Mediterranean Sea

an almost landlocked, intercontinental sea of the Atlantic Ocean, with which it is connected in the west by the Strait of Gibraltar. Parts of the Mediterranean are known by other names, such as the Ligurian, Tyrrhenian, Adriatic, Ionian, and Aegean seas. The Mediterranean basin also includes the Sea of Marmara, the Black Sea, and the Sea of Azov.

Physical geography, GENERAL CHARACTERISTICS. The Mediterranean Sea lies between Europe, Africa, and Asia. The seas of the Mediterranean basin wash the coasts of Spain, France, Italy, Malta, Yugoslavia, Albania, Greece, Bulgaria, Rumania, the USSR, Turkey, Cyprus, Syria, Lebanon, Israel, Egypt, Libya, Tunisia, Algeria, and Morocco. In the northeast the Mediterranean is linked with the Sea of Marmara through the Dardanelles and further on with the Black Sea through the Bosporus. In the southeast the Suez Canal links it with the Red Sea.

Extending over an area of 2.5 million sq km, the Mediterranean has a water volume of 3,839,000 cu km, an average depth of 1,541 m, and a maximum depth of 5,121 m. Its mountainous shores generally have straight abrasion coastlines. In low-lying areas, lagoon-estuary or delta shorelines predominate. A Dalmatian coastline is typical of the eastern Adriatic. The largest gulfs are those of Valencia, Lions, Genoa, Taranto, Sidra, and Gabès. The major islands are the Balearic Islands, Corsica, Sardinia, Sicily, Crete, and Cyprus. The principal rivers flowing into the Mediterranean are the Ebro, Rhône, Tiber, Po, and Nile; they have a combined annual runoff of about 430 cu km.

GEOLOGICAL STRUCTURE AND TOPOGRAPHY OF THE BOTTOM. The floor of the Mediterranean Sea is divided into several basins with relatively steep continental slopes descending to depths of 2,000–4,000 m. Along the coasts the basins are fringed by a narrow strip of shelf that widens only between the coasts of Tunisia and Sicily and within the Adriatic.

In terms of geomorphology, the Mediterranean may be divided into Western, Central, and Eastern basins. The Western, or Algerian-Provencal, Basin has a maximum depth of more than 2,800 m and includes the basins of the Alboran, Balearic, and Li-gurian seas, as well as the Tyrrhenian Basin, whose depth exceeds 3,600 m. The Central Basin, which descends to more than 5,100 m, comprises the Central Depression and the basins of the Adriatic and Ionian seas. The Eastern, or Levantine, Basin, with a maximum depth of about 4,380 m, includes the basins of the Levantine, Aegean, and Marmara seas. The floor of some of the depressions is covered by Neogene-Anthropogene sedimentary and volcanic strata. In the Balearic and Ligurian seas the strata are up to 5–7 km thick. Among the Messinian (Upper Miocene) deposits of the Western Basin there is a salt-bearing evaporite stratum more than 1.5–2 km thick that forms typical salt tectonic structures. Several large faults with extinct and active volcanoes extend along the edges and down the center of the Tyrrhenian Basin. Some of the volcanoes form large underwater mountains, such as the Lipari Islands and the Vavilov Volcano. Whereas the volcanoes along the margins of the Tyrrhenian Basin (those in the Tuscan Archipelago and Pontine Islands, Vesuvius, and the Lipari Islands) extrude acidic and alkaline lavas, the volcanoes in the central part of the Mediterranean Sea emit more abyssal, basic lavas (basalts).

Parts of the Central and Eastern (Levantine) basins are filled with sedimentary strata, including thick deposits of river sediment, especially from the Nile. Geophysical studies of these basins have led to the discovery of the Hellenic Trough and the Central Mediterranean Ridge, a large arch rising to 500–800 m. Off the coast of Cyrenaica, at the foot of the continental slope, lies the Libyan Trough, only slightly filled with sediment.

The basins of the Mediterranean Sea differ greatly as to the time of their formation. Although a significant part of the Eastern (Levantine) Basin originated in the Mesozoic, the Algerian-Provencal Basin was formed in the late Oligocène and early Miocene, and some Mediterranean basins developed in the early and middle Miocene or in the Pliocene. Shallow basins existed over much of the Mediterranean Sea by the late Miocene (Messinian Age). During the deposition of salts in the Messinian Age, the Algerian-Provencal Basin was roughly 1–1.5 km deep. The salts accumulated because of strong evaporation and the concentration of brine as sea water flowed into the enclosed body through a strait that existed south of Gibraltar.

