430 
PACIFIC SCIENCE, Vol. XXII, October 1968 
sea cliff. On northeastern Ta’u the historic vil- 
lage of Fitiiuta is located on post-erosional lava 
flows comprising the Fitiiuta Formation, which 
have built out a platform of nearly 1 mi 2 sea- 
ward of the old sea cliff. Even though the lavas 
in this area appear to be quite young, they prob- 
ably were erupted at least 1,500-2,000 years 
ago, prior to settlement by the Polynesians, be- 
cause, according to Samoan legends, Fitiiuta 
was the first village to be settled in these islands 
and possibly in all of Samoa. There are no 
Samoan legends that mention volcanic eruptions 
on Ta’u Island. 
Thus, the rocks exposed on Ta’u Island can be 
placed in the following units, in approximate 
order of decreasing age (see legend for Ta’u 
Island, page 434) : 
( 1 ) Lata Formation 
(a) extra-caldera member, consisting of 
pre-caldera and post-caldera deposits of the 
Lata shield, the latter including both pre-ero- 
sional and post-erosional volcanism 
(b) intra-caldera member 
(2) Tunoa Formation 
(3) Luatele Formation 
(4) Faleasao Formation 
(5) Fitiiuta Formation 
(6) Intrusive rocks, mainly basaltic dikes 
associated with Lata caldera 
(7) Sedimentary deposits, including allu- 
vium, landslide debris, beaches, marshes, and 
so forth 
EXTRA-CALDERA MEMBER OF LATA FORMA- 
TION: The lava flows and pyroclastic deposits 
associated with the building of the Lata shield 
were erupted prior to, during, and after collapse 
of its caldera. Those rocks not deposited within 
the caldera itself belong to the extra-caldera 
member of Lata Formation. Pre-caldera rocks are 
exposed in the high fault scarp on the southern 
part of the island. The base of the volcano is 
about 9,000 feet below sea level, giving a total 
vertical thickness of about 12,000 feet. Be- 
cause of the dense vegetation on the flanks of 
the shield, it is impossible to distinguish pre- 
caldera lavas from the post-caldera lavas which 
form most of the surface of the Lata shield. 
However, the rocks cut by deep valleys on the 
north shore are certainly pre-caldera lavas, al- 
though late flows may have filled the floors in 
a few of these valleys at lower elevations. There- 
fore pre-caldera lavas, since they cannot be 
distinguished from post-caldera lavas, are not 
mapped separately except in a few areas where 
exposures are adequate. 
Some of the post-caldera vents are shown on 
the geologic map (Fig. 2) ; undoubtedly there 
are many others that were not discovered in the 
dense jungle. Lava flows associated with these 
vents are extremely difficult to delineate. The 
youthful appearance of numerous post-caldera 
cinder cones and the fact that many flows are 
found spilling over the sea cliffs indicates that 
the post-caldera volcanism was in part contem- 
poraneous with the post-erosional volcanism at 
Fitiiuta and at Faleasao. Judging from the con- 
sistent height of the sea cliff surrounding the 
entire island, there must have been an extensive 
period of volcanic quiescence after the forma- 
tion of the Tunoa and the Luatele shields. 
Therefore, post-caldera volcanism was more 
active prior to the formation of the sea cliff 
and again after its development. Lavas from the 
post-caldera cones on the Lata shield flowed 
over the sea cliff. Few can be traced to their j 
source, and no major erosional unconformity 
like that represented by the lava-mantled sea 
cliff can be found to separate pre-erosional lavas 
from post-erosional deposits on the upper flanks 
of the Lata shield. Wherever possible, post- 
caldera cones and lava flows, whether pre-ero- 
sional or post-erosional, are mapped separately j 
from the rest of the post-caldera rocks. Lavas 
that have flowed over the sea cliff, as well as the 
vents from which they were erupted, are ob- i 
viously post-erosional. 
The contact between the extra-caldera rocks 
of the Lata shield and those of the Tunoa I 
shield (shown on Fig. 3) is based on the topo- 
graphic expression of the two shields, because 
there is no petrographic distinction between 
rock types. The contact between post-caldera 
rocks of the Lata shield and lavas of the Luatele j 
shield is also based on topography. Even though 
the Luatele lavas are quite characteristic, soil 
cover and dense jungle obscure outcrops along ; 
most of the contact. 
The extra-caldera lavas are dominantly olivine 
basalt with lesser amounts of picrite-basalt and 
feldspar-phyric basalt at low elevations. Aa and 
pahoehoe flows are interbedded, with aa flows 
