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PACIFIC SCIENCE, VoL XX, July 1966 
required only a correction for the heading error 
resulting from towing the polarizing head in a 
north-south direction 100 ft behind the plane. 
The average heading error varied from 30 to 
40 gammas. Because the normal magnetic field 
gradient was low, no significant error would 
have been introduced if this effect had been 
neglected. 
The total force magnetic field out to 150 miles 
north of Maui was surveyed by the EJ. S, Coast 
and Geodetic Survey ship "Surveyor,” whereas 
the remainder of the offshore areas adjacent to 
the islands were surveyed using the University 
of Hawaii RV "Teritu.” However, aeromag- 
netic profiles were also taken over the sea tracks 
of the "Surveyor” out to 50 miles north of 
Maui. Although the aircraft was flown at 8,000 
ft above sea level and the ship observations 
were made a few feet below sea level, no sig- 
nificant differences in values were observed be- 
tween the airborne and seaborne data. This lack 
of difference in values can be attributed to the 
great depth of the anomalous geologic bodies 
lying below the ocean floor which cause ocean 
magnetic anomalies. 
Possible Origin of Hawaiian Magnetic 
Anomalies 
Magnetic anomalies result from changes in 
the magnetic characteristics of rock masses 
which, in general, can be related to the percent- 
age of magnetite and ilmenite present. As these 
two minerals are present to some extent in most 
igneous rocks, the natural thing to expect in the 
magnetic study of an oceanic archipelago of 
volcanic origin, such as the Hawaiian Islands, is 
a composite anomaly pattern. The basic com- 
ponent would be that portion which can be 
related directly to the size and geometry of the 
volcanic mass rising from the sea floor and the 
strength and inclination of the earth’s magnetic 
field, and on this would be superimposed the 
effect of local variations in types of lava present 
and intrusions within the volcanic pile. 
Even a casual inspection of the regional mag- 
netic map (Fig. 1) shows that the island mass 
effect of the Hawaiian Islands is of such sec- 
ondary importance as to be lost in the over- 
riding magnetic effects originating from other 
geologic causes. This empirical observation is 
further substantiated by quantitative calculation, 
which indicates that only about a 6 0 gamma 
effect is to be expected for the island mass. 
Similarly, local variations in the lavas present 
do not appear to be too significant in terms of 
changes in the anomaly pattern. Although there 
may be petrologic significance in the somewhat 
smaller magnitude anomalies observed on the 
island of Hawaii as compared with other, older 
islands, this could also be the result of higher 
temperatures at depth in this island, which is the 
only one now characterized by active volcanism. 
Probable areas of abyssal intrusion defined by 
either surface fracture systems or volcanic centers 
of eruption are associated everywhere with the 
magnetic anomalies which occur mostly as 
dipole pairs. It is significant, though, that only 
primary central vent areas and rift (fracture) 
zones that were the source of the bulk of the 
volcanic rocks forming the islands are marked 
by magnetic anomalies. Secondary centers of 
eruption, such as Diamond Head on Oahu, are 
not defined magnetically. In connection with 
rift zone type of anomalies, Figure 1 shows 
that most of the rift zone type anomalies do not 
terminate at the physical boundaries of the 
islands on which they occur. Some extend for 
considerable distances into the adjacent oceanic 
area. This suggests that the rift zone type anom- 
alies may well be independent of the geology 
of the islands and are related to intrusions at 
depth in crustal fractures. Along these rifts, 
and locally at the intersections of crosscutting 
crustal fractures, magma penetrated to the ocean 
floor to initiate a series of seamounts that devel- 
oped into the Hawaiian Islands. Because each 
locus of magmatic intrusion (whether now de- 
fined geologically by a major volcanic mountain 
peak, such as Mauna Kea on Hawaii, a sub- 
merged seamount, a deeply dissected vent area 
recognizable only through its associated dike 
complex and boundary faults, such as the 
ancient Waianae caldera and the present day 
Koolau range on Oahu or the Molokai Fracture 
Zone on the ocean floor) requires a similar 
theoretical contrast in magnetic susceptibility, 
it is probable that the controlling lithology at 
depth is much the same in each case and repre- 
sents some differentiate of what originally was 
probably mantle material. This conclusion is 
based, in part, on depth analysis as the source 
of the magnetic anomalies as well as the seismic 
