The term "pothole" has been applied to a
variety of depressions by many writers. The term
is used here to describe depressions which are
abraded and scoured by water and grinding
tools under the influence of currents that produce
a vortex motion. Elston (1918) referred
to holes developed in this manner as "normal
potholes," and Alexander (1932) used also the
term "eddy holes" to describe depressions
formed by this mechanism

Abbott, Agatin T.

Pottratz, Steven W.

Pacific Science

ScholarSpace at University of Hawai'i at Manoa

Marine Pothole Erosion, Oahu, Hawaii!AGATIN T. ABBOTT2 and STEVEN W. POTTRATZ3THE TERM "pothole" has been applied to avariety of depressions by many writers. The termis used here to describe depressions which areabraded and scoured by water and grindingtools under the influence of currents that pro-duce a vortex motion. Elston (1918) referredto holes developed in this manner as "normalpotholes," and Alexander (1932) used also theterm "eddy holes" to describe depressionsformed by this mechanism.Fluvial and marine potholes and solutionfeatures found on the Hawaiian Islands havebeen described in general by Wentworth(1944). In the present study, marine potholesdeveloped primarily by wave abrasion along theshorelines of Oahu are treated in greater detail.Our purpose is to demonstrate that wave scourin the drilling of marine potholes plays a sig-nificant role in the erosion and reduction ofportions of Oahu's shores, and that the type ofrock determines the type of pothole formed.Wiens (1957) was impressed by the significantrole of mechanical abrasion in the erosion ofshorelines of Pacific islands.GENERAL SETTINGLocalities selected for study provide a varietyof conditions in rock type, structural attitudes,and exposure to wave attack. At seven locationsaround the perimeter of the island, the dimen-sions of more than 140 potholes were measured,and the relationship between the form of thepothole and the kind of rock was recorded(Tables 1, 2, and 3). These specific sections ofcoastline are shown in Fig. 1.Three major rock types exposed along Oahu'scoasts are being actively drilled. These rocks arebasalt (Fig. 2), volcanic tuff (Fig. 3), and1 Hawaii Institute of Geophysics Contribution No.253. Manuscript received September 26, 1968.2 Department of Geosciences, University of Hawaii,Honolulu, Hawaii 96822.3 San Joaquin Delta College, Stockton, California95204.lithified carbonate sand beach deposits, calledbeachrock (Fig. 4).MECHANISM OF DRILLINGThe action of waves in producing potholes isquite similar to that of running water in a streamchannel, although the stream-formed varietyhas received more attention in the literature.From observing wave action, it is evident thatthe incoming surge of the wave is the majorenergy source for swirling the pebbles aroundin the depressions. Some additional swirlingtakes place as the wave recedes, but this actionis less vigorous. There is no reason to suspectthat the behavior of the grinding pebbles ismuch different from that observed in laboratoryexperimentation by Alexander (1932, p. 318).Some potholes contain no grinders, and it ap-pears likely that hydraulic action by heavy waveswash and cavitation (Barnes, 1956) are effec-tive in forming potholes even though constantpebble abrasion is lacking (Fig. 5). Alexander(1932, p. 334) makes a point of this in speak-ing of stream-eroded potholes:The adhesion of water to rock is greater than thecohesion of water, from which it follows that all rocksare covered with a fixed layer of water. This layer ofwater not only protects the rock from the effect ofwater flowing over it, but also permits many varietiesof algae to secure a "foothold." This algal growthwhich often covers the rocks in rapids [or in waveswash] still further protects them from wear. Forerosion to occur the water itself must strike againstthe rock surface with sufficient force to break throughthe fixed layer, or it must dash or roll against the rocksurface some solid particle which will break throughand produce wear. A stream [waves] that does notcarry tools therefore does not erode except under fallsor in precipitous rapids [or heavy surf]. Much of theerosion in plunge pools and gouge holes is of the tool-less type, although tools when present greatly increasethe effect.GENERAL DESCRIPTION OF POTHOLESMost individual potholes along the shorelinetend toward circular to elliptical shapes. In276Marine Pothole Erosion-ABBoTT AND POTTRATZIEi<!iI BASALTI!iI!lll TUFF&ill CALCAREOUSBEACH ROCK277I'""T-~----,---,-----,.