15. OCEANIC ISLANDS, SEAMOUNTS, GUYOTS AND ATOLLS 



H. W. Menard and H. S. Ladd 



1. Introduction 



Oceanic islands, in general, are small in surface area, rise from the deep sea 

 and lie far from continental shores. This definition excludes large continental 

 islands such as Greenland, Iceland, the British Isles, Sicily, Madagascar, the 

 islands of Indonesia, New Guinea and the "island continent" of New Zealand. 

 It leaves in a borderline category such chains of islands as the Aleutians, 

 Japan and other arcuate groups. Typical oceanic islands are volcanic. They 

 may be isolated — at least as far as surface expression is concerned — or they 

 may form part of a chain. Many of those lying in the warmer seas are encircled 

 by reefs. 



Seamounts, like oceanic islands, rise from the deep-sea floor and many of 

 them lie far from continental shores. They, too, represent materials extruded 

 from volcanic centers but not extruded in sufficient quantity to build their 

 masses above sea-level. Again, like oceanic islands, they may be isolated or 

 arranged in chains. 



CJuyots are flat-topped seamounts. They appear to have reached the surface 

 of the sea at one stage in their development, at which time they were bevelled 

 by wave action before sinking to their present levels below the sea. Atolls are 

 reef-capped seamounts or guyots without exposed volcanic cores. 



The distribution of islands and atolls has long been known but a better 

 understanding of their distribution and their origin has been obtained during 

 the past 10 years with the rising flood of echo-soundings. Many hundreds of 

 thousands of kilometers of sound tracks have been recorded by oceanographic 

 vessels operated by institutions in the United States, the Soviet Union, Sweden, 

 Denmark and Great Britain. 



In the case of guyots, the sounding data have been supplemented by in- 

 formation from dredging and bottom coring. The slopes of atoUs have likewise 

 been dredged and drill holes have been put down through their islets. Detailed 

 studies of exposed reef surfaces have also been carried out. 



Space does not permit full coverage of all recently acquired data but an 

 attempt is made to summarize available information, particularly as regards 

 its bearing on distribution, age and origin. In discussing the four major features, 

 maps showing general distribution are given and typical examples are selected 

 for brief description. 



2. Oceanic Islands 



Many of the oceanic islands that lie far from land are not as isolated as they 

 appear on maps of the Earth's surface. Bathymetric charts show that they are 

 high points on mid-oceanic ridges or rises that divide the Atlantic, Indian 

 and South Pacific Oceans (Ewing and Heezen. 1956; Menard, 1959). In this 

 category are the Azores, Ascension and St. Helena in the Atlantic, Prince 



[MS received June, 1960] 365 



