Strait of Gibraltar
Around 5.97 million years ago, the connection between the Mediterranean Sea and the Atlantic Ocean along the Betic and Rifan Corridor was progressively restricted until its total closure. This event caused the salinity of the Mediterranean to rise periodically 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 to precipitate many mineral salts in layers on the seabed. The resultant accumulation of various huge salt and mineral deposits about the Mediterranean basin are directly linked to this era. It is believed that this process took a short time by geological standards lasting around 640,000 years. Approximately 5.33 million years ago the Atlantic, Mediterranean connection was completely reestablished through the Strait of Gibraltar by the Zanclean flood. The erosion produced by the incoming waters seems to be the main cause for the present depth of the Strait at the narrows and at the Camarinal Sill.
The Strait has been identified as an Important Bird Area by BirdLife International because of the hundreds of thousands of seabirds which use it every year to migrate between the Mediterranean and the Atlantic. Significant numbers of Scopoli's and Balearic shearwaters, Audouin's and lesser black-backed gulls, razorbills, and Atlantic puffins traverse these waters annually. A resident orca pod of some 36 individuals lives around the Strait one of the few that are left in Western European waters. The pod may be facing extinction in the coming decades due to long term effects of PCB pollution. These marine mammals rely on the specific currents and fish populations found within the narrow passage connecting two major bodies of water.
Evidence of the first human habitation of the area by Neanderthals dates back 125,000 years ago. It is believed that the Rock of Gibraltar may have been one of the last outposts of Neanderthal habitation in the world with evidence of their presence there dating to as recently as 24,000 years ago. Archaeological evidence of Homo sapiens habitation of the area dates back thousands of years. The relatively short distance between the two shores has served as a quick crossing point for various groups and civilizations throughout history including Carthaginians campaigning against Rome and Romans travelling between the provinces of Hispania and Mauritania. Vandals raided south from Germania through Western Rome and into North Africa in the 5th century while Moors and Berbers arrived in the 8th, 11th centuries. Beginning in 1492 the Strait began to play a certain cultural role in acting as a barrier against cross-channel conquest and the flow of culture and language that would naturally follow such a conquest.
Water flows through the Strait more or less continuously both eastwards and westwards. A smaller amount of deeper saltier and therefore denser waters continually flow westwards as the Mediterranean outflow while a larger amount of surface waters with lower salinity and density continually flow eastwards as the Mediterranean inflow. These general flow tendencies may be occasionally interrupted for brief periods by temporary tidal flows depending on various lunar and solar alignments. At the Strait's far western end is the Camarinal Sill the Strait's shallowest point which limits mixing between the cold less saline Atlantic water and the warmer more saline Mediterranean waters. The Mediterranean waters are so much saltier than the Atlantic waters that they sink below the constantly incoming water and form a highly saline thermohaline layer of bottom water. This layer of bottom-water constantly works its way out into the Atlantic as the Mediterranean outflow.
During the Second World War German U-boats used the currents to pass into the Mediterranean Sea without detection by maintaining silence with engines off. From September 1941 to May 1944 Germany managed to send 62 U-boats into the Mediterranean. All these boats had to navigate the British-controlled Strait of Gibraltar where nine U-boats were sunk while attempting passage and 10 more had to break off their run due to damage. Following the Spanish coup of July 1936 the Spanish Republican Navy tried to blockade the Strait of Gibraltar to hamper the transport of Army of Africa troops from Spanish Morocco to Peninsular Spain. On the 5th of August 1936 the so-called Convoy de la Victoria was able to bring at least 2,500 men across the Strait breaking the republican blockade.
The United Kingdom claims around Gibraltar on the northern side of the Strait putting part of it inside British territorial waters. As this is less than the maximum it means according to the British claim that part of the Strait lies in international waters. The ownership of Gibraltar and its territorial waters is disputed by Spain. Similarly Morocco disputes Spanish sovereignty over Ceuta on the southern coast. There are several islets such as the disputed Isla Perejil that are claimed by both Morocco and Spain. Under the United Nations Convention on the Law of the Sea vessels passing through the strait do so under the regime of transit passage rather than the more limited innocent passage allowed in most territorial waters. Therefore a vessel or aircraft has the freedom of navigation or overflight for the purpose of crossing the strait of Gibraltar.
Discussion between Spain and Morocco of a tunnel under the strait began in the 1980s. In December 2003 both countries agreed to explore the construction of an undersea rail tunnel to connect their rail systems across the Strait. While the project remained in a planning phase Spanish and Moroccan officials met to discuss it occasionally including in 2012. Those talks led to nothing constructive happening but in April 2021 ministers from both countries agreed to a joint intergovernmental meeting to be held in Casablanca in the coming months. Earlier in January 2021 the UK government had studied plans for a tunnel to link Gibraltar with Tangiers that would replace the Spanish-Moroccan project that until then had had no tangible results after over 40 years of discussions. Some studies have proposed the possibility of erecting tidal power generating stations within the Strait to be powered from the predictable current at the Strait.
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Common questions
When did the Messinian salinity crisis occur in the Strait of Gibraltar?
The Messinian salinity crisis occurred around 5.97 million years ago when the connection between the Mediterranean Sea and the Atlantic Ocean progressively restricted until total closure. This event lasted approximately 640,000 years before the Atlantic-Mediterranean connection was completely reestablished by the Zanclean flood around 5.33 million years ago.
What marine life inhabits the waters of the Strait of Gibraltar today?
A resident orca pod of some 36 individuals lives around the Strait of Gibraltar, which is one of the few remaining pods in Western European waters. Hundreds of thousands of seabirds including Scopoli's shearwaters, Balearic shearwaters, Audouin's gulls, lesser black-backed gulls, razorbills, and Atlantic puffins use the area annually to migrate between the Mediterranean and the Atlantic.
How long has human habitation existed in the Strait of Gibraltar region?
Evidence of the first human habitation of the area by Neanderthals dates back 125,000 years ago with their presence there dating as recently as 24,000 years ago. Archaeological evidence of Homo sapiens habitation of the area dates back thousands of years while various groups including Carthaginians, Romans, Vandals, Moors, and Berbers have crossed the strait throughout history.
Why does water flow continuously through the Strait of Gibraltar in both directions?
Water flows more or less continuously eastwards and westwards because a smaller amount of deeper saltier and denser waters continually flow westwards as the Mediterranean outflow while a larger amount of surface waters with lower salinity and density continually flow eastwards as the Mediterranean inflow. The Camarinal Sill at the far western end limits mixing between the cold less saline Atlantic water and the warmer more saline Mediterranean waters creating distinct layers.
What role did the Strait of Gibraltar play during World War II naval operations?
During the Second World War German U-boats used the currents to pass into the Mediterranean Sea without detection from September 1941 to May 1944 when Germany managed to send 62 U-boats into the region. Nine U-boats were sunk while attempting passage through the British-controlled Strait of Gibraltar and 10 more had to break off their run due to damage before reaching their destination.