Geology of Manu’a Islands, Samoa — S tice and McCoy 
437 
two and possibly three normal faults formed 
major benches on the caldera floor. The highest 
bench, at the base of the fault scarp that rises 
vertically 1,370 feet to Lata Mountain, is com- 
posed of a sequence of approximately horizontal 
pahoehoe flows. Intercalated with them is a bed 
of ash 3-4 feet thick which is composed of indi- 
vidual laminae less than 0.5 inch thick. The ash 
is basaltic glass. The flows both overlying and 
underlying the ash are 1-4 feet thick and are 
mostly vesicular olivine basalts with some 
oceanites. The vesicles in several surface flows 
of the caldera floor are filled with limonite- 
stained clay. These vesicle fillings may be the 
result of alteration and deposition by rising 
gases and hot solutions in the vent area of the 
volcano, or simply the result of ordinary weath- 
ering and poor drainage of nearly horizontal 
flows, although in other volcanic rocks in the 
Manu’a Islands, the vesicles are not filled. 
tunoa formation: The small shield on 
northwestern Ta’u was built predominantly by 
thin-bedded pahoehoe flows and less abundant 
interbedded aa flows with average dips of less 
than 10°. The summit of this shield collapsed, 
and the central depression was partly filled by 
ponded lava flows and pyroclastic deposits. 
Subsequently, the western portion of the shield 
has been eroded away so that it is nearly bisected 
by a sea cliff. The formation is named after 
Tunoa Ridge, a curvilinear escarpment approx- 
imately 200 feet high which forms the eastern 
rim of the collapsed area. 
The shield-building lava flows are mainly 
aphanitic basalt with some flows of olivine 
basalt and at least one 10-foot-thick flow of 
oceanite, but they are too deeply weathered to 
reveal much detail in composition and structure. 
The floor of the depression, on the other hand, 
is covered by pahoehoe flows with well-pre- 
served ropy surfaces and tumuli, and by some 
aa flows still fresh in appearance. Most of these 
lavas are vesicular olivine basalt with feldspar 
microphenocrysts less than 1 mm long. At least 
five vents were located on the floor of the 
depression; undoubtedly there are others, but 
the dense vegetation makes their discovery 
more or less accidental. The vents are low cin- 
der cones less than 30 feet high and about 200 
feet in diameter. Some of the flows from these 
cinder cones contain small dunite xenoliths, 
which are usually less than 1 cm across. Two 10- 
foot-thick pahoehoe flows containing dunite 
xenoliths are exposed in the sea cliff behind 
Ta’u Village. Underlying those lavas is a pala- 
gonitized tuff bed of unknown thickness. 
The southern half of the cinder cone that 
stands on the northern edge of the escarpment 
near Tulatula is missing. Either half of the cone 
has slumped down the cliff during or after its 
formation, or it has been eroded back at about 
the same rate as the fault scarp. The former 
explanation seems more likely, because the cin- 
der cone should erode much more rapidly than 
the lava flows exposed beneath it. The western 
wall of the pit crater at Fogapo’a has either been 
eroded away or been cut off by faulting during 
the collapse of the shield. Because the floor of 
the pit crater is at the same elevation as that of 
the Tunoa depression, it was probably filled in 
by later lavas ponding within this larger col- 
lapsed area. 
luatele formation : As at Tunoa, a secon- 
dary shield on the northeast side of the island 
has collapsed to form a depression known to 
the Samoans as Luatele. On the topographic 
map of the Manu’a Islands, published in 1963 
by the U. S. Geological Survey, this place is 
erroneously called "Judd’s Crater,” but no one 
on the island knows it by that name. There- 
fore, this depression will be referred to as 
Luatele. The lava flows and pyroclastic de- 
posits that form this shield comprise the Luatele 
formation. 
The Luatele shield is made up almost en- 
tirely of thin-bedded pahoehoe flows of vesi- 
cular olivine basalt containing olivine pheno- 
crysts up to 4 mm in diameter. Only one dunite 
xenolith about 0.5 inch across was found in the 
lavas. The flows of this shield vary in thickness 
from less than a foot to 3 or 4 feet. The dip 
ranges from 3-4° near the summit to 6-8° 
farther down the flanks. 
The collapsed area is only about 0.3 mile in 
diameter, and at its deepest point is 400 feet 
deep. The depression has been partly filled 
with ponded lavas, but the nature and thick- 
ness of these deposits were difficult to determine 
due to poor exposures. Less than 500 feet north- 
east of Luatele is a small pit crater, Lualaitiiti, 
