Geomorphic features of Niue Island: chasms, caves and other karst varieties

International฀Scientific฀Council฀For฀Island฀Development c/o฀UNESCO 1,฀rue฀Miollis 75015฀Paris France www.insula.org Edited฀by: James฀P.฀Terry฀and฀Warwick฀E.฀Murray Edtition฀reviewed฀by: Cipriano฀Marín Giuseppe฀Orlando Layout:฀ Luis฀Mir Front฀Cover:฀‘Ngutu฀Ana’ Oil฀Painting฀by฀Mark฀Cross,฀Niue฀Artist This฀is฀Anaana฀Cave฀in฀the฀cliffs฀of฀Avaiiki฀on฀the฀north฀west฀coast฀of฀Niue Reproduced฀from฀the฀original฀with฀kind฀permission฀of฀Mark฀Cross 2 Niue Island GEOGRAPHICAL฀PERSPECTIVES฀ ON฀THE฀ROCK฀OF฀POLYNESIA Edited฀by: James฀P.฀Terry฀and฀Warwick฀E.฀Murray International฀Scientific฀Council฀For฀Island฀Development฀ 3 GEOMORPHIC FEATURES OF NIUE ISLAND: CHASMS, CAVES AND OTHER KARST VARIETIES JAMES฀P.฀TERRY Geography Department, the University of the South Pacic Suva, Fiji Introduction Niue฀has฀a฀unique฀and฀attractive฀landscape.฀Because฀the฀island฀is฀an฀uplifted฀coral฀ atoll,฀it฀has฀a฀central฀depression฀surrounded฀by฀a฀rocky฀rim,฀dropping฀off฀in฀sheer฀ cliffs฀to฀the฀sea.฀Superimposed฀on฀this฀morphology฀is฀an฀assemblage฀of฀interesting฀ smaller฀landscape฀features,฀some฀of฀which฀have฀been฀sculpted฀from฀the฀limestone฀ bedrock฀ by฀ solution฀ and฀ may฀ be฀ called฀ karst฀ landforms฀ (Figure฀ 1).฀This฀ chapter฀ describes฀these฀features฀and฀explains฀why฀Niue’s฀raised฀reef฀landscape฀should฀be฀ regarded฀as฀a฀terrain฀of฀geomorphological฀signiicance. Characteristics of Niue’s carbonate geology Composition Limestone฀is฀a฀bio-sedimentary฀rock฀made฀from฀the฀calcareous฀skeletons฀of฀marine฀ organisms.฀There฀are฀two฀principal฀modes฀of฀formation:฀the฀growth฀of฀coral฀reefs฀and฀ the฀sedimentation฀of฀skeletons฀of฀marine฀creatures.฀Niue฀is฀an฀uplifted฀coral฀atoll฀and฀ its฀limestone฀geology฀occurs฀in฀three฀facies฀(Cliff฀Ollier฀pers.฀comm.).฀There฀is฀a฀reef฀ facies฀with฀corals฀in฀their฀original฀growth฀position฀in฀the฀Mutalau฀Reef.฀There฀is฀also฀ a฀fore-reef฀facies฀which฀is฀a฀conglomerate฀of฀chunks฀broken฀from฀and฀accumulated฀ in฀front฀of฀the฀living฀reef฀while฀it฀was฀still฀submerged.฀Fore-reef฀conglomerate฀is฀ well฀exposed฀in฀the฀cliffs฀and฀caves฀north฀of฀Mutalau฀village฀and฀in฀the฀cliffs฀at฀Ana฀ Ana฀lookout.฀These฀two฀limestone฀ facies฀comprise฀skeletons฀of฀corals,฀ crustose฀algae฀and฀foraminifera.฀A฀ lagoon฀ facies฀ of฀ poorly฀ cemented฀ coralline฀sands฀covers฀most฀of฀the฀ island฀ in฀ the฀ central฀ depression.฀ This฀ facies฀ is฀ a฀ partly฀ lithiied฀ lagoonal฀ deposit,฀ rich฀ in฀ shells฀ of฀ marine฀gastropods฀and฀bivalve฀molluscs,฀coralline฀algae฀and฀echinoids฀ Figure฀2.฀Partly฀lithiied฀calcarenite฀deposits฀in฀the฀Mutalau฀ Lagoon,฀rich฀in฀shells฀of฀marine฀gastropods฀and฀bivalve฀mol(Figure฀2). luscs,฀coralline฀algae฀and฀echinoids.฀Photo฀by฀Cliff฀Ollier. 75 76 NIUE฀ISLAND Figure฀1.฀฀Schematic฀cross-section฀through฀Niue,฀showing฀the฀geomorphic฀features฀described฀in฀the฀text. GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES The฀ texture฀ of฀ the฀ limestones฀ varies฀ from฀ hard฀ and฀ dense,฀ to฀ soft,฀ chalky฀ and฀ sugary฀ (Jackson฀ and฀ Hill฀ 1980).฀ The฀ colour฀ is฀ white,฀ sometimes฀ creamy฀ or฀ tan,฀ weathering฀to฀grey฀at฀exposures.฀The฀rock฀is฀faulted,฀jointed฀and฀porous,฀so฀rainwater฀percolates฀easily,฀giving฀vertical฀drainage฀and฀no฀runoff. Mineralogy Niue’s฀carbonate฀bedrock฀has฀interesting฀mineralogy.฀Calcite฀and฀aragonite฀(both฀ CaCO3)฀are฀the฀primary฀minerals฀of฀the฀coastal฀rocks,฀but฀the฀bulk฀of฀the฀inland฀ limestone฀has฀been฀altered฀by฀a฀geochemical฀process฀called฀dolomitisation.฀This฀is฀ the฀replacement฀of฀calcium฀carbonate฀by฀the฀mineral฀dolomite,฀which฀has฀a฀higher฀ magnesium฀content฀(CaMg(CO3)2),฀and฀occurs฀when฀magnesium-rich฀groundwaters฀seep฀through฀limestone฀bedrock.฀According฀to฀Friedman฀and฀Sanders฀(1967),฀a฀ source฀of฀the฀magnesium฀needed฀for฀the฀dolomitisation฀process฀is฀hypersaline฀brine.