The present depth of the Tyrrhenian Basin is the result of a subsidence of the floor during the Pliocene and Anthropogene (in the last 5 million years). Several other basins were also formed by the same relatively rapid subsidence. The formation of the Mediterranean basins is believed to be related either to the moving apart of the continental crust or to the compression of the crust and its subsidence. Some scientists think that the basins are relict basins, the remains of the ancient Tethys Sea. Geosynclinal development continues in certain parts of the basins.

Many parts of the sea floor are promising for petroleum and gas exploration, especially the area of the salt domes. On the shelf, petroleum and gas deposits are associated with Mesozoic and Paleogene beds.

CLIMATE. The climate of the Mediterranean Sea is determined by its position in the subtropical belt. Its specific characteristics make it a distinct type of climate, which has come to be called Mediterranean climate, marked by mild wet winters and hot dry summers. The trough of low atmospheric pressure that forms over the sea during the winter brings changeable weather with frequent storms and abundant precipitation. Cold north winds lower the air temperature. Local winds develop, such as the mistral over the Gulf of Lions and the bora in the eastern Adriatic. In summer the crest of the Azores anticyclone affects most of the Mediterranean Sea, causing predominantly clear weather with few cloudy days and little precipitation. Dry fogs and a dusty haze carried from Africa by the southerly sirocco wind are observed during the spring and summer months. Stable northerly winds, called etesian winds, develop in the eastern part of the sea.

The mean air temperature in January varies from 14°–16°C along the southern coast to 7°–10°C in the north; in August the mean temperature ranges from 22°–24°C in the north to 25°–30°C in the south. Evaporation from the surface of the sea reaches 1,250 mm a year (3,130 cu km). The relative humidity, 50–65 percent in summer, rises to 65–80 percent in winter. The incidence of cloudy days varies from 0–30 percent in summer to 60 percent in winter. The annual precipitation averages 400 mm (approximately 1,000 cu km). It ranges from 1,100–1,300 mm in the northwest to 50–100 mm in the southeast, with the minimum occurring in July and August and the maximum in December.

HYDROLOGICAL REGIME. The sea’s hydrological regime is governed by the rapid rate of evaporation and general climatic conditions. Because the expenditure of fresh water exceeds intake, the sea level is lowered, causing a steady influx of less saline surface waters from the Atlantic Ocean and Black Sea. Highly saline waters flow out along the bottom of the straits because of the differences in water density. Most of the water exchange occurs through the Strait of Gibraltar, where the surface current brings in 42,320 cu km of Atlantic water annually, and the lower current carries out 40,800 cu km of Mediterranean water. The corresponding annual inflow and outflow through the Dardanelles are 350 cu km and 180 cu km.

The circulation of water in the Mediterranean Sea is chiefly the result of wind action. The main, almost zonal, Canaries Current carries waters largely of Atlantic origin along Africa from the Strait of Gibraltar to the shores of Lebanon. In addition, there are cyclonic gyres in certain seas and basins to the left of the current. To depths of 750–1,000 m the water transfer is in one direction, with the exception of the Levantine intermediate counter-current, which carries Levantine waters from Malta westward to the Strait of Gibraltar along the coast of Africa. The velocities of the established currents are 0.5–1.0 km/hr on the open sea and up to 2–4 km/hr in some straits. In February the water temperature at the surface increases from north to south—from 8°–12° to 17°C in the eastern and central Mediterranean and from 11° to 15°C in the west. In August the average water temperature ranges from 19°C in the north to 25°C in the south; in the extreme east it rises to 27°–30°C.

Rapid evaporation is responsible for the sea’s high salinity. From west to east the salinity increases from 36%c to 39.5%c. The water density at the surface varies from 1,023–1,027 g per cu cm in summer to 1,027–1,029 g per cu cm in winter. During winter cooling, intensive convective mixing develops in areas of greater density, leading to the formation of highly saline and warm intermediate layers in the Eastern Mediterranean and highly saline and warm deep layers in the northern part of the Western Mediterranean, and in the Adriatic and Aegean seas. With respect to bottom temperature and salinity, the Mediterranean Sea is one of the warmest and most saline seas of the world ocean (12.6–13.4°C and 38.4–38.7).

