SECT. 2] OCEANIC ISLANDS, SEAMOUNTS, GUYOTS AND ATOLLS 3G7 



In the Indian Ocean there are a nnmber of isolated volcanic islands that are 

 essentially similar to those of the Atlantic. Prince Edward and Marion Islands 

 (known collectively as the Prince Edward Islands) lie southeast of the Cape 

 of Good Hope. These islands rise from a deep-ocean ridge that is a continuation 

 of the Mid- Atlantic Ridge around Africa into the Indian Ocean (Ewing and 

 Heezen, 1956). Marion Island, the larger of the two, has an area of 70 mi^ and 

 an elevation of 4200 ft; Prince Edward has an area of 16 mi^ and a height of 

 2370 ft. Like all volcanic islands in the cooler, reefiess seas, these islands are 

 strongly cliffed. They are composed of olivine basalt flows on which lie younger 

 cones of scoria but neither island shows evidence of volcanic activity at the 

 present time. 



Two other examples of cliffed volcanic islands are Amsterdam and St. Paul, 

 to the northeast of the Prince Edward Islands. These two also rise from a 

 submarine ridge near the middle of the Indian Ocean (Menard, 1959, fig. 2). 



In the Pacific, the Tuamotu (Paumotu) Islands rise from a mid-ocean ridge 

 (Menard, 1959, fig. 2) but they are capped by limestone and have had a more 

 eventful history. These low islands, in Dana's words, form "a vast island 

 cemetery, where each atoll marks the site of a buried island" (Dana, 1849, 

 p. 134). 



The Hawaiian Islands are gently sloping, shield volcanoes spaced along the 

 crest of a broad swell 2400 km in length, extending from northwest to south- 

 east. The swell is nearly 800 km wide and is partly surrounded by a deep (Dietz 

 and Menard, 1953), but these structures are independent of the mid-oceanic 

 ridge. The islands have been built up largely by flows of olivine basalt. The 

 largest island, carrying the only presently active volcanoes, lies at the south- 

 east end of the chain. Early observers recognized that there appears to have 

 been a progressive extinction of volcanic action from northwest to southeast 

 (Dana, 1849). Today the northwestern end of the chain consists of coral reefs 

 (Macdonald, 1956). 



The age of the Hawaiian Islands is uncertain. Most geologists believe that 

 they were built above the sea in Tertiary times and that the entire area has 

 been deeply submerged (Stearns, 1945, 1946). Both conclusions have very 

 recently been confirmed by dredging Miocene reef faunas on terraces deep 

 on the flanks of Oahu. Cretaceous rocks have recently been dredged from the 

 submerged Mid-Pacific Mountains less than 500 miles to the southwest (Hamil- 

 ton, 1956) and there are biological data suggesting that the islands are older 

 than is indicated by their meager fossil record (Ladd, 1960). 



Other island groups in the central and west-central Pacific are spaced along 

 lines that essentially parallel that of Hawaii. These include the Tubuai (Austral), 

 Society, Cook, Samoa, Tuamotu and Line Islands (Macdonald, 1956). All of 

 these, except the Tuamotus and the Line Islands, are primarily volcanic. 



Most oceanic islands that lie within an area 30°N and 30°S are encircled by 

 coral reefs. Water temperature in these low latitudes appears to be the chief 

 control of reef growth. Reefs are best developed where the mean annual tem- 

 perature is about 25°C ; they are poorly developed or absent in areas where 



