302 
PACIFIC SCIENCE, VoL XX, July 1966 
North Lanai Volcanic Vent Zone are small in 
amplitude, 25 gammas for the former and 50 
gammas for the latter. These two volcanic vent 
zones also probably represent shallow sources 
of volcanic activity. Geologically, the West 
Lanai Volcanic Zone is located in the area of 
the Southwest Rift Zone of Stearns (1940). 
Island of Oahu 
GEOLOGY AND GEOLOGIC STRUCTURE: Oahll 
was built by lavas erupted from two centers — 
the Waianae Volcano and the Koolau Volcano. 
Three groups of lavas form the Waianae vol- 
canic range. The older lavas, probably of late 
Tertiary age, appear to be largely pahoehoe 
basalts, while the late stage eruptions produced 
large cinder cones and some alkalic basalts. The 
Waianae Volcano, like other Hawaiian vol- 
canoes, produced only small amounts of ash, 
and the lavas were extruded both from a central 
vent and from fissures. Dikes and rifts are 
numerous in the Waianae caldera and range in 
thickness from a few inches to several feet. 
Stearns (in Stearns and Vaksvik, 1935), after 
a study of the rift zones on Kilauea and Mauna 
Loa and in the Waianae caldera, noted that in 
almost all cases of rifting in the Hawaiian vol- 
canoes the magma is confined to fissure zones 
that rise from the magma reservoir to the sur- 
face. Concentration of dike rocks in certain 
zones such as these probably could produce the 
elongate magnetic trends observed over such 
rift zones. Three dike systems have been 
mapped by Stearns (1939), and it will be seen 
that all of these rift zones lie within the boun- 
daries of the Waianae Primary Rift Zone anom- 
aly (Fig. 28). Furthermore, judging from the 
magnetic trend map of the seaward magnetic 
anomalies (Fig. 9), the Waianae Primary Rift 
Zone extends offshore west and south of Oahu. 
The Koolau volcanic range is composed of 
the Koolau, Kailua, and Honolulu series. Both 
