SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA FLOOR 



261 



the general trend. Large volcanic seamounts are also common. The Murray 

 Fracture Zone may be taken as an example. It extends along the sea floor in 

 an easterly direction from near the Hawaiian Islands to Southern California, 

 or about 3000 km. The western section of the zone is composed of two asym- 

 metrical elevations with the steep sides facing and a band of narrow troughs 

 and ridges between (Fig. 24). The total relief in a cross-section 100 km wide is 



Santa Inez Mts. 



4000 



Fig. 24. North-south profiles of the Murray Fracture Zone between Hawaii and Southern 

 CaUfornia. (After Menard, 1955.) 



about 1600 m, and most of the changes in depth are abrupt. South of the regular 

 ridges and troughs, and parallel to them, is a great range of submarine volcanoes 

 called the Moonless Mountains because none of them has ever been above sea- 

 level, although several are more than 3000 m high. In the central section of the 

 Murray Fracture Zone, the northern asymmetrical irregular ridge disappears, 

 and the relief of the southern ridge becomes greater. In addition, a regional 

 change in depth appears, such that the sea floor, for hundreds of kilometers 

 north of the fracture zone, is about 400 m deeper than the sea floor for an equal 

 distance to the south (Fig. 25). A single seamount rises above the crest of the 

 ridge to give a local relief of 7450 m. This seamount, Erban Guyot, has a flat 

 top from which beach gravels and fossils have been dredged. Although it is 

 now 450 m beneath the sea surface, it apparently was an island some time 



