RED SEA, a narrow strip of water extending S.S.E. from Suez to the Strait of Bab el-Mandeb in a nearly straight line, and separating the coasts of Arabia from those of Egypt, Nubia and Abyssinia. Its total length is about 1200 m., and its breadth varies from about 250 m. in the southern half to 130 m. in 27° 45' N., where it divides into two parts, the Gulf of Suez and the Gulf of Akaba, separated from each other by the peninsula of Sinai.
The Gulf of Suez is shallow, and slopes regularly down to the northern extremity of the Red Sea basin, which has a. maximum depth of 640 fathoms, and then over a shoal of 60 fathoms goes down to 1200 fathoms in 22° 7' N. The Gulf of Akaba is separated from the Red Sea by a submarine bank only 70 fathoms from the surface, and in 28° 39' N. and 34° 43' E. it attains the depth of 700 fathoms. South of the 1200fathom depression a ridge rises to 500 fathoms in the latitude of Jidda, and south of this again a similar depression goes down to 1190 fathoms. Throughout this northern part, i.e. to the banks of Suakin and Farsan in 20° N., the loo-fathom line keeps to a belt of coral reef close inshore, but in lower latitudes the shallow coral region, 300 m. long and 70 to 80 m. across, extends farther and farther seaward, until in the latitude of Hodeda the deep channel (marked by the loo-fathom line) is only 20 m. broad, all the rest of the area being dangerous to navigation, even for small vessels. In the middle of the gradually narrowing channel three depressions are known to exist; soundings in two of these are: i 1 io fathoms in zo° N. and 890 fathoms in 16° N., a little to the north of Massawa. To the north-west of the volcanic island of Zebayir the depth is less than 500 fathoms; the bottom of the channel rises to the ioofathom line at Hanish Island (also volcanic), then shoals to 45 fathoms, and sinks again in about the latitude of Mokha in a narrow channel which curves westward round the island of Perim (depth 170 fathoms), to lose itself in the Indian Ocean. This western channel is 16 m. wide in the Strait of Bab el-Mandeb; the eastern channel of the strait is 2 m. broad and 16 fathoms deep.
Murray estimates the total area at 158,750 sq. m., and its volume at 67,700 cub. m., giving a mean depth of 375 fathoms. Karstens gives the area at 448,810 sq. kilometres (130,424 sq. geographical m.) and the volume at 206,901 cub. kilometres (32,413 cub. geographical which gives a mean depth of 252 fathoms. depth. Both these computations, however, were made before the date of the Austrian exploring expeditions (1896-98). Bludau's measurements give the total area draining to the Red Sea at about 255,000 sq. geographical m. Kriimmel's more recent calculations (see Ocean) give values somewhat higher than those of Karstens.
The Red Sea is formed by a line of fracture, probably dating from Pliocene times, crossing the centre of a dome of Archean rocks, on both flanks of which, in Egypt and Arabia., rest Secondary and Tertiary deposits. The granite rocks forming the core of the dome appear at the surface on the Red Sea coast, at the western end of the transverse line of heights crossing Nejd. Along the line of fracture traces of volcanic activity are frequent; a group of volcanic islands occurs in 14° N., and on Jebel Teir, farther north, a volcano has only recently become extinct. The margin of the Red Sea itself consists, on the Arabian side, of a strip of low plain backed by ranges of barren hills of coral and sand formation, and here and there by mountains of considerable height. The greater elevations are for the most part formed of limestones, except in the south, where they are largely volcanic. The coasts of the Gulf of Akaba are steep, with numerous coral reefs on both sides. On the African side there are in the north wide stretches of desert plain, which towards the south rise to elevated tablelands, and ultimately to the mountains of Abyssinia. The shores of the Red Sea are little indented; good harbours are almost wanting in the desert regions of the north, while in the south the chief inlets are at Massawa, and at Kamaran, almost directly opposite. Coral formations are abundant; immense reefs, both barrier and fringing, skirt both coasts, often enclosing wide channels between the reef and the land. The reefs on the eastern side are the more extensive; they occur in places as much as 25 m. from the land. It has long been known that the whole Red Sea area is undergoing gradual elevation, and much has been done in recent years in investigating the levels of raised beaches found in different localities., In the northern part, down to almost 19° N., the prevailing winds are north and north-west. The middle region, to 14°-16° N., has variable winds in an area of low barometric pres- sure, while in the southern Red Sea south-east and east winds prevail. From June to August the north west wind blows over the entire area; in September it retreats again as far as 16° N., south of which the winds are for a time variable. In the Gulf of Suez the westerly, or "Egyptian," wind occurs frequently during winter, sometimes blowing with violence, and generally accompanied by fog and clouds of dust. Strong north-north-east winds prevail in the Gulf of Akaba during the greater part of the year; they are weakest in April and May, sometimes giving place at that season to southerly breezes. The high temperature and great relative humidity make the summer climate of the Red Sea one of the most disagreeable in the world.