฀ Such฀brines฀can฀be฀produced฀by฀the฀partial฀evaporation฀of฀sea฀water,฀for฀example฀ in฀supratidal฀lakes฀or฀shallow฀coastal฀lagoons฀that฀are฀subjected฀to฀evaporitic฀conditions฀(Murray฀1969).฀These฀environmental฀conditions฀can฀be฀envisaged฀as฀the฀preNiue฀ atoll฀ irst฀ emerged฀ above฀ sea-level฀ some฀ half฀ a฀ million฀ years฀ ago,฀ and฀ the฀ central฀lagoon฀became฀progressively฀cut฀off฀from฀the฀surrounding฀ocean.฀Quaternary฀glacio-eustatic฀low฀sea฀levels฀may฀likewise฀have฀contributed฀to฀dolomitisation฀ (Wheeler฀et฀al.฀1999).฀Recent฀work฀by฀Ebrahim฀(1999)฀on฀Tarawa฀atoll,฀Kiribati,฀ has฀also฀shown฀that฀dolomitisation฀can฀occur฀in฀atoll฀lagoons฀that฀are฀not฀cut฀off฀ from฀the฀ocean,฀and฀other฀workers฀suggest฀that฀microbial฀processes฀may฀also฀be฀ important.฀For฀recent฀ideas฀on฀dolomitisation,฀readers฀are฀directed฀to฀Purser฀et฀al.฀ (1994).฀ Weathering A฀ primary฀ mechanism฀ of฀ landscape฀ denudation฀ on฀ limestone฀ in฀ humid฀ environments฀is฀chemical฀weathering฀by฀solution.฀This฀is฀because฀calcite฀is฀attacked฀by฀the฀ natural฀acidity฀of฀rainwater.฀Dolomite฀is฀less฀soluble฀than฀calcite,฀but฀the฀principle฀ is฀the฀same.฀The฀solution฀process฀can฀be฀considered฀in฀stages฀as฀follows: Stage฀1:฀Carbon฀dioxide฀gas,฀present฀in฀the฀atmosphere฀and฀in฀soil฀air฀spaces,฀dissolves฀in฀rainwater฀to฀form฀a฀weak฀solution฀of฀carbonic฀acid:฀ CO2฀(g)฀+฀H2O฀(l)฀↔฀H2CO3฀(aq)฀฀฀1 1 (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution 77 NIUE฀ISLAND Stage฀2:฀When฀the฀carbonic฀acid฀solution฀comes฀into฀contact฀with฀limestone,฀it฀ reacts฀to฀form฀calcium฀bicarbonate,฀which฀immediately฀dissociates฀into฀ions฀of฀calcium฀and฀hydrogen฀carbonate฀in฀aqueous฀solution: H2CO3฀(aq)฀+฀CaCO3฀(s)฀↔฀Ca2+฀(aq)฀+฀2HCO3-฀(aq) ฀฀฀฀฀฀฀฀฀฀฀(calcite) ฀฀ For฀dolomite฀the฀equation฀is: 2H2CO3฀(aq)฀+฀CaMg(CO3)2฀(s)฀↔฀Ca2+฀(aq)฀+฀Mg2+฀(aq)฀+฀4HCO3-฀(aq) ฀ ฀฀฀฀฀฀฀฀฀฀฀฀(dolomite) (The฀double฀headed฀arrows฀in฀the฀equations฀indicate฀that฀these฀chemical฀reactions฀ are฀reversible,฀i.e.฀carbon฀dioxide฀can฀be฀degassed฀from฀carbonic฀acid฀and฀calcium฀ carbonate฀precipitated฀out฀of฀solution,฀which฀we฀shall฀see฀later฀is฀important฀for฀the฀ formation฀of฀speleothems) Stage฀3:฀The฀ions฀of฀calcium,฀magnesium,฀and฀hydrogen฀carbonate฀in฀solution฀are฀ transported฀away฀by฀water฀percolating฀downwards฀through฀joints฀and฀issures฀in฀the฀ bedrock.฀ Over฀a฀very฀long฀period,฀continued฀solution฀of฀limestone฀may฀eventually฀lead฀to฀ the฀development฀of฀a฀suite฀of฀associated฀landforms,฀known฀collectively฀as฀a฀karst฀ landscape.฀The฀name฀comes฀from฀the฀slavic฀word฀‘kras’,฀meaning฀rock,฀and฀was฀ originally฀applied฀to฀the฀extensive฀limestone฀landscape฀of฀the฀Dinarai฀Planina,฀bordering฀the฀Adriatic฀Sea฀in฀the฀former฀Yugoslavia.฀Nowadays,฀the฀term฀karst฀is฀used฀ more฀widely฀by฀geomorphologists฀to฀describe฀any฀terrain฀that฀is: 1.฀฀ underlain฀by฀soluble฀carbonate฀rocks฀such฀as฀limestones฀(CaCO3,฀calcium฀carbonate)฀and฀dolomite฀(CaMg(CO3)2,฀calcium฀magnesium฀carbonate) 2.฀฀ dominated฀ by฀ solution฀ processes฀ in฀ humid฀ regions,฀ or฀ in฀ arid฀ regions฀ where฀ solution฀features฀have฀been฀inherited฀from฀a฀wetter฀period฀in฀the฀past, 3.฀฀ an฀ assemblage฀ of฀ landforms฀ exhibiting฀ an฀ absence฀ of฀ surface฀ drainage฀ and฀ a฀ predominance฀of฀vertical฀and฀underground฀drainage฀instead. Conditions for karstification For฀any฀limestone฀area฀to฀be฀karstiied,฀there฀are฀certain฀geologic,฀climatic฀and฀topographic฀conditions฀that฀must฀be฀met,฀and฀we฀should฀consider฀these฀before฀describing฀ Niue’s฀landforms฀in฀detail.฀The฀geological฀conditions฀are฀that: 78 GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES 1.฀ the฀limestone฀should฀be฀extensive 2.฀ the฀rock฀must฀be฀of฀considerable฀thickness,฀typically฀tens฀to฀hundreds฀of฀metres,฀ These฀two฀spatial฀criteria฀are฀important฀because฀large฀solution฀features฀such฀as฀ cave฀systems฀(see฀later)฀cannot฀develop฀on฀thin฀or฀restricted฀rock฀outcrops. 3.฀ the฀bedrock฀should฀be฀highly฀permeable,฀to฀allow฀the฀penetration฀and฀free฀circulation฀of฀rainwater 4.