Geology of Manu’a Islands, Samoa- — Stice and McCoy 
441 
total relief of about 12,000 feet. The caldera 
apparently has no southern rim, but seaward 
from it is a huge depression or "valley” that 
extends to more than 8,000 feet below sea 
level. The numerous small faults in the two 
large, seaward-sloping, downfaulted blocks at 
each side of the caldera could have resulted 
from tension produced by gravity collapse on 
the southern slope of the volcano (Figs. 3 and 
5 ) . On the north coast, however, the dips of the 
subaerial lavas are about the same as the 
bathymetric slope, and so there is no need to 
postulate extensive slope failure there. 
The sea cliff along the south coast may be a 
fault line scarp. The dikes, described above, 
and the numerous faults parallel to the coast- 
line, support this conclusion. Perhaps the area 
of gravity collapse (Fig. 5) contains a series of 
steep faults below sea level. Near the mouth of 
Laufuti Stream there are two small normal 
faults which offset some of the dikes in the dike 
complex; these dikes strike N 60° E and dip 
39° S with a 5 -foot vertical displacement. 
It is possible that the Laufuti dike complex, 
the dikes exposed in the cliff between Papao- 
toma and Si’ufa’alele points, and the vent at 
Lotoaise Point are related to normal faulting 
due to gravity collapse, rather than being as- 
sociated with rift zones or the caldera of the 
Lata shield. A gravity collapse of such mag- 
nitude could force magma from its chamber to 
the surface via avenues formed by tensional 
faulting and fracturing of the collapsed area. 
If this gravity collapse did occur, it probably 
was not a single short event, but took place 
slowly over a long period of time, perhaps even 
continuing at the present time. Some residents 
of Ta’u state that they feel earthquakes every 
few years. 
The crescent-shaped cliff bounding the Tunoa 
depression (Fig. 4, middle ) suggests an original 
circular depression typical of calderas. If the 
escarpment is projected seaward the diameter of 
the depression is approximately 1.5 miles. The 
escarpment varies from 200 to 300 feet in 
height and has an average slope of about 34°. 
The southern end of the scarp merges with the 
slope of the Lata shield, late flows of which 
apparently have buried the southern slope of 
the Tunoa shield. No indication of the sea- 
ward prolongation of the escarpment was found 
in the sea cliff south of Ta’u Village, where it 
might be expected to occur, nor is the northern 
seaward extension of the escarpment exposed, 
because the sea cliff there has subsequently been 
buried by eruptions of the To’a tuff cone. 
Gravity measurements (Machesky, 1965) show 
no anomalous high such as is commonly asso- 
ciated with Hawaiian calderas (Strange, Wool- 
lard, and Rose, 1965). Nevertheless, the atti- 
tudes of the beds both within and outside the 
depression are evidence for partial filling of the 
depression formed by shield collapse. Cinder 
cones at the top and base of the escarpment, as 
well as the pit crater at Fogapo’a, indicate that 
faulting associated with the collapse of the 
Tunoa shield provided avenues along which 
magma was forced to the surface. 
Fig. 5. Schematic diagram illustrating "gravity collapse” and possible associated volcanic activity. 
