366 MENAKD AND LADD [CHAP. 15 



ICdward aiul St. l*aul Islands in the Indian Ocean, and tlie Tuaniutu Islands in 

 the Pacific. 



Oceanic islands of a second type occur in the Pacific. These are volcanic and 

 are arranged in linear groups that are independent of the mid-ocean rise. The 

 Hawaiian, Marquesan, Samoan and Society Islands are examples of this class. 

 The submarine ridges from which they rise may be caused in part by faulting 

 l)ut the islands and their ridges may be explained solely as the result of i)iling 

 up of lava flows between closely spaced vents. Other linear groups in the Pacific, 

 such as the Tubuai (Austral) Islands, are made up of relatively isolated vol- 

 canoes rising directly from the dee])-sea floor. The volcanoes in these chains 

 appear to be too widely spaced or have not been productive enough to pile 

 up lavas between them to build a ridge (Menard, 1959). Most oceanic islands 

 lying in tropical latitudes are encircled by coral reefs. On some of these the 

 volcanic foimdation is partly or entirely covered by elevated shallow-water 

 hmestones. 



Arcuate groups of islands are widely developed in the western Pacific. Two 

 types are recognized: (1) the continental arcs, such as the Kamchatka-Kuril- 

 Hokkaido Arc, the Philippine Arc and others lying between them, and (2) the 

 "oceanic arcs" typified by the Marianas and Palau (Dietz, 1954), The first type 

 is closely tied geologically to the continents and is not considered here. The 

 "oceanic arcs", despite their name, are in a borderline category, because they 

 lie outside the andesite line that encloses the Pacific Basin and they include 

 rocks regarded as "continental". The arcs are believed to be tied to the tecto- 

 gene, the postulated down-buckle in the Earth's crust into subcrustal material. 

 Each arc is bordered by a deep trench on the convex side, the islands capping 

 parts of a submarine ridge on the active or concave side. Saipan (Cloud et al., 

 1956) and Guam (Tracey et al., in press) are tvpical Mariana islands and both 

 have been studied in detail. They are composed of volcanics (andesites, basalts) 

 and are partially covered by limestones (mostly shallow water) that range in age 

 from Eocene to Recent. Yap, an island forming part of a small arc between the 

 Marianas and Palau, is unflerlain by schists and other rocks of unknown age 

 overlain locally by Tertiary limestones (Cole, Todd and Johnson, 1960). 



A typical example of an isolated volcanic island in the Atlantic is Ascension. 

 It lies almost midway between Africa and South America, being 1500 km from 

 the nearest point on the African coast. Ascension is roughly triangular in plan, 

 measuring about 10 km on a side with an area of about 100 km- (Daly, 1925). 

 It rises to a height of 858 m. It is composed entirely of volcanic rock (except 

 for some diatomaceous material in one of the craters) and shows well preserved 

 tuff cones, but there is no volcanic activity at the present time. The island tops 

 a broad cone that rises from a depth of nearly 3000 m near the center of the 

 Mid-Atlantic Ridge. 



Ascension is only one of a number of isolated, reefless, volcanic islands that 

 rise from the Atlantic Ridge. St. Helena, 1200 km to the southeast, is slightly 

 larger but similar in essential features. Others include Tristan da Cunha and 

 Clough Island. 