฀ the฀rock฀should฀be฀hard,฀so฀that฀physical฀weathering฀will฀be฀subordinate฀to฀chemical฀weathering. Niue’s฀ island฀ geology฀ meets฀ the฀ above฀ criteria.฀ None฀ of฀ the฀ 259฀ km2฀ area฀ of฀ raised฀atoll฀reef฀is฀overlain฀by฀other฀geology,฀and฀Quaternary฀uplift฀has฀exposed฀a฀ thickness฀of฀up฀to฀68฀m฀of฀rock฀above฀sea-level.฀The฀coral฀limestone฀is฀also฀permeable฀as฀mentioned฀earlier. Important฀climatic฀conditions฀for฀karstiication฀are฀the฀availability฀of฀moderate฀to฀high฀ annual฀precipitation,฀in฀combination฀with฀high฀temperatures,฀so฀that฀solution฀can฀readily฀ take฀place.฀Niue’s฀humid฀tropical฀climate฀has฀2050฀mm฀of฀rainfall฀per฀year฀and฀average฀ temperatures฀above฀20°C.฀This฀favours฀the฀growth฀of฀rainforest฀vegetation฀which฀assists฀ solution฀because฀plant-root฀respiration฀and฀the฀decomposition฀of฀organic฀humus฀raises฀ the฀concentration฀of฀carbon฀dioxide฀in฀the฀soil฀above฀normal฀atmospheric฀concentrations.฀Diffusion฀of฀this฀additional฀CO2฀into฀iniltrating฀rainwater฀makes฀the฀soil฀moisture฀ a฀more฀aggressive฀carbonic฀acid,฀increasing฀solution฀of฀the฀bedrock. The฀main฀topographic฀condition฀for฀karstiication฀is฀suficient฀elevation฀to฀permit฀ free฀ vertical฀ drainage฀ and฀ circulation฀ of฀ the฀ groundwater.฀ Niue’s฀ 68฀ m฀ elevation฀ above฀sea-level฀promotes฀good฀percolation฀and฀vertical฀drainage. Niuean karst in context Important฀ studies฀ of฀ geomorphology฀ on฀ islands฀ with฀ carbonate฀ bedrocks฀ include฀ Borneo฀ in฀ the฀ East฀ Indies฀ (Wilford฀ and฀Wall฀ 1967),฀ Jamaica฀ and฀ Puerto฀ Rico฀ in฀ the฀West฀Indies฀(Pfeffer฀1997),฀the฀Trobriand฀Islands฀of฀Papua฀New฀Guinea฀(Ollier฀ 1975),฀Makatea฀in฀the฀Tuamoto฀archipelago฀(Montaggioni฀et฀al.฀1985),฀Tongatapu฀ and฀‘Eua฀in฀Tonga฀(Lowe฀and฀Gunn฀1986),฀and฀Santo฀island฀in฀Vanuatu฀(Strecker฀ et฀al.฀1987).฀The฀Paciic฀Island฀examples฀mainly฀describe฀geomorphic฀features฀that฀ have฀formed฀on฀reef฀limestones,฀but฀these฀studies฀still฀remain฀a฀minority฀compared฀ to฀ karst฀ studies฀ on฀ non-reef฀ types฀ of฀ limestone.฀ Since฀ Niue฀ is฀ a฀ large฀ emerged฀ atoll,฀it฀provides฀an฀excellent฀location฀to฀investigate฀karst฀development฀over฀a฀large฀ expanse฀of฀coralline฀limestones. 79 NIUE฀ISLAND Other฀raised฀atoll฀islands฀in฀the฀Paciic฀with฀well฀developed฀solutional฀features฀ include฀Banaba฀and฀Nauru฀in฀Micronesia.฀These฀islands฀can฀be฀distinguished,฀however,฀ because฀ their฀ karst฀ surfaces฀ developed฀ beneath฀ a฀ layer฀ of฀ rock฀ phosphate,฀ formed฀ from฀ the฀ guano฀ of฀ enormous฀ seabird฀ rookeries฀ that฀ were฀ present฀ before฀ human฀ occupation.฀ Niue฀ does฀ not฀ have฀ an฀ extensive฀ covering฀ of฀ phosphate฀ rock฀ because฀of฀the฀absence฀of฀an฀upwelling฀current฀of฀deep฀nutrient-rich฀ocean฀waters฀ to฀support฀a฀large฀seabird฀rookery,฀as฀was฀the฀case฀on฀Banaba฀and฀Nauru฀(Stoddart฀ and฀Scofin฀1983).฀ Features of Niuean karst An฀absence฀of฀drainage Niue’s฀landscape฀has฀no฀rivers฀or฀streams,฀nor฀the฀associated฀features฀of฀luvial฀geomorphology฀such฀as฀valleys,฀terraces฀and฀deltas.฀This฀is฀due฀to฀the฀high฀bedrock฀permeability,฀which฀promotes฀vertical฀drainage฀instead฀of฀a฀surface฀drainage฀pattern.฀ Continental฀or฀large฀island฀karst฀terrains฀elsewhere฀in฀the฀world฀may฀often฀exhibit฀ surface฀drainage฀and฀luvial฀landforms฀because฀rivers฀can฀rise฀on฀adjacent฀non-permeable฀rock฀types฀and฀then฀cross฀a฀limestone฀area,฀sometimes฀through฀gorges฀that฀ have฀been฀deepened฀by฀solution.฀Niue’s฀landscape฀has฀developed฀in฀isolation฀from฀ such฀inluences฀as฀there฀are฀no฀impermeable฀rocks.฀There฀is฀one฀dry฀valley฀on฀Niue,฀ extending฀from฀the฀depression฀in฀the฀island฀interior,฀through฀the฀raised฀atoll฀rim฀at฀ Aloi฀(see฀Chapter฀by฀Nunn฀and฀Britton),฀but฀this฀is฀a฀relict฀reef฀passage฀connecting฀ the฀former฀lagoon฀to฀the฀sea,฀and฀is฀therefore฀not฀a฀luvial฀feature. Basin-and-rim฀topography The฀central฀topographic฀depression฀or฀basin฀on฀Niue฀slopes฀gently฀to฀a฀base฀34฀m฀ above฀ sea-level฀ (Paulay฀ and฀ Spencer฀1992).