SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 265 



separated by a central asymmetrical depression, and a smaller frontal scarp is 

 found 50 miles southeast of the main Walvis Ridge Scarp. Walvis Ridge can be 

 traced soutliwestward as a large feature over 1000 fm high for 1500 miles and 

 its tectonic trend continues for 2000 miles further in a line of similar but lower 

 asymmetrical scarps and ridges. It is in many ways similar to the Mendocino 

 Escarpment of the North Pacific. Other such ridges are now known in the 

 Indian Ocean. There are several similar ridges in the vicinity of Madagascar 

 and Madagascar itself may be a feature very similar to the Walvis Ridge, 

 with its characteristic asymmetrical shape. 



In the Arctic Ocean the Lomonosov Ridge (Fig. 27) is of similar form and 

 in common with the other ridges of tliis group is devoid of seismic activity 

 (Heezen and Ewing, 1960; Dietz and Shumway, 1960). Although both the 

 Lomonosov and Walvis Ridges are flanked by huge escarpments over 2000 fm 

 high, the surface of each ridge is relatively smooth and stands in strong contrast 

 to the intensely disrupted relief of the mid-oceanic ridges or of the abyssal hills. 

 This smoothness of the relief and the lack of earthquakes indicates that these 

 aseismic ridges, now tectonically stable, have probably been stable for some 

 time. A core taken from the southern scarp of the Walvis Ridge penetrated a 

 condensed section of Tertiary oozes and ended in Upper-Cretaceous Globigerina 

 ooze (D. B. Ericson, i7i litt.). This would seem to suggest that the Walvis Ridge 

 has existed in essentially the same form since the Cretaceous and that the 

 thinness of the condensed section resulted in slumping of material from the 

 southern side of the Walvis Ridge ; this slumping has continued since late 

 Cretaceous and hence it suggests that the scarp has existed since late Cretaceous 

 time. 



The Muscarene Ridge in the Indian Ocean lies parallel to the mid-oceanic ridge 

 mid-way between Madagascar and the mid-oceanic ridge. Granite rocks out- 

 crop in the Seychelles Islands. This bank, flat-top j)ed at about 100 fm and over 

 100 miles wide, droj)s abruptly to abyssal depths on either side. The topography 

 of the Muscarene Ridge is massive and blocky, and it, too, is devoid of the in- 

 tensely disrupted small-scale relief typical of the mid-oceanic ridges. However, 

 dredging from the crest of the Mendocino Ridge and a core taken from the 

 eastern Bermuda Rise, coupled with the results of seismic-refraction studies, 

 indicate that these features are composed of basic rocks. Thus, the Muscarene 

 Ridge may, with Madagascar, form an independent class of aseismic ridges. 

 They may, in fact, be fragments of continents. 



j. Seamounts, seamount groups and linear archipelagos (see Chapter 15) 



A seamount is by definition any submerged peak over 500 fm high. Seamounts 

 are distributed through all the physiographic provinces of the oceans. Sea- 

 mounts sometimes occur randomly scattered, but more often lie in linear rows 

 (Fig. 28). It seems safe to conclude that virtually aU conical seamounts are 

 extinct or active volcanoes. The Kelvin Seamount Group, an 800-mile-long line 

 of seamounts, stretches out from the vicinity of the Gulf of Maine toward the 

 Mid-Atlantic Ridge (Fig. 3). The Atlantis-Great Meteor Seamount Group 



