Geology of Manu’a Islands, Samoa — Stice and McCoy 
443 
has not been demolished by storm waves during 
historic time, but archaeological excavations 
have exposed older habitations covered by sand 
and gravel (W. Kikuchi, personal communica- 
tion, 1966). 
Around the island of Ta’u there is a nearly 
continuous fringing coral reef. Nowhere is the 
reef front more than 800 feet from shore. The 
island of Tutuila also has a narrow fringing 
reef, but soundings clearly indicate that a 
drowned barrier reef extends more than a mile 
offshore (U. S. Coast and Geodetic Survey, 
Chart 4190, 1962). The only offshore sound- 
ings for the Manu’a Islands were completed in 
1939, and are sparse except between Ta’u and 
Olosega islands. Moreover, their accuracy is 
questionable; the islands themselves are posi- 
tioned 1.7 miles farther west than is shown on 
more recent charts. The absence of any indica- 
tion of a submerged reef around Ta’u may be 
merely the result of sketchy data. 
The reef flat contains calcareous sand, coral, 
and coralline algae in patches, whereas the fore- 
reef is composed of prolific colonies of corals 
and algae. At various places along the reef front 
there are surge channels about 15-25 feet wide 
and 9-15 feet deep. Corals found on Ta’u in- 
clude Acropora, Pocillopora, Millepora, Ma- 
endra, Favia, Psammocora, Goniopora, Pavona, 
Favites, and Goniastrea. H dime da, Porolithon, 
Goniolithon, and other calcareous algae are 
more abundant on the reef flat than on the fore- 
reef. 
Geologic History 
There is little evidence to indicate the age of 
the volcanic formations. Judging from the 
present stage of erosion of the island, the rapid 
extrusion of shield-building basalts along the 
crest of the Samoan Ridge began during Plio- 
cene time. Perhaps by early Pleistocene time the 
Lata shield had built considerably above sea 
level, and volcanism subsided, allowing time 
for erosion between some succeeding flows. The 
summit of the shield collapsed, and the caldera 
was partly filled with lavas and pyroclastic ma- 
terial. This collapse may have been accompanied 
or followed by gravity collapse of the southern 
portion of the shield, involving vertical dis- 
placement of up to 1,400 feet. Lavas from post- 
caldera cones mantled the shield. 
After collapse of the Lata shield, possibly in 
middle Pleistocene time, the Tunoa shield was 
formed by rapid extrusion of lavas, until its 
summit collapsed and the resulting caldera was 
partly filled with lava and pyroclastic material. 
At about the same time, but perhaps a little 
later, the Luatele shield was also built up until 
its summit collapsed and the crater was partly 
filled. Volcanism then became so infrequent 
during late Pleistocene time that a sea cliff ap- 
proximately 200 feet high was cut around the 
island. 
Continued volcanism from post-caldera cones 
such as Lepu’e, Olomatimu, and Olomanu dur- 
ing Recent time mantled most of the Lata 
shield, several of the flows spilling over the 
cliff into the sea. The Faleasao and To’a tuff 
cones and the lava flows at Fitiiuta built out in 
front of the former sea cliff. The most recent 
volcanism in Manu’a was a submarine eruption 
about 1866 between Ta’u and Olosega islands 
(Friedlander, 1910). 
No definite evidence was found to indicate 
relative changes in sea level. The 5 -foot bench 
in the tuff complex could be explained as due to 
lithification resulting from proximity to sea 
level, and the 15 -foot constructional bench 
could have been formed by storm waves, but 
the eroded beachrock, as well as the 5- and 15- 
foot benches, may be indications of a more 
recent higher stand of the sea. The narrow 
fringing reef may indicate submergence rapid 
enough to drown any former barrier reefs. 
GEOLOGY OF OFU AND OLOSEGA ISLANDS 
Nature and Distribution of Rock Types 
GENERAL statement: Ofu and Olosega 
islands are a complex of volcanic cones that 
have been buried by lava flows from two co- 
alescing shields. One shield is centered off the 
northwest coast of Olosega near Sili Village, and 
the other is centered at A’ofa on the northern 
coast of Ofu. Older cones, approximately aligned 
along the crest of the Samoan Ridge, include a 
small cinder cone at Tauga Point on north- 
western Ofu, a nearby tuff cone at the western 
end of Samo’i beach, a composite cone exposed 