-------,,...,u·OICAUAI.t1 .AHU~ MOLDK'"Do'0 ;t:),lUI"AWAII.7FIG. 1. Location map showing seven areas along Oahu's coastline where marine potholes were observed.basaltic lava rock, the potholes occur as indi-vidual depressions with wide interareas (Figs.6 and 7). In tuff or calcareous beachrock, thepotholes frequently coalesce. By the process ofcoalescence an intricate network of potholes isdeveloped to form an extremely irregular sur-face (Fig. 8). The remaining ridges separatingthe potholes which have not joined become verythin and, in general, lie parallel to the directionof surge motion which is usually perpendicularto the trend of the shoreline.Some potholes are interconnected by shallowchannels which serve as drainage outlets for ex-cess water after the wave surge has receded(Fig. 8). Sand grains and small pebbles arefound in some channels, suggesting that abrasionand scouring are important processes in thedeepening and widening of these outlets. Thechannels vary in depth and width but generallymaintain dimensions which are less than thoseof the connected potholes.Elongate troughs, with potholes in the channelbottom (Fig. 9), are developing along the shorearound Diamond Head and around Koko Head,(Honolulu Series tuff cones). These troughscommonly follow joints and fractures in thetuff. Once a channel has been enlarged alonga fracture, the incoming wave swash follows aconfined path. Much of the energy of the waveis concentrated in this trough each time a wavebreaks over the shore. This concentration ofenergy establishes ideal conditions for potholedrilling in the channel bottom.DEVELOPMENT ON DIFFERENT ROCK TYPESAND ATTITUDESThe shapes of potholes and the rate at whichthey are scoured are directly influenced by therock types mentioned earlier. On Oahu's shores,potholes are most numerous per unit of area oncalcareous beachrock deposits. Fewer potholesdevelop per unit area in basalt than in eitherbeachrock or tuff. Tuff shows high susceptibilityto the formation of potholes where the sea-ward dip of the tuffaceous beds is gentle, as is278 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 2. Pothole in massive basalt. Cobble of basalt is wedged in hole and cutting occurs around it. (Area 3.)the case along the south coast of Diamond Headand along the southwest shore of Koko Head.But where the seaward dip steepens, as along thenortheast shore of Koko Head and on the south-western side of Diamond Head, pothole forma-tion is poor or absent (Fig. 10).A distinct wave-cut bench about 5 feet abovepresent sea level extends along the Koko Headshorelines (Stearns, 1961, p. 7). In HanaumaBay the bench truncates steeply dipping layersof tuff. Differential erosion on the bench re-sulted in a series of tiny parallel ridges andvalleys which represent the edges of the trun-cated layers of tuff. The strike of these smallridges and intervening valleys approximatelyparallels the shoreline. Hence, when wavesbreak and surge across the bench, the flow pat-tern is quite irregular. This dissipation of theenergy associated with the inland-moving surgehinders pothole development. Where the dip ofthe layers of tuff or beachrock is gentle ornearly flat, as it is on the west side of KokoHead, potholes are fairly common (Fig. 6).Along shores composed of beachrock, pot-holes and solution pits are very numerous(Wentworth, 1944). Unquestionably, potholeabrasion plus the chemical attack of salt waterand salt spray operate together in producingcavities, holes, pits, pans, and depressions in thecalcareous beachrock. In the zone of active wavesurge, the insides of the holes are polishedsmooth as the result of pothole abrasion. Abovethis zone the insides of the holes are unpolishedand rough and spiny (Figs. 11 and 12). Thebeachrock on the northwest shore of Oahu isriddled with potholes. One section (Fig. 1,Area 