Geology of Manu’a Islands, Samoa — Stice and McCoy 
429 
the much higher (1,500-2,000 feet) curvilinear 
precipices on the north and south central shores 
suggest an origin in a double evisceration 
through (a) volcanic explosion, (b) faulting 
into a sink analogous to Kilauea or Mokuaweo- 
weo in Hawaii, or (c) large-scale landsliding 
due to foundering of large parts of the vol- 
cano. The first hypothesis he considered im- 
probable, but he could not decide whether the 
foundering was due to collapse both to the 
north and to the south, or to landsliding. 
(3) The lavas of Ta’u are relatively fresh, 
whereas deep weathering has laterized the flows 
on Ofu and Olosega. 
From only one or two days’ observations, 
Stearns (1944) produced a remarkably accurate 
geologic sketch map of the Manu’a Islands. He 
divided the rock units into pre-caldera volcanic 
deposits, a dike complex and post-caldera vol- 
canic deposits. In disagreeing with Daly’s state- 
ment that the Ofu-Olosega volcano was a cone 
of the explosive type, Stearns stated that pyro- 
clastic beds are no thicker or more numerous 
than around the main vents of many basaltic 
volcanoes. According to Stearns, the steeply 
dipping pre-caldera lavas of the Ofu-Olosega 
cone indicate that they plunged into deep water 
and mantled a steep-sided submarine cone, 
probably largely of pyroclastic material, the 
calderas being formed in part by collapse over 
a magma reservoir and in part by landslides of 
the steep underlying ash beds. He also sug- 
gested the possibility that the 2,000-foot cliff 
on the north side of Ta’u may be due to fault- 
ing related to another caldera offshore. 
Machesky (1965) conducted a gravity survey 
of the Manu’a Islands. The results of this work 
showed a Bouguer anomaly of up to — |— 290 
milligals over the center of the main caldera 
on Ta’u Island. The highest Bouguer anomaly 
on Ofu and Olosega islands (-(-310 milligals) 
was recorded over an intrusive gabbro plug at 
Fatuaga Point on eastern Ofu. Contouring of 
the corrected gravity anomalies shows a general 
concentric decrease away from these two centers. 
GEOLOGY OF TA’U ISLAND 
Nature and Distribution of Rock Types 
general statement: Lavas issuing from 
vents on the crest of the Manu’a Ridge gradu- 
ally built above sea level to form Ta’u Island. 
The history of the early shield-building stage 
is not revealed in the present exposures, but 
there was probably a long period of relatively 
quiet, frequent thin lava flows emanating from 
rift zones which steadily built up a basal shield 
volcano, as in the Hawaiian Islands. The lavas 
which built the main shield of Ta’u Island 
above sea level are exposed in the 1,400-foot 
escarpment that extends to the summit of the 
island, Lata Mountain (3,056 feet). The rocks 
comprising this shield, hereafter called the 
Lata shield, belong to the Lata formation. 
The summit of the shield collapsed to form 
a caldera, and somewhat more explosive erup- 
tions from cinder cones within the caldera and 
on the flanks of the volcano continued to build 
up the island. However, the eruptions were not 
as frequent during this stage, and erosion be- 
came more effective. There are local erosional 
unconformities in the lower sections exposed on 
Ta’u Island, indicating a long history of inter- 
mittent lava flows even before the formation of 
the caldera. 
Two smaller shields built out the northeast- 
ern and northwestern portions of the island. The 
Tunoa shield, on the northwest, is located along 
the regional rift zone. Luatele shield is part of 
a minor rift zone extending down the north- 
eastern slope of the main Lata shield. These 
smaller shields are composed mainly of thin- 
bedded olivine basalt pahoehoe flows with av- 
erage dips of 5-10°. The summits of both 
shields collapsed to form depressions which 
were partly filled by deposits of subsequent 
volcanism. Small pit craters are associated with 
both the Tunoa and the Luatele shields. The 
Tunoa and Luatele formations are composed 
of the rocks of their respective shields, including 
the deposits within the collapsed areas. 
After the formation of these shields vol- 
canism probably subsided considerably and ero- 
sion became more pronounced. The lava flows 
were so infrequent that a sea cliff about 200 feet 
high developed around Ta’u Island. Post-ero- 
sional lava flows occasionally spilled over this 
cliff from cones on the flanks above it; in two 
places, extensive post-erosional volcanism built 
large areas of land in front of the sea cliff. 
On the northwest corner of the island the 
Faleasao Formation consists of a tuff complex, 
approximately 1 mi 2 in area, which buried the 
