268 



HEEZEN AND MENABD 



[chap. 12 



seamouiits are included with the islands. Consequently, they mark a zone of 

 ■weakness in the crust, rather than a single major fault. The center of volcanic 

 activity appears to migrate in a regular manner from one end of a zone of 

 weakness to the other. An outstanding example of apparent shifting is the 

 Hawaiian archipelago where former volcanoes at the northern end are now 

 atolls, the central section is made up of inactive volcanoes, and the southern 

 end has active volcanoes. The Austral Islands, in the drowned mid-Pacific 

 mountains, have had a more comjilicated history with at least two periods of 

 vulcanism. Sprinkled among the Austral Islands are five guyots at depths of 



ft ssr 



600 • -I 



NAUTICAL MILES 





{ 



\ — *v- 



- 1800 



2400 



1800 



i.jii mi uiii^jiav 



2400' 



MfiROUF'^aS IS^AMDS 



■aiSBMMM 



1800 



2400 



1800 r 



s*^ 



Sc 



Fig. 30. Echograms of a typical archipelagic apron. The figiu-e shows a well-developed 

 archipelagic plain. Note the contrast of the archipelagic-plain topography with the 

 blocky topography of the mid -oceanic ridge in the central east Pacific. Note abyssal 

 hills in Profile B. "(After Menard, 1956.) 



1000 to 1500 m. Clearly, they are former active volcanoes which were islands but 

 which became inactive and subsided long before the period of vulcanism in 

 wliich the existing islands were formed. 



The enormous load of a group of volcanic islands might be expected to 

 depress the crust of the earth, and a moat or encircling depression around the 

 southern Hawaiian Islands has been attributed to this cause (Dietz and Menard, 

 1953). The moat is only slightly deeper than average for the region but it, in 

 turn, is encircled by a broad arch, which makes the moat more conspicuous 

 (Fig. 24). Most archipelagos are not encircled by moats that are evident in the 

 topogra])hy of the sea floor. However, abnormal thicknesses of several kilo- 

 meters of volcanic rock lie under the sea floor in the vicinity of many archi- 

 pelagos. Inasmuch as the sea floor is not elevated, the upper surface of the 



