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PACIFIC SCIENCE, Vol. XIX, July 1965 
1. Outpourings of tholeiitic basalts along a 
major rift zone. 
2. Volcanic activity subsides. 
3. Explosive volcanic activity increases with 
outpourings of andesitic basalts. 
4. Collapse and burial of the crater. 
5. Period of submergence followed by emer- 
gence. 
6. Renewed volcanic activity with small out- 
pourings of alkalic basalts to recent times. 
The Bouguer gravity anomaly map (Fig. 1) 
in general corroborates Stearns’ conclusions. As 
the Bouguer anomaly values vary from +233 
on the southeast to +293 near the center of 
the main caldera area, they are very similar to 
those found in Hawaii. Stearns advocated a 
single large caldera in the vicinity of Pago 
Pago, whereas the present gravity survey sug- 
gests there may be two separate areas of erup- 
tion within the area of this single large caldera. 
However, because of the ruggedness of the jun- 
gle terrain and lack of time, it was not possible 
to make a firm case for this hypothesis. The sec- 
ondary caldera proposed by Stearns near the 
eastern end of the island is not substantiated 
by the gravity data. However, it is possible that 
another caldera may occur offshore from the 
western end of the island. Well-developed rift 
zones trending N 60°E and S 60°W are readily 
apparent from the contoured Bouguer anomaly 
map. The gravity gradient averages 10 mgal per 
mile, southwest from the primary caldera, and 
increases to 20 mgal per mile near the coastline. 
A low anomalous area is indicated slightly south 
and east off the east end of the island, and an- 
other offshore low is suggested south of the west 
end of the island. However, their geological sig- 
nificance is not known. 
Ofu and Olosega 
Together these islands have an area of 3-5 sq 
miles. They have a bar-bell shape and are sep- 
arated by a narrow channel that can be waded 
during periods of low tide. The bathymetry 
falls off to the north and south, with shallow 
areas extending east and west of the islands. 
The highest point on Ofu is 1590 ft, and on 
Olosega, 2095 ft. 
According to McCoy (1965), the predomi- 
nant exposed rock types are alkalic basalts, and 
no trachytes are observed. Small exposures of 
more basic basalts, thought to be more recent 
in age, also outcrop. It is possible that tholei- 
itic basalts are present but, if so, they are effec- 
tively masked by more recent alkalic volcanics. 
Here, as on Tutuila, the gravity data substan- 
FlG. 1. Bouguer anomaly map of Tutuila I., American Samoa. 
