A Reconnaissance Gravity Survey of the Island of Molokai, Hawaii 1 
James G. Moore 2 and Harold L. Krivoy 3 
During December, January, and February, 
1962-1963, 72 gravity measurements were made 
with a La Coste and Romberg meter ( G-8 ) on 
the island of Molokai. Stations were located and 
elevations determined on 1:24,000 scale mod- 
ern topographic maps. A gravity station (No. 
23) at the Molokai airport (Fig. 1), first es- 
tablished by R. R. MacDonald, was used as the 
primary base during the course of the survey. 
Because of limited time and the inaccessible 
nature of much of the island, especially the 
eastern part, the present survey is of a recon- 
naissance nature only. However, enough data 
are available to outline roughly the general 
gravity field of West Molokai Volcano and to 
permit speculation on that of East Molokai 
Volcano. 
The help of the following residents of Molo- 
kai is gratefully acknowledged: Edward Bur- 
lem, Superintendent of Kalaupapa Settlement; 
James Linebaugh and Sakuichi Nakamura, Soil 
Conservation Engineers; and Henry Meyer, 
Operations Manager of Molokai Ranch. 
GENERAL GEOLOGY 
Molokai, situated southeast of Oahu, is the 
fifth largest of the Hawaiian Islands and con- 
sists of two shield volcanoes arranged along an 
east-trending line (Fig. 2). West Molokai Vol- 
cano is built mainly of thin basalt flows which 
dip gently from the high point of the volcano. 
It is flatter than most Hawaiian volcanoes and 
apparently the summit was never indented by a 
caldera (Stearns and Macdonald, 1947:15). 
K-Ar ages (McDougall, 1964) indicate that 
the last eruptions of West Molokai Volcano oc- 
curred about 1.8 million years ago. 
Two rift zones marked by broad ridges radi- 
ate from the summit of the western volcano, 
1 Publication authorized by the Director, U. S. Geo- 
logical Survey. 
2 U. S. Geological Survey, Menlo Park, California. 
3 U. S. Geological Survey, Denver, Colorado. 
one to the northwest and the other to the south- 
west. Numerous dikes have been mapped 
(Stearns and Macdonald, 1947) along the coast 
from 1 to 5 miles east of the northwest cape of 
the island along the trend of the northwest rift 
zone. No dikes have been mapped on the south- 
west rift, but a few dikes are exposed along the 
south coast and may define a possible southeast 
rift zone of the volcano. 
On the northeast side of West Molokai Vol- 
cano are many faults which are downthrown 
chiefly toward the northeast (Fig. 2). They in- 
dicate collapse of the northeast slope and sum- 
mit area of the volcano, apparently before East 
Molokai Volcano had attained its present size. 
East Molokai Volcano is composed of two 
volcanic series, a lower unit of olivine basalts, 
and a relatively thin upper unit dominantly of 
mugearite and trachyte. The latest lavas are dis- 
tinctly younger than those of West Molokai Vol- 
cano and overlap its east flank. K-Ar ages (Mc- 
Dougall, 1964) of the exposed rocks of East 
Molokai Volcano are 1.3-1. 5 million years. 
These lavas were erupted from a central cal- 
dera complex, and from the west and east rift 
zones which intersect at the summit. These rift 
zones are marked by dikes and cinder cones 
(Stearns and Macdonald, 1947); Macdonald 
( 1956:16) has suggested that another rift zone 
extends south from the summit. 
On the north shore of East Molokai Volcano 
several faults dip south and are downthrown on 
their south side (Fig. 2). Especially thick lava 
flows are present here, where lava flowing north 
from the summit of the volcano ponded on the 
downthrown blocks. 
The latest volcanic activity on the island of 
Molokai occurred north of the great sea cliff 
on the north side of East Molokai Volcano. A 
small vent 1 mile east of Kalaupapa extruded 
olivine basalt lava which built the low Kalau- 
papa peninsula nearly 3 miles north of the base 
of the sea cliff. 
343 
