Ofu and Olosega are parts of a volcanic doublet in the Manu‘a Group of the Samoan Islands-part of American Samoa. The twin islands, formed from shield volcanoes, have a combined length of 6km. They are geographic volcanic remnants separated by the narrow 137m wide Asaga strait, a natural bridge of shallow coral reef. Before 1970, one had to wade between the two islands at low tide; now a single-lane road bridge over the strait connects villages on Ofu island with those on Olosega.
The highest peak on Ofu is Mount Tumutumu (491m, also referred to as Tumu) and the highest elevation on Olosega is Mount Piumafua (629m). The most recent volcanic eruption took place in 1866, 3km south east of Olosega. Archaeology field work carried out in the 1980s yielded pre-historic evidence including ceramics, adzes, shell and bone which have been significant in furthering understanding of the ancient history of the Samoa Islands and Polynesia.
This included samples of red-slipped plainware ceramics that appeared to be in the tradition of Lapita culture. The work, carried out by a team that included Pacific archaeology specialist Patrick Vinton Kirch, focused on a site called To'aga (site AS-13-1), a 2km coastal stretch on the south coast of Ofu. The results showed continuous human habitation of about 3,000 years.