The dominant relief feature of the gulf's terrain is the Sheba Ridge, an extension of the Indian Ocea....

The dominant relief feature of the gulf's terrain is the Sheba Ridge, an extension of the Indian Ocean ridge system, which extends along the middle of the gulf. The rough topography of the ridge includes a well-defined median valley that is continually offset by faults running approximately northeast to southwest. The largest of these faults forms the Alula-Fartak Trench, in which is found the gulf's maximum recorded depth of 17,586 feet (5,360 m). The Sheba Ridge is flanked on both sides by sediment-filled basins that reach depths of 13,000 feet (3,900 m) at the mouth of the gulf. To the west, the ridge gives way to a relatively shallow east�west-trending valley known as the Tadjoura Trench. The main factor in the gulf's geologic formation is the spreading of the seafloor away from the Sheba Ridge axis. The African continent and the Arabian Peninsula split initially along their present margins either in the late Eocene epoch (57.8 to 36.6 million years ago) or else in the ! Oligocene epoch (36.6 to 23.7 million years ago). They have since drifted apart in a direction parallel to the gulf's faults. The intensive exchange of water between the Red Sea, the Gulf of Aden, and the Arabian Sea, as well as strong evaporation and monsoon (rain-bearing) winds that constitute part of the airflow, all assist in the formation of a complex water structure. The surface layer is highly saline, and eddies complicate its flow pattern. Surface temperatures of the gulf's waters are generally between 77� and 88� F (25� and 31� C). The gulf's marine life is rich in both the quantity and the variety of its species. Seasonally variable upwelling of waters in the coastal zone provides the surface layer with a considerable supply of nutrient elements, which produce an abundant growth of plankton. Sardines and mackerel abound in these areas of upwelling. The main open-sea fish are dolphin, tuna, billfish, and sharks. Whales are frequently sighted. The gulf provides a br! eeding ground for sea turtles, and rock lobster are abundant. ! Despite a lack of large-scale commercial fishing facilities, the coastline supports many isolated fishing towns and villages. Local fishing takes place close to the shore; sardines, tuna, kingfish, and mackerel make up the bulk of the annual catches. Crayfish and sharks are also fished locally, while survey ships have occasionally pulled in exceptional catches of fish. The morphology, geology, and geophysics of the gulf and its surrounding areas are comprehensively covered in various articles in the Philosophical Transactions of the Royal Society, Series A, vol. 267, no. 1181 (1970), which also contains up-to-date bathymetric and magnetic charts of the gulf itself. The hydrological structure of the water mass is described by V.A. Khimitsa in Oceanology, 8:318�322 (1968); while its physical and chemical properties are covered in the University of California, Institute of Marine Resources Report, I.M.R. Reference 67�12 (1967).