276 HEEZEN AND MENARD [CHAP. 12 



over the ridge. The wholly submerged mountains in the Tuamotu archipelago 

 bear a close resemblance in every detail of topography except that the ancient 

 islands of the mid-Pacific were not capped by reef corals which grew upward as 

 the islands subsided. The mid-oceanic ridge in most places in the ocean has an 

 average shape of a broad, gentle, more or less fractured arch. But in the Central 

 and North Pacific, the aseismic median elevations are narrow and steep-sided. 

 Do these different types of topography represent different stages of develop- 

 ment? If they do, the broad, fractured arches may be an early stage, and the 

 narrow ridges may be a later one. This is suggested by the fact that the broad 

 fractured arches are the locus of a continuous narrow band of earthquakes and 

 active, or recently extinct, volcanoes. In addition, the heat flow from the earth 

 is high and variable under the broad fractured arch, suggesting that very 

 powerful deforming forces are active. Central and North Pacific narrow ridges 

 show no such signs of crustal instability. There are no earthquakes, active 

 volcanoes, or abnormal values of heat flow. On the contrary, deep guyots and 

 atolls bear witness to long-continued quiescence and subsidence. 



c. Mid-oceanic plateaus 



In the vicinity of the Azores Islands in the North Atlantic and near the 

 Crozet Islands in the Indian Ocean, the mid-oceanic ridge passes along one 

 flank of a large mid-oceanic plateau. The Azores Plateau has an area of 52,000 

 mi2 and surrounds the Azores Islands. The depth of this plateau is generally 

 less than 1000 fm. The Azores Islands are oriented S to SE-N to NW along a 

 topographic trend striking off toward the Straits of Gibraltar. This topographic 

 connection between the Azores Plateau and the Southern Iberian Peninsula has 

 been called the Azores-Gibraltar Ridge, which Heezen et al. (1959) consider as 

 a poorly developed mid-oceanic ridge, of the same general class as the Mid- 

 Atlantic Ridge. 



The Azores Plateau itself merges with the rift mountains of the Mid- Atlantic 

 Ridge. The sea-floor topographic trends of the eastern part of the plateau are 

 parallel to the known tectonic and volcanic trends of the Mid- Atlantic Ridge. 

 The Azores Plateau or bulge is generally considered to be a highly fractured 

 tectonic uplift in which vulcanism has played a very smaU part (Cloos, 

 1939). 



The topography of the Azores Plateau is somewhat less irregular than that of 

 the Mid- Atlantic Ridge. Although prominent topographic trends cross the area, 

 isolated steep-sided volcanoes occur and long linear scarps are common. The 

 minor relief on the surface of the plateau is relatively gentle. So little work has 

 been done in the area of the Crozet Plateau that it is impossible to delineate 

 the sea-floor features. One echogram taken midway between Prince Edward 

 Island and the Crozet Islands reveals that the surface of the plateau is remark- 

 ably smooth, in fact much smoother than the Azores Plateau. The origin of this 

 smooth topography is difficult to ascertain. However, it seems likely that an 

 original irregular reHef nuist underlie the smooth surface, which was smoothed 

 by redistribution of sediments by strong bottom currents. The surface of the 



