Geologic Implications of Magnetic Surveys — Malahoff and Wqollard 
271 
refraction measurements which indicate the rift 
zone type anomalies originate from depths 
ranging from 4-10 km below sea level. The 
failure to obtain magnetic anomalies over the 
late-stage centers of volcanic activity, such as 
Diamond Head and Koko Head on Oahu which 
were centers of alkalic basalt extrusion, prob- 
ably lies not so much in the difference in the 
mineralogic constituents of the extruded lavas, 
but rather in the difference in susceptibility 
contrast between the rock at depth representing 
the source magma and the enclosing rock. 
Whereas the primary intrusions appear to have 
had their magma source in the mantle and were 
emplaced in the crust, the late-stage intrusions 
could well have been derived from shallow 
magma chambers that developed (Eaton, 1962) 
within the volcanic pile itself. Thus, the com- 
position of the magma and its equivalent rock 
magnetic susceptibility could be essentially the 
same as that of the primary enclosing tholeiitic 
basalt, and the alkalic basalt would be the result 
of in situ differentiation through the gravity 
separation of early formed olivine, as suggested 
by Macdonald et al. (I960). 
Under these conditions there would be no 
contrast in magnetic susceptibility either at the 
surface over the vent or at the depth of the 
magma chamber, since the bulk of the available 
iron would be in the form of nonmagnetic 
silicates rather than oxides. Although these ob- 
servations do not identify the exact lithologic 
character of the rock material causing the ob- 
served anomalies, it does appear to be an intru- 
sive which contains a higher percentage of 
magnetite and possibly ilmenite than does the 
enclosing crustal rock. Because the associated 
gravity anomalies all indicate that these intru- 
sives must also have a density of 3.2 gm/cc, it is 
probably very similar to peridotite. However, 
until one or more anomalous areas, such as the 
Koolau caldera on Oahu, are drilled, no real 
answer can be given to this problem. 
Magnetic Properties of Rocks of the Hawaiian 
Islands Used in the Reduction of Magnetic Data 
As susceptibility and the natural remanent 
magnetization of rocks are essential factors in 
the interpretation of the total force magnetic 
anomalies, it might be well to review the data 
for the Hawaiian Islands. Studies of this nature 
on the Hawaiian rocks have been carried out 
by Doell and Cox (1963), Decker (1963), and 
Tarling (1963) as well as by the senior author 
of this paper. The results of all these determi- 
nations are summarized in Table 1. 
As the table shows, there are two groups 
within the extrusive basaltic rocks that appear 
to have greatly differing susceptibilities and 
intensities of natural remanent magnetization. 
The first group, those having low magnetic 
susceptibilities, are predominantly olivine-rich 
rocks, in which olivine makes up more than 
15% of the total weight of the rock sample. 
Rocks in this group include those from Hualalai 
Volcano on the island of Hawaii, which have 
susceptibilities that average 0.41 X 10“ 3 cgs 
units and intensities of remanent magnetization 
that range from 0.5 to 5.0 X 10“ 3 cgs units, 
and samples of garnet peridotite from Salt Lake 
Crater on Oahu, which range in susceptibility 
from 0.4 to 0.5 X 10“ 3 cgs units and have an 
intensity of remanent magnetization which 
averages between 1.0 and 2.0 X 10 -3 cgs units. 
In the second group, those having a high 
magnetic susceptibility, are the olivine-poor 
lavas, such as those found on the island of 
Hawaii. These olivine-poor lavas have an aver- 
age susceptibility of 2.5 X 10~ 3 cgs units and 
natural remanent magnetization of 10.0 X 10 -3 
cgs units. 
Similarly, intrusive rocks show extensive vari- 
ations in magnetic properties. One dike rock 
sample collected on East Maui had a suscepti- 
bility of 6.8 X 10~ 3 cgs units and a natural 
remanent magnetization of approximately 
100 X 10“ 3 cgs units. On the other hand, fine- 
grained dike rocks collected near the lao 
Needle, West Maui, had an average suscepti- 
bility of only 0.12 X 10“ 3 cgs units and a 
remanence of 3.0 X 10~ 3 c gs units. These low 
values of magnetic properties of West Maui 
intrusive rocks could account perhaps for the 
reversed dipole effects in the magnetic field 
observed over West Maui. However, most of 
the intrusive rocks sampled in the Hawaiian 
Islands have intensities of remanent magnetiza- 
tion that are, on the average, higher by 
5 X 10“ 3 to 10 X 10 -3 cgs units, and suscepti- 
bilities that are higher by 2 X 10“ 3 cgs units 
than the basaltic lavas which they intrude. 
Altogether, 40 samples of basalt were col- 
