Submarine Canyons and the Shelf along the North Coastof Molokai Island, Hawaiian Ridge'

CHRISTOPHER C. MATHEWSON2

ABSTRACT: The north insular shelf of Molokai is a smooth plain, gently dippingseaward, with three slight steps, one occurring between the 30- and 60-foot isobaths,one between the 150- and 180-foot isobaths, and one near the 300-foot isobath.The shelf break occurs near the 500-foot isobath. Off East Molokai Volcano theshelf is cut by eleven submarine canyons; along West Molokai it is unbroken exceptfor one canyon. About half the canyons have bowl-shaped heads ; the remainderhave V-shaped heads. The canyons originate about 1 mile offshore. Seismic reflec­tion data show that the insular shelf is covered by a thin veneer of sediments, 0.005to 0.025 seconds of reflection time, thickening seaward. The veneer is underlain byanother series of reflectors, the deepest being 0.05 seconds 1 mile from shore and0.25 seconds 3 miles from shore. The Molokai submarine canyons appear to haveoriginated from subaerial erosion, which was followed by island subsidence withsediment deposition on the shelf and transport in the canyons. The geomorphologyof the north slope of Molokai appears to have developed through erosion anddeposition operating upon a subsiding volcanic island, rather than through theaction of a giant submarine landslide .

DURING 1968 the U. S. Coast and GeodeticSurvey Ship "McArthur" carried out detailedbathymetric surveys along the north coast ofMolokai Island (Fig. 1). From the bathymetricdata the author was able to contour the bathym­etry at the same interval as the island topog­raphy and hence to study the geomorphology ofthe island in the transition into the shelf, andof the valleys into the submarine canyons.Using the University of Hawaii's R/V "Mahi,"seismic reflection data were obtained to studythe sedimentary regimes over the shelf (Fig. 1) .The aim of this paper is to describe and discussthe relationship between the geomorphology ofthe island of Molokai and the submarine geol­ogy along the northern slope of the island.

The geology of Molokai Island was studiedby Stearns and Macdonald (19 47) . Previousstudies of the submarine canyons by Shepardand Dill (1966) described the topography and

1 Hawaii Institute of Geophysics Contribution 315.Manuscript received November 19, 1969.

2 Environmental Science Services Administration,Joint Tsunami Research Effort, Hawaii Institute ofGeophysics . Present address : USC&GS, OSS Ocean­ographer, 1801 Fairview Avenue E., Seattle , Wash­ington 98102 .

sediments of the canyons. Malahoff and Wool­lard (1966) carried out magnetic surveys of thestudy area, Strange et al. (1965) made somegravity observations, and Kroenke (1965) re­ported the results of two seismic reflectionsurvey lines that were run near there .

GEOMORPHOLOGY OF MOLOKAI

Molokai comprises two major geomorphicprovinces : West Molokai Dome, and East Molo­kai Dome (Stearns and Macdonald, 1947). Th eWest Molokai Dome was built by late Tertiaryvolcanism and is generally flat and subduedcompared with the East Molokai Dome. Vol­canism took place along a southwest-trendingrift zone that appears to extend out to sea alongthe crest of Penguin Bank and along a north­west-trend ing rift. Hoolehua Plain, lying be­tween the domes, is a smooth plain built bylava flows from the East Molokai Volcano thatwere ponded against the West Molokai Dome.East Molokai Dome, the largest geomorphicprovince, is an upper Tertiary volcanic domewhose crest is now 4,970 feet high . The EastMolokai volcanism appears to have originatedfrom a center vent and from two rift zones, one

235

FIG . 1. Bathymetric survey line density and seismic reflection lines along the north coast of Molokai Island, Hawa ii.

20°/5'

10'

..: ~..

157°15' 10' 05' 157" 55'

I

IsIIIII

SURVEY DENSITY AND TRACKLINESR/V Mahi USC 8 GSS MeArthur

line $pOCinq- WOLCAN I _ <50 m

-- MOLeAN 1I. ~ 50·100 1'11;Z~ IOO- 200 m

:::::::::: ZOO -300 m

so'

N'-"0\

-e>()......>Tj

156°40'......()

C/l()......tTlZ().tTl

<::~N......

