Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
289 
it, the magnetic anomaly is almost nonexistent. 
We can deduce from this association that the 
vent material beneath the caldera is dense and 
nonmagnetic or partially magnetic, as would 
be the case with a partially molten vent complex. 
5. The "Hilina Primary Rift Zone” has been 
named after the Hilina Fault System, with which 
it apparently has a direct association. It is sur- 
prising that such a prominent surface feature as 
the Kilauea Southwest Rift has no magnetic 
anomalies associated with it. In fact, it cuts right 
across the zone of strong east-west striking 
anomalies. It can only be concluded from this 
evidence that the Kilauea Southwest Rift is only 
a superficial feature. The Hilina trend has a 
distinct, normally-polarized magnetic anomaly 
of 450 gammas peak-to-peak associated with it. 
This anomaly could be the result of intrusion 
along the Hilina Fault System, or it could be 
due to an ancient volcanic complex now sub- 
merged beneath the covering lavas of the 
Kilauea series. 
6. The "Honuapo Primary Rift Zone” 
crosses the shoreline of the island of Hawaii 
in the neighborhood of the town of Honuapo. 
As seen in Figure 11, this rift zone has a vent 
tube magnetic dipole anomaly associated with 
it. No doubt this broad dipole marks the center 
of an extinct volcanic vent now buried. 
It is important to note that the primary rift 
zones described above are not linear and, in 
places, are sharply bent, suggesting that there 
has been intrusion along intersecting fractures. 
In all cases, the rift zones probably exist, but 
they have not been marked on Figure 11. How- 
ever, the general strike of the primary rift zones 
is east-west and the bending is probably the 
result of local differential tectonic movement on 
cross faults. As it is unlikely that two cross- 
cutting sets of fractures would be open at the 
same time, intrusions were not necessarily con- 
temporaneous, but could have taken place in 
two stages, with the short flexure offsets in the 
dominant east-west trends representing leakage 
of magma into the joining fractures as they 
open up with a change in regional stress pattern. 
Although the point cannot be proved, it is not 
unlikely that vents on the primary rift zones 
developed at points of weakness where two 
sets of crustal fractures intersect. 
Under this concept, the volcanoes which 
formed the island are secondary features super- 
imposed on primary crustal rifts. In this respect, 
the writers are in agreement with the theory 
of Betz and Hess (1942), which proposes a 
fissure eruption origin for the Hawaiian Islands. 
However, as is evident, there is no single fault 
zone forming fissures from which magma 
erupted in mass. 
Inasmuch as on Oahu the primary rift 
zones are oriented northwest-southeast and strike 
parallel to the axis of the Hawaiian Ridge 
rather than east-west, as they do on Hawaii, 
Maui, and Molokai; and inasmuch as both 
trends are present on Kauai, it appears that 
not only are there two sets of primary fractures 
associated with the Hawaiian Islands, but also 
that intrusion into them must have been gov- 
erned by a change in regional stress pattern 
whereby the northwest-southeast sets were closed 
after the development of Oahu. Although the 
primary rifts are oriented east-west, it is the 
continuation of the Ridge along this same gen- 
eral strike that constitutes the principal argu- 
ment for the centers of volcanism in Hawaii 
being localized at points of weakness where the 
earlier, now-closed, northwest-southeast frac- 
tures intersect the east-west fractures that stand 
out so prominently in the magnetic anomaly 
pattern. 
The lack of negative anomalies and the ab- 
sence of subdued positive anomalies above the 
summit of Mauna Loa, which is periodically 
active, appear to substantiate the existence of a 
secondary shallow magma chamber as postu- 
lated by Eaton (1962). Similarly, the lack of 
any pronounced magnetic anomalies beneath the 
Kilauea caldera suggests the existence of such 
a chamber. In this respect, these two volcanoes 
appear to differ from the other Hawaiian vol- 
canoes, all of which have marked dipole anoma- 
lies associated with the vents. 
QUANTITATIVE INTERPRETATION OF THE 
MAGNETIC ANOMALIES OVER THE ISLAND OF 
Hawaii: Depth, size, and shape estimates of the 
volcanic pipe zones, together with comparable 
model studies, are presented in Table 2. 
As indicated earlier, rocks from 30 exposures 
were sampled on the island of Hawaii and 
analyzed in the laboratory for susceptibility, 
