs. Fringing reefs are situated only a few metres from the coast and often are built on the rocks. If there is a lagoon between the coast and the reef, that is to say, an area separated from the sea, the reef is called barrier reef.

The largest known barrier reef is the Australian Great Barrier Reef. It extends between 30 to 250 metres from the Australian coast and is 1600 kilometres long. It is interrupted only in few places by river deltas. Rivers bring freshwater, and thatís why the corals die in those areas.

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