>'"0~......\0-..j0

Submarine Canyons off Molokai-MATHEwsON

21° 14'

21° 10'~~~(tf.

156°50'

o-Scale in Miles

237

21° 14'

FIG. 2. Topographic and bathymetric map of the Pelekunu Canyon area. Topographic contour interval, 60and 300 feet ; bathymetr ic contour interval, 30 and 60 feet, corrected. After Mathews on (1969) .

238

trending eastward and the other northwestward.Sea cliffs are found along the north ern (wind­ward) side of both domes. Those along EastMolokai have a maximum relief of about 3,600feet, whereas along West Molokai the maximumis about 500 feet.

PACIFIC SCIENCE, Vol. 24, April 1970

The East Molokai Dome is an asymmetricalshield volcano with east-west elongation andthin -bedded basaltic aa and pahoehoe lava flowsdipping from 2° to 15° away from the centralcaldera (Stearns, 1966). However, in the lowerpart of the high northern sea cliff, the beds dip

FIG. 3. Aer ial photographs of the north coast of East Molokai, Up per photo shows the sea cliff ( 2,500feet, from 2 miles offshore) . Lower ph oto shows the U-shaped canyons (1 ,000 feet , from 0.5 mile) , and thevery steep gradient tributaries at the mouth of the canyons. The mountain tops are obscured by the clouds.Ph otographs by the author.

157· IS' 157· 10' IS7" OS' IS7 "

Impartont Note

This map i s no t to beused fo r nov igation.

INorth Shore, Island of Molokai

State of HawaiiBathymetric Map

Contour Interval 60 FeetSecondary Contour 30 Feet

Christopher C. Mathewson Lt.lUSESSA JTREUniversity of Hawaii

May 1969a I 2.. .. I

Scole in Miles

· o~

(>~")(J

. ./)'----- - - - - - - - -f- - - - - - -- - - - --- --_+_~~~~~==_--t----___:;_\(>~&&\.... '" ,":)

FI G. 4. Bathymetri c map of the north insular shelf of Molokai Island, Hawaii.

157 0 156055' 1560 50 ' 156045' 156· 40'

