336 HEEZEN AND LAUGHTON [CHAP. 14 



certainly identical in oriirin to the abyssal ])lains of" the ocean-basin floor, lie 

 in diverse tectonic and ])hysiographic settings. They are only indicative of 

 identical smoothing mechanisms whereby the bottoms of basins become flat. 



4. Archipelagic Plains 



Menard (1956) originaUy proposed the term "archipelagic apron" for the 

 smooth sea floor around islands or island gronps which "has the form of an 

 apron" (cf. Chapter 12). Although most archipelagic aprons were found to be 

 smooth, some were relatively rough and, therefore, Menard made the ])ro vision 

 in his definition that an archipelagic apron "need not have a smooth surface". 

 In view of this latter provision, it seemed necessary to propose the term 

 "archi])elagic plain" for those portions of an archipelagic ajjron which are 

 smooth. An archipelagic plain is a smooth portion of the deep-sea floor which 

 lies at the base of the pedestal of an island group or island chain. The charac- 

 teristically low gradients and smoothness of archipelagic plains are extremely 

 similar to abyssal plains, Menard has discovered dozens of archipelagic plains 

 in the Pacific around the Hawaiian Islands, the Mid-Pacific Mountains, and the 

 Marshall, Gilbert and Marquesas archipelagos. As Menard points out, aprons 

 "often exhibit the features used elsewhere as criteria for partial burial of a 

 hilly terrain, namely, steep-sided hills rising abruptly from smooth plains, 

 apparent ponding of sediment behind ridges acting as dams and the absence of 

 deeps below the level of the smooth surface. Consequently, it appears probable 

 that the smooth aprons have buried an older terrain of hills." 



The fact that there are archipelagic aprons which lack a smooth surface 

 suggests that there was a rough-hewn apron or flange-like feature lying around 

 the base of the island groups, which originally did not have a smooth surface, 

 and, in many cases, this rough flange or apron may have been later smoothed 

 by sedimentation. Menard points out that the total volume of material con- 

 tained in that portion of an archipelagic apron which lies above the general 

 level of the sea floor is many times the total volume of the island pedestal it 

 surrounds. For this reason, it seems unlikely that the total volume of material 

 in an archipelagic apron has accumulated through turbidity-current deposition 

 of sediments derived from the pedestal. Likewise, it seems clear that the 

 archipelagic ])lains are similar, if not identical in origin, to the abyssal plains. 

 Both were ])robably produced by the burial of former relief by turbidity- 

 current deposition. 



Almost all the archipelagic plains of the world are found in the central 

 Pacific. However, archipelagic plains are found in the Atlantic surrounding 

 the pedestal of Bermuda and indications of similar features are found sur- 

 rounding the oceanic islands of the South Atlantic and the Indian Ocean. If 

 we accept the limits of the Pacific archipelagic aprons shown by Menard (1958), 

 we must conclude that the area of the archii)elagic aprons of the Pacific is 

 nearly equal to the area of abyssal plains in the Atlantic. However, Menard 

 does not state what percentage of the total area of the archipelagic aprons 



