Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
305 
three major primary rift zone anomalies cross 
the island. These are designated in this paper 
as the North Kauai Primary Rift Zone, the 
Waimea Primary Rift Zone, and the Koloa 
Primary Rift Zone (Fig. 30). One volcanic 
vent zone is indicated on each of these primary 
rift zones. All three vent zones are normally 
polarized with maximum peak-to-peak ampli- 
tudes of the magnetic anomalies ranging from 
300 gammas (North Kauai Volcanic Vent 
Zone), to 400 gammas (Waimea Volcanic 
Vent Zone), to 500 gammas (Koloa Volcanic 
Vent Zone). All three primary rift zones strike 
in a general westerly direction, converging 
towards the western portion of the island. It is 
significant that the Niihau Primary Rift Zone 
anomaly originating over the island of Niihau 
converges with the Koloa Primary Rift Zone 
anomaly. This association suggests a common 
rift zone origin for the two islands — or, more 
likely, that Niihau was formed as a result of 
southward branching of the Koloa Primary Rift 
Zone. It is also significant that, as in all the 
Hawaiian Islands, the primary rift zones origi- 
nate in the ocean and cross the island without 
interruption. 
The Waimea Volcanic Vent Zone coincides 
with the magma conduit which is defined geo- 
logically and is believed to be the source for 
the lavas which built the Kauai shield. The 
geologic analysis indicates that the vent zone 
is approximately 11 km long and 6.5 km wide, 
with its upper surface buried about 1.6 km 
beneath the peak of Mt. Waialeale and its base 
Fig. 30. Sketch of the primary rift zones and volcanic pipe zones of the island of Kauai. 
