Geology of Manu’a Islands, Samoa — Stice and McCoy 
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behind Faleasao Village and on the east side 
of Si’ulagi Point, 
FITIIUTA FORMATION: Post-erosional lavas 
which erupted from at least two vents and ex- 
tended the northeast corner of Ta’u Island be- 
yond the old sea cliff comprise the Fitiiuta 
formation (Fig. 4C). Fitiiuta Village is situated 
on these flows. The fresh appearance of num- 
erous tumuli on the lava surfaces indicates a 
Recent age. The sea is eroding a 150-foot-high 
cone, Maluatia Hill, revealing its internal struc- 
ture. The cone is composed of cinder, beneath 
which a sequence of thin-bedded pahoehoe 
flows and thick lenses of red cinder extends 
from about 90 feet down to 30 feet above sea 
level. This sequence is in turn underlain by a 
polygonally jointed flow 30-40 feet thick. The 
pahoehoe flows conform somewhat to the topog- 
raphy of the hill, but with a gentler dip. 
Therefore, they probably came from the same 
vent, and may have been an earlier part of the 
eruption that ejected the overlying cinder. 
Cinder and scoria indicate the proximity of 
another vent inland to the southwest, near the 
base of the old sea cliff behind the village. This 
vent, along with Maluatia Hill, the Lualaitiiti 
pit crater, and the central depression of the 
Luatele shield, lie along a line trending N 4l° 
E from the caldera of the Lata shield. 
The Fitiiuta lavas are almost entirely pahoe- 
hoe flows of olivine basalt. These lavas are 
aphanitic to finely porphyritic with phenocrysts 
of olivine 1-4 mm in diameter and plagioclase 
of less than 1 mm. The lavas contain abundant 
dunite xenoliths ranging from less than 0.1 
inch to 0.3 inch across. Nearly all of the inclu- 
sions are entirely olivine; augite is rare. 
intrusive rocks: The only exposure of a 
major intrusive complex on Ta’u Island is a 
swarm of dikes and sills near the mouth of 
Laufuti Stream on the southern side of the 
island. Several dikes also crop out parallel to 
the cliff at Vailolo’atele near the southwestern 
tip of the island. Only a few widely scattered 
radial dikes were found in the high escarpment 
on southern Ta’u. Most of the dikes are less 
than 2 feet thick ; none were found to exceed 4 
feet. Many of them were magnetic enough to 
deflect a compass needle. 
Virtually all of the hundreds of dikes exposed 
along Laufuti Stream strike N 70°-90° W and 
dip 80°-90° S. Most are only 2 or 3 feet thick. 
They are composed of dense basalt and olivine 
basalt, and a few are oceanite. Some of the 
dikes are vesicular, indicating that they were 
intruded near the surface. The selvage is 
usually less than 1 inch thick. Sometimes vesicles 
are concentrated near this chilled contact zone. 
A few multiple dikes are exposed in the steep 
sides of Laufuti stream valley, about 100 yards 
from its mouth. Olivine basalt sills were found 
associated with the dike complex. The sills are 
usually about 2 feet thick and have a maximum 
lateral extent of less than 30 feet. Only two or 
three small radial dikes were seen exposed in 
the northern wall of the 1,300-foot fault scarp. 
A dozen or so thin (1.5-2 feet) dikes are 
exposed in the cliff between Papaotoma and 
Si’ufa’alele points, at the southwest corner of 
the island. These dikes parallel the cliff which 
merges with the sea cliff at Tali’i, 0.6 mile to 
the northeast. In the same area at the base of 
the cliff, a Recent vent has extruded pahoehoe 
basalt flows which form Lotoaise Point. The 
horizontal flows of olivine-rich basalt on Leatu- 
toga Point only 0.1 mile north are probably 
from the same source. Apparently the lava has 
flowed out over the reef. 
NONCALCAREOUS SEDIMENTARY DEPOSITS: 
Rock waste forming the talus at the base of the 
old sea cliff is the most prominent noncalcare- 
ous sediment. Much of the talus has been de- 
posited by landslides, but many boulders have 
become dislodged one at a time, rolling down to 
the base of the sea cliff. Generally the talus is 
heavily covered with vegetation. The alluvium 
deposited by the streams is a similar type of 
rock waste, except for the absence of soil and 
vegetative cover. Boulders up to 10 feet in di- 
ameter comprise the bulk of the alluvial deposits. 
These boulders, especially the larger ones, usu- 
ally are from the dense portions of aa flows; 
most of them are picrite-basalt. 
The cobbles and pebbles in the stream beds 
have usually been formed by chipping and 
breaking of the boulders. Most of the granules 
in the stream bed occur as angular chips of 
non-vesicular or poorly vesicular flow rock. 
Less frequently the granules are large pheno- 
