The Wallis Islands comprise Uvea, the main island, and 22 islets enclosed by or lying upon a nearly circular barrier reef about 200 miles west of the Samoan group in the Southern Pacific Ocean. The highest point in the group, about 470 feet above sea level, and several crater lakes exist on Uvea Island.
The high islands are composed of olivine basaltic lavas and pyroclastics, except for one cinder cone and its associated flows of oligoclase andesite on Uvea Island. The low islands either are composed of calcareous sand or are erosional remnants of tuff cones and lava domes.
Uvea Island was built by the coalescence of lava flows from 19 volcanic vents. The vents comprise 15 flat shield-shaped lava cones, 3 consolidated ash cones, and 1 cinder cone. Except for two Recent lava cones barely covered with soil, the bulk of the island is composed of deeply weathered middle Pleistocene(?) volcanics. Lavas of intermediate age do not exist. Limestone ejecta in the tuff cones indicate that the whole group is built on a submerged reef, probably about 180 feet below sea level. This reef presumably rests on a Tertiary basaltic volcano. Definite evidence of emerged shore lines at 25 and 5 feet above mean sea level exists, as well as evidence suggestive of higher stands.