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GeologyoftheDeep
Submarinevolcanoesformislandsaroundtheworld
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BYRYANSCHLEETER
Thursday,July25,2013
FromHawaiitoIndonesiatoIceland,hundredsofislandsacrosstheglobehavebeenformedbysubmarinevolcanoes.
Submarinevolcanoesareexactlywhattheysoundlike—volcanoeslocatedbeneaththeocean’ssurface.
Becausetheyeruptintowaterinsteadofair,submarinevolcanoesbehavequitedifferentlythanterrestrialvolcanoes.For
instance,it’suncommonforsubmarinevolcanoestohaveexplosiveeruptions.Thesheerweightofthewaterabovethem
createsveryhighpressure,usuallyresultinginwhatareknownaspassivelavaflowsalongtheseafloor.Mostsubmarine
eruptionsdonotdisturbtheoceansurface.
CharlesMandevilleistheprogramcoordinatorfortheVolcanoHazardsProgramoftheUnitedStatesGeologicalSurvey
(USGS).Heandhiscolleaguesareresponsibleformonitoringall169activevolcanoes—allterrestrial—intheUnitedStates.
PriortojoiningtheUSGS,Mandevillefocusedhisresearchonsubmarinevolcanology,becominganexpertonthefamous
1883eruptionoftheislandofKrakatoainIndonesia.
AccordingtoMandeville,therearetwomainfactorsthatcontributetosubmarinevolcanoeseventuallyformingislands:the
supplyofmagmaandtectonicactivity.
“Thefirstthingyouneedisasupplyofmagma,”hesays.“Typically,intheoriginofmostoceanicislandvolcanoesor
submarinevolcanoes,youneedtomelttheEarth’smantle.”
Mostvolcanicislandsoriginatefrompassivelavaflowsontheseafloor.Thesepassiveflowshardenintorockandbuildup
theheightoftheunderwatermountainovermillionsofyears.Eventually,somevolcanoesreachheightsabovetheseafloor
wherelowerpressureallowsforexplosiveeruptions.Submarinevolcanoesthatdonotreachsealevelarecalledseamounts.
Inadditiontomagmasupply,platetectonicsplayalargepartindeterminingwhichsubmarinevolcanoeswilleventuallyform
islands.Tectonicactivitycansometimes“taketheislandvolcanoawayfromthesourceofmagmathatoriginatesinthe
mantle,becausethetectonicplatethatthevolcanoisgrowinguponismoving,”saysMandeville.
VolcanicIslandEcosystems
Formedfromnothingbutrock