SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 273 



in the crest provinces in the Eastern Pacific and in some places in the 

 Atlantic give values several times greater than have been obtained in normal 

 ocean-basin floor or continental areas (Von Herzen, 1959). A large positive 

 magnetic anomaly is often but not always associated with the rift valley and 

 some seismic-refraction measurements suggest a crust intermediate in com- 

 position between the oceanic crust and the mantle. The crest provinces of the 

 mid-oceanic ridge can be traced directly into the rift valleys, rift mountains 

 and high plateaus of Africa (Figs. 34 and 35) and of the western United States. 

 The faulted topography characteristic of the crest provinces is well developed 

 on an unnamed ridge trending NW from southern Chile. However, much of 

 the Pacific portion of the system is smoother than the remainder of the mid- 

 oceanic ridge. 



b. Flank provinces - 



The flank provinces of the Mid-Atlantic Ridge can usually be divided into 

 somewhat arbitrary steps or ramps, each bounded by prominent scarps. Parts of 

 the flank provinces, particularly the upper step south of the Azores, are charac- 

 terized by smooth-floored intermontane valleys ; but, in general, the relief of 

 the entire Mid- Atlantic Ridge is highly irregular and characterized by lack of 

 any smooth areas (Fig. 35). Thus, these intermontane valleys stand out very 

 distinctly. Photographs, cores and dredgings indicate that the crest of the Mid- 

 Atlantic Ridge north of the Azores is being denuded of its sediments. These 

 sediments, eroded from the crest provinces and deposited on either side, 

 probably account for the smoothing of the relief of the flanks and the filling of 

 the intermontane basins. Rocks dredged from the rift valley in the North 

 Atlantic consist of fresh basalts and fresh-to-weathered serpentines and gabbros. 

 Several large boulders of basalt dredged from the rift valley have recently been 

 dated by the potassium-argon method and found to be less than 10,000,000 

 years old (G. Erikson, in press). 



The serpentines, gabbros and peridotites dredged from the rift valley, and 

 those which protrude above sea-level at St. Paul's Rocks, would seem to support 

 Hess's (1955) theory that the ridge has been domed up by serpentinization of 

 the upper mantle in a long linear belt lying under the Mid-Oceanic Ridge. On 

 the other hand, the seismic velocity of 7.2-7.4 km/sec observed in the 

 mid-oceanic ridges is remarkably similar to the seismic velocity found in the 

 low-velocity channel lying 120 km below the continents and some 50 km 

 beneath the ocean basins (Gutenberg, 1959). 



The mid-oceanic ridge is conspicuously centered in the Atlantic and Indian 

 Oceans, where it is called the Mid-Atlantic and Mid-Indian Ocean Ridges. In 

 the South Pacific the position relative to the margin of the basins is less obvious 

 but it can be determined by an arbitrary method. The technique used is to 

 accept the 1000-m isobath as the margin of the ocean basin and to draw a family 

 of lines approximately parallel and equidistant from a margin by swinging arcs 

 on a globe (Fig. 30). Intersections of arcs with equal radii are connected to 

 established median lines. Consequently, any major convexity of margin of an 



