A Reconnaissance Gravity Survey of the Island of Molokai, Hawaii'

JAMES G. MOORE2 and HAROLD 1. KRIVOy3

DURING DECEMBER, January, and February, one to the northwest and the other to the south­ 1962-1963,72 gravity measurements were made west. Numerous dikes have been mapped with a La Coste and Romberg meter ( G-8) on (Stearns and Macdonald, 1947 ) along the coast the island of Molokai. Stations were located and from 1 to 5 miles east of the northwest cape of elevations determined on 1:24,000 scale mod­ the island along the trend of the northwest rift ern topograph ic maps. A gravity station (N o. zone. No dikes have been mapped on the south ­ 23) at the Molokai airport (Fig. 1), first es­ west rift, but a few dikes are exposed along the tablished by R. R. MacDon ald, was used as the south coast and may define a possible southeast primary base during the course of the survey. rift zone of the volcano. Because of limited time and the inaccessible On the northeast side of West Molokai Vol­ nature of much of the island, especially the cano are many faults which are downthrown eastern part, the present survey is of a recon­ chiefly toward the northeast (Fig. 2 ). They in­ naissance nature only. However, enough data dicate collapse of the northeast slope and sum­ are available to outline roughly the general mit area of the volcano, apparently before East gravity field of West Molokai Volcano and to Molokai Volcano had attained its present size. permit speculation on that of East Molokai East Molokai Volcano is composed of two Volcano. volcanic.series, a lower unit of-olivine -basalts, The help of the following residents of Molo­ and a relatively thin upper unit dominantly of kai is gratefully acknowledged: Edward Bur­ mugearite and trachyte. The latest lavas are dis­ lem, Superintendent of Kalaupapa Settlement; tinctly younger than those of West Molokai V01­ James Linebaugh and Sakuichi Nakamura, Soil cano and overlap its east flank. K-Ar ages (Me­ Conservation Engineers ; and Henry Meyer, Dougall, 1964 ) of the exposed rocks of East Operations Manager of Molokai Ranch. Molokai Volcano are 1.3-1.5 million years. These lavas were erupted from a central cal­ GENERAL GEOLOGY dera complex, and from the west and east rift Molokai, situated south east of Oahu, is the zones which intersect at the summit. Th ese rift fifth largest of the Hawaiian Islands and con­ zones are marked by dikes and cinder cones sists of two shield volcanoes arranged along an (Stearns and Macdonald, 1947) ; Macdonald east-trending line (Fig. 2). West Molokai Vol­ (1956:16) has suggested that another rift zone cano is built mainly of thin basalt flows which extends south from the summit. dip gently from the high point of the volcano. On the north shore of East Molokai Volcano It is flatter than most Hawaiian volcanoes and several faults dip south and are downrhrown on apparently the summit was never indented by a their south side ( Fig. 2 ). Especially thick lava caldera (Stearns and Macdonald, 1947:15). flows are present here, where lava flowing north K-Ar ages (McDougall, 1964) indicate that from the summit of the volcano ponded on the the last eruptions of West Molokai Volcano oc­ downrhrown blocks. curred about 1.8 million years ago. The latest volcanic activity on the island of Two rift zones marked by broad ridges radi­ Molokai occurred north of the great sea cliff ate from the summit of the western volcano, on the north side of East Molokai Volcano. A small vent 1 mile east of Kalaupapa extruded 1 Publication autho rized by the Director, U. S. Geo­ olivine basalt lava which built the low Kalau­ logical Survey. 2 U. S. Geological Survey, Menlo Park, California. papa peninsula nearly 3 miles north of the base 3 U. S. Geological Survey, Denver, Colorado. of the sea cliff.

