Strait of Gibraltar

The Strait of Gibraltar (Arabic: مضيق جبل طارق‎, romanized: Maḍīq Jabal Ṭāriq; Spanish: Estrecho de Gibraltar, Archaic: Pillars of Hercules), also known as the Straits of Gibraltar,[1] is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Gibraltar and Peninsular Spain in Europe from Morocco in Africa.

Not to be confused with Gibraltar.
Strait of Gibraltar
The Strait of Gibraltar as seen from space.
The Iberian Peninsula is on the left and North Africa on the right.
LocationAtlantic Ocean – Mediterranean Sea
Coordinates35°58′N 5°29′W
TypeStrait
Native nameمضيق جبل طارق  (Arabic)
Estrecho de Gibraltar  (Spanish)
Basin countries
Min. width14.3 km (8.9 mi)
Max. depth900 metres (2,953 ft)

The two continents are separated by 14.3 kilometres (8.9 miles; 7.7 nautical miles) of ocean at the Strait's narrowest point. The Strait's depth ranges between 300 and 900 metres (980 and 2,950 feet; 160 and 490 fathoms)[2] which possibly interacted with the lower mean sea level of the last major glaciation 20,000 years ago[3] when the level of the sea is believed to have been lower by 110–120 m (360–390 ft; 60–66 fathoms).[4] 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.

Names and etymology

The name comes from the Rock of Gibraltar, which in turn originates from the Arabic Jabal Ṭāriq (meaning "Tariq's Mount"),[5] named after Tariq ibn Ziyad. It is also known as the Straits of Gibraltar, the Gut of Gibraltar (although this is mostly archaic),[6] the STROG (STRait Of Gibraltar) in naval use,[7] and Bāb al-Maghrib (Arabic: باب المغرب‎), "Gate of Morocco" or "Gate of the West".

In the Middle Ages, Muslims called it Az-Zuqāq (الزقاق), "the Passage", the Romans called it Fretum Gaditanum (Strait of Cadiz),[8] and in the ancient world it was known as the "Pillars of Hercules" (Ancient Greek: αἱ Ἡράκλειοι στῆλαι, romanized: hai Hērákleioi stêlai).[9]

Location
Europe (left) and Africa (right)

On the northern side of the Strait are Spain and Gibraltar (a British overseas territory in the Iberian Peninsula), while on the southern side are Morocco and Ceuta (a Spanish autonomous city in northern Africa). Its boundaries were known in antiquity as the Pillars of Hercules.

Due to its location, the Strait is commonly used for illegal immigration from Africa to Europe.[10]

Extent

The International Hydrographic Organization defines the limits of the Strait of Gibraltar as follows:[11]

On the West. A line joining Cape Trafalgar to Cape Spartel.
On the East. A line joining Europa Point to P. Almina.

Geology
A view across the Strait of Gibraltar taken from the hills above Tarifa, Spain

The seabed of the Strait is composed of synorogenic Betic-Rif clayey flysch covered by Pliocene and/or Quaternary calcareous sediments, sourced from thriving cold water coral communities.[12] Exposed bedrock surfaces, coarse sediments and local sand dunes attest to the strong bottom current conditions at the present time.

Around 5.9 million years ago,[13] the connection between the Mediterranean Sea and the Atlantic Ocean along the Betic and Rifan Corridor was progressively restricted until its total closure, effectively causing 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 and occurred regularly 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 between 500,000 and 600,000 years.

It is estimated that, were the Strait 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,[13] and such an event would lay down mineral deposits like the salt deposits now found under the sea floor all over the Mediterranean.

After a lengthy period of restricted intermittent or no water exchange between the Atlantic Ocean and Mediterranean basin, approximately 5.33 million years ago,[14] the Atlantic-Mediterranean connection was completely reestablished through the Strait of Gibraltar by the Zanclean flood, and has remained open ever since.[15] The erosion produced by the incoming waters seems to be the main cause for the present depth of the Strait (900 m (3,000 ft; 490 fathoms) at the narrows, 280 m (920 ft; 150 fathoms) at the Camarinal Sill). The Strait is expected to close again as the African Plate moves northward relative to the Eurasian Plate, but on geological rather than human timescales.