~--L~~+~~121. 15'I ---l------------------t---:?---~{o~CJ<:,~4,

~~~

\e'+­~e

600

Cave

. I shelf of Molokai Island, Hawaii,ap of the north InSU ar

Submarine Canyons off Molokai-MATHEWSON 239

Se Q le yel Ston ds8° to 25° toward the south, suggesting that anearly volcanic source may lie submerged on theinsular shelf . The north side of the Dome isdeeply incised by numerous northward-flowingstreams (Fig. 2), with very steep gradientsaveraging about 1/3 (18 ° slope). The streamvalleys are generally U-shaped throughout withvery steep walls (Fig. 3). Near the mouths ofmajor streams, the stream gradient abruptlyreduces to about 1/30 (2 ° slope).

300

600

900

1200

1500

l EGEND

A= 157" 15' .

B"1 57" 10'

C ' 15 7"05'

D-15 7"00'E -156·55'

F =156°50 '

G ' 156"4 5 '

-------. Nor l h

2700 ---- - - - - - - - - - ---------'

FIG. 5. North-south bathymetric profiles across thenorth shelf of Molok ai Island, showing the ancientstands of sea level.

The Submarine Canyons

The East Molokai Shelf lying off the EastMolokai Dome, is cut by eleven submarinecanyons, whereas the West Molokai Shelf offthe West Molokai Dome is cut by only one.

3 Sediment thickness based on round-trip traveltimes as measured on the seismic reflection record andan assumed velocity of sound in the sediments of5,000 feet per second.

4 Cruises MOLCAN I and MOLCAN II were sup­ported by the Joint Tsunami Research Effort (]TRE) ,ESSA, at the Un iversity of Hawaii.

Ver t ica l Exog gerat ion 11:\

INe cr tee l Milu

1800

2100

2400

1966) related to the North Molokai VolcanicPipe Zone. From these geophysical data itwould appear that the pinnacles mark the centerof volcanism of the East Molokai Volcano.

The entire shelf is covered by a thin veneerof recent sediments which are highly reflectiveand conformable to the basement reflector andcharacteristically appear as two distinct reflec­tors, with a total thickness of about 0.005secondsf (12 .5 feet) near shore, thickening toabout 0.025 seconds (62.5 feet) offshore. Theveneer is underlain by another series of reflec­tors that also increase in thickness with in­creasing distance from shore. The deepest re­flector is the lower limit of the veneer near theshore and is 0.23 seconds (575 feet) below thesurface of the veneer about 3 miles offshore(Fig. 7) .4

The Shelf

The north insular shelf of Molokai Island isa smooth plain gently dipping toward the seawith three slight steps (Fig. 4). The shelf dipsat about 2° between the shoreline and the

, 30-/60-foot isobath step, then at about a 1° dipto the 150-/180-foot isobath step, which is ashort segment of the shelf profile that also dipsabout 2°. Between this step and the 300-footisobath the shelf dips at about 10 . At about the300-foot isobath the dip of the shelf decreasesslightly to about a 0.75 ° dip, conti~1Uing at thisflatter dip to the shelf break, which occurs atabout the 500-foot isobath.

The three steps are remnants of ancient ter­races of Molokai. A total of six submergedterraces, representing former sea level stands,have been identified around the Hawaiian Is­lands: the Koko at 15 feet, the Waipio at 60feet, the Penguin Bank at 180 feet, the Marnala­Kahipa at 300 feet, the Lualualei at from 1,200to 1,800 feet, and the Waho at 3,600 feet be­low sea level (Easton, 1965; Ruhe et aI., 1965 ;Stearns, 1966, 1967) . The submerged 15-foot,60-foot, 180-foot, and 300-foot ancient terracesare appar ent in bathymetric profiles across theNorth Molokai Shelf (Fig. 5) .

An anomalous bathymetric area off EastMolokai, about 1.5 miles north of PelekunuBay and about 6 miles east of Kalaupapa Penin­sula, is characterized by numerous small, steep­sided pinnacles (Fig. 6). These small featureshave a relief of about 75 feet and are probablysubmerged stacks. The area occupied by thepinnacles corresponds to a +270 mgal Bouguergravity anomaly maximum reported by Strangeet aI. (1965), and to an inflection point inthe magnetic anomaly (Malahoff and Woollard,

SUBMARINE GEOLOGY OF THE NORTH COAST

240

FIG. 6. D etailed bathym etric map of the NorthMolokai Volcanic Pipe Zone. Contour inter val 30feet, corrected.

This study shows that, except for the twocanyons off Kalaupapa, the East Molokai sub­marine canyons originate about 1 mile offshore,that the West Molokai submarine canyon orig­inates about 3 miles offshore, and that the shelfbetween the canyon heads and the shoreline isgenerally smooth. About one half of the canyonshave typical bowl-shaped heads, the remainderhave V-shaped heads. Some of the canyons withbowl-shaped heads actually have two or more

