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PACIFIC SCIENCE, Vol XXII, October 1968 
crysts of olivine, augite, or plagioclase. Some 
of the finest gravels are made up almost entirely 
of plagioclase phenocrysts as, for example, in 
’Ao’auli stream bed downstream from exposed 
flows containing plagioclase phenocrysts up to 
2 inches long. 
CALCAREOUS SEDIMENTARY DEPOSITS: Most 
of the coastline of Ta’u Island is fringed by 
long, narrow beaches that are usually 40-100 
feet wide at mean sea level. Beachrock composed 
of cemented calcareous sand is common on these 
beaches near sea level and offshore. A fringing 
coral reef nearly surrounds the island. 
Sand samples collected at sea level from most 
of the beaches around the island and offshore at 
Ta’u and Faleasao villages vary in median grain 
size from 0.29 mm to 3.50 mm. Nearly all of 
the samples are well sorted (only 4 of 43 an- 
alyzed samples had 0$ > 1.3) • Samples from the 
beach and the reef flat at Ta’u Village are very 
well sorted, possibly due to the strong currents 
of up to 3.8 feet/second that flow periodically 
across this reef. 
The noncalcareous material in these samples 
is mostly lithic fragments of lava rock with 
occasional mineral grains of olivine, augite, and 
magnetite. The calcareous material is mainly 
fragments of calcareous algae, foraminifera, 
coral, mollusk shells, and crustacean skeletons in 
approximate order of abundance. Samples from. 
Faga on northern Ta’u have the highest per- 
centage of noncalcareous grains, up to 32 per 
cent. Several streams along this coast provide 
abundant volcanic detritus. Most other beaches 
on Ta’u contain more than 95 per cent calcareous 
material. The highest calcareous content (over 
99 per cent CaC0 3 ) was found in sand collected 
from Tufu on the southeastern tip of the island. 
Major Structures 
Ta’u Island represents the remnant of a con- 
structional dome with two lesser shields located 
along northwest- and northeast-trending rift 
zones. The northwest rift zone, along which lie 
the Tunoa shield and the Faleasao tuff complex, 
extends seaward to Ofu and Olosega islands as 
the regional Samoan Ridge. Bathymetric data 
(Fig. 2) indicate a dozen or more volcanic 
cones located along the crest of this ridge; one 
cone erupted about 1866 (Friedlander, 1910). 
Water depth over the ridge crest nowhere ex- 
ceeds 750 feet, and in places the water is only 
125 feet deep. Midway between Ta’u and 
Olosega islands, there may be a rift zone that 
trends approximately north-south, cutting across 
the Samoan Ridge. The northeast rift zone, 
along which lie the Luatele shield, the Fitiiuta 
lavas, and a line of cones on the flank of the 
Lata shield, continues at least 3.6 miles offshore, 
beyond which soundings are sparse. 
The caldera apparently was not formed at the 
exact summit of Lata shield but was located 
slightly to the south. The beds on the southeast 
and west sides of the shield have an average dip 
of about 15°, conforming to the ground slope in 
that area. Within the collapsed summit area, two 
major benches are present — a higher one at 
Afuatai, and a lower one at ’Ele’elesa (Fig. 4, 
top photo'). The lower bench contains three 
large pit craters and at least one cinder cone 
(see Fig. 3). The upper bench is covered with 
thin-bedded, horizontal flows of oceanite and 
olivine basalt with a few small areas that are 
mantled by a 3-foot bed of fine-grained tuff 
with laminae less than 0.5 inch thick. No vents 
or pit craters were seen on this bench. 
Because of the relatively steep (10-17°) sea- 
ward dips of the beds at Li’u and Tali’ i and the 
series of faults paralleling the sea cliff, the 
bench at Afuatai and the narrower benches to 
the southwest at Leavania and Tali’i, and to the 
southeast at Li’u, probably represent the former 
summit of the volcano which has dropped ver- 
tically as much as 1,300 feet. The lower bench 
at ’Ele’elesa and Leatutia, the pit craters and the 
cinder cone, probably represents the original 
caldera of the volcano. If so, the caldera was a 
little more than a mile in diameter and was 
partly filled with volcanic material to its present 
depth of 300 feet. Later collapse has dropped 
the adjacent summit area to form the present 
bench at Afautai, as well as the Leavania-Tali’i 
and the Li’u slopes. These areas have therefore 
been shown on the geologic map as extra- 
caldera deposits (Fig. 3). 
The offshore soundings (Fig. 2), though 
very sparse, also suggest the possibility of 
large-scale foundering on the southern slopes 
of Ta’u Island. The ridges on the east and west 
sides of the depression slope downward about 
15° from their summit to the ocean floor, for a 