฀ It฀ is฀ called฀ the฀ Mutalau฀ Lagoon฀ and฀ is฀ interpreted฀ as฀ the฀ former฀ atoll฀ lagoon.฀This฀ is฀ surrounded฀ by฀ a฀ rim,฀ called฀ the฀ Mutalau฀Reef,฀which฀rises฀to฀68฀m฀and฀is฀interpreted฀as฀the฀former฀atoll฀reef.฀Since฀ the฀basin-and-rim฀morphology฀is฀inherited฀from฀atoll฀emergence,฀it฀is฀not฀wholly฀of฀ karst฀origin.฀However,฀there฀is฀evidence฀that฀the฀central฀depression฀has฀been฀deepened฀further฀under฀subaerial฀conditions฀by฀solution฀weathering.฀The฀clue฀lies฀in฀the฀ extent฀of฀the฀veneer฀of฀poorly฀cemented฀coral฀sands,฀which฀cover฀approximately฀ 40%฀ of฀ the฀ basin฀ loor฀ (Wright฀ and฀ van฀Westerndorp฀ 1965).฀ In฀ modern฀ shallow฀ atoll฀lagoons,฀sand฀deposits฀blanket฀most฀of฀the฀lagoon฀loor฀between฀isolated฀coral฀ heads.฀Schoield฀(1959)฀suggests฀that฀the฀‘missing’฀coral฀sand฀in฀Niue’s฀emerged฀ lagoon฀has฀been฀preferentially฀removed฀by฀solution฀since฀island฀uplift.฀ 80 GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES Pinnacles The฀Mutalau฀Reef฀has฀been฀exposed฀to฀subaerial฀weathering฀for฀the฀longest฀period฀ on฀Niue,฀possibly฀700,000฀years฀since฀emergence฀(Fieldes฀et฀al.฀1960),฀and฀has฀been฀ deeply฀dissected฀by฀solution,฀producing฀abundant฀pinnacle฀formations.฀The฀pinnacles฀are฀isolated,฀irregularly฀shaped,฀steep-sided฀blocks฀of฀limestone,฀with฀grooved฀ and฀ pitted฀ surfaces,฀ sometimes฀ resembling฀ large฀ jagged฀ tombstones฀ standing฀ 2-3฀ m฀high.฀Many฀individual฀pinnacles฀may฀represent฀residual฀coral฀heads,฀and฀would฀ have฀a฀spatial฀pattern฀corresponding฀to฀the฀original฀coral฀head฀distribution฀in฀the฀ reef฀(John฀Barrie฀pers.฀comm.).฀As฀the฀difference฀in฀elevation฀between฀the฀highest฀ remnants฀of฀reef฀limestone฀and฀the฀average฀height฀of฀Mutalau฀Reef฀is฀approximately฀ 17฀m฀(Schoield฀1959),฀the฀surface฀lowering฀rate฀by฀solution฀is฀estimated฀at฀24฀mm฀ per฀thousand฀years.฀ Niue’s฀best฀known฀pinnacle฀surface฀is฀found฀on฀the฀23฀m฀marine฀terrace,฀called฀the฀ Aloi฀ Terrace,฀ on฀ the฀ south฀ east฀ coast฀ near฀ Togo฀ (Figure฀ 3).฀ The฀ exposed฀ rock฀ surface฀ has฀ an฀ erratic฀ topography฀ of฀ solution฀ pits฀ and฀sharp฀edged฀points฀ and฀ is฀ well-developed฀ because฀of฀its฀exposed฀ windward฀ location,฀ where฀sea฀spray฀assists฀ normal฀ bedrock฀ solu- Figure฀3.฀Highly฀dissected฀and฀razor฀sharp฀karrenield฀surface฀on฀the฀south฀east฀ coast฀near฀Togo.฀Photo฀by฀Patrick฀Nunn. tion฀by฀rainwater. Dolines Dolines฀are฀closed฀surical฀depressions,฀circular฀to฀oval฀in฀plan,฀ranging฀from฀several฀ metres฀to฀tens฀of฀metres฀across.฀There฀are฀different฀types฀depending฀on฀the฀mode฀of฀ formation.฀Solution฀dolines฀develop฀by฀pronounced฀surface฀solution฀at฀a฀joint฀intersection฀and฀are฀usually฀conical฀in฀shape,฀whereas฀collapse฀dolines฀form฀when฀the฀ roof฀of฀underground฀caverns฀fall฀in,฀giving฀steep฀walls.฀Niue’s฀dolines฀are฀shallow฀ with฀steep฀walls฀and฀loose฀angular฀rock฀debris฀on฀the฀loor,฀indicating฀a฀collapsedcavern฀origin.฀Fine฀dolines฀occur฀at฀the฀entrances฀to฀Ulupaka฀and฀Anataloa฀caves.฀ There฀ are฀ not฀ suficient฀ dolines฀ on฀ Niue฀ to฀ produce฀ the฀ pockmarked฀ surface฀ or฀ 81 NIUE฀ISLAND ‘moonscape’฀effect฀ seen฀ in฀ classic฀ areas฀ of฀ karst,฀ e.g.฀ Croatia฀ in฀ Europe฀ and฀ the฀ Marianna฀region฀of฀Florida,฀USA.฀This฀is฀because฀the฀poorly฀cemented฀coralline฀ sands฀on฀the฀loor฀of฀Mutalau฀Lagoon฀are฀not฀suited฀to฀doline฀formation฀compared฀ to฀hard฀and฀jointed฀bedrock,฀as฀there฀is฀more฀even฀solution฀throughout฀the฀deposit฀ and฀because฀such฀materials฀cannot฀support฀steep-sided฀doline฀walls. Chasms Around฀Niue’s฀coastline฀are฀a฀series฀of฀steep-walled฀chasms฀that฀cut฀deeply฀into฀the฀ Aloi฀terrace.฀These฀impressive฀features฀form฀interconnected฀systems฀up฀to฀500฀m฀ long฀and฀25฀m฀deep,฀and฀were฀interpreted฀by฀Schoield฀(1959)฀as฀solution฀channels฀ along฀fault฀zones฀that฀run฀sub-parallel฀to฀the฀coast.฀He฀suggested฀the฀faults฀were฀ formed฀as฀tension฀cracks฀during฀slumping฀of฀some฀outer฀sections฀of฀the฀Niue฀volcano฀on฀which฀the฀carbonate฀cap฀has฀grown.