PACIFIC SCIENCE, Vol. 24, April 1970

small, less than 0.1 mile long, V-shaped or U­shaped canyons as tributaries (Fig. 3) . Thesetributaries acting together form the genera lbowl-shaped head of the major canyon. Thestudy by Shepard and Dill (1966) , extendingfarther offshore than the surveys by the USC&GSShip "McArthur," shows that the East Molokaicanyons have typical V-shaped profiles and noindication of broad flat floors; that they appearto be fairly straight, of rather even gradient,and terminate at depths of about 6,000 feet; andthat a direct relationsh ip between the submarineand subaerial portions of each canyon systemappears to exist. This canyon/submarine canyoncorrespondence appears to hold for Pelekunu,Halawa, Kawainui, Waikolu, and N aiwa sub­marine canyons. East-west profiles across thePelekunu subaerial submar ine canyon system( Fig. 8 ) appear to confirm this connection be­tween the subaerial canyon and the submarinecanyon which can be traced across the shelfbetween the shore and the canyon head. Thelong profile of the Pelekunu Canyon system issmooth and does not show any distinct nickpoints . Kalaupapa Peninsula, north of Wai­hanau Canyon, has two submarine canyons thatbegin within 0.25 mile of the peninsula shore­line. It is possible that these two submarinecanyons were filled during the eruption of

0,-- - - - --.--- - - - - - - - - - - - - --- - ----- -.------,0

09

01

02 r-.~ , I \'

\ .03

(f)co

is 05uw(f)

~~\L0\ . V10'------- - 156°55 W

"" ~~ \ g

~ \~lINE 5

NI1

"i

156°51'

f-

1200 ~u,

2400

FIG. 7. Tracin gs of the seismic reflection records obtained along lines 3, 5, 9, and 10, cruise MOLeAN I,superimposed so that the sedimentary sequences can be related. See Figure 1 for line position s.

Submarine Canyons off Molokai-MATHEWSON 241

FIG. 8. East-west profiles, spaced 0.5 nauticalmil es apa rt, across the Pelekunu Canyon System show­ing the apparen t continuation of the canyon axis acrossthe shelf .

Kauhako Volcano, which formed KalaupapaPeninsula, and could therefore be related to theland canyons inland of the peninsula (Shepardand Dill, 1966) .

There appears to be no evidence of sedimentsdeposited along the axis of the submarinecanyons (F ig. 7), suggesting that the canyonsare primarily sediment-transport areas, whilethe shelf is a depositional area. The submarinecanyons probably carry most of the material thatis transported into the sea from the subaerialcanyons through turbidity currents , eventuallysettling in the Hawaiian Deep. Periodic tur­bidite flows may act to keep the submarine

4200' 156°50'

3600'-

3000'

2700'

1800'

1200'

1200'

1800'-

08 5'

21°100'

'"'0.215-'

canyons clear of sediments delivered by thebottom currents from the island shoreline .

GEOMORPHOLOGICAL HISTORY OF THENORTH SLOPE

Moore ( 1964) suggested that the Oahu­Molokai seamount group, located about 60 milesnorth of Molokai, was formed through twogiant landsl ides, one from the northeast coastof Oahu and the other from the north coast ofMolokai. This study does not lend support tothat content ion, since the evidence outlinedabove suggests that the north slope of Molokaihas developed thr ough island subsidence ac­companied by subaerial erosion, wave erosion,sediment deposition, and sediment transport.

It is well known that high islands in trade­wind regions interrup t the normal tradewindpattern and cause heavy rainfall on the wind­ward sides of the islands. East Molokai Domemay have been as much as 15,000 feet highbefore erosion. Heavy rainfall on the relatively .steep northern slope of Molokai Island couldtherefore be expected to have formed manystraight subaerial canyons having steep gra ­dients . Similarly, such an island would alsointerrupt the normal wind-driveri wave systems,resultin g in heavy wave erosion along the wind ­ward shoreline, and wave erosion could. also beexpected to have cut back into the lava andthus have developed the insular shelf. Withcontinu ed subsidence of the island, the sub­aerially form ed canyons would eventually be­come submarine canyons, and the ' submergedwave-cut bench or shelf would begin to collectsediments , as appears to be the case along thenorth shore of Molokai. Sediments transpo rteddown the subaerial canyons would probablycontinue to move down the submarine canyonsin the form of turbidity currents and be de­posited in the Hawaii an Deep .