343 344 PACIFIC SCIENCE, Vol. XIX, July 1965

in fact, is largely within the area of extensive faulting on the northeast flank of the volcano. Of t I ces 80gg oge and Te rminal roo m Much of this central positive gravity anomaly Gar oges lies in the area covered by the younger lavas of IX I East Molokai Volcano. The relationship of the : ol __ Lcmp- 1 post fault system on the northeast flank of W est : Bog : Molokai Volcano to the gravity-defined vol­ I I t ru c k I : .. I dr lve- "'- canic center suggests that these faults, prob­ L ..9~a...!.~.!..!~ l.!.."':'~~ __ -J way I Fence ably caused by gravity sliding to the north, en­ Ap ron croached on the volcanic center in much the FIG. 1. Plan of Molokai airport. X, site of gravity same way as the H ilina fault system is encroach­ station No. 23. ing on the south flank of Kilauea Volcano (Moore and Krivoy, 1964:2043). Gravity noses trending northwest and south ­ GRAVITY west from this central gravity high ( Fig. 3) ABouguer gravity map ( Fig. 3) was pre­ presumably define the major rift zones of West pared for the island from gravity measurements Molokai Volcano. A gravity nose extending to from 72 stations. The table of pr incipal facts the east may represent an east rift zone of the is reported elsewhere (Hawaii Inst, Geoph., west volcano, but in this region of overlap the 1965, Table 3) . As with gravity surveys of the possibility that this structure belongs to East island of Hawaii (Kinoshita er aI., 1963) , a Molokai Volcano cannot be elimi nated . Control density of 2.3 gl ee was used for the Bouguer of gravity measurements on the south coast of corrections. N o systematic-tidal; drift, or ter­ West Molokai is insufficient to test the presence rain corrections have been made. of a southeast rift zone. Like other Hawaiian volcanoes, W est Molo­ The gravity measurements on East Molokai kai Volcano shows a well-marke d gravity high Volcano are few and are restricted chiefly to the near the center. Th is anomaly pr esumably rep­ south and east coastal region and to the Kalau­ resents the higher percentage of denser intrusive papa peninsula to the north . N o gravi ty meas­ rocks and perhaps ponded lavas associated wit h urements were made near the mapped caldera the magma reservoir and central conduit of the complex and the character of the central gravity volcano. This anomaly, however, lies consider­ configuration is not known. Terrain corrections ably east of the high part of the volcano and, have been computed for four stations on East

KALAUPAPA

...... -...... -. - ••~ -- -. _ EAST -» _.~~~~ ~~~- ---- .-- .-- .­ -- VOLCANO VOLCANO X 4970 ft.

EXPLANATION ~ 0> Fa ul t s , bal l den otes down f h r ow n side, st i ppled e r e e : cal de ra com p le x :) M i l es Ri f t zo ne s '--'--'-~--'-.....' _-. Line s ep o r ot l n q lovo s of two vo I co no e s 157 000'

FIG. 2. Geologic sketch map of the island of Molo kai. After Stearn s and Macdonald (1947) and Stearns (1946) . Gravity Survey of Molokai-MooRE and KRIVOY 345

-6 9 3 70 /': ' 2 '13 /. 6 /, EX PLAN AT ION 12 240 _ ...-!./'o ' 2 5 Grav it y st a tion ( no.In ta bl e n " ; - _-; I 7 o 5 Miles Gr av ity co nt our , d c s h e d whe r e u nce r ta i n, ,~~-'-~...I' In t e r v a l 10 mlil igais 157°00 '

FIG. 3. Bouguer gravity anomaly map of the island of Molokai, Hawaii.

Molokai. These stations with terrain correcti ons KINOSHITA; W . T., H. 1. KRIVOY, D. R. MA­ in milligals are: No. 3, +1.6; No. 38, +16.5; BEY, and R. R. MACDoNALD. 1963. Gravity No. 49, + 4.6; and No. 55, +0.8. The terrain survey of the island of H awaii. U. S. Geo!' effects are particularly large in the central, rug- Survey ~rof. Paper 47?-C:Cl1 4--C116. . ged part of East Molokai,and here uncorrected values must be used with caution. MACDONALD, G. A. 1956. The structure of Ha­ The gravity data on the south and east flanks waiian volcanoes. K. Nederlandsch Geo!'­ of East Molokai Volcano are insufficient to Mijn. Genoorschap Verh ., Geo!' Ser., Dee! define any rift zones. Although the coverage is 16:274--295. incomplete, the data suggest that the rift zone McDOUGALL, IAN. 1964. Potassium-argon ages on the west side of the volcanic center extends from lavas of the Hawaiian Islands. Geo!' west-northwest from the summit of East Molo­ Soc. Am. Bul!. 75:107-128. kai Volcano through the Kalaupapa peninsula. Th e vent which extruded the lava flows that MOORE, ]. G., and H. 1. KRIVOY. 1964. The built the Kalaupapa peninsula was pr esumably 1962 flank eruption of Kilauea volcano and fed by magma moving from the volcanic center structure of the east rift zone. ]. Geoph. Res. through this rift zone. 69: 2033-2045.

REFERENCES STEARNS, H. T. 1946. Geology of the·Hawaiian Islands. Hawaii Div. Hydrog. Bull. 8. 106 pp. HAWAII INSTITUTE OF GEOPHYSICS. 1965. Data from gravity surveys over the Hawaiian - -- and G. A. MACDONALD. 1947. Geology Arch ipelago and other Pacific islands. Hawaii and ground-water resources of the island of Inst. Geoph. Rep t. 65-4, Mar ch, 1965, 10 Molokai, Hawaii.H awaii Div. Hydrog, Bul!. tables. 11. 113 pp.