Gravity Relations in Samoa and Society Is— Machesky 
369 
Fig. 2. Bouguer anomaly map of Ofu and Olosega Is., American Samoa. 
date the geologic information. The gravimetric 
center of the caldera lying between the two 
islands ( Fig. 2 ) is slightly east of the geologic 
center. The major portion of the structure lies 
offshore to the north of the islands. The Bou- 
guer anomaly contours directly parallel the 
major rift zones which trend in southwest and 
southeast directions. The Bouguer anomaly 
values range from +260 on the east to +312 
mgal in the center of the caldera area. A low 
anomalous area is suggested off the west end of 
Ofu Island. The maximum observed gradient is 
20 mgal per mile. Thus, there is evidence that 
the island platform mass is tholeiitic basalt and 
that the feeder pipe contains ultrabasic rock. 
Tau 
Tau has an area of 15 sq miles, and the high- 
est point of 3056 ft is found near the islands 
center. The water depth increases to the north 
and south, with a ridge area extending east and 
west from the land mass. Fringing reefs sur- 
round the island. The topography, as on Tutuila, 
and on Ofu and Olosega, is rugged with pre- 
cipitous cliffs occurring on the north sides 
whereas the southern slopes are usually more 
gentle. Dense jungle vegetation covers most of 
the interior, making overland traverses ex- 
tremely difficult. The geology according to Gary 
Stice and Floyd McCoy (personal communica- 
tion) is similar to that observed on Ofu and 
Olosega. As seen from the northwest, Tau pre- 
sents a perfect outline of a shield-type volcano. 
The Bouguer gravity anomaly map (Fig. 3) 
depicts a single closure located in the south 
central portion of the island. The anomaly val- 
ues vary from +250 to +290 mgal. The anom- 
aly pattern is roughly circular in shape and its 
center coincides with the caldera which has rift 
zones extending to the northeast and the north- 
west from its center. The maximum anomaly 
gradient of 23 mgal per mile occurs to the 
north. The gravimetric center of the caldera was 
not reached, but the maximum anomaly value 
would appear to be near + 300 mgal Although 
the geologic indications are that the center of 
the caldera occurs offshore a short way south 
of the center of the island, the gravity data sug- 
gest that the intrusive center lies directly north- 
ward toward the land mass. As on the other 
islands studied in the Samoan group, the grav- 
ity data indicated that the island platform is 
