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Encyclopedia Britannica - Main :: MEC-MIC |
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MEDITERRANEAN SEA . The Mediterranean is all that remains of a great ocean which at an early geological epoch, before the formation of the Atlantic, encircled half the globe along a line of latitude
elevation
enclosed by the Apennines, the mountains of northern Africa, and of southern and south-eastern Spain (Cordillere betfque). (2) The Adriatic, occupying the space between the Apennines and the Dinaric group (Suess compares the Adriatic to the valley of the Brahmaputra). (3) A part surrounded by the fragments of the Dinaro-Taurus arch, especially by Crete and Cyprus. This includes the Aegean and the Black Sea, and its margin skirts the south coast of Asia Minor. These three parts belong strictly to Eurasia. (4) Part of the coastal region of Indo-Africa, terraced downwards in successive horizontal
Gozo
Western Mediterranean 841,593 sq. kin. Sicilian-Ionian basin 767,658 Greece and Levant basin 769,652 Adriatic Sea 130,656 Total . 2,509,559 ,, A more recent
The physical divisions of the Mediterranean given above hold good in describing the form of the sea-bed. The western Mediterranean is cut off by a bank crossing the narrow strait between Sicily and Cape Bon, usually known as the Adventure Bank, on which the depth is nowhere 200 fathoms. The mean depth of the western basin is estimated at 881 fathoms, and the deepest sounding recorded is 2040 fathoms. In the eastern Mediterranean the mean depth is nearly the same as in the western basin. The Sicilian-Ionian basin has a mean depth of 885 fathoms, and the Levant basin, 793 fathoms. Deep water is found close up to the coast of Sicily, Greece, Crete and the edge of the African plateau. The steepest slope observed occurs off the island of Sapienza, near Navarino, where 1720 fathoms has been obtained only *_o miles from land. In 1897 the ship " Washington " obtained depths of 2220 fathoms in the middle of the eastern Mediterranean; and the Austrian expeditions in the " Pola " discovered in the " Pola Deep " (35 44' N., 21 45' E.), south-west of Cape Matapan, a maximum depth of 2046 fathoms. Between these two deep areas a ridge
ridge
Depth. Area. Volume. Fathoms. Sq. Miles. Cub. Miles. o- 100 201,300 80,950 100- 500 251,65o 220,850 500-1000 81,300 189,200 1000-2000 263,250 217,050 Over 2000 15,500 1,750 813,000 709,800 Volume. Mean Depth. Cub. Km. Fathoms. Western Mediterranean 1,356,512 881 Sicilian-Ionian basin 1,242,549 885 Levant 1,116,599 793 Adriatic Sea 31,844 133 Krummel gives the total volume of the basin as 4,249,020 cubic kilometres or 1,019,400 cubic statute miles, and the mean depth as 782 fathoms. (See OCEAN.) Meteorology.As already stated, the " Mediterranean region " forms a distinct climatic unit, chiefly due to the form and position of the Mediterranean Sea. The prevailing winds in this region, which the sea traverses longitudinally, are westerly, but the sea itself causes the formation of bands of low barometric pressure during the winter season, within which cyclonic disturbances frequently develop, while in summer the region comes under the influence of the polar margin of the tropical high pressure belt. Hence the Mediterranean region is characteristically one of winter rains, the distinctive feature becoming less sharply defined from south to north, and the amount of total annual fall increasing in the same direction. The climate becomes more continental in type from west to east, but there are great local irregularitiesthe elevated plateaus of Algeria and Spain cause a rise of pressure in winter and delay the rainy
spring , especially in the coastal regions, and this is exaggerated in the eastern region by local land winds, which replace the cool sea-breezes of summer: overcoats are ordinarily worn in Spain and Italy till July, and are then put aside till October. Local winds form an important feature in nearly all the coast climates of the Mediterranean, especially in winter, where they are primarily caused by the rapid change of temperature from the sea to the snow-clad hinterlands. Cold dry winds, often of great violence, occur in the Rhone valley (the Mistral), in Istria, and Dalmatia (the Bora), and in the western Caucasus. In summer a north-west " trade " wind, the Maestro, occurs in the Adriatic. The Sirocco is a cyclonic wind characteristic of the winter rainy
Temperature.The mean surface temperature of the waters of the Mediterranean falls from south-east, where it is over 70 F., to north-west, the average at the coast of the Gulf of Lyons being. 6o,. The isothermal of 65 runs from Gibraltar to the north of Sardinia, and thence by the Strait of Messina to the Gulf of Corinth. A similar distribution is found too fathoms from the surface, temperature falling from 6o in the Levant to 55 east of Gibraltar. At 200 fathoms temperature falls in the same way from 58 to S5, but below 250 fathoms temperatures are practically uniform to the bottom, 55.5 in the western basin and 56.5 in the eastern. The bottom temperature observed in the Pola Deep was 56.3. Salinity.In the extreme west the salinity of the. surface water is about 36.3 per mille, and it increases eastwards to 37.6 east of Sardinia and 39.0 and upwards in the Levant. Observations of salinity in the depths of the western Mediterranean are very deficient, but the average is probably between 38o and 38.5. In the eastern basin the " Pola " expedition observed salinities of 38.7 to 39o to the east of a line joining Cape Matapan with Alexandria, and 38.2 to 38.7 to the west of it. The salter waters apparently tend to make their way westwards close to the African coast, and at the bottom the highest salinities have been observed south of Crete. Evnitzki states that the saltest water of the whole basin occurs in the Aegean Sea. Circulation.There is little definite circulation of water with1fi the Mediterranean itself. In the straits joining it with the Atlanti e and the Black Sea the fresher surface waters of these seas flow inwards to assist in making good the loss by evaporation at the surface of the Mediterranean, and in both cases dense water makes its way outwards along the bottom of the channels, the outflowing currents being less in volume and delivery than the inflowing. Elsewhere local surface currents are developed, either drifts due to the direct action of the winds, or streams produced by wind action heaping water up against the land; but these nowhere rise to the dignity of a distinct current system, although they are often sufficient to obliterate the feeble tidal action characteristic of the Mediterranean. Dr Natterer, the chemist of the " Pola expeditions, has expressed the opinion that the poverty of the pelagic fauna is solely due to the want of circulation in the depths.Deposits.A great part of the bottom of the Mediterranean is covered with blue muds, frequently with a yellow upper layer containing a considerable proportion of carbonate of lime, chiefly shells of pelagic Foraminifera. In many parts, particularly in the eastern basin, a calcareous or siliceous crust, from half an inch to three inches in thickness, is met with ; and Natterer suggested that the formation of this crust may be due to the production of carbonate of ammonium where deposits containing organic matter are undergoing oxidation, and the consequent precipitation of carbonate of lime and other substances from the waters nearer the surface. This view, however, has not met with general acceptance. (H. N. D.) End of Article: MEDITERRANEAN SEA If you wish, you can link directly to this article.
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