฀Another฀possibility฀is฀movement฀and฀ settling฀of฀large฀structural฀blocks฀of฀reef฀limestone฀owing฀to฀the฀characteristically฀ greasy฀nature฀of฀the฀geological฀boundary฀between฀coral฀reef฀and฀underlying฀volcanic฀ rocks.฀ This฀ may฀ have฀ developed฀by฀subaerial฀weathering฀ of฀the฀volcanic฀rocks฀to฀clays,฀ perhaps฀ when฀ the฀ Niue฀ volcano฀ irst฀ grew฀ up฀ to฀ the฀ ocean฀ surface฀ in฀ the฀ Late฀ Miocene,฀or฀possibly฀in฀periodsof฀glacio-eustatic฀low฀sea฀ levels.฀ A฀ greasy฀ carbonate/ volcanic฀ boundary฀ has฀ been฀ documented฀beneath฀the฀reef฀ limestone฀on฀Christmas฀Island฀ Figure฀4฀A฀painting฀by฀Mark฀Cross,฀Niue฀artist,฀of฀Matapa฀Chasm฀on฀ in฀ the฀ Indian฀ Ocean฀ (Barrie฀ the฀north฀west฀coast.฀This฀spectacular฀chasm฀with฀a฀deep฀but฀sheltered฀ pool฀of฀clear฀ocean฀water฀is฀a฀good฀spot฀for฀diving฀and฀is฀popular฀with฀ 1967).฀ A฀ further฀ possible฀ tourists. cause฀of฀slumping฀is฀the฀lack฀ of฀a฀solid฀foundation฀to฀parts฀of฀the฀reef฀which฀originally฀grew฀over฀unconsolidated฀ deposits฀of฀coral฀sands฀rather฀than฀irm฀limestone฀bedrock. Some฀chasms฀are฀open฀to฀the฀sea,฀such฀as฀Matapa฀on฀the฀north฀west฀coast฀(Figure฀ 4).฀This฀ example฀ has฀ breccia฀ exposed฀ in฀ the฀ chasm฀ loor,฀ thought฀ to฀ indicate฀ its฀ fault฀origin.฀Other฀chasms฀like฀Togo฀and฀Vaikona฀in฀south฀east฀Niue฀are฀not฀open฀to฀ the฀sea,฀but฀contain฀pools฀supplied฀by฀the฀underground฀aquifer฀(Jacobson฀and฀Hill฀ 82 GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES 1980).฀The฀pools฀are฀brackish฀because฀these฀chasms฀are฀located฀at฀the฀aquifer฀transition฀zone฀from฀fresh฀to฀salt฀water. Caves฀and฀caverns Some฀of฀Niue’s฀most฀striking฀geomorphology฀is฀seen฀by฀venturing฀underground฀into฀ the฀honeycomb฀of฀caves฀and฀caverns฀that฀riddle฀the฀island.฀Caves฀are฀natural฀subterranean฀chambers฀in฀the฀limestone฀bedrock,฀sometimes฀joined฀to฀the฀surface฀by฀small฀ openings฀or฀larger฀passages฀branching฀from฀the฀base฀of฀dolines.฀Different฀types฀of฀ limestone฀caves฀are฀classiied฀according฀to฀their฀position฀with฀respect฀to฀present฀or฀ palaeo-levels฀of฀the฀water฀table. Phreatic฀zone฀caves฀are฀those฀formed฀by฀solution฀below฀the฀water฀table฀as฀slowly฀ migrating฀groundwater฀enlarges฀bedding฀planes฀and฀joints฀into฀a฀network฀of฀interconnecting฀galleries฀and฀shafts.฀Epiphreatic฀zone฀caves฀are฀formed฀by฀the฀combination฀of฀solution฀and฀hydraulic฀action฀(erosion฀by฀the฀force฀of฀lowing฀water)฀by฀ lateral฀low฀at฀the฀level฀of฀the฀water฀table.฀These฀caves฀are฀often฀recognisable฀by฀ their฀smooth฀cylindrical฀walls.฀These฀caves฀are฀lowered฀as฀the฀percolating฀waterexploits฀deeper฀routes฀through฀the฀bedrock.฀Vadose฀zone฀caves฀are฀formed฀by฀solution฀ along฀fractures฀and฀bedding฀planes฀above฀the฀water฀table,฀and฀may฀have฀irregular฀ shapes฀and฀deposits฀of฀rock฀debris฀due฀to฀wall฀and฀roof฀collapse. Around฀Niue’s฀clifline฀are฀ numerous฀ caves฀ which฀ have฀ been฀ exposed฀ by฀ marine฀ erosion฀ and฀ show฀ the฀ intricate฀ solution฀ features฀ typical฀ of฀ formation฀ within฀ the฀ phreatic฀zone฀(Jacobson฀and฀ Hill฀1980),฀or฀are฀the฀outlets฀ of฀ epiphreatic฀ caves฀ formed฀ at฀the฀top฀of฀an฀old฀water฀table฀ Figure฀5.฀Two฀distinct฀levels฀of฀uplifted฀epiphreatic฀caves฀at฀Palaha.฀ (Figure฀ 5).฀ Since฀ tectonic฀ Photo฀by฀Cliff฀Ollier. uplift฀ has฀ stranded฀ many฀ caves฀above฀the฀present฀water฀table,฀these฀are฀strictly฀relict฀phreatic฀or฀epiphreatic฀ caves.฀These฀caves฀should฀not฀be฀confused฀with฀rock฀overhangs฀or฀notches฀found฀ at฀ different฀ heights฀ on฀ cliff฀ faces฀ that฀ were฀ eroded฀ by฀ wave฀ action฀ and฀ marine฀ organisms฀during฀Quaternary฀stillstands฀of฀sea-level.฀The฀true฀caves฀can฀be฀distinguished฀by฀their฀smooth฀sided฀passages฀extending฀far฀back฀into฀the฀cliff฀rock. 83 NIUE฀ISLAND Away฀ from฀ the฀ coast,฀ caves฀ accessible฀ from฀ the฀ surface฀ are฀ generally฀ of฀ the฀ vadose฀type.฀There฀may฀be฀relict฀epiphreatic฀caves฀deeper฀in฀the฀bedrock,฀but฀so฀ far฀there฀are฀no฀written฀accounts฀of฀any฀deep฀underground฀expeditions.฀Many฀of฀ the฀surface฀caves฀are฀small,฀simple฀chambers.