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PACIFIC SCIENCE, VoL XXII, October 1968 
in the cliff above. The To’aga composite cone is 
formed of interbedded aa flows, pahoehoe flows, 
cinder, and tuff. In the cliff, the exposures are 
disappointingly few due to the dense vegetation, 
but an ash bed representing the former surface 
of the To’aga cone is seen to be overlain by on- 
lapping lavas from A’ofa shield. 
Exposed in the cliff behind Va’oto on Ofu is 
a bed of red ash up to 20 feet thick. It extends 
up the slope of the ridge to Lepu’a. A lower 
impermeable tuff bed provides a spring in the 
cliff behind Va’oto that is utilized for drinking 
water in Ofu Village. If the thick red ash bed 
in this area is the same as the uppermost bed of 
the composite cone exposed in the cliff behind 
To’aga, then this cone must have extended at 
least to Va’oto, where west-dipping beds still 
occur, giving a minimum diameter at sea level 
of 1.5 miles. The summit of the cone was 
probably located about 0.4 mile due east of 
Tumu Mountain at approximately 700 feet 
above sea level. 
Another larger cone of at least the same 
height was centered offshore between Ofu and 
Olosega, about 1,500 feet due east of Fatuaga 
Point, where a related gabbroic intrusive plug 
is exposed. A tuff bed in the cliff on the north 
side of Olosega Village apparently marks the 
eastern surface slope of this cone, giving it a 
minimum diameter at sea level of 1.8 miles. The 
eastern portion of the cone was buried by aa 
flows of the Sili shield. A large percentage of 
the total volume of the cone is explosion breccia 
consisting almost entirely of fist-sized blocks of 
at least six distinct lithologies — vesicular pa- 
hoehoe basalt, ankaramite, dense dike rock, 
olivine basalt, and two types of aphanitic basalt. 
The matrix, comprising less than 5 per cent of 
the total rock, is composed of very well indu- 
rated vitric ash that contains a few fragments 
of olivine crystals 1-2 mm in diameter. The 
cone is probably the oldest feature now exposed 
in the Manu’a Islands. 
Near Tauga Point, at the western end of 
Samo’i beach on Ofu, an old tuff cone has been 
covered by a series of about six aa flows of the 
A’ofa shield. The northern half of the tuff cone 
has been eroded away by wave action, and a 
horizontal flow of basalt that was ponded within 
its crater now forms a bench about 15 feet above 
sea level. A 10-foot-thick yellow lapilli tuff bed, 
dipping 20° away from the center of the cone, 
pinches out 500 feet to the east and overlies a 
-[-5 0-foot-thick unstratified, palagonitized tuff. 
An adjacent but slightly later small cinder 
cone at Tauga Point has also been buried by 
later aa flows from the A’ofa shield. Numerous 
ribbon and spindle bombs occur in this cone. 
At Maga Point, on the southern tip of 
Olosega, another old tuff cone has been buried 
by later flows of the Sili shield. The northern 
slope of this cone is 34° ; its summit rose at 
least 250 feet above the present sea level. The 
lower beds of this cone are lapilli tuff with 
horizons locally rich in basalt blocks up to 6 
inches across, and the upper 50 feet is com- 
posed of cinder and red ash. A dense flow of 
hawaiite, 35 feet thick, was ponded within the 
crater of the cone. A series of thin pahoehoe 
flows of the Sili shield has overridden the cone 
and now forms Maga Point. This sequence is 
overlain by interbedded aa and pahoehoe flows 
from the Sili shield, with aa flows becoming 
increasingly dominant up-section. 
The cones at To’aga and Fatuaga definitely 
pre-date the two shields. The cones at Tauga, 
Samo’i, and Maga are all overlain by lava flows 
from the shields — but they could be parasitic 
cones on the flanks of the shields. However, 
since no evidence was found to suggest that the 
cones were underlain by lava flows from either 
of the shields, they are considered to pre-date 
the shields and are included as part of the Asaga 
Formation. 
TUAFANUA FORMATION: The Tuafanua For- 
mation comprises the two shields centered at 
Sili and A’ofa, which buried the older cones of 
the Asaga Formation. Tuafanua is the area on 
the north shore of Ofu near the intersection of 
the coalescing shields. The pre-caldera lavas of 
the A’ofa shield are predominantly thin-bedded 
pahoehoe flows with many interbedded aa flows 
and occasional thin beds of ash and tuff. 
The Sili extra-caldera member on Olosega 
also consists mainly of olivine basalt. The 
lowermost exposure of the pre-caldera lavas 
from the Sili shield is found near Leaumasili 
Point, on the northern tip of the island. A se- 
quence of pahoehoe flows (1-2 feet thick), 
with a few interbedded aa flows up to 10 feet 
thick, are cut by several dikes 1-4 feet thick 
