308 
PACIFIC SCIENCE, Vol. XX, July 1966 
Geologically, the local anomalies are associated 
with centers of volcanism, but most of the elon- 
gate anomalies do not have surface geologic 
counterparts and are believed to represent in- 
trusions of mantle rock into rift type fractures 
in the upper mantle and overlying crust. 
The majority of the prominent magnetic 
anomalies defining trends of crustal rifts strike 
parallel to one of the two directions of the 
Hawaiian Ridge. One direction parallels the 
east-west strike of the Molokai Fracture Zone. 
The other direction parallels the west north- 
west-east southeast strike of the crest of the 
Hawaiian Ridge. Inasmuch as the trends paral- 
lel to the Hawaiian Ridge are truncated by those 
parallel to the Molokai Fracture Zone, the 
Fig. 33. Total force magnetic map of the island of Niihau, based on aeromagnetic profiles flown at 8,000 
ft. Contour interval at 50 gammas. 