฀Others฀have฀numerous฀branching฀passages,฀either฀wide฀and฀cavernous฀or฀narrow฀with฀low฀ceilings.฀Often฀there฀are฀pools฀ of฀water฀on฀the฀loor.฀The฀most฀striking฀features฀in฀these฀caves฀are฀their฀attractive฀ speleothem฀formations฀(see฀next฀section). An฀alternative฀origin฀for฀many฀of฀the฀caves฀on฀Niue฀is฀that฀they฀are฀simply฀tectonic฀ features,฀ and฀ may฀ be฀ called฀ fracture฀ caves฀ (Ford฀ and฀Williams฀ 1989).฀This฀ idea฀ is฀ based฀ on฀ the฀ Quaternary฀ uplift฀ history฀ of฀ Niue,฀ and฀ suggests฀ that฀ during฀ atoll฀emergence,฀sea-level฀fall฀reduced฀buoyant฀support฀of฀the฀fore-reef฀submarine฀ slopes,฀leading฀to฀massive฀slope฀failures฀and฀fracturing฀of฀the฀coral฀cap฀on฀top.฀The฀ surface฀exposure฀of฀these฀tectonic฀fractures฀has฀caused฀them฀to฀be฀(mis)interpreted฀ as฀ vadose฀ zone฀ caves.฀ We฀ may฀ accept฀ the฀ tectonic฀ theory฀ of฀ fracturing,฀ but฀ rainwater฀ would฀ still฀ exploit฀ the฀ fractures฀ as฀ easy฀ routes฀ through฀ the฀bedrock,฀encouraging฀solution฀ and฀cave฀formation. Speleothems Speleothem฀is฀a฀generic฀name฀given฀ to฀the฀large฀variety฀of฀cave฀features฀ produced฀ by฀ calcite฀ precipitation,฀ some฀of฀which฀are฀seen฀in฀Figure฀ 6.฀Speleothems฀form฀when฀percolating฀water฀that฀is฀highly฀charged฀ with฀ dissolved฀ calcium฀ carbonate฀ inds฀its฀way฀into฀an฀air-illed฀chamber.฀ On฀ entering฀ an฀ open฀ space,฀ some฀of฀the฀carbon฀dioxide฀in฀the฀ water฀is฀released฀by฀degassing฀(see฀ earlier฀formulae).฀The฀decrease฀in฀ CO2฀ content฀ reduces฀ the฀ water’s฀ ability฀to฀hold฀all฀its฀solute฀material,฀and฀calcium฀carbonate฀is฀precipitated฀out฀of฀solution. 84 Figure฀6.฀Mr.฀Tali฀Magatogia,฀leader฀of฀Tali’s฀Cave฀Tours,฀stands฀ next฀to฀some฀ine฀speleothems฀in฀Ulupaka฀cave. GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES Stalactites฀ and฀ stalagmites฀ are฀ features฀formed฀by฀precipitation฀on฀the฀roof฀ and฀ loor฀ of฀ caves฀ by฀ dripping฀ water.฀ These฀ may฀ join฀ up฀ over฀ time฀ to฀ form฀ pillars.฀ In฀ some฀ of฀ Niue’s฀ caves,฀ stalactites฀and฀stalagmites฀are฀so฀abundant฀ that฀groups฀resembling฀organ฀pipes฀are฀ common.฀Many฀have฀pure฀internal฀crystal฀ structures฀ and฀ emit฀ resonant฀ notes฀ are฀if฀they฀are฀gently฀tapped.฀There฀are฀ also฀ fairly฀ abundant฀ helictites,฀ literally฀ ‘spiral’฀stalactites฀which฀actually฀grow฀ in฀all฀directions฀(Figure฀7).฀Thousands฀ of฀ broken฀ stalactites฀ litter฀ cave฀ loors฀ in฀ many฀ places฀ as฀ a฀ result฀ of฀ vandalism,฀although฀some฀may฀also฀have฀been฀ broken฀ from฀ their฀ original฀ position฀ as฀ the฀loor฀of฀a฀cave฀subsides฀through฀the฀ Figure฀7.฀An฀interesting฀helictite฀formation฀in฀Ulupaka฀cave. roof฀of฀a฀younger฀cave฀below. If฀water฀lows฀down฀the฀side฀of฀a฀cave฀wall฀or฀along฀the฀loor฀then฀calcite฀precipitation฀gives฀smooth฀lowstones.฀Horizontal฀lowstone฀is฀seen฀in฀some฀of฀the฀coastal฀ caves฀ north฀ of฀ Mutalau.฀ Elsewhere฀ limestone฀ steps฀ or฀ gours฀ are฀ common.฀These฀ form฀where฀small฀pools฀of฀water฀are฀enclosed฀by฀a฀level฀surrounding฀rim฀of฀limestone.฀As฀water฀overlows฀the฀pool฀it฀precipitates฀calcite,฀so฀any฀breaks฀are฀soon฀ healed฀and฀the฀rim฀gradually฀grows฀upwards฀and฀outward. Caves฀and฀archaeology Caves฀are฀often฀important฀archaeological฀sites,฀and฀those฀on฀Niue฀are฀no฀exception.฀This฀is฀because฀caves฀provide฀environments฀with฀relatively฀stable฀microclimates฀ that฀ are฀ generally฀ little฀ affected฀ by฀ the฀ day-to-day฀ extremes฀ of฀ the฀ weather฀outside.฀Cave฀deposits,฀and฀any฀evidence฀of฀human฀occupation฀that฀they฀ may฀ contain,฀ such฀ as฀ bones,฀ fossils,฀ middens฀ and฀ artefacts,฀ can฀ therefore฀ be฀ well฀ preserved฀ for฀ a฀ considerable฀ time,฀ especially฀ if฀ the฀ cave฀ is฀ dry.฀ Careful฀ excavation฀of฀these฀remains฀gives฀valuable฀information฀about฀the฀diet,฀lifestyle,฀ health฀and฀diseases,฀tools฀and฀traditions฀of฀the฀people฀from฀times฀before฀written฀ records฀were฀kept. 85 NIUE฀ISLAND On฀Niue,฀shallow฀nearsurface฀ caves฀ were฀ used฀ as฀sites฀for฀burial฀or฀body฀ disposal฀ in฀ pre-historic฀ times.