From the data presented in this study, fromdata obtained on two north-south profiles of thenorth slope of Molokai (Fig. 9), and from thestudy by Shepard and Dill ( 1966) , a structuralprofile of the north slope of Molokai was con­structed (Fig. 10). The suggestion that thesubmarine canyons were subaerially formed issupported by the long pr ofile of the canyonaxes. The flatter gradient in the waterline sec-

242 PACIFIC SCIENCE, Vol. 24, April 1970

Or---------- - --- -- - ---, tion of the profile is probably the result ofsubaerial erosion that has now nearly reachedequilibrium and the result of wave erosion andsediment deposition that has occurred duringstages of island submergence. The continuationof this grad ient along the surface of the shelfwithout a nick point suggests that submarineerosion is not significant. The similarity betweenthis profile and the profile thr ough the 3,600­foot shelf suggests that their origins are alsosimilar. The canyons terminate at about a 6,000­foot depth, which is about 2,000 feet less thanthe amount of inferred island subsidence ( Fum ­moto and Woollard, 1965; Strange et al., 1965) ,suggesting that the entire canyon system wasdeveloped subaerially and then drowned.

Support for the suggestion that the submarin ecanyons are now only turb idity current channelsis seen in the seismic reflection data. Sedimentsdo not appear in the axis of the canyons,whereas they do appear as conformable layersin the intra canyon areas. The intracanyon sed­iments thicken with increasing distance fromthe shore and therefore increase the canyon wallheights . The primary source of these sedimentsappears to be the entire eroded coast of Molokaiwhich has acted, and still is acting, as a linesource. These sediments are probably derivedthrough wave erosion from the shore and trans­ported offshore by bottom currents, in thatway producing the conformable sedimentary se­quences that cover the entire shelf. Since thereappear to be no sediments in the canyon axes,the canyons must therefore be sediment trans­port channels.

The slope of North Molokai suggests thatsubsidence may have continued at a uni formrate until the - 3,600-foot level, when the firstapparent stand of sea level occurred. This standappears to have lasted for a long period of timebecause a shelf about 2 to 4 miles wide wasformed in the interval. Seismic reflection ev­idence suggests that coral reefs may have formedon this shelf. Age estimates of the shelf rangefrom 2 to 50 million years (Mathewson andMalahoff, 1969 ). During that period erosioncontinued to cut the subaerial canyons which arenow drowned, while deposition continued toincrease the submarine canyon wall heights andto build out the shelf and slope. Turbiditycurrents continued to flow down the canyonsinto the Hawaiian Deep .

eo Le ve l

+ r=- N

MOLCAN IIo 5L..............~

Ncuticcl Miles

Vertical Emqqercton I I I

~

=auw

<J)

,-,

\

, - 120 0 - 180 0 Fool Seo LevelSubmarine canyon~, Stand ( Luo tuc te t)

Axis "\

\ -,-,

' ~, - 360 0 Fool She lf.... , ( Waho),

?" ,,

0_ JNautical Miles

48 2

72 3

96 4

()Q

24rr- - r-- - - - - - - - - - - - - - --,

120"------------ ------ ---'

FIG. 9. Tracings of the seismic reflection recordsobtained .on two north-s outh lines during cruiseMOL CAN II . See Figur e 1 for line positions,

120L-- - - - - - - - - - - - - - - - - -'

24

~ ISUbm~ - Luoluolei,SeoLevel Siondcon~~~n e "\ 1200 - 1800

24 - I ~ "

7 2

oQ

9 6

z 48Q....'">"'..J

"'

.....

FIG. 10 . Composite profile of the present northslope of Molokai Island showing the sedimentarysequences as inferred from the seismic reflection data,and the average axis of the submarine canyons asobtained in these bathymetric surveys and fromShepard and Dill (1 966) . Verti cal exaggeration:11 to 1.

Submarine Canyons off Molokai~MATHEWSON

