Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
277 
(Arthur D. Raff, personal communication) had 
shown to be magnetically disturbed. These 
measurements were carried out on the "Sur- 
veyor.” 
The total magnetic intensity map of the area 
studied is shown in Figure 4. This map shows 
a striking convergence of anomalies and steep 
magnetic gradients immediately north of Maui. 
Farther north, beyond the nearshore anomalies 
which are associated with elements of the 
Molokai Fracture Zone, and north of latitude 
21°4l' the magnetic field over the Flawaiian 
Rise is smooth, with a uniform gradient of 6 
gammas per degree of latitude. Although mag- 
netic anomalies are observed north of 21°40' 
which are of the same wavelength (20 km or 
greater) as those observed south of this latitude, 
the amplitudes of these anomalies to the north 
do not exceed 100 gammas. 
Of local significance in this survey are the 
two distinct dipole anomalies associated with 
the Molokai Fracture Zone north of Maui. One 
lies 20 miles north of Maui, where a 1,300- 
gamma peak-to-peak anomaly occurs. The other 
lies 20 miles northwest of Maui, where a 1,200- 
gamma peak-to-peak anomaly occurs. 
By using depth and susceptibility contrast 
estimations coupled with two-dimensional model 
studies, the following geologic analyses were 
determined. The dipole anomaly centered at 
156°15'W and 21°10'N, and here named the 
"Flawaiian Deep magnetic anomaly,” appears 
to be caused by an intrusive body some 25 km 
wide and 65 km long, striking approximately 
east-west. The top of the anomalous body lies 
at a depth of about 8.5 km below sea level and 
it appears to extend vertically downward to a 
depth of 17.5 km. The rock associated with this 
body appears to have a susceptibility that is 
greater by 18.0 X 10 -3 cgs units than that of 
the surrounding crustal rock. As indicated, geo- 
graphically this anomaly is situated directly 
above the crustal downwarp and bathymetric low 
termed the Hawaiian Deep. It is also situated 
directly above a small bathymetric feature with- 
in the Hawaiian Deep that varies from 10-20 
km in width and in height from 600-1,600 m 
(see Figs. 5 and 6). It is significant that the 
area of the shallow "Moho” depth of 5.8-7 km 
recorded by Shor and Pollard (1964) and 
Western Geophysical Company (unpublished) 
lies on the western end of this anomaly and 
over the center of the disturbing body as defined 
by the point of inflection of the magnetic anom- 
aly (Fig. 7). Thus, there is a reasonable argu- 
ment that the anomalous Moho depth and the 
magnetic anomaly are related to the same cause. 
Considering the uncertainty in the depth anal- 
ysis of magnetic anomalies, and the fact that the 
induced upper pole may not correspond to the 
actual upper surface of the body, there is also 
reasonable agreement with the seismic depth of 
5.8-7 km and the magnetic depth determination 
of 8.5 km. As the anomaly is normally polar- 
ized, and the combined magnetic and seismic 
data show that the disturbing rock mass is not 
only more magnetic than the surrounding 
crustal rock but also extends well below normal 
mantle depths (12 km) and has a normal 
mantle velocity, it must represent an intrusion 
of mantle material into the crust and not repre- 
sent a crustal displacement as postulated by 
Shor and Pollard. 
Similarly, the normally polarized magnetic 
anomaly which is centered on 156°10'W and 
21°05'N appears to be related to an intrusion 
that is 35 km long and 24 km wide. This 
anomalous region strikes southwest and abuts 
against the island of Maui. Its top appears to be 
located at about 9.0 km below sea level. The 
rocks causing the anomaly appear to have a 
greater than the surrounding rocks by about 
11.0 X 10 -3 cgs units. 
As indicated earlier, no significant magnetic 
anomalies are observed, or are to be expected, 
in association with the Hawaiian Ridge itself. 
Similarly, the magnetic effect of bathymetric 
features at a depth of 13,000-18,000 ft below 
the plane of observation is observed to be negli- 
gible. All the observed magnetic anomalies ap- 
pear to have resulted from intrusive rock sources. 
The Regional and Residual Magnetic Field 
North of Maui 
The observed regional magnetic gradient 
north of Maui, as deduced from Figure 4, is 6 
gammas per minute of latitude; and that deter- 
mined south of the Hawaiian Islands, from data 
taken by the U. S. Naval Oceanographic Office 
(1962), is 5 gammas per minute of latitude. 
Removal of the regional magnetic field from 
the total intensity magnetic field of the ocean 
