The Paleomagnetic Significance of Aeromagnetic Surveys 
of the Hawaiian Islands 1 
Alexander Malahoff and William E. Strange 
ABSTRACT : Aeromagnetic surveys of the Hawaiian Islands have revealed that the 
primary magnetic anomalies associated with the islands are dipole anomalies caused 
by the intrusive rocks of the volcanic centers and rift zones. Comparisons of the 
direction of magnetization indicated by the dipole anomalies with results of lab- 
oratory measurements on lavas show that in many cases the lavas possess reverse 
polarization while the intrusive rocks are normally polarized. These results must be 
taken into account when interpreting the magnetic field of submerged marine vol- 
canic features such as seamounts and when establishing periods of reversal in the 
earth’s magnetic field. 
An aeromagnetic survey covering the major 
islands at the southern end of the Hawaiian 
chain has recently been completed with flight 
lines approximately 1 mile apart. The structural 
and geologic implications of this survey are dis- 
cussed in detail by Malahoff and Woollard (in a 
forthcoming issue of Pacific Science ), to which 
paper the reader is referred for examination of 
the actual anomaly contour maps. The dominant 
magnetic anomalies observed over the islands 
were found to be positive-negative anomaly 
pairs — typical dipole anomalies. Such anomalies 
might be expected to result from bodies with 
near vertical sides magnetized parallel to the 
present earth’s field, which, in this area, has an 
inclination of 30-40°. From a comparison of 
the location of the dipole anomalies with the 
geologic and gravimetric data on the islands, 
it is apparent that they are caused by the in- 
trusive rocks associated with the volcanic cen- 
ters and major rift zones on the islands. Such 
a result is in agreement with laboratory deter- 
minations of susceptibility and intensity of 
remnant magnetization of some Hawaiian rocks 
made by the authors. These measurements 
show that the intensity of remnant magneti- 
zation is much greater in most intrusive rocks 
of the Hawaiian Islands than in the lavas. In 
both types of rocks the intensity of remnant 
magnetization greatly exceeded that of induced 
1 Hawaii Institute of Geophysics Contribution No. 
100 . 
magnetization — by a factor of 1:10 in olivine- 
poor samples. Several model computations 
showed that it is possible to explain the ob- 
served aeromagnetic anomalies by assuming that 
the intrusive rocks were either normally or in- 
versely polarized in a direction nearly parallel 
to the present earth’s field. This is in agree- 
ment with measurements by Tarling (1963) of 
direction of remnant magnetization carried 
out on surface samples, primarily lavas, which 
also indicated directions of remnant magneti- 
zation nearly parallel to the present earth’s 
field. Because the magnetic anomalies caused by 
the remnant magnetization of the intrusive 
rocks are dipole anomalies, it is possible to 
determine by inspection whether the intrusive 
rocks are normally or inversely polarized. This 
gross direction of remnant magnetization (nor- 
mal or reverse) is given in Table 1, along with 
the results obtained by McDougall and Tarling 
( 1963 ) , and Doell and Cox ( 1963 ) , and meas- 
urements made by the writers on surface sam- 
ples of both extrusive and intrusive rocks. 
The paleomagnetic and age dating results 
obtained by McDougall and Tarling ( 1963 ) 
from Hawaiian lavas have been utilized by 
Cox, Doell, and Dalrymple (1964) in con- 
junction with data from other areas to estab- 
lish alternating periods of normality and re- 
versal in the earth’s magnetic field during the 
last four million years. The reality of these 
reversals, their length (if they exist), and the 
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