Gravity Investigations on Leeward Islands — Kroenke and Woollard 
365 
of Sand I. to +306 mgal on the north. The 
gravity gradient on Sand I. averages 10 mgal per 
mile. The contours indicate a single high with 
a maximum value of about +320 mgal. The 
gravity high appears to be positioned midway 
between Sand I. and Eastern I. 
Johnston 
Located south of Hawaii, at l6°45'N, 169° 
31'W, Johnston I. is not a part of the Hawaiian 
Ridge but can be considered geologically as the 
northernmost of the Line Islands. On the basis 
of bathymetric contours it appears to lie on the 
northern end of a continuous shoal area extend- 
ing southeastward from Johnston I. to Pal- 
myra I. North of Johnston the shoal area has 
a northeast-southwest trend. Johnston I. itself 
is elongated in a northeast-southwest direction 
parallel to the barrier reef that exists to the 
west and northeast of the island. 
As seen from Figure 6, the minimum gravity 
value of +240 mgal is obtained on the south- 
west end of the island and values increase to 
+ 251 mgal towards the northeast. Projecting 
the gravity gradient of 5 mgal per mile would 
predict a maximum Bouguer anomaly value of 
+ 270 mgal at the center of a gravity high 
lying to the northeast of the island. 
SUMMARY 
All islands along the Hawaiian Ridge are 
characterized by high gravity values with Bou- 
guer values approaching +300 mgal, includ- 
ing those islands represented as atolls having 
a thick coral cap, such as Midway, where an 
actual value of +306 mgal was observed. All 
have pronounced gradients of about 10 mgal 
per mile that appear to be related to a pri- 
mary volcanic pipe rather than to the topo- 
graphic configuration. Despite the geologic in- 
dications of greater subsidence toward the 
northwest along the Hawaiian Ridge there is 
no indication of any systematic change from 
southeast to northwest in Bouguer anomaly val- 
FlG. 6a. Johnston I. and reef. 
