SECT. 2] THE MID-OCEANIC RIDGE 399 



with the known position of the Easter Island Ridge and permits an inference 

 of continuity of the ridge in this httle known area. 



Near Easter Island the earthquake belt splits, sending one branch southeast 

 in the direction of southern Chile, and another northwards toward the Gulf of 

 California. The existence of a ridge between Easter Island and Chile, predicted 

 on the basis of the earthquake belt (Ewing and Heezen, 1956), has been 

 confirmed by an I.G.Y. expedition of the Scripps Institution (Fisher, 1958). 



The earthquake belt follows a well defined ridge north of Easter Island at 

 least to the Equator. From the Equator to the mouth of the Gulf of California, 

 the epicenter belt seems to follow a ridge similar to other mid-oceanic ridges, 

 but Menard (1955), who has studied much unpublished and unavailable data, 

 has come to the diametrically opposed view that the area is dominated by 

 east-west-trending "fracture zones", and that no distinctive topographic 

 feature is associated with the epicenter belt. This problem is possibly clarified 

 by a later publication (Menard, 1960; and Fig. 9) which gives much informa- 

 tion about the Pacific fracture zones and shows a continuation of the Easter 

 Island Ridge ("East Pacific Rise") from Easter Island through the Gulf of 

 California and western North America into the northeastern Pacific. Menard 

 pointed out that the ridge is smoother in the southeast Pacific than in the 

 Atlantic. 



The offshore belt parallels the coast, at a distance of 100 miles from the 

 continental slope, until it again extends into the continent along the linear 

 Lynn Canal of southern Alaska. Menard found the topographic trend to be 

 oriented parallel to the epicenter belt. 



Apparently one end of the mid-oceanic ridge system is found in the neigh- 

 borhood of the Gulf of Alaska. 



6. Iceland and the Norwegian Sea 



The belt of epicenters follows the crest of the Mid- Atlantic Ridge northward 

 through the central graben of Iceland (Rutten and Wensink, 1960) into the 

 Norwegian Sea, where it follows the median ridge system (Fig. 5). Soundings 

 are rare in these northern waters, but those reported by Boyd (1948) give 

 several clear indications of a central rift valley (Fig. 10). The line of epicenters 

 clearly follows this rifted ridge through the Norwegian Sea (Fig. 11). Although 

 some authors (Stocks, 1950) believe that the median ridge of the Norwegian 

 Sea merges with the continental slope in the vicinity of Bear Island, the epi- 

 center map suggests an alternative interpretation according to which the ridge 

 bends to the north and parallels the western margin of the Barents continental 

 slope west of Bear Island and Spitsbergen. 



The recent work of Rutten and Wensink (1960) has further described the 

 central graben of Iceland. It has long been recognized that essentially all 

 earthquakes and all modern, and Recent, vulcanism in Iceland have been 

 restricted to this fault-bounded continuation of the mid-oceanic rift. Bernauer 

 (1943) described gaping fissures known by the local name of gja which occur 



