Straits of Gibraltar

The Straits of Gibraltar from space

The Straits of Gibraltar are a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates the Iberian peninsula of Europe from Morocco in Africa.

The Straits are the only natural passage for any ship to leave the Mediterranean Sea: the only other outlet at all is the Suez Canal far to the east. Thus the Straits of Gibraltar are a vital passage for world trade and a strategic point of supreme importance. It is for this reason that Gibraltar has assumed such an important role in history; its guns commanding the vital strait.

Europe and Africa are separated by 7.7 nautical miles of sea at the straits' narrowest point. Ferries cross between the two continents every day in as little as 35 minutes.

The depth of the Straits ranges between 150 and 500 fathoms. The shallowest part is known as the Camarinal Sill at the far western end of the strait, which geologists believe once formed a natural dam holding the Atlantic waters back from entering the Mediterranean basin. When breached, the dam would have seen the creation of a massive waterfall lasting for centuries until the basin filled and the Mediterranean Sea formed.

The Mediterranean, bottled up behind the Straits, has barely any tide, though the great Atlantic tides, trying to force the narrow passage twice a day, produce dramatic currents in the Strait itself.

Name of the Straits

The name of the Straits comes from Gibraltar, Britain's fortress peninsula which is the most prominent seamark overlooking the waters. Although the name of Gibraltar, from which the straits are named in English (and Spanish) is itself from the Arabic Jebel Tariq ("Tariq's mountain"), the Arabic name for the Strait is Bab el-Zakat, meaning "Gate of Charity.".

These waters were known to the ancients as the "Pillars of Hercules" (Greek Ηράκλειες Στήλες); One pillar was Gibraltar and the other Mount Abyla (Gebel Musa, on the Moroccan shore).

Location and extent

Africa and Europe, from Gibraltar

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

The International Hydrographic Organization, in its endeavours to define the indefinable, set the limits of the Straits of Gibraltar as follows:

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

Due to its location, the Straits are commonly used for illegal immigration from Africa to Europe.

Bird migrations

The Strait has been identified as an "Important Bird Area" by BirdLife International because hundreds of thousands of seabirds use it every year to pass between the Mediterranean and the Atlantic, including large numbers of Cory's and Balearic Shearwaters, Audouin's, Yellow-legged and Lesser Black-backed Gulls, Razorbills and Atlantic Puffins.

History

Historic map of the Straits of Gibraltar by Piri Reis

Evidence of the first human habitation of the area by Neanderthals dates back to 125,000 years ago and it is believed that the Rock of Gibraltar may have been one of the last outposts of Neanderthal habitation in the world; evidence of their presence there dates to as recently as only 24,000 years ago. Archaeological evidence of Homo sapiens habitation of the area dates back to around 40,000 years ago.

To the Greeks and the Romans the Straits of Gibraltar marked the end of the world: the untamed ocean lay beyond. This concept was more symbolic than real however, ships did sail past the Pillars of Hercules to Lusitania and the African coast and indeed the Romans were familiar with the Fortunatae insulae, the Canary Islands, which lie beyond. To the Vikings, Norvasund was the gateway to the riches of the Mediterranean lands.

The Straits are both a barrier and a link between Europe and Africa. In Roman days the two sides were distinct under one empire, though in time Christian Roman culture prevailed on both sides of the sea, even after both shores were held by Germanic successor kingdoms after the fall of the Empire. The Arab invasions of North Africa split the two again. At the Straits though the Moors crossed to subdue Spain to found a kingdom on both sides of the sea: only in 1492 did the Spaniards wholly drive them out, and the Straits became the frontier between two very different cultures.

In 1704, an English-Dutch force captured Gibraltar from Spain, a rock fortress which the Spaniards had held for a little over 200 years. It has been held by Britain ever since. Gibraltar has enabled Britain in less settled times to control the narrow waters and therefore the sea lanes into and out of the Mediterranean.

During the Second World War, German U-boats repeatedly tried to pass the Straits under the noses of British guns; they used the strong currents drift into the Mediterranean Sea silently. From September 1941 to May 1944, 62 German U-boats managed to get into the Mediterranean, notwithstanding British command of the Straits; 9 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.

Communications

American Shipping off the Rock of Gibraltar (Ivan Aivazovsky)

The Strait is an important shipping route from the Mediterranean to the Atlantic Ocean. Ferries operate between Gibraltar and Tangier, and also between Spain and Africa.

Special flow and wave patterns

The Strait of Gibraltar serves to directly link the Atlantic Ocean to the Mediterranean Sea. As such, 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

Internal waves caused by the Straits

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 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 strait.

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 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 500 fathoms. The Mediterranean outflow water layer can be traced for thousands of miles west of the strait, before completely losing its identity.

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. 62 U-boats made it into the Mediterranean although the Straits were under British control; 9 U-boats were sunk while attempting passage and 10 more had to break off their run due to damage. Of the U-boats attempting to cross back into the Atlantic, every one was sunk.

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 60 miles, and sometimes create interference patterns with refracted waves.

Outside links

("Wikimedia Commons" has material about Straits of Gibraltar)

• Project for a Europe-Africa permanent link through the Strait of Gibraltar — United Nations Economic and Social Council, 2001. Accessed 26 February 2006.
• "Solitons, Strait of Gibraltar". NASA Earth Observatory. http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16581. Retrieved 2006-05-24. 
• "Internal Waves, Strait of Gibraltar". NASA Earth Observatory. http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=4707. Retrieved 2006-05-24. 
• Climate Control Requires a Dam at the Strait of Gibraltar - American Geophysical Union, 1997