Friday, April 2, 2010

The Strait of Gibraltar

The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain from Morocco. The name comes from Gibraltar, which in turn originates from the Arabic Jebel Tariq , albeit the Arab name for the Strait is Bab el-Zakat or "Gate of Charity". It is also erroneously known as the Straits of Gibraltar, in naval use and as "Pillars of Hercules" in the ancient world.
Europe and Africa are separated by 7.7 nautical miles (14.24 km) of ocean at the strait's narrowest point. The Strait's depth ranges between 300 and 900 metres (980 and 3,000 ft) which possibly interacted with the lower mean sea level of the last major glaciation 20,000 years before present when the level of the sea was believed to be 110 to 120 metres (361 to 394 ft) lower. Ferries cross between the two continents every day in as little as 35 minutes. The Spanish side of the Strait is protected under El Estrecho Natural Park.
Around 5.9 million years ago, the connection between the Mediterranean Sea and the Atlantic Ocean along the Bethic and Rifan Corridor was progressively restricted until its total closure, effectively causing the salinity of the Mediterranean to periodically fall within the gypsum and salt deposition range, during what is known as the Messinian Salinity Crisis. In this water chemistry environment, dissolved mineral concentrations, temperature and stilled water currents combined properly and occurred regularly to precipitate many mineral salts in sea floor bedded layers. The resultant accumulation of various huge salt and mineral deposits about the Mediterranean basin are directly linked to this era. It is not believed this process took a long time geologically, lasting only 500-600 thousand-years, for it is further estimated that were the straits closed even at today's higher sea level, most water in the Mediterranean basin would evaporate within only a thousand years; as it is believed to have done then, and such an event would lay down similar mineral deposits as those such as the fabulous salt mines now found under the sea floor off Sicily. After a lengthy period of restricted intermittent or no water exchange between the Atlantic Ocean and Mediterranean basin, approximately 5.33 million years ago, the Atlantic-Mediterranean connection was completely reestablished through the Strait of Gibraltar, and has remained open ever since. It is expected that the strait will close again as the African Plate moves northward relative to the Eurasian Plate[citation needed], but on geological rather than human timescales.

Through the strait, water generally flows more or less continually in both an eastward and a westward direction. A smaller amount of deeper saltier and therefore denser waters continually work their way westwards , while a larger amount of surface waters with lower salinity and density continually work their way eastwards . These general flow tendencies may be occasionally interrupted for brief periods to accommodate temporary tidal flow requirements, depending on various lunar and solar alignments. Still, on the whole and over time, the balance of the water flow is eastwards, due to an evaporation rate within the Mediterranean basin higher than the combined inflow of all the rivers that empty into it. The shallow Camarinal Sill of the Strait of Gibraltar, which forms the shallowest point within the strait, acts to limit mixing between the cold, less saline Atlantic water and the warm Mediterranean waters. The Camarinal Sill is located at the far western end of the straits.
The Mediterranean waters are so much saltier than the Atlantic waters, that they sink below the constantly incoming water and form a highly saline layer of bottom water. This layer of bottom-water constantly works its way out into the Atlantic as the Mediterranean outflow. On the Atlantic side of the strait, a density boundary separates the Mediterranean outflow waters from the rest at about 100 metres (330 ft) depth. These waters flow out and down the continental slope, losing salinity, until they begin to mix and equilibrate more rapidly, much further out at a depth of about 1,000 metres (3,300 ft). The Mediterranean outflow water layer can be traced for thousands of kilometres west of the strait, before completely losing its identity.
Internal waves are often produced by the strait. Like traffic merging on a highway, the water flow is constricted in both directions because it must pass over a shallow submarine barrier, the Camarinal Sill. When large tidal flows enter the Strait and the high tide relaxes, internal waves are generated at the Camarinal Sill and proceed eastwards. Even though the waves may occur down to great depths, occasionally at the surface the waves are almost imperceptible, at other times they can be seen clearly using satellite imagery. These internal waves continue to flow eastward and to refract around coastal features. They can sometimes be traced for as much as 100 kilometres (62 mi), and sometimes create interference patterns with refracted waves.

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