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PACIFIC SCIENCE, VoL XX, July 1966 
Ridge, but also the only prominent magnetic 
anomaly which is inversely polarized, it is of 
special interest. This reversal in the magnetic 
field observed over the Koolau caldera can be 
explained either by the intrusion of weakly 
magnetized volcanic rocks within the Koolau 
Rift Zone or by a temporary reversal of the 
earth’s magnetic field during the solidification 
of Koolau intrusive rocks. The explanation is 
not obvious, as studies of the extrusive rocks 
give conflicting data. The Honolulu series, for 
example, does not exhibit reversed magnetic 
polarization. Also, dike rocks collected within 
the Koolau caldera by the writers show normal 
directions of polarization in the laboratory. 
However, their intensities of remanent magne- 
tization are lower than are those of the sur- 
rounding basalts. On the other hand, the results 
of polarization studies by McDougall and Tar- 
ling (1963) indicate that the Koolau series of 
basalts are inversely polarized. 
As the magnetic anomaly across the Koolau 
caldera (Fig. 3) shows that the point of inflec- 
tion of the dipole is centered over the middle 
of the Koolau caldera, the inverse polarization 
is not a surficial effect but is one extending to 
depth. A gravity analysis (Strange, 1964) of 
the gravity high over the caldera requires a rock 
density of 3.2 gm/cc extending from a depth 
of 1 km to at least 16 km, and horizontal 
dimensions expanding with depth, as shown in 
Figure 3. This corroborates closely the seismic 
analysis by Adams and Furumoto (1965). The 
high seismic velocities and high densities suggest 
that the disturbing rock mass is a peridotite. 
However, it is not clear whether the inverse 
magnetic polarization is related to diamagnetism 
or to a past reversal in the earth’s magnetic 
field. That the observed low susceptibilities for 
olivine-rich rocks could account for the anomaly 
is shown by the theoretical profile for a peri- 
dotite-filled caldera (Fig. 3): the computed 
profiles fit the observed profile within 50 gam- 
mas. A magnetization contrast of 15 X 10 -3 
cgs units would give excellent agreement be- 
tween the observed and computed profiles. If 
this is the case, one then has to account for 
most other vent zones having the caldera "pipe” 
filled with highly magnetic rock having a sus- 
ceptibility of approximately 2 X 10“ 3 cgs units. 
On a statistical basis, the diamagnetic explana- 
tion appears to be less reasonable than a reversal 
in polarity. Only by drilling to the source rock, 
however, will the explanation be determined. 
The geologic analysis of the Koolau Volcanic 
Vent Zone magnetic anomaly, on the basis that 
it is a ferromagnetic body inversely polarized, 
indicates that it is approximately 12 km wide at 
a depth of 1.6 km and extends to a depth of 
approximately 16 km. The intrusive rock hav- 
ing reversed polarity has a magnetic suscepti- 
bility of 20 X 10“ 3 cgs units. 
The analysis of the magnetic field over the 
Waianae caldera shows that the Waianae Vol- 
canic Vent Zone averages 9 km in width at a 
depth of 800 m and extends to a depth of 5 km. 
The rocks occupying the vent zone are normally 
polarized with a magnetization contrast of 
9.0 X 10“ 3 cgs units. 
Island of Kauai 
geology: According to Macdonald et al. 
(I960), Kauai is one of the oldest of the 
Hawaiian Islands. It consists principally of a 
shield volcano built up from the sea floor by 
innumerable eruptions of thin lava flows from 
a central vent and rift zones (Fig. 30). Activ- 
ity started in the Kauai Volcano in early or 
middle Pliocene times. Growth of the shield 
was rapid and was completed before the end 
of the Pliocene. Towards the end of its growth, 
the summit of the shield collapsed and formed 
a large central caldera. A smaller caldera in the 
southeast portion of the island may or may not 
have had a contemporaneous origin. Later flows 
filled the grabens that formed after the caldera 
collapsed. The flows that built up the shield 
volcano as well as the later flows that filled the 
caldera are composed predominantly of olivine 
basalt. 
Thus, the volcanic shield is made up of a 
basaltic sequence, termed the Waimea Canyon 
series, which is divided into four formations. 
The eastern part of the shield is veneered by 
later lavas of the Koloa series, which were 
erupted after a long period of erosion and con- 
tinued through most of the Pleistocene epoch. 
Dikes occur in all the formations over most of 
the island with a dominant east-northeast trend 
(Macdonald et al., I960). However, no well- 
developed dike complexes, like those found on 
the other islands, are observed. 
magnetic relations: The total intensity 
magnetic map of Kauai (Fig. 31) shows that 
