384 



MENARD AND I.ADD 



[chap. 15 



The net rate of subsidence of atolls and guyots in geological time can be 

 determined from the depth to datable horizons. For guyots, the depth to be 

 considered is not the depth of the dredge haul but the break in slo])e at the edge 



300- 

 200- 



100- 



If 



5^f/? 



Of 



!hl%s 



ir 



AHy 



llMe 



TOTAL NUMBER Of ISLANDS IN DATED GROUPS AT ANY TIME 



:ioo- 



o 0- 



LOW ISLANDS AND BANKS 

 VOLCANIC ISLANDS 



100- 



0- 



UNOATED ATOLLS 



GUYOTS 



_r 



ATOLLS 



-100 



10 20 30 40 50 60 70 80 90 100 



Plei5t-Plio| Miocene | Oliq | Eocene | Poleoc U Creloceous | 



YEARS « lO'' 



Fig. 10. Numbers and ages of islands, atolls and related features. 



10 20 30 40 50 60 70 80 90 100 110 



Pleisl-Plio| Miocene! Olig | Eocene | Poleoc u Creloceous | L Creloceous 



YEARS A 10* 



Fig. 11. iX'ptlis ami agos of atolls aii<l guyols. 



of the flat top, because this represents sea-level when truncation began. Avail- 

 able dates suggest a constant rate of subsidence of 2 cm per 1000 years for the 

 last 100 million years (Fig. 11). The relative subsidence of widely separated 

 volcanic islands of various sizes at a constant net rate seems consistent with a 

 rapid accumulation of juvenile water and a rise in sea-level relative to the sea 



