28 



The topography of the Atlantic Ocean bottom is characterized by its division into 

 East and West Atlantic Troughs by the Atlantic Ridge. This ridge begins at Iceland ; 

 from the Iceland shelf it runs south-westward as the narrow Reykjanaes Ridge whose 

 bottom form was fixed by the soundings of the "Meteor" (Bathymetric chart by 

 Defant, 1930, 1931, 1936). At 5 1 ° N. the ridge broadens out somewhat towards the 

 west (Telegraph Plateau), and then runs into the Azores Plateau which can be regarded 

 as a great extension of the central ridge to the east and south-east. The ridge then 

 narrows and remains at a depth of 2500-3500 m and apart from St Paul Island 

 supports no islands as far as the equator. At 7-8° N. 36° W. there is a gap which 

 reaches to a depth of 4400 m. The greatest gap is, however, on the equator near 

 the Romanche Trench (see p. 27). South of this the ridge is broad and rounded and 

 carries the islands Ascension (height 860 m), Tristan da Cunha (2329 m), Gough 

 (1335 m) and Bouvet (935 m). St. Helena belongs to a minor ridge farther to the east. 

 These extended minor ridges are peculiar to the section of the ridge between 0° and 

 20 "" S. The South Atlantic Ridge is connected west of Bouvet Island by the Atlantic 

 Indian Ridge to the Crozet and Kerguelen Ridges of the Indian Ocean. The Atlantic 

 Ridge extends over 20,300 km and is by far the longest underwater mountain system 

 on the Earth. 



The Eastern and the Western Atlantic Basins are further divided by transverse 

 ridges. An outline of the main division is shown in Plate 2 where the geographical 

 arrangement of the basins is particularly clearly shown. A special characteristic of the 

 Atlantic Ocean is that it is completely closed in the north towards the Arctic Sea and 

 the Norwegian Sea below a depth of about 500 m. This has far-reaching oceano- 

 graphic consequences. In contrast to this nearly complete blocking of the deeper layers 

 to the north, the Atlantic in the south is completely open down to great depths to the 

 Atlantic-Antarctic Basin. 



There are topographical differences between the eastern and the western troughs 

 that have a considerable effect on the oceanographic structure. The transverse ridges 

 are not as well developed in the western trough as in the eastern, and particularly the 

 Rio Grande Ridge, which is somewhat better developed, has deep openings that per- 

 mit continuous communication from the Atlantic-Antarctic Basin through the Ar- 

 gentina Basin, the Brazil Basin and the Guiana Basin to the North America Basin 

 below 4000 m. In the eastern trough, on the other hand, the Whalefish Ridge, which 



