288 
PACIFIC SCIENCE, Vol. XX, July 1966 
N 
Fig. 14. Map of the island of Hawaii showing 
surface rift zones. (From Stearns and Macdonald, 
1946.) 
vents are located on the east-west primary rift 
anomaly zones. As discussed previously, these 
primary rift zones, defined by elongate dipole 
anomalies, are not confined to the island mass 
but extend into the adjacent ocean. Thus, on the 
island of Hawaii, moving south from Kohala 
Mountain, we have: 
1. The "Kohala Primary Rift Zone," on 
which is located the volcanic center of Mt. 
Kohala. 
2. The "Mauna Kea Primary Rift Zone," 
which strikes in a general east-west direction 
and on which are located the Mauna Kea and 
Hualalai vents. This primary rift zone is located 
some 20 miles south of the "Kohala Primary 
Rift Zone." The northeast rift on Mauna Kea, 
as shown by Stearns and Macdonald (Fig. 14), 
appears on the main limb of the primary rift 
zone, as does the west rift. However, although 
the south rift is reflected in the magnetic anoma- 
lies as a southward bay, it is improbable that it 
has any significant depth or magnetization, as 
the local anomaly is only -}-80 gammas. The 
primary rift zone apparently enters the area of 
Mauna Kea along the northeast rift, and changes 
direction beneath the west rift of Mauna Kea. 
The trend then experiences a southward deflec- 
tion between the mountains of Mauna Kea and 
Hualalai and enters beneath the summit of 
Hualalai Volcano, continuing to strike westward 
out to sea. The southeast rift of Hualalai Vol- 
cano is defined by a -|- 50-gamma anomaly, 
which disappears in the Mauna Loa Primary Rift 
Zone. However, the magnetic anomaly is too 
low, in view of the anomalies produced by the 
volcanic pipe complexes, to warrant an assump- 
tion that the Mauna Loa and Mauna Kea pri- 
mary rift zones are interconnected at shallow 
depth along this rift. The northeast rift appears 
as a short low positive local magnetic anomaly 
of -f- 50 gammas, which persists for only 12 km 
northeast of the summit of Hualalai Volcano. 
The northwest rift is difficult to interpret in 
terms of local residual magnetic anomalies, be- 
cause it lies directly along the negative limb of 
the Hualalai magnetic anomaly. 
3. The "Mauna Loa Primary Rift Zone" 
strikes from Cape Kumukahi westward and is 
marked by a magnetic low of — 950 gammas 
located 2 miles north of the trend. The magnetic 
low may be interpreted as being caused by a flat 
shallow source of molten nonmagnetic magma 
(located at a depth of 5-10 km from the sur- 
face) with dimensions of 8 km by 2 km. 
However, because the contours of this anomaly 
do not close seaward, no definite statements on 
size or shape of the cause of this anomaly can 
be made. Some curious branches of this rift zone 
bifurcate so that the volcanic pipe complex of 
Mauna Loa Volcano is located on a southwestern 
extension of the rift zone. A secondary pipe 
complex of Mauna Loa is located 10 km south 
of Mauna Loa’s summit, and probably accounts 
for the southern extension of the -j-330 milligal 
Bouguer gravity anomaly on the gravity anomaly 
map of the island of Hawaii (Kinoshita et al., 
1963). Similarly, another pipe complex is lo- 
cated 15 km northeast of the summit of Mauna 
Loa, on a branch of the Mauna Loa Primary 
Rift Zone. 
4. The "Kilauea Primary Rift Zone" appears 
as an east-west striking feature 8 km south of 
Cape Kumukahi, curves southward and joins an 
indistinct east-west striking rift zone 10 km 
south of Kilauea caldera. It is difficult to locate 
the Kilauea caldera on any of the primary rift 
zones. Judging from the strike of these rift 
zones, it appears that Kilauea Volcano originated 
in the zone of coalition between the southeast 
branch of the Mauna Loa Rift Zone and the 
two Kilauea rift zones. Although Kilauea caldera 
has a distinct gravity anomaly associated with 