Biodiversity

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, including significant numbers of Scopoli's and Balearic shearwaters, Audouin's and lesser black-backed gulls, razorbills, and Atlantic puffins.[16]

A resident killer whale 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.[17]

History
Main articles: History of Gibraltar, History of Spain, and History of Morocco
Historic map of the Strait of Gibraltar by Piri Reis

Evidence of the first human habitation of the area by Neanderthals dates back to 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.[18] Archaeological evidence of Homo sapiens habitation of the area dates back c. 40,000 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, Romans travelling between the provinces of Hispania and Mauritania, Vandals raiding south from Germania through Western Rome and into North Africa in the 5th century, Moors and Berbers in the 8th–11th centuries, and Spain and Portugal in the 16th century.

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. In that year, the last Muslim government north of the Strait was overthrown by a Spanish force. Since that time, the Strait has come to foster the development of two very distinct and varied cultures on either side of it after sharing much the same culture for over 300 years from the 8th century to the early 13th century.

On the northern side, Christian-European culture has remained dominant since the expulsion of the last Muslim kingdom in 1492, along with the Romance Spanish language, while on the southern side, Muslim-Arabic/Mediterranean has been dominant since the spread of Islam into North Africa in the 700s, along with the Arabic language. For the last 500 years, religious and cultural intolerance, more than the small travel barrier that the Strait presents, has come to act as a powerful enforcing agent of the cultural separation that exists between these two groups.

The small British enclave of the city of Gibraltar presents a third cultural group found in the Strait. This enclave was first established in 1704 and has since been used by Britain to act as a surety for control of the sea lanes into and out of the Mediterranean.

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 5 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.[19]

Communications
3-d rendering, looking eastwards towards the Mediterranean.

The Strait is 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
Main article: Strait of Gibraltar crossing

In December 2003, Spain and Morocco agreed to explore the construction of an undersea rail tunnel to connect their rail systems across the Strait. The gauge of the rail would be 1,435 mm (4 ft 8.5 in) to match the proposed construction and conversion of significant parts of the existing broad gauge system to standard gauge.[20] While the project remains in a planning phase, Spanish and Moroccan officials have met to discuss it as recently as 2012,[21] and proposals predict it could be completed by 2025.

Special flow and wave patterns

The Strait of Gibraltar links the Atlantic Ocean directly to the Mediterranean Sea. This direct linkage creates certain unique flow and wave patterns. These unique patterns are created due to the interaction of various regional and global evaporative forces, tidal forces, and wind forces.

Inflow and outflow
The Strait of Gibraltar with the Mediterranean Sea in upper right. Internal waves (marked with arrows) are caused by water flowing through the Strait (bottom left, top right).

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 (the Mediterranean outflow), while a larger amount of surface waters with lower salinity and density continually work their way eastwards (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. 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.[22] 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 warm 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, both warm and salty) 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 m (330 ft; 55 fathoms) 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 m (3,300 ft; 550 fathoms). The Mediterranean outflow water layer can be traced for thousands of kilometres west of the Strait, before completely losing its identity.

Simplifed and stylized diagram of currents at the Camarinal Sill

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.[23] 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. No U-boats ever made it back into the Atlantic and all were either sunk in battle or scuttled by their own crews.[24]

Internal waves

Internal waves (waves at the density boundary layer) 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 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 the waves are almost imperceptible at the surface, at other times they can be seen clearly in 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 km (62 mi; 54 nmi), and sometimes create interference patterns with refracted waves.[25]

Territorial waters

Except for its far eastern end, the Strait lies within the territorial waters of Spain and Morocco. The United Kingdom claims 5.6 km (3.5 mi; 3.0 nmi) around Gibraltar on the northern side of the Strait, putting part of it inside British territorial waters. As this is less than the 22.2 km (13.8 mi; 12.0 nmi) 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.[26] There are several islets, such as the disputed Isla Perejil, that are claimed by both Morocco and Spain.[27]

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.[26][28]

Power generation

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.

In the 1920s and 1930s, the Atlantropa project proposed damming the Strait to generate large amounts of electricity and lower the sea level of the Mediterranean by several hundreds of meters to create large new lands for settlement.[29] This proposal would however have devastating effects on the local climate and ecology and would dramatically change the strength of the West African Monsoon.

