Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
307 
Niihau originated. The dike zones mapped by 
Stearns on Niihau have the same strike as the 
Niihau Primary Rift Zone and the largest con- 
centration of the dikes occurs within the boun- 
daries of the trend. 
The horizontal dimensions of the vent zone 
are 8 km by 8 km and the top surface of the 
anomalous body is located 0.8 km beneath the 
surface and its base at 6.0 km. The dike rocks 
that are exposed above the vent zone probably 
are representative of the anomalous volcanic 
rocks occurring within the deeper portions of 
the pipe zone. This conclusion stems from the 
apparent association between the Niihau dike 
swarms and the Niihau Volcanic Vent Zone. 
Rocks within the vent area appear to have a 
magnetization contrast of 8.0 X 10 -3 cgs units 
with the surrounding basalts. 
CONCLUDING REMARKS 
The airborne magnetic study reported here 
has shown that the area on and adjacent to the 
Hawaiian Islands is characterized by two types 
of magnetic anomalies: those that are elongated 
and extend for several tens of kilometers, and 
those that are centered over local areas. In all 
cases the local type anomalies are superimposed 
on the axes of the elongated type anomalies. 
Fig. 32. Sketch of the primary rift zone and volcanic pipe zone of the island of Niihau. 
