SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA MiOOR 



267 



at least from late Cretaceous to the present day, and it seems to have been 

 concentrated in different areas at different times. This is indicated by distribu- 

 tion maps of guyots and existing islands. At present, oceanic islands are 

 concentrated in the southwestern Pacific, whereas in the past they were also 

 common in the northeastern part. 



That island groujjs lie along straight lines has long been recognized as 

 geologically significant. Modern oceanographic exploration has added little 

 information about the islands, except to show that there are similar groups of 

 marine volcanoes which are wholly submerged, and that some of the seamounts 

 formerly were islands. The deej)-sea floor in the vicinity of island groups has 



A 



eOO tn 



1800 



2400 

 600 i 



1200 ; 



I8Q0 - i 



2400 I- 

 1200 c 



!, 



1800 5 



2400 

 1200 c 



I80O *; 

 2«ao 



Fig. 



29. Echograms of typical seamounts rising above abyssal hills in the Baja California 

 seamount province. (After Menard, 1955.) 



been relatively unknown until recently, however, and it has proved rewarding 

 to study it. Most linear groups such as the Hawaiian, Marquesas, Samoan and 

 Society Islands stand above a high ridge which may be caused in a small part 

 by faulting but appears to be explainable solely as the result of piling up of 

 lava flows between closely spaced volcanoes. Some linear groups, including the 

 Austral Islands, are relatively isolated volcanoes rising directly from the deep- 

 sea floor. The volcanoes of the latter groups are too widely spaced or have not 

 been productive enough to pile up lavas between them and to build a ridge. 

 Although they are linear on a small-scale map, the archipelagos in detail are 

 spread in a band that may be one-fourth as wide as the length, if adjacent large 



