The Geology of the Austral or Tubuai Islands (Southern Pacific)

Abstract

The following paper embodies the geological results of the St. George Expedition to the South Pacific Ocean, organized by the Scientific Expeditionary Eesearch Association, in so far as they relate to the Austral or Tubuai Group. This group (see map, fig. 1, p. 292), which constitutes part of the Etablissements Francais de l9Oceanie, extends for a distance of about 750 nautical miles between lat. 21° 44′ S. and 27° 55′ S., and between long. 143° 26′ W. and 154° 48′ W. It includes seven islands, as well as several isolated rocks and shoals, which are arranged along lines having a west-north-westerly and east-south-easterly trend. These lines are continued westwards in the Cook Group, and are parallel to the trend-lines in the Society Islands on the north, and the Tuamotu Archipelago on the north-east. Soundings are few and scattered in these regions, but the general depth of the ocean appears to be between 1900 and 2500 fathoms, with a bottom of red clay. The expedition spent several weeks in this group in April and May, 1925. The islands are mainly volcanic, with or without coral-reefs. Little work on the geology of the region appears to have been done in the past. Four of the islands (Rapa, Marotiri, Rurutu, and Rimatara) are described here, and references to previous work on them will be found in the following pages. The others have received only the scantiest attention in geological literature. Rapa Island is situated in lat. 27° 35′ south, long. 144° 17′ west.