To a depth of 50–60 m the water is relatively transparent and has a deep blue color. Tides are mainly semidiurnal and less than 1 m high, but fluctuations in sea level may reach 4 m when tides are combined with wind surges, as in the Gulf of Genoa and on the northern coast of Corsica. Strong tidal currents are observed in narrow straits, such as the Strait of Messina. The maximum turbulence occurs in winter, when the wave height reaches 6–8 m.

PLANT AND ANIMAL LIFE. The Mediterranean Sea is deficient in phytoplankton and zooplankton, which has resulted in relatively low populations of the larger animals, including fish, that feed on them. The quantity of phytoplankton in the surface layers is only 8–10 mg per cu m, and at depths of 1,000–2,000 m it is 10 or 20 times less. The algae are chiefly peridinians and diatoms.

The fauna of the Mediterranean Sea consists of a great variety of species, none of which occur in great numbers. The sea is inhabited by dolphins, one species of seal (monk seal), sea turtles, and 550 fish species, including sharks, mackerel, herring, anchovies, mullet, dolphinfish, tuna, bonitos, and horse mackerel. About 70 fish species are endemic, among them skates, several species of anchovy, bullheads, blennies, wrasses, and needlefish. The most valuable edible mollusks are the oyster, Mediterranean-Black Sea mussel, and date mussel. Among common invertebrates are octopuses, squids, Sepia, crabs, and spiny lobsters. The sea contains numerous species of jellyfish, siphonophores, salpas, and pyrosomes. In some areas, particularly in the Aegean Sea, there are sponges and red corals.

ECONOMIC GEOGRAPHY. Densely populated since ancient times, the countries along the Mediterranean, especially those along its northern shores, are highly developed economically. The Mediterranean basin accounts for more than one-tenth of the population of the capitalist world and produces approximately one-tenth of its industrial output. The Mediterranean countries produce a smaller proportion of the world agricultural output (about 1/15), although they are among the leading producers of citrus fruit (one-third of the world harvest), cotton, and oil crops. The Mediterranean Sea holds a special place in international trade and economic relations. Located at the junction of Europe, Asia, and Africa, the Mediterranean Sea is an important transportation route for Europe’s maritime trade with Asia, North Africa, Australia, and Oceania. Important trade routes connecting the Soviet Union with foreign countries cross the Mediterranean Sea. Moreover, major coastal shipping lines link the Black Sea ports with other Soviet ports.

The importance of Mediterranean shipping for the Western European countries is steadily growing as these countries become increasingly dependent on imported raw materials. The total cargo handled by Mediterranean seaports almost doubled in ten years, reaching 780 million tons in 1971, of which European Mediterranean ports handled 480 million tons, or about one-third of the total cargo turnover of Western European ports. The Mediterranean Sea is especially important for petroleum shipping. It is the key “petroleum” route between Western Europe and the Middle East. The role of such southern ports as Marseille, Trieste, and Genoa in supplying Western Europe with petroleum is steadily growing; the southern ports handled 40 percent of Western Europe’s petroleum imports in 1972. Of the 250 million tons of petroleum imported through the Mediterranean ports, about half comes from North Africa and the remainder from the Middle East. Pipelines connect the Mediterranean ports with the Western European countries, including Austria, West Germany, France, and Switzerland, and with the petroleum deposits of the Middle East and North Africa. Large shipments of various types of raw materials, metal ores, bauxite, and agricultural products pass through the Suez Canal, by which Western Europe maintains its ties with Asia and Australia.

The largest ports are Marseille and its outer harbors (cargo turnover of 109 million tons) in France; Genoa (62 million tons), Augusta (42 million tons), and Trieste (38 million tons) in Italy; and al-Sidr (46 million tons) and Marsa al-Burayqah (33 million tons) in Libya.

Many industrial enterprises have been built on the shores of the Mediterranean and on its islands. Chemical and metallurgical industries have developed using raw materials delivered by sea. Between 1960 and 1975 the Italian islands of Sardinia and Sicily, the mouth of the Rhone in France, and other regions became major centers of the chemical industry. Petroleum and gas extraction has begun on the Mediterranean shelf in the northern Adriatic, along the coast of Greece, and elsewhere. Compared to other basins of the Atlantic Ocean, fishing is of secondary importance in the Mediterranean. The industrialization of the coastline, the growth of cities, and the development of recreation areas are polluting the coastal waters. The most famous resorts are on the Riviera in France and Italy, on the Levantine coast, and on Spain’s Balearic Islands.