Geology of Manu’a Islands, Samoa — Stice and McCoy 
455 
downslope and seems to have ponded against 
an obstruction, perhaps a buried cone beneath 
Le’ala Point or another cone offshore from there 
along the crest of the Samoan Ridge. Offshore 
soundings in that area are sparse but do sug- 
gest the presence of a cone as shown in cross 
section B-B' on Figure 8. 
Pyroclastic deposits are concentrated on Ofu 
and Olosega within a zone between sea level 
and an altitude of about 500 feet. Phreatomag- 
matic explosions commonly produce pyroclastic 
cones near sea level; these pyroclastic deposits 
represent some of the oldest rocks exposed on 
Ofu and Olosega, suggesting little if any subsi- 
dence of these islands since their formation. If 
a caldera existed in the Sili shield, its floor may 
never have extended above sea level. The north- 
ern boundary may always have been much lower 
than the southern rim, leaving the southern 
wall exposed to wave attack. The dike complex 
of eastern Ofu may have been related to the 
collapse of the Sili shield, or it may be an 
expression of volcanism along the regional rift 
zone of the Samoan Ridge. 
In the high cliff behind To’aga, occasional 
dikes can be seen trending approximately paral- 
lel to the cliff face. Several large normal (?) 
faults can be seen in this cliff, and also in the 
cliff along the northern coast of Ofu near 
Oneonetele. The bedding in the cones has suf- 
fered large displacements, but there is no surface 
expression of the faults, nor could the direction 
or amount of their displacement be measured. 
These faults may have been related to the col- 
lapse that formed the A’ofa caldera. 
Less than one-half of the A’ofa caldera is 
now present above sea level. Offshore sound- 
ings (Fig. 7) have not been made in sufficient 
detail to determine whether the northern half 
has simply been eroded away or whether foun- 
dering occurred. Small faults downthrown to 
the north within the A’ofa caldera indicate that 
at least minor faulting has been involved. 
Soundings are not complete enough to indi- 
cate the nature of the ocean bottom in the huge 
embayment between southeast Ofu and south- 
west Olosega (Fig. 6, top). As Daly (1924) 
suggested, some type of foundering probably 
occurred in this area, but there is no evidence 
for another caldera. Perhaps gravity collapse 
similar to that suggested for Ta’u is responsible 
for the formation of the high cliffs. 
Geomorphology 
streams and valleys: The stream valleys 
on southwestern Ofu and southeastern Olosega 
extend away from the former summits of the 
shields in a radial drainage pattern. Streams 
within the A’ofa caldera drain the intra-caldera 
area and empty into the sea along the cliffed 
north coast. Because these islands are both 
lower in elevation and smaller in area than 
Ta’u, there is considerably less rainfall and 
resultant runoff. Therefore, the streams are 
neither as large nor as numerous as those on 
Ta’u. All streams on Ofu and Olosega are inter- 
mittent, flowing only after a downpour. The 
stream valleys are all youthful and nowhere 
exceed 50 feet in depth. 
beaches and coasts : After cessation of vol- 
canic activity on Ofu and Olosega an extensive 
cliff 200-400 feet high was carved into the 
island by the sea. Behind Ofu Village the cliff 
is only about 80 feet high due to protection 
from wave attack afforded by the tuff cone off- 
shore. The much higher cliffs along the north- 
ern and southern coastlines originated by fault- 
ing and/or foundering but have certainly been 
modified by marine erosion. The sea cliffs are 
usually slightly higher than those on Ta’u, per- 
haps because a longer period of time has elapsed 
since there was frequent volcanic activity, or 
because a larger amount of more easily eroded 
pyroclastic material was deposited near sea level 
on Ofu and Olosega. 
Landsliding and subsequent removal of the 
talus by wave action are also important erosional 
processes, especially along the high cliffs of the 
northern and southern coasts. The numerous 
fan-shaped landslides and fresh scars in the 
cliffs above indicate that this process is opera- 
tive at the present time. A landslide at Pouono, 
the southern part of Olosega Village, is so 
fresh in appearance that it probably occurred 
since the area was inhabited by the Samoans. 
Individual blocks also work loose from the 
cliff face and fall, forming talus slopes that 
extend almost continuously around the islands 
at the base of these cliffs. The reef flats are lit- 
