Strait of Gibraltar
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The Strait of Gibraltar (Arabic: مضيق جبل طارق, Spanish: Estrecho de Gibraltar) is the strait that connects the Atlantic Ocean from the Mediterranean Sea and separates Spain from Morocco. The name comes from Gibraltar, which in turn originates from the Arabic Jebel at-Tariq (جبل طارق) meaning mountain of Tariq. It refers to the Ummayad Berber general Tariq ibn-Ziyad who led the Islamic conquest of Hispania in 711. It is also known as the Straits of Gibraltar or Strog (Strait Of Gibraltar), the latter being used in naval circles.
On the northern side of the Strait is Spain and Gibraltar, while on the southern side is Morocco and Ceuta, a Spanish exclave in North Africa. Its boundaries were known in antiquity as the Pillars of Hercules. There are several small islands, such as the disputed Isla Perejil, that are claimed by both Spain and Morocco.
About 6 million years ago, the Strait closed, effectively turning the Mediterranean into a huge salty lake that eventually dried up, in what is known as the Messinian Salinity Crisis.
The Strait has a depth of approximately 300 metres, and is about 14 kilometres (8.7 miles) wide at its narrowest point.
For a number of years, the Spanish and Moroccan governments have been jointly investigating the feasibility of a tunnel underneath the strait, similar to the Channel tunnel between the UK and France. The idea of a tunnel for petrol/diesel powered vehicles was renounced because of the currently insurmountable engineering challenge of ventilation to remove exhaust gases from automobiles from a tunnel some 14 km (9 mi) long. A new three-year study for a railway tunnel was announced in 2003.
In addition, a group of American and British engineers have studied the feasibility of building a bridge to span the straits.  Such a bridge would be of a combination suspension-truss design and would dwarf any existing bridge in height (over 900 metres). The 1979 science fiction novel The Fountains of Paradise by Arthur C. Clarke featured such a bridge.
In the event that the earth's general sea level rises significantly due to global warming, then a barrage would become an option worth studying. Such a barrage would incorporate locks to pass ships, a road and rail link to connect the continents, and a hydroelectric power plant to generate power from the flow which would be generated by the Mediterranean's excess evaporation. See also Atlantropa for a discussion of such a project that was never undertaken (although Atlantropa had many other goals and requirements).
To prevent the Mediterranean becoming inexorably saltier, a large pipe would be led from the deep part of the dam, down into the depths of the Atlantic ocean. Because of the salinity difference, the outward flow of deep Mediterranean water would not require pumping.
 Inflow and outflow
On a net basis, water continually flows into (eastward through) the Strait of Gibraltar, due to an evaporation rate within the Mediterranean basin higher than the combined inflow of all the rivers that empty into it. The sill of the Strait of Gibraltar acts to limit mixing between the cold, less saline Atlantic water and the warm Mediterranean waters. The latter are so much saltier that they sink below the constantly incoming Atlantic water and form a highly saline (thermohaline, both warm and salty) bottom water, called the Mediterranean Outflow. A density boundary separates the layers at about 100 m depth. It flows out and down the continental slope, losing salinity, until it equilibrates after mixing at a depth of about 1000 meters. The Mediterranean outflow water can be traced for thousands of kilometers before losing its identity.
Internal waves (waves at the density boundary layer) are common in 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, internal waves are set off at the Camarinal Sill as the high tide relaxes. The waves -- sometimes with heights up to 100 m -- travel eastward. Even though the waves occur at great depth and the height of the waves at the surface is almost nothing, they can be traced in the sunglint because they concentrate the biological films on the water surface, creating slight differences in roughness. The waves flow eastward, refract around coastal features; can be traced for as much as 150 km, and sometimes create interference patterns with refracted waves.
 External links
- Climate Control Requires a Dam at the Strait of Gibraltar — American Geophysical Union, 1997. Accessed 26 February 2006.
- Project for a Europe-Africa permanent link through the Strait of Gibraltar — United Nations Economic and Social Council, 2001. Accessed 26 February 2006.
- Map of Morocco — Multimap.com, 2006. Accessed 26 February 2006.
- (Spanish) Estudios Geográficos del Estrecho de Gibraltar — La Universidad de Tetuán and La Universidad de Sevilla. Accessed 26 February 2006.
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