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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 to the Mediterranean Sea and separates Spain from Morocco. The name comes from Gibraltar, which in turn originates from the Arabic Jebel 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 in naval use.

There are 13 km (8 miles) of ocean separating Europe from Africa at the strait's narrowest point. The strait depth ranges between . A ferry commutes between the two continents. The Spanish part of the strait is protected under the El Estrecho Natural Park.

Location

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..

Due to its location, the strait is widely used for illegal immigration from Africa to Europe.

Geology

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. At the Miocene/Pliocene boundary, approximately 5.33 million years ago, the Strait opened up for the last time, and has remained open since.

Communications

The Straits are an important shipping route from the Mediterranean to the Atlantic. There are ferries that operate between Spain and Morocco across the strait, as well as between Spain and Ceuta and Gibraltar to Tangier.

Tunnel across the strait

In December 2003, Spain and Morocco agreed to explore the construction of an undersea rail tunnel to connect their rail systems. The gauge of the tunnel would be to match the proposed construction and conversion of significant parts of the existing broad gauge system to standard gauge.

Inflow and outflow

On a net basis, water continually flows 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 depth. It flows out and down the continental slope, losing salinity, until it equilibrates after mixing at a depth of about . 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 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 , and sometimes create interference patterns with refracted waves.