the southern portions of the Pacific, Atlantic, and Indian oceans and their tributary seas surrounding Antarctica. Unbroken by any other continental landmass, the Southern Ocean's narrowest constriction is the Drake Passage, 600 miles (about 1,000 km) wide, between South America and the tip of the Antarctic Peninsula. The structure of the ocean floor includes a continental shelf usually less than 160 miles (about 260 km) wide that attains its maximum width of more than 1,600 miles (2,600 km) in the vicinity of the Weddell and Ross seas. There are oceanic basins farther north that are as much as 14,800 feet (4,500 metres) deep, defined by oceanic rises and often marked by ranges of abyssal hills. There are also narrow oceanic trenches with high relief, such as the South Sandwich Trench on the eastern side of the South Sandwich Islands. Other relief features include oceanic plateaus that rise from the oceanic basins to depths of less than 6,650 feet (2,000 metres) below sea level and form rather flat regions, which are often covered by relatively thick sedimentary deposits. The most extensive such plateau is the Campbell, or New Zealand, Plateau, which rises southeast of New Zealand and extends southward beyond the Campbell Islands
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