The mean annual temperature of the surface waters near the head is 77° F.; it rises to 80° in about 22° N., to 84° in 16° N., and drops again to 82° at the Strait of Bab el-Mandeb. T Daily variations of temperature are observable to a depth of over 50 fathoms. Temperature is, on the whole, higher near the Arabian than the Egyptian side, but it everywhere diminishes with increase of depth and latitude, down to 380 fathoms from the surface; below this depth a uniform constant temperature of 70.7° F. is observed throughout. In the Gulf of Suez temperature is relatively low, falling rapidly from south to north. The waters of the Gulf of Akaba are warmer towards the Arabian than the Sinai coasts; a uniform temperature of 70.2° is observed at all depths below 270 fathoms.
The salinity of the waters is relatively great, the highest recorded being 42.7 per mille (Gulf of Suez), and the lowest 36.2 (Perim harbour). The distribution is, speaking Salinity. generally, the opposite to that of temperature; salinity increases from the surface downwards, and from the south northwards, and it is greater towards the western than the eastern side. This statement holds good for the Gulf of Suez, in which the water is much salter than in the open;sea; but in the Gulf of Akaba the distribution is exceedingly uniform, nowhere differing much from an average of 40.6 per mille.
The movements of the waters are of great irregularity and complexity, rendering navigation difficult and dangerous. Two features stand out with special distinctness: the ex- Circula- change of water between the Red Sea and the Indian tion. Ocean, and the tidal streams of the Gulf of Suez. From the observations of salinity it is inferred that a surface current flows inwards to the Red Sea in the eastern channel of the Strait of Bab el-Mandeb, while a current of very salt water flows outward to the Indian Ocean, through the western channel, at a depth of 50 to loo fathoms from the surface. In the Gulfs of Suez and Akaba, almost the only part of the Red Sea in which tidal phenomena are well developed, a sharply defined tidal circulation is found. Elsewhere the surface movements at least are controlled by the prevailing winds, which give rise in places to complex "transverse" currents, and near the coast are modified by the channels enclosed by the coral reefs. During the prevalence of the north and north-west winds the surface level of the northern part of the Red Sea is depressed by as much as 2 ft. The great evaporation going on from the surface probably causes a slow vertical circulation in the depth, the salter colder waters sinking, and ultimately escaping to the Indian Ocean. Extensive collections of the deposits forming the bed were made by the expeditions of the Austrian ship "Pola" (1896 and 1898). These were analysed by Dr K. Natterer, whose conclusions, however, have been disputed by a number of other investigators. The zoological collections of the "Pola" expeditions show that certain well-defined districts are extremely rich in plankton, while others are correspondingly poor; and it appears that the latter occur in districts surrounded by currents of relatively low temperature, while the richer parts are where the movements of water are blocked by irregularities in the coast-line.
Authorities. - A. Issel, Morfologia e genesi del Mar Rosso. Saggio di Paleogeografia, Congresso Geogr. Ital. (Florence, 18 99); "Die Korallenriffe der Sinai-Halbinsel," Abhandl. Math.-phys. Gesell. Wiss., vol. xiv. (Leipzig, 1888); Meteorological Charts of the Red Sea (Meteorological Office, 1895); Report of the Voyage of the Russian Corvette "Vitiaz" (1889); "Berichte der Commission fiir oceanographische Forschungen," 6th series, 1898 in vol. LXV. of the Denkschriften der K.K. Akademie der Wissenschaften (Vienna); also various notes and preliminary reports in the Sitzungsberichte of the Vienna Academy of Sciences; Report of the Voyage of H.M.S. "Challenger," " Oceanic Circulation," p. 30; J. Hann, Klimatologie (1897), vol. iii. p. 76. (H. N. D:)
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