Aeromagnetic Surveys — Malahoff and Strange 
391 
TABLE 1 
Summary of Geomagnetic Results 
FORMATION 
POLARITY 
l 1 
POLARITY 
2 2 
POLARITY 
3 3 
POLARITY 
4 4 
K-AR AGE 1 
M.Y. 
Hawaii (historic) 
N 6 
N 
N 
historic 
Hana (East Maui) 
N 
N 
*> 
1 
0.4 
Kula (East Maui) 
N 
N 
N 
0.86, 0.43 
Honomanu (East Maui) 
R 6 
N 
J 
1 
0.86 
Honolua dike rocks 
R 
1 
Honolua (West Maui) 
R 
R 
i i s -t-n no- 
Wailuku dike rocks 
N 
R 
[ 
1 . 1 j — u.uz 
Wailuku (West Maui) 
R 
R 
1 
1.29 ±0.03 
Kahoolawe 
N 
Lanai 
R 
N 
East Molokai 
R 
N 
1.3-1. 5 
West Molokai 
R 
N 
1.85 ±0.01 
Koolau dike rocks (East Oahu) 
N 
l „ 
Koolau ( East Oahu ) 
R 
N and R 
r R 
2.2-2. 5 
i 
r n 
1 
1 
2.7 6 ±0.02 
N 
1 
2.84 ±0.02 
Waianae (West Oahu) 
R 
► N 
2.95 ±0.06 
i 
IN 
1 
3.27 ±0.04 
Koloa (Kauai) 
NandR 
N and R 
N 
Napali (Kauai) 
N 
N 
N 
4. 5-5. 6 
Niihau 
N 
1 As determined by McDougall and Tarling (1963) primarily from extrusive material. 
2 As determined by Doell and Cox (1963). 
3 As determined by present authors on rock samples, using an astatic magnetometer. 
4 As determined by total magnetic intensity maps (intrusive rocks only). 
6 Normal polarization. 
6 Reversed polarization. 
period of time over which the change from 
one polarity to the other took place, are ex- 
tremely important in establishing a model for 
the main magnetic field of the earth. It is im- 
portant, therefore, to explain the differences 
shown in Table 1 between the direction of 
magnetization of the bulk of the intrusive ma- 
terial, as revealed by the aeromagnetic survey, 
and that of the surface samples of the lavas. 
A number of possible explanations exist. The 
age of the bulk of the intrusive material usu- 
ally is unknown and lavas of both normal and 
reverse polarization but different ages are some- 
times found to be associated with the same vol- 
canic center. The most obvious explanation for 
this difference would be that, for many centers 
of eruption, the bulk of the intrusive material 
solidified at a different time than did the lavas, 
whose directions of magnetization are different 
from the directions of magnetism of the in- 
trusive material. At the present time it is not 
possible to confirm or disprove this or any 
other explanation for the differences. Detailed 
measurements of the ages and directions of 
remnant magnetization of the intrusive rocks 
of islands and determination of the types of 
magnetic minerals present are first necessary. 
It is worth pointing out, however, that differ- 
ences between the direction of polarization of 
intrusives and extrusives which are almost cer- 
tainly not due to age differences have been 
noted previously. 
The aeromagnetic survey results also bring 
out several important facts which affect the 
interpretation of airborne or seaborne magnetic 
surveys over seamounts, guyots, and similar 
oceanic volcanic features. It has been common 
practice to assume that the magnetic field asso- 
ciated with a seamount is caused by uniform 
magnetization of the entire bathymetric feature. 
