Analysis of Gravity Field — STRANGE, WOOLLARD, and Rose 
389 
It is interesting to note the large cross- 
sectional dimensions which must be chosen for 
the volcanic pipes and rift zones at depth in 
order to explain the observed gravity anomalies. 
On the basis of seismic activity and tilt data 
Eaton (1962) has postulated that the magma 
which is erupting today on the island of Hawaii 
originates at some 60 km below sea level and 
rises through a conduit to a magma chamber 
a few kilometers below the surface, where it 
remains until sufficient force is built up to 
cause an eruption. He was able also to show 
that the lava migrated laterally from the magma 
chamber beneath the central pipe to erupt along 
the rift zone. From this one might postulate 
that the rift zones are rather surfkial features 
formed when the central throat of the eruptive 
center became clogged with solidified material. 
Flowever, the gravity data indicate that the 
rift zones are of two distinctly different types. 
Rift zones such as those in Haleakala crater on 
Maui have little gravity or magnetic effect and 
apparently are rather superficial features. How- 
ever, rift zones with a large gravitational effect, 
such as the northwest rift zone of the Koolau 
volcanic center, are very basic structural features 
extending downward at least to the Mohorovicic 
discontinuity. Such an interpretation is also 
substantiated by the magnetic results obtained 
by Malahoff and Woollard (referenced above). 
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