With the advent of a rise in sea level orrenewed island subsidence, or both, the slopeabove the Waho shelf began to form . Thepresent -1,200- to -1,800-foot terrace wasformed during this period when island sub­s!dence probably halted for a short period oftime. The present geomorphology of the islandand the shelf was probably formed through acomplex series of island submergences andemergences, as is suggested by the numerouselevated and submerged stands of sea level re­ported in Stearns (1966). The sediments de­rived from the island through wave erosion areprobably the sediments that form the sedimen­tary veneer seen in the seismic profiler recordsand the deeper reflectors detected on the outershelf. The sediments derived from the landcanyons probably still continue to travel to theHawaiian Deep as turbidity currents flowingthrough the submarine canyons. The inner shelfnow appears to be covered only by a thinsedimentary veneer (0.005 seconds reflectiontime thick at 0.5 mile offshore), suggesting thatthis area is within or at least near the waveerosion zone.

CONCLUSIONS

From bathymetric and seismic profiler ev­idence the geomorphology of the north slopeof Molokai Island appears to have developedthrough the processes of erosion and deposition

. operating upon a subsiding volcanic island witha steep north slope. The canyon systems wereapparently cut by subaerial erosion prior to thesinking of the island, and after submergence thesubmarine canyons acted as turbidity currentchannels . At the present time subaerially erodedmaterial continues to travel down the canyonsand out into the Hawaiian Deep as turbiditycurrents. Wave erosion, operating along the en­tire north shore, cut the sea cliffs and acts as aline sediment source supplying sediment to theentire shelf. The sediment is then transportedalong the sea floor and deposited on the outershelf. These sediments, which are seen in theseismic reflection profiler data, have built upthe submarine canyon walls. The geomorphologyof the present day shelf and the submerged3,600-foot shelf suggests that the geologic pro­cesses operating on the north coast of Molokaihave been uniform throughout most of thehistory of the area.

243

ACKNOWLEDGMENTS

e e The offic~rs a~d men of the USC&GS ShipMcArthur carried out the bathymetric survey,

and Commander R. L. Newsom made the dataavailable to the author. The officers and men ofthe R/V "Mahi" and many graduate students inOceanography assisted in the collection of theseismic reflection data. The figures reflect theskills of Gary Kajiwara. Janet Mathewson spentmany hours working on the data : Ethel McAfeegave editorial guidance in the preparation of thepaper for publication. Dr. James Andrews andDr. Alexander Malahoff critically reviewed thisp~per. To all, the author expresses his appreci­ation.

LITERATURE CITED

EASTON, W. H. 1965. N ew Pleistocene shorelines in Hawaii. Geological Society of Amer­ica Cordilleran Section, 61st Annual Meet­ing, Program, p. 21.

FURUMOTO, A. S., and G. P. WOOLLARD.1965. Seismic refraction studies of the crustalstructure of the Hawaiian Archipelago. Pa­cific Science, vol. 19, no. 3, pp . 315-319.

KROENKE, L. W . 1965. Seismic reflectionstudies of sediment thicknesses around theHawaiian Ridge. Pacific Science, vol. 19, no.3, pp . 335-338.

MALAHOFF, A., and G. P. WOOLLARD. 1966.Magnetic surveys over the Hawaiian Islandsand their geologic implications. Pacific Sci­ence, vol. 20, no. 3, pp. 265-311.

MATHEWSON, CHRISTOPHER C. 1969. Bathym­etry of the north insular shelf of MolokaiIsland, Hawaii from surveys by the USC&GSS "McArthur." Data Report No . 14,Hawaii Institute of Geophysics Report HIG­69-21, 9 pp., 5 figs., 1 map.

MATHEWSON, CHRISTOPHER c., and ALEX­ANDER MALAHOFF. 1969. Ancient shelf alongthe Hawaiian Ridge (Abstract) . Fall Meeting(San Francisco) , A.G.u., December 1969.

MOORE, J. G. 1964. Giant submarine landslideson the Hawaiian Ridge. U. S. GeologicalSurvey, Professional Paper 501-D , pp . D95­D98.

RUHE, R. V., J. M. WILLIAMS, and E. L. HILL.1965. Shorelines and submarine shelves,Oahu, Hawaii. Journal of Geology, vol. 73,pp. 485- 497.

244

SHEPARD, F. P., and R. F. DILL. 1966. Sub­marine canyons and other sea valleys. RandMcNally and Co., Chicago. 381 pp .

STEARNS, H. T. 1966. Geology of the State ofHawaii. Pacific Books, Palo Alto, California.266 pp .

- - - 1967. Geology of the Hawaiian Islands.State of Hawaii Dept. of Land and N aturalResources, Bulletin 8; supp lement by G. A.Macdonald.

PACIFIC SCIEN CE, Vol. 24, April 1970

STEARNS, H. T., and G. A. MACDONALD. 1947.Geology and groundwater resources of theisland of Molokai, H awaii. H awaii Divisionof Hydrography, Bulletin 11. 113 pp. (geo­logic map enclosed) .

STRANGE, W . E., G. P. WOOLLARD, and J. c., ROSE. 1965. An analysis of the grav ity field

over the Hawaiian Islands in terms of crustalstructure. Pacific Science, vol. 19, no. 3,pp . 381-389.