SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA FLOOR 



275 



tica ; but at about 55° south latitude, the crest bends sharply to the east. 

 Apparent fault scarps suggest that the bend is an offset caused by faulting or else 

 a discontinuity along the margin of a large crustal block. It is only in the Central 

 and North Pacific that an obvious correlation between the geometric median line 



140° 160° 180° 160° 140° 120° 100° 80° 60° 40° 20' 



20° 40° 60° 80° 100° 



140° 160° 180° 160° 140° 120° 100° 80° 60 40 20 



20° 40° 60 80° 100° 



Fig. 37. Position of the crest of the mid-oceanic ridge in ocean basins compared with the 

 center hne of the ocean basins as defined by an arbitrary method discussed in 

 the text. (After Menard, 1959.) 



and a broad median elevation is not found. However, there is a correlation with 

 another type of submarine topography — long, narrow, steep-sided ridges capped 

 with atolls and seamounts. The median line of the whole basin trends north- 

 west from Easter Island to the Tuamotu Islands and the Line Islands to 

 Johnston Island. Tuamotu Ridge has an area of 200,000 km^ shoaler than 3 km, 

 and about 100 atolls and seamounts rise above the ridge. Some of the sea- 

 mounts are guyots or former islands, now submerged. The Line Islands rise 

 from the Christmas Island Ridge, which also has an extensive area, perhaps 

 100,000 mi^ above 3 km in depth, and many seamounts and guyots between 

 the islands. From Johnston Island, the median line has three spurs. The north- 

 eastern spur is not related to any conspicuous topography. The northern spur 

 lies somewhat east but parallel to the Emperor Seamounts, which are a line of 

 large guyots rising from a ridge with about 1 km relief. The western spur 

 corresponds to the Marquis-Necker Rise, which, in its eastern half, the Mid- 

 Pacific Mountains, has an area of 60,000 km^ shoaler than 3 km. The Mid-Pacific 

 Mountains include about 50 seamounts, mostly guyots, distributed broadly 



