444 
PACIFIC SCIENCE, Vol. XXII, October 1968 
in the cliff behind To’aga on southeastern Ofu, 
an explosion breccia cone with an associated 
intrusive plug at Fatuaga Point on eastern Ofu, 
and another tuff cone at Maga Point on the 
southern tip of Olosega. Rocks of the older 
cones comprise the Asaga Formation, and those 
forming the two coalescing shields belong to 
the Tuafanua Formation. 
The subaerial part of the islands consists pre- 
dominantly of lava flows of the two shields. 
Deeply lateritized flows on southwestern Ofu 
dip south westward away from a volcanic center 
that lay just north of the present northern shore- 
line. The summit of the shield collapsed to form 
a caldera, the fault boundaries of which are 
exposed on the north coast, and the boundaries 
extend inland as a steep crescentic escarpment 
within which nearly horizontal lava flows form 
a gently sloping platform known as A’ofa. Ex- 
tended seaward, the caldera boundaries form a 
crude circle with a diameter of about 1 mile. 
The caldera is hereinafter referred to as the 
A’ofa caldera, and the surrounding shield as 
the A’ofa shield. Vertical dikes in the sea cliff 
behind Samo’i parallel the boundary of the 
caldera and are probably related structurally to 
the caldera collapse. 
The presence of a second shield is suggested 
by the dips of lava flows on Olosega, and by 
numerous dikes striking northeast and dipping 
northwestward in the high cliff behind Sili 
Village (Fig. 6, top and middle). The strike of 
the dikes gradually changes from east-northeast 
near the western tip of Olosega to nearly north 
at the northern tip. The center seems to have 
been beneath the ocean northwest of Sili, and 
the shield will be referred to as the Sili shield. 
Whether or not a caldera existed in this shield 
is uncertain, but a suggestion of one is seen in 
the submarine topography (Fig. 7). However, 
no definite evidence was found to establish the 
presence of two shields, either from geologic 
field mapping or from gravity measurements 
(Machesky, 1965). Because the presence of two 
smaller eruptive centers seems more likely, the 
shield-building lavas on Ofu are tentatively 
mapped separately from those on Olosega. 
The A’ofa caldera was partly filled with thick 
olivine basalt, hawaiite, and ankaramite flows. 
Ponded lavas buried a small intra-caldera cinder 
cone at the mouth of Sinapoto Stream. Some of 
the parasitic cones on the flanks of the volcano, 
as well as some of the uppermost thick an- 
karamite flows, may be post-caldera deposits. 
Local erosional unconformities stratigraphi- 
cally low in the pre-caldera section at Tafalau 
on eastern Olosega indicate a period of decline 
of volcanic activity. Some of the lavas in this 
area lie unconformably over weathered lava 
flows and ash beds, dipping as steeply as 24°. A 
later, more extensive period of quiescence per- 
mitted the carving of deep valleys and the 
formation around the islands of a sea cliff about 
300 feet high. Following the formation of the 
sea cliff, two or three thick hawaiite lavas flowed 
down old valleys on the southwest side of Ofu 
and formed Nu’upule Rock and the ridges be- 
hind Ofu Village. Nu’utele and Nu’usilaelae 
islets, off the west end of Ofu, are remnants of 
a Recent tuff cone. The Nu’u Formation consists 
of these post-erosional rocks, which were de- 
posited after the formation of the sea cliff. High 
cliffs truncate the lava flows on southeastern 
Ofu and southwestern Olosega, and there may 
have been foundering along these coasts similar 
to that suggested for Ta’u Island. 
Thus, the rocks exposed on Ofu and Olosega 
islands can be placed in the following units, 
in approximate order of decreasing age (see 
legend for Ofu and Olosega islands, page 454). 
(1) Asaga Formation 
(2) Tuafanua Formation 
(a) A’ofa extra-caldera member 
(b) A’ofa intra-caldera member 
(c) Sili Member 
(3) Intrusive rocks, mainly dikes associated 
with the Tuafanua Formation and the plug asso- 
ciated with the Fatuaga breccia cone 
(4) Nu’u Formation 
(5) Sedimentary deposits, including allu- 
vium, landslide debris, beaches, marshes, etc. 
asaga formation: Nearly all of the cones 
included in this formation can be seen from 
Asaga Strait, which separates Ofu and Olosega 
islands. The To’aga composite cone and the 
Fatuaga breccia cone, probably the oldest fea- 
tures exposed on Ofu and Olosega, can be seen 
only in the high, nearly vertical cliffs on eastern 
Ofu. No outcrop could be examined closely, but 
numerous talus blocks scattered along the shore 
give some indication of the rock types present 
