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PACIFIC SCIENCE, Vol. XX, July 1966 
Measurements of dike rocks collected in the 
islands have shown that, in general, the suscep- 
tibility and remanent magnetization in these 
intrusive rocks are higher than that in the sur- 
rounding extrusive rocks by a factor of two. 
These relations verify the results of theoretical 
analyses of the anomalies, and suggest that most 
of the magnetic anomalies in the Hawaiian 
Islands are explainable by having magnetic in- 
trusive rocks occupy the rift zones and the 
primary volcanic vents responsible for the for- 
mation of the islands. 
Island of Hawaii 
geology of Hawaii: Since the geology of the 
island of Hawaii has been described in detail by 
Stearns and Macdonald (1946), it is reviewed 
here only briefly. The rocks constituting this 
island are basalts and their differentiates, whose 
magnetic susceptibilities vary between 2.6 X 
10“ 3 and 0.2 X 10“ 3 cgs units. Because the 
intensity of remanent magnetization is approxi- 
mately 10 times the numerical value of the 
susceptibility, it has not been possible, in gen- 
eral, to discriminate between the magnetic effects 
of the individual formations. As will be seen, 
the principal magnetic anomalies are associated 
primarily with intrusive features such as centers 
of volcanism and dikes. 
Although the island is only 93 miles long 
and 76 miles wide, Stearns and Macdonald 
identify five major volcanic centers: Kohala 
Mountain, Mauna Kea, Hualalai, Mauna Loa, 
and Kilauea. The earliest eruptions appear to 
have taken place in Tertiary time. 
Hualalai Volcano was active from Tertiary 
to Recent time and has erupted basalts and 
trachyte along three rift zones. In 1801 an 
eruption produced olivine basalt with a large 
proportion of ultrabasic to dioritic inclusions. 
Kohala Mountain is built largely of olivine 
basalts, tholeiitic basalts, and ash erupted along 
three rift zones trending across the summit of 
the volcano. Most of the activity was during the 
Middle Pleistocene. Caldera faults defining a 
shallow graben containing alkalic basalt now 
mark the summit area. 
Mauna Kea, the highest of the volcanoes, is 
composed of tholeiitic basalt with a capping of 
alkalic basalt and ash, erupted along three rift 
zones trending away from the summit. The 
volcano was active from the Pleistocene to Re- 
cent, and the summit is now marked by several 
large cinder cones. 
Mauna Loa, the largest and second highest 
volcano in Hawaii, is located adjacent to Mauna 
Kea. It is active periodically and has erupted 
olivine basalt along two rift zones. A large 
caldera marks the summit. 
Kilauea is the smallest and currently the most 
active of the volcanoes. It is located at the inter- 
section of two rift zones. 
Although, as seen, rift zones defined by 
surface fissures and chains of volcanic cones 
which usually intersect at the summits are as- 
sociated with all the volcanoes, most of these 
surface fissures do not have magnetic anomalies 
associated with them. Thus the surface fissures 
appear to be superficial features. Figure 11 
shows their locations and identifies the indi- 
vidual volcanoes with which they are associated. 
It is along these rifts that most of the recent 
flank eruptions have occurred. Their locations 
suggest that they have originated from the 
dilational forces associated with the develop- 
ment of the individual volcanic shields. 
Similarly, there are normal faults (Fig. 12) 
which appear to be superficial features that have 
resulted from the rapid growth of the volcanic 
shields. These are of three types: (1) circular 
or concentric faults originating around pit 
craters (calderas), (2) faults parallel to rift 
zones, and (3) faults near the coast, dipping 
seawards. The horizontal extent of these faults, 
which are generally less than 10 miles in length, 
is small. Continuous strike directions are un- 
common. 
A comparison between the superficial struc- 
tural patterns and deep-seated features such as 
the primary rifts and volcanic vents on the 
island of Hawaii can best be made by compar- 
ing Figures 11 and 12 with 13, which presents 
the total force intensity magnetic map of the 
island. As seen from Figure 13, the summits of 
all five volcanoes on the magnetic map are 
marked by dipole anomalies polarized normally ; 
i.e., with the positive pole to the south and the 
negative pole to the north. Because at the mag- 
netic latitude of Hawaii a dipole effect will be 
produced by nearly vertically sided bodies whose 
vertical dimensions are in excess of the minimum 
horizontal dimension by a factor of two or 