฀ In฀ 1974-75,฀ the฀ Niue฀ Island฀ Archaeological฀ Survey,฀ carried฀ out฀ by฀ M.฀ Trotter฀ of฀ the฀ Canterbury฀ Museum฀ in฀ Christchurch฀ (Trotter฀ 1979),฀ uncovered฀ many฀ interesting฀deposits฀in฀59฀ separate฀burial฀sites฀(both฀ caves฀ and฀ rock฀ shelters)฀ (Figure฀8).฀Many฀human฀ bones฀were฀found,฀sometimes฀ burnt,฀ and฀ often฀ inside฀ stone฀ enclosures,฀ underneath฀ piles฀ of฀ stones,฀or฀on฀top฀of฀special฀stone฀burial฀mounds.฀ Radiocarbon฀ dating฀ of฀ Figure฀8.฀Known฀burial฀sites฀in฀caves฀and฀rock฀shelters. bones฀from฀Foukula฀and฀ Paluki฀caves฀gave฀ages฀of฀210±60฀years฀and฀370±40฀years฀before฀present.฀Artefacts฀ discovered฀ alongside฀ human฀ remains฀ include฀ cowry฀ and฀ cone฀ shells,฀ basalt฀ adze฀ heads,฀coconut฀scrapers,฀fragments฀of฀wood,฀and฀ornaments฀such฀as฀shell฀necklace฀ pieces฀and฀pendants. Some฀caves฀also฀show฀good฀evidence฀that฀early฀Niuean฀people฀occupied฀them.฀ A฀good฀example฀is฀Anakula฀cave฀on฀the฀edge฀of฀the฀Mutalau฀Reef,฀where฀excavations฀revealed฀stratiied฀deposits฀containing฀layers฀of฀charcoal฀and฀ash฀and฀pieces฀ of฀ shell฀ from฀ edible฀ types฀ of฀ shellish.฀These฀ occupational฀ deposits฀ indicate฀ that฀ people฀ frequently฀ sheltered฀ in฀ caves฀ to฀ make฀ ires฀ and฀ cook฀ food,฀ possibly฀ over฀ periods฀of฀several฀months฀(Walter฀and฀Anderson฀1995).฀Elsewhere,฀in฀passages฀leading฀to฀underground฀pools,฀“smoothly฀worn฀rocks฀and฀blackened฀ceilings”฀(Trotter฀ 1979:22)฀suggest฀that฀deeper฀caves฀had฀been฀explored฀and฀were฀used฀as฀a฀source฀of฀ water฀in฀the฀past,฀perhaps฀in฀times฀of฀drought.฀ 86 GEOMORPHIC฀FEATURES฀OF฀NIUE฀ISLAND:฀CHASMS,฀CAVES฀AND฀OTHER฀KARST฀VARIETIES Conclusions: the significance of Niuean karst Niue฀has฀a฀variety฀of฀landscape฀features,฀formed฀in฀part฀by฀solution฀of฀the฀limestone฀ bedrock,฀ including฀ pinnacle฀ formations,฀ depressions,฀ steep-walled฀ chasms฀ and฀ networks฀of฀caves฀that฀are฀ornately฀decorated฀with฀speleothems.฀This฀geomorphology฀is฀ important฀for฀several฀reasons.฀First,฀there฀are฀still฀relatively฀few฀accounts฀of฀karst฀landscapes฀on฀oceanic฀islands.฀Second,฀the฀bedrock฀comprises฀interesting฀kinds฀of฀limestones,฀including฀fossil฀reefs฀and฀poorly฀cemented฀coralline฀sands,฀that฀emerged฀above฀ sea-level฀ during฀ the฀ Quaternary฀ when฀ the฀ pre-Niue฀ atoll฀ was฀ raised฀ up฀ by฀ tectonic฀ forces.฀The฀limestones฀were฀dolomitised฀by฀the฀addition฀of฀magnesium฀from฀hypersaline฀brine฀as฀the฀former฀lagoon฀was฀progressively฀cut฀off฀from฀the฀open฀ocean.฀ Third,฀Niue฀does฀not฀display฀a฀classical฀karst฀landscape,฀which฀adds฀to฀its฀attraction฀ as฀a฀focus฀for฀study.฀The฀geomorphological฀features฀are฀best฀explained฀by฀the฀solutional฀ imprint฀on฀a฀carbonate฀landscape฀controlled฀by฀tectonism,฀submarine฀mass฀movements฀ and฀intermittent฀emergence.฀So,฀therefore,฀the฀island’s฀central฀depression฀is฀inherited฀ from฀the฀atoll฀structure,฀but฀has฀been฀over-deepened฀by฀solution.฀The฀best฀pinnacles฀ have฀developed฀on฀exposed฀seaward฀terraces฀originally฀cut฀by฀marine฀erosion฀during฀ stillstands฀of฀sea-level฀during฀the฀uplift฀history.฀In฀Niue’s฀฀hasms฀and฀caves,฀solution฀ and฀precipitation฀processes฀have฀left฀their฀impression,฀although฀these฀features฀are฀often฀ associated฀with฀bedrock฀fractures,฀the฀origin฀of฀which฀remains฀unclear. Forth,฀the฀geomorphic฀features฀of฀Niue฀are฀a฀valuable฀component฀of฀the฀island’s฀ cultural฀heritage.฀Chasms฀leading฀down฀to฀the฀sea฀provided฀access฀for฀early฀Polynesian฀ settlers,฀ and฀ are฀ still฀ used฀ as฀ sea฀ tracks฀ for฀ traditional฀ ishing฀ activities฀ in฀ the฀absence฀of฀sheltered฀bays.฀Niue’s฀caves฀were฀important฀burial฀sites฀in฀prehistoric฀times฀and฀have฀yielded฀archaeological฀remains฀providing฀clues฀about฀the฀early฀ people.฀In฀modern฀times,฀eco-tourism฀is฀growing฀as฀an฀important฀sector฀of฀the฀economy,฀beneiting฀in฀part฀from฀visitor฀interest฀in฀the฀limestone฀terrain.฀Current฀ecotourist฀activities฀include฀diving฀the฀chasms,฀guided฀tours฀among฀the฀pinnacles฀and฀ explorations฀through฀accessible฀cave฀systems.