1) has several hundred potholes in a stripof shoreline 25 feet wide and approximatelyMarine Pothole Erosion-ABBoTT AND POTTRATZ 279FIG. 3. Pothole in volcanic tuff. Honolulu Volcanic Series on face of Diamond Head. Pebbles are coral andtuff. (Area 5.)300 feet long. Many of the potholes are elon-gate, with the maximum axis oriented perpen-dicular to the trend of the beach.GRINDING TOOLSThe lithology of the grinding tools is anotherimportant factor. On Oahu three types of rockserve as grinding tools in the potholes. Theseare (1) calcareous material, including fragmentsof coral reef and beachrock, (2) volcanic tuff,and (3) basalt. Although calcareous materialrests in many potholes scoured in basalt, it isdoubtful, because of its comparative softness,that it played a significant role in the drillingprocess. Pebbles of dense basalt· found in alltypes of potholes in all types of rock un-doubtedly provide the most effective grindingtools.PROCESS OF DEVELOPMENTFor this study more than 140 potholes weremeasured in basalt, tuff, and calcareous beach-rock. For each pothole, the diameter and depthwere recorded and the diameter-depth ratio wascomputed. The purpose of these measurementswas to' determine whether lateral widening orvertical deepening in each of the three rocktypes is the dominant process in the drilling ofthe marine potholes. In elongated potholes, themaximum and minimum diameters were mea-sured and an average circular dimension deter-mined. Measurements and ratios are presentedin Tables 1, 2, and 3.In potholes in basalt (Table 1), the con-clusions are not so well defined as in the othertwo rock types mentioned below. Ratios in ba-salt varied from 0.7 to 2.7. Of the potholes280 PACIFIC SCIENCE, Vol. XXIII, July 1969TABLE 1 TABLE 2DIMENSIONS (IN INCHES) AND DIAMETER-DEPTHRATIOS OBTAINED FOR POTHOLES CUT INBASALT; OAHU, HAWAIIDIMENSIONS (IN INCHES) AND DIAMETER-DEPTHRATIOS OBTAINED FOR POTHOLES CUT IN TUFF;OAHU, HAWAIICIRCULAR MAXIMUM MINIMUMDIAMETER DIAMETER DIAMETER DEPTH RATIOCIRCULAR MAXIMUM MINIMUMDIAMETER DIAMETER DIAMETER DEPTH RATIO• ( ) = Circular diameter computed.19351420(26) 32 21(14) 16 1234818112115181719(14) 17 1013112073060Average ratio = 1.1measured in basalt, 45 per cent had diametersless than their depth, 40 per cent had diametersgreater than their depth, and 15 per cent haddiameters approximately equal to their depth.All ratios combined for potholes in basalt showan average value of 1.1.Similar measurements and ratios were ob-tained from potholes in volcanic tuff. The dia-meter exceeded the depth in all holes excepttwo, where the depth and diameter were ap-proximately equal (Table 2). The range ofratios extends from 1.0 to 2.5, with an averagevalue of 1.6. As is the case in beachrock, therate of lateral cutting seems to exceed the rate ofdeepening.Data from potholes developed on calcareousbeachrock (Table 3) show that in every hole• ( ) = Circular diameter computed.2.52.11.51.81.52.02.01.71.11.31.61.51.72.01.41.21.71.11.41.01.61.41.01.3128941261411131210141871424720818249121575121418241218Area 51813Area 6(30)* 42(17) 22147 918(12) 1828(18) 231415(16) 2022(30) 36(14) 16(19) 202812(22) 24121840131219Average ratio = 1.62.72.21.01.02.02.51.01.91.81.11.11.50.80.90.80.90.81.00.81.10.80.70.90.71.01.31.11.10.91.01.31.21.00.90.91.30.80.90.90.70.80.91.01.10.71.10.93579329961064112312151118116614142120153113232511298201316181918271715111910286689613151220Area 36Area 779132121111524( 8)*1179659(17)1111769(20)10(13 )9189( 7)5(10)(12)15(20)Marine Pothole Erosion-ABBoTT AND POTTRATZ 281TABLE 3 TABLE 3 (continued)DIMENSIONS (IN INCHES) AND DIAMETER-DEPTHRATIOS OBTAINED FOR POTHOLES CUT INBEACHROCK; OAHU, HAWAIICIRCULAR MINIMUM MAXIMUMDIAMETER DIAMETER DIAMETER DEPTH RATIOArea 2 (continued)* ( ) = Circular diameter computed.CIRCULAR MAXIMUM MINIMUMDIAMETER DIAMETER DIAMETERthe diameter exceeded the depth. The ratio ofdiameter to depth ranged from 1.3 to 4.0, withan average value of 2.1. This suggests that inbeachrock lateral undercutting of the potholewalls occurs more commonly than downcutting.These observations indicate that the diameter-depth ratio is controlled by lithology-bearingout the conclusions of Alexander (1932, p.329). Potholes along a shoreline containingvery gently seaward-dipping stratified rocks gen-erally have a greater diameter-depth ratio thanis found in massive basalt. The apparent explan-ation is that, in the process of lateral extensionof the pothole walls, in finely stratified rockssuch as tuff and beachrock, the grinding toolsattack the strata edgewise where some are lessresistant. The dense, more uniform, unstratified* ( ) = Circular diameter computed.2.22.42.22.02.11.32.11.51.41.61.82.62.52.53.21.61.41.91.42.31.21.71.91.31.92.21.71.4455566798777685710181399111110161011107111814Area 431917912111013( 8)15(14)111113181520161114(35) 3919(21) 23111920133122(19) 2514Average ratio = 2.12.22.82.52.42.33.01.91.82.02.22.21.62.03.72.02.52.32.22.71.51.71.81.61.72.02.52.12.32.22.01.91.61.62.71.71.71.81.72.11.52.11.42.74.02.01.81.63.5RATIO54453465345833347436695769111257911119713131111151111367552DEPTH148710911109211324141811191519Area 1109Area 220914131413141618111421302819262228202624(11)*(11)6(12)71211969( 11)(13)6(11 )610(16)( 9)8910(16)812(12)(24)23(27)1114(17)1818(24)12(22)23(19)2322(23)(16)8(24)149( 8)7282 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 4. Pothole in calcareous beachrock; flakes and grains of carbonate rock resting in hole. (Area 4.)Marine Pothole Erosion-ABBoTT AND POTTRATZ 283FIG. 5. Potholes in beachrock, formed largely by hydraulic action of heavy surf; few if any grindingpebbles present. (Area 4.)284 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 6. Potholes in basalt. Note wide spacing. Cobble of basalt and fragments of coral rest in larger hole.(Area 3.)Marine Pothole Erosion-ABBoTT AND POTTRATZ 285FIG. 7. Potholes in basalt. (Area 3.)286 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 8. Potholes scoured in volcanic tuff. Note polishing of interiors and coalescence. Pebbles are composedof tuff, coral, and basalt. (Area 5.)Marine Pothole Erosion-ABBoTT AND POTTRATZ 287FIG. 9. Elongate trough following joint in gently dipping volcanic tuff. Pothole at landward end of trough.P~bbles are composed of coral, tuff, and basalt. (Area 6.)288 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 10. Comparison of active potholing in flat-lying, light-colored beachrock in foreground with no potholeaction in more steeply dipping dark-colored volcanic tuff in background. (Area 5.)Marine Pothole Erosion-ABBoTT AND POTTRATZ 289FIG. 11. Above constant surf zone in beachrock, combined action of solution processes and potholing toform cavities. (Area 4.)290 PACIFIC SCIENCE, Vol. XXIII, July 1969FIG. 12. Pothole abrasion predominant in surf zone; solution cavities predominant to the right of ham-mer. (Area 2.)basalt wears away slightly faster at the bottom ofthe pothole where downcutting pebble abrasionexceeds lateral cutting.SUMMARYAlong certain sections of the shorelines ofOahu, erosion is being hastened by wave-inducedpothole drilling. Most potholes occur in theactive wave-surge zone and are drilled in basalt,volcanic tuff, and calcareous beachrock. Rocktype is an important factor in controlling thediameter-depth ratio. In basalt the average is1.1, in volcanic tuff it is 1.6, and in beachrock2.1.The writers wish to thank Dr. Gordon A.Macdonald for his reading of the manuscriptand for his excellent suggestions and comments.LITERATURE CITEDALEXANDER, H. S. 1932. Pothole erosion. Jour-nal of Geology, vol. 40, no. 4, pp. 305-337.BARNES, H. L. 1956. Cavitation as a geologicalagent. American Journal of Science, vol. 254,pp. 493-505.ELSTON, E. D. 1918. Potholes, their variety,origin, and significance, II. Scientific Monthly,vol. 6, pp. 37-53.STEARNS, H.t. 1961. Eustatic shorelines onPacific islands. Zeitschrift fur Geomorpholo-gie, vol. 3, pp. 3-16.WENTWORTH, C. K. 1944. Potholes, pits, andpans; subaerial and marine (Hawaii). Journalof Geology, vol. 52, pp. 117-130.WIENS, H. J. 1957. The role of mechanicalabrasion in the erosion of coral reefs and landareas. Pacific Science Congress (1957), vol.12, pp. 361-366.

Marine Pothole Erosion, Oahu, Hawaii

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Marine Pothole Erosion, Oahu, Hawaii

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