The Geology of the Manu’a Islands, Samoa 1 
Gary D. Stice 2 and Floyd W. McCoy, Jr. 3 
ABSTRACT: The Manu’a Islands are a group of three islands — Ta’u, Ofu, and 
Olosega — that were built by volcanic activity along the crest of the easternmost 
portion of the Samoan Ridge. Ta’u Island represents the largest volcanic center, 
where aa and pahoehoe flows of non-porphyritic basalt, olivine basalt, picrite basalt, 
and feldspar-phyric basalts constructed a volcanic shield more than 3,000 feet above 
sea level. The present-day total thickness of this volcanic material is over 12,000 
feet, as measured from the ocean floor to the summit of Ta’u, Lata Mountain (3,056 
feet). Dips of the lava flows frequently exceed 30°, but average 20-25°. Summit 
collapse formed a caldera that became partially filled with ponded lavas and pyro- 
clastic deposits which accumulated to a thickness of over 1,000 feet. From the 
summit area, two rift zones radiate to the northeast and northwest, the latter 
coinciding with the trend of the Samoan Ridge. Two smaller shields are located 
along these rift zones. Following a period of extensive erosion, the northeast corner 
of the island was built out by dunite-bearing lava flows, upon which the village of 
Fitiiuta now stands. A tuff complex containing large dunite xenoliths and coral 
blocks extended the northwest corner of the island near the village of Faleasao, 
burying a former sea cliff. 
Ofu and Olosega islands represent a complex of at least six volcanic cones aligned 
along the regional rift of the Samoan Ridge. Two of these cones developed as 
shields that later coalesced and buried the older cones of largely pyroclastic material. 
The shields are composed mainly of aa and pahoehoe flows of non-porphyritic 
basalt, olivine basalt, and picrite-basalt, with hawaiites occurring in the uppermost 
portion of the shield on Ofu. More than 11,000 feet of volcanic material are 
represented by these shields, as measured from the ocean floor to the present-day 
summits of Ofu at Tumu (1,621 feet), and of Olosega at Piumafua (2,095 feet). 
These summits represent the approximate former summits of both shields. Average 
dips of the lava flows are 10-20°, locally becoming considerably steeper. Summit 
collapse of both shields produced calderas, one on northern Ofu and another off 
northwestern Olosega near Sili Village. The caldera on northern Ofu was partly 
filled by the ponding of olivine basalt, hawaiite, and ankaramite lava flows. The 
floor of the other depression, Sili caldera, lies offshore and may never have been 
exposed above sea level. Following a period of quiescence and erosion probably 
contemporaneous with that on Ta’u Island, recent volcanism built a lapilli tuff cone, 
the remnants of which form Nu’utele and Nu’usilaelae islets. Volcanism has con- 
tinued to the present day with a submarine eruption reported between Olosega and 
Ta’u islands in 1868. 
Narrow fringes of calcareous beach deposits and fringing coral reefs nearly 
surround each of the three islands. High cliffs around the islands are the result of 
marine erosion. Benches at +15 and -|-5 feet above present sea level may indicate 
former higher stands of sea level. 
The Samoan archipelago lies about 2,200 
miles southwest of Honolulu and about 500 
1 Hawaii Institute of Geophysics Contribution No. 
233. Manuscript received October 26, 1967. 
2 Hawaii Institute of Geophysics, University of 
Hawaii, Honolulu, Hawaii 96822. 
miles northeast of Fiji. Politically, Samoa is 
divided into Western Samoa, which includes the 
two largest islands of Savai’i and Upolu, and 
American or Eastern Samoa (Fig. 1). Tutuila, 
3 Woods Hole Oceanographic Institution, Woods 
Hole, Massachusetts 02543. 
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