฀ For฀all฀these฀reasons,฀Niue’s฀special฀type฀of฀karst฀has฀both฀scientiic฀and฀heritage importance,฀ and฀ deserves฀ recognition฀ as฀ an฀ exceptional฀ Paciic฀ Island฀ carbonate฀ landscape. References Barrie,฀J.฀(1967)฀Geology฀of฀Christmas฀Island.฀Unpublished฀Bureau฀of฀Mineral฀Resources฀Record฀No.฀1967/37,฀ Canberra,฀Australia. Ebrahim,฀M.฀(1999)฀Carbonate฀Sedimentation฀and฀Recent฀Inluence฀of฀Human฀Activities฀on฀Tarawa฀Atoll,฀Republic฀ of฀Kiribati.฀Volume฀I.฀Unpublished฀PhD฀thesis,฀Victoria฀University฀of฀Wellington,฀New฀Zealand. 87 NIUE฀ISLAND Fieldes,฀M.,฀Bealing,฀G.,฀Claridge,฀G.G.,฀Wells,฀N.฀and฀Taylor,฀N.H.฀(1960)฀Mineralogy฀and฀radioactivity฀of฀Niue฀ Island฀soils.฀New฀Zealand฀Journal฀of฀Science,฀3,฀658-675. Ford,฀D.฀and฀Williams,฀P.฀(1989)฀Karst฀Geomorphology฀and฀Hydrology.฀Chapman฀&฀Hall. Jacobson,฀G.฀and฀Hill,฀P.J.฀(1980)฀Hydrogeology฀of฀a฀raised฀coral฀atoll฀-฀Niue฀Island,฀South฀Paciic฀Ocean.฀BMR฀ Journal฀of฀Australian฀Geology฀and฀Geophysics,฀5,฀271-278. Montaggioni,฀L.F.,฀Richard,฀G.,฀Bourrouilh-Le฀Jan,฀F.,฀Gabrié,฀C.,฀Humbert,฀L.,฀Monteforte,฀M.,฀Naim,฀O.,฀Payri,฀ C.฀and฀Salvat,฀B.฀(1985)฀Geology฀and฀marine฀biology฀of฀Makatea,฀an฀uplifted฀atoll,฀Tuamotu฀archipelago,฀central฀ Paciic฀Ocean.฀Journal฀of฀Coastal฀Research,฀1,฀165-171. Ollier,฀C.D.฀(1975)฀Coral฀island฀geomorphology฀-฀the฀Trobriand฀Islands.฀Zeitschrift฀für฀Geomorphologie,฀N.F.฀19,฀ 164-190. Paulay,฀G.฀and฀Spencer,฀T.฀(1992)฀Niue฀Island:฀geologic฀and฀faunatic฀history฀of฀a฀Pliocene฀atoll.฀Paciic฀Science฀ Association฀Information฀Bulletin,฀44,฀3-4. Pfeffer,฀K.-H.฀(1997)฀Palaeoclimate฀and฀tropical฀karst฀in฀the฀West฀Indies.฀Zeitschrift฀für฀Geomorphologie,฀S.B.฀108,฀ 5-13. Purser,฀B.,฀Tucker,฀M.฀and฀Zenger,฀D.฀(Eds)฀(1994)฀Dolomites.฀A฀Volume฀in฀Honour฀of฀Dolomieu฀International฀ Association฀of฀Sedimentologists,฀Special฀Publication฀No.21,฀Blackwell฀Scientiic฀Publications,฀UK. 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Wheeler,฀C.W.,฀Aharon,฀P.฀and฀Ferrell,฀R.E.฀(1999)฀Successions฀of฀Late฀Cenozoic฀platform฀dolomites฀distinguished฀by฀ texture,฀geochemistry,฀and฀crystal฀chemistry:฀Niue,฀South฀Paciic.฀Journal฀of฀Sedimentary฀Research฀69,฀239-255. Wilford,฀G.E.฀and฀Wall,฀J.R.D.฀(1967)฀Karst฀topography฀in฀Sarawak.฀Journal฀of฀Tropical฀Geography,฀21,฀44-70. Wright,฀A.C.S.฀and฀van฀Westerndorp,฀F.J.฀(1965)฀Soils฀and฀agriculture฀of฀Niue฀Island.฀New฀Zealand฀Soil฀Bureau฀ Bulletin,฀17. Acknowledgements Figure฀1฀was฀drawn฀by฀Susanna฀Xie฀of฀Xivine฀Studio,฀Suva,฀and฀Figure฀8฀by฀James฀Britton,฀former฀Director฀of฀the฀ GIS฀Unit฀of฀the฀USP฀Geography฀Department.฀The฀base฀map฀of฀cave฀locations฀used฀for฀Figure฀8฀was฀kindly฀supplied฀ by฀Matt฀McIntyre,฀formerly฀of฀the฀Niue฀Lands฀Department.฀Mark฀Cross,฀a฀Niue฀artist,฀is฀thanked฀for฀permission฀ to฀use฀his฀painting฀of฀Matapa฀Chasm฀for฀Figure฀4.฀Karen฀Tamate,฀Cassygina฀Tukiuha,฀Sidney฀Aue,฀Misa฀Kulatea฀ and฀Tali฀Magatongia฀gave฀much฀assistance฀in฀the฀ield.฀Professors฀Paul฀Williams฀and฀Patrick฀Nunn฀are฀thanked฀for฀ sharing฀their฀experiences฀of฀Niue฀and฀for฀many฀insights฀on฀interpreting฀the฀Niuean฀landscape.฀Fr.฀John฀Bonato฀is฀ also฀thanked฀for฀information฀on฀limestone฀and฀dolomite฀weathering฀processes.฀I฀am฀especially฀grateful฀to฀Professor฀Cliff฀Ollier฀of฀the฀Australian฀National฀University฀and฀Mr฀John฀Barrie฀of฀Avien฀Mining,฀and฀formerly฀of฀the฀ Australian฀Bureau฀of฀Mineral฀Resources,฀Canberra.฀Both฀have฀considerable฀experience฀of฀geological฀work฀on฀many฀ carbonate฀ islands฀ in฀ the฀ Paciic฀ and฀ Indian฀ Oceans,฀ including฀ Niue.฀They฀ freely฀ gave฀ so฀ much฀ of฀ their฀ time฀ in฀ useful฀discussions฀and฀help฀with฀the฀script฀that฀they฀probably฀deserve฀to฀be฀co-authors฀of฀this฀chapter.฀Mr฀Barrie฀ also฀provided฀access฀to฀early฀air฀photos฀and฀other฀useful฀unpublished฀archive฀materials.฀Financial฀support฀for฀this฀ research฀from฀the฀University฀of฀the฀South฀Paciic฀is฀gratefully฀acknowledged. 88