Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
301 
shield, and very little pyroclastic material ap- 
pears to have been associated with the eruptions. 
magnetic relations: The three rift zones 
as described by Stearns are all reflected by mag- 
netic anomalies (Fig. 27). Three major pri- 
mary rift zone anomalies and two major volcanic 
vent zone anomalies are indicated. The prom- 
inent North Lanai Primary Rift Zone anomaly 
(Fig. 10) appears to be a member of the 
Molokai Fracture Zone system. The westward 
portion of the North Lanai Primary Rift Zone 
anomaly coincides with the Northwest Rift 
Zone of Steams. Similarly, the South Lanai 
Primary Rift Zone anomaly coincides with the 
faulted South Rift Zone of Stearns. The West 
Lanai Primary Rift Zone has no apparent surface 
expression. 
The South Lanai Volcanic Vent Zone anom- 
aly, as elsewhere, probably reflects the intrusive 
rocks from which the majority of the lavas of 
Lanai originated. This vent zone is also marked 
by a pronounced gravity high. Though the 
geologic extent of the vent zone is broad (12 
km long, 6.5 km wide) the total amplitude of 
the associated magnetic anomaly is low (150 
gammas peak-to-peak) . A depth analysis of this 
anomaly indicates that the top of the disturbing 
body lies at a depth of only about 0.8 km below 
the surface and appears to have a thickness of 
only about 2-5 km. The probable magnetization 
contrast with the surrounding basalts is low 
and of the order of 2. 0-5.0 X 10 -3 cgs units. 
Similarly, the magnetic anomalies designated 
as the West Lanai Volcanic Vent Zone and the 
Fig. 27. Total force magnetic map of the island of Lanai, based on aeromagnetic profiles flown at 8,000 
ft. Contour interval at 25 gammas. 
