Geology of Manu’a Islands, Samoa — Stice and McCoy 
447 
that are parallel to, or dip steeply toward, the 
caldera boundary to the west. In other places the 
pre-caldera pahoehoe flows vary from 1 to 6 feet 
thick but are generally less than 3 feet. The 
flows usually dip less than 10° away from the 
former summit northwest of Sili Village. 
Interbedded aa flows exposed in the steep 
cliffs behind Sili and Olosega villages generally 
increase up-section in number and thickness 
from 2 or 3 feet to more than 20 feet. As on 
Ta’u, some of the steeply dipping aa flows 
(locally up to 30°) have as much as 10 feet of 
clinker associated with only 1 foot of the mas- 
sive central portion of the flow. 
Thick aa flows comprise most of the upper 
800 feet of a 2,100-foot section which extends to 
the summit at Piumafua Mountain. The dips of 
these later flows are relatively steep, ranging 
between 15° and 20°. A series of 3 or 4 flows 
of hawaiite with a total thickness of over 7 5 feet 
are the highest flows in the section that could be 
closely examined. The rock is quite fresh except 
along some joints that are partly coated with 
purplish-black manganese oxide. A few expo- 
sures near the summit are probably the same 
thick aa flows exposed in the cliff behind Sili 
Village, but they are too deeply weathered to be 
identified. 
Thus, all of the upper 600 feet of the shield 
may be capped with flows of hawaiite. Picrite- 
basalts do not seem to be as abundant on 
Olosega as on Ofu, but a few oceanites and 
ankaramites are exposed along the eastern coast 
of Olosega. One of these ankaramite flows con- 
tains a concentration of 90 per cent augite 
phenocrysts up to 0.5 inch long in the frothy 
1-inch thick surface crust of the flow. Table 2 is 
a stratigraphic section of the Sili extra-caldera 
member at Tafalau on eastern Olosega. 
The extra-caldera lava flows of the A’ofa 
shield are very similar to those of the Sili Mem- 
ber, consisting mainly of basalt or olivine basalt 
except for a few flows of feldspar-phyric basalt 
(Fig. 6, bottom ), picrite-basalt, and hawaiite. 
Macdonald (1944) described a hawaiite col- 
lected by Stearns from a talus block at the base 
of the sea cliff near Tauga Point on Ofu. This 
talus block must have come from the dense, 
thick aa flow near the top of the sea cliff, which 
represents the upper portion of the shield. The 
thick ankaramite and olivine basalt flows at 
TABLE 2 
Stratigraphic Section of the Sili Extra-caldera 
Member in the Cliff behind Tafalau, Olosega 
TOP 
THICKNESS 
(feet) 
Nonporphyritic, weathered vesicular pa- 
hoehoe flows 1-7 feet thick, dipping 
12°SE 
90 
Olivine basalt, moderately vesicular aa 
flows 5-7 feet thick with clinker beds 
3-5 feet thick, dipping 24°E 
30 
Olivine basalt with feldspar microlites 
forming a dense, massive flow 
15 
Nonporphyritic, vesicular aa flows 0.5- 
3 feet thick with clinker beds 0.5-7 
feet thick, dipping 24°E 
20 
Moderately vesicular aa flows of olivine 
basalt 4-7 feet thick, clinker beds 3-4 
feet thick, with much red cinder and 
ash, dipping 24°E 
35 
Brown palagonitized vitric crystal tuff 
with olivine and augite crystals, lying 
unconformably on lower flows, tuff 
dipping 24°E 
5 
Minor angular unconformity due to ero- 
sion 
Nonporphyritic basalt with feldspar mi- 
crolites forming dense aa flows 3-5 
feet thick, clinker beds 1 foot thick, 
dipping 30°E 
30 
Nonporphyritic, vesicular pahoehoe flows, 
0.5-4 feet thick, dipping 30°E includ- 
ing: 
ankaramite gradational to olivine- 
augite basalt 
40 
olivine basalt with rare augite 
phenocrysts 
10 
basalt containing small laths of 
feldspar phenocrysts randomly 
oriented and rare olivine pheno- 
crysts 
10 
Talus of blocks at base of cliff 
65 
Total thickness of section 
350 
Tumu Mountain, the summit of Ofu, are essen- 
tially horizontal and probably represent nearly 
the original summit of the A’ofa shield. 
Pyroclastic deposits are interbedded in the 
A’ofa extra-caldera member. On northwestern 
Ofu a palagonitized yellow lapilli tuff more 
than 4 feet thick, an unstratified tuff more than 
50 feet thick, a few thin red ash beds, and 
cinder in talus are present. Red cinder found in 
the soil 700 feet due north of Tumu Mountain 
is probably from an old post-caldera vent in that 
area, and may have been a source for the thick 
flows of the Nu’u Formation. A few intercalated 
