Magnetic Surveys Over the Hawaiian Islands and 
Their Geologic Implications 1 
Alexander Malahoff and G. P. Woollard 
ABSTRACT: A geophysical and geological analysis is made of a total field mag- 
netic survey of the major islands of Hawaii. It is established that the regional distor- 
tion of the earth’s normal magnetic field due to the topographic mass of the Hawaiian 
Ridge rising in places to over 30,000 ft above the ocean floor seldom exceeds 150 
gammas. On each island, local magnetic anomalies having the form of lenticular and 
circular dipoles are found. The lenticular dipole anomalies appear to be related to 
crustal rifts that have been invaded by magmatic material of mantle origin, and the 
circular dipole anomalies are associated with primary areas of volcanic eruption. 
Although the inferred crustal rifts have surface geologic expression in some areas, 
such as the Koolau Mountains on Oahu, for the most part they do not. Furthermore, 
offshore magnetic data indicate that these features extend beyond the islands and out 
into the adjacent, deep-water, oceanic area where they can be traced for 100 miles 
or more. The most pronounced of these features is associated with the ocean floor 
Molokai Fracture Zone, which magnetically extends across the Hawaiian Ridge 
without interruption for an unknown distance to the west. The circular dipole 
anomalies appear to represent the effect of intrusions in volcanic pipes or vents 
rising from these crustal rifts which strike essentially east-west on the islands of 
Hawaii, Lanai, Maui, and Molokai, and west northwest— east southeast on Oahu, 
Kauai, and Niihau. With two exceptions, all of the anomalies indicate normal 
polarization conformable with the earth’s present field. 
During the year 1964, the authors carried out 
the first of a series of planned magnetic surveys 
over the Hawaiian Ridge and adjacent oceanic 
area. The area covered extends from the island 
of Kauai on the north to the island of Hawaii 
on the south. 
In this present paper, the magnetic results 
are examined on both a qualitative and a quan- 
titative basis as to their relation to the centers 
of volcanism which built the Hawaiian Ridge 
and to the primary geologic tectonic trends hav- 
ing surface expression or bathymetric expres- 
sion on the ocean floor. As will be shown, 
good correlations exist between the pattern of 
magnetic anomaly values and the volcanic fea- 
tures of the islands as well as the oceanic rifts 
having bathymetric expression. In order to 
minimize the magnetic effects of local changes 
in geology, soils, and the terrain associated with 
1 Hawaii Institute of Geophysics, Contribution No. 
148. Manuscript received July 20, 1965. 
mountains such as Mauna Kea (elevation 
13,796 ft), the magnetic profiles were flown at 
least 2,000 ft above the ground surface. Al- 
though a complex pattern of magnetic anom- 
alies is obtained because of the low magnetic 
latitude of Hawaii, the interpretation is straight- 
forward. Depth and size estimations were 
based on the interpretive procedures of Vacquier 
et al. (1951) as well as on the basis of mag- 
netic susceptibility-remanence measurements. 
These results were then compared with those 
determined from other geophysical measure- 
ments and the geologic probability of the anom- 
alous bodies assessed. Finally, the magnetic 
effects of the derived geologic bodies were 
computed, using a two-dimensional, high-speed 
computer program and the derived theoretical 
profiles compared with those observed. All the 
profiles used in these comparisons were cor- 
rected for terrain. Because the regional mag- 
netic gradient at the low magnetic latitude of 
Hawaii does not exceed 6 gammas per mile, it 
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