See also

References
  1. "Strait | Meaning of Strait by Lexico". Lexico Dictionaries | English. Retrieved 2020-04-28.
  2. See Robinson, Allan Richard and Paola Malanotte-Rizzoli, Ocean Processes in Climate Dynamics: Global and Mediterranean Examples. Springer, 1994, p. 307, ISBN 0-7923-2624-5.
  3. Würm glaciation
  4. Cosquer cave
  5. Chisholm, Hugh, ed. (1911). "Gibraltar" . Encyclopædia Britannica. 11 (11th ed.). Cambridge University Press. p. 938.
  6. Google Books Ngram Viewer results "Strait of Gibraltar/Gut of Gibraltar"
  7. See, for instance, Nato Medals: Medal for Active Endeavor, awarded for activity in the international water of the Mediterranean and STROG.
  8. Pamphlet of the Museum of the Castle of Guzman el Bueno, [El Ayuntamiento de Tarifa] accessed 16 November 2016.
  9. Strabo Geographia 3.5.5.
  10. "Migration Information Source – The Merits and Limitations of Spain's High-Tech Border Control". Migrationinformation.org. Retrieved 2011-07-15.
  11. "Limits of Oceans and Seas, 3rd edition" (PDF). International Hydrographic Organization. 1953. Retrieved 7 February 2010.
  12. De Mol, B., et al. 2012. Ch. 45: Cold-Water Coral Distribution in an Erosional Environment: The Strait of Gibraltar Gateway, in: Harris, P.T., Baker, E.K. (Eds.), Seafloor geomorphology as benthic habitat: GEOHAB Atlas of seafloor geomorphic features and benthic habitats. Elsevier, Amsterdam, pp. 636–643
  13. Messinian salinity crisis#Evidence
  14. At the Miocene/Pliocene boundary, c. 5.33 million years before the present
  15. Cloud, P., Oasis in space. Earth history from the beginning, New York: W.W. Norton & Co. Inc., p. 440. ISBN 0-393-01952-7
  16. "BirdLife Data Zone". datazone.birdlife.org. Retrieved 2019-02-23.
  17. editor, Damian Carrington Environment (2018-09-27). "Orca 'apocalypse': half of killer whales doomed to die from pollution". The Guardian. ISSN 0261-3077. Retrieved 2019-02-23.CS1 maint: extra text: authors list (link)
  18. "Last of the Neanderthals". National Geographic. October 2008. Retrieved 2009-12-29.
  19. Antony Beevor (2006) [1982]. The Battle for Spain. Orion. ISBN 978-0-7538-2165-7.
  20. "Europe-Africa rail tunnel agreed". BBC News.
  21. "Tunnel to Connect Morocco with Europe". bluedoorhotel.com. February 17, 2012. Archived from the original on 2012-11-04.
  22. Soto-Navarro, Javier; Criado-Aldeanueva, Francisco; García-Lafuente, Jesús; Sánchez-Román, Antonio (2010-10-12). "Estimation of the Atlantic inflow through the Strait of Gibraltar from climatological and in situ data". Journal of Geophysical Research. 115 (C10): C10023. doi:10.1029/2010JC006302. ISSN 0148-0227.
  23. Paterson, Lawrence. U-Boats in the Mediterranean 1941–1944. Chatham Publishing, 2007, pp. 19 and 182. ISBN 9781861762900
  24. "U-boat war in the Mediterranean". uboat.net. Retrieved 2011-07-15.
  25. Wesson, J. C.; Gregg, M. C. (1994). "Mixing at Camarinal Sill in the Strait of Gibraltar". Journal of Geophysical Research. 99 (C5): 9847–9878. Bibcode:1994JGR....99.9847W. doi:10.1029/94JC00256.
  26. Víctor Luis Gutiérrez Castillo (April 2011). The Delimitation of the Spanish Marine Waters in the Strait of Gibraltar (PDF) (Report). Spanish Institute for Strategic Studies. Retrieved 5 July 2019.
  27. Tremlett, Giles, "Moroccans seize Parsley Island and leave a bitter taste in Spanish mouths", in The Guardian, 13 July 2002.
  28. Donald R Rothwell. "Gibraltar, Strait of". Oxford Public International Law. Oxford University Press. Retrieved 6 July 2019.
  29. "Atlantropa: A plan to dam the Mediterranean Sea". Xefer blog. 16 March 2005. Retrieved on 13 August 2012.
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