Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
303 
Fig. 29- Total force magnetic map of the island of Oahu, based on aeromagnetic profiles flown at 10,000 
ft. Contour interval at 50 gammas. 
the Koolau and Kailua series were erupted from 
the Koolau Volcano, and the Kailua series rep- 
resents a hydrothermally altered intra-caldera 
group. Dikes are very common in the Kailua 
series, which occupies the Koolau caldera, and 
form a complex with younger dikes intruding 
into older ones. Many of the dike breccias and 
flows in the Koolau caldera are hydrothermally 
altered. It is believed that rocks of both the 
Kailua and Koolau volcanic series were erupted 
from the fissure zones of the Koolau Volcano. 
Fissure eruptions also characterized the building 
up of the Koolau volcanic shield. As in the 
Waianae area, the Koolau Primary Rift Zone 
anomaly (Fig. 28) coincides with the rift and 
dike zones of the Koolau Volcano, and, as with 
the Waianae Primary Rift Zone, is not confined 
by the shores of the island of Oahu. 
magnetic relations: The magnetic field of 
the island of Oahu (Fig. 29) is relatively sim- 
ple. There are two primary rift zone anomalies, 
the Koolau and the Waianae, on each of which 
is located a large and distinct volcanic vent 
zone anomaly. These correlate with the Waianae 
and Koolau volcanic calderas. Both of these two 
volcanic centers are marked by distinct positive 
gravity anomalies (Woollard, 1951; Strange, 
1964) . However, the Koolau caldera, which is 
marked by a large amplitude 1,200-gamma 
peak-to-peak magnetic anomaly, is inversely 
polarized, whereas the Waianae caldera is 
marked by a 650-gamma, normally-polarized 
magnetic anomaly. The Koolau caldera has 
also been studied by seismic measurements 
(Adams and Furumoto, 1965 ; Furumoto et ah, 
1965) , which show high velocity rock (7.5 
km/sec) at a depth of only 1600 m. 
Inasmuch as the Koolau caldera marks not 
only the site of one of the largest magnetic 
anomalies observed so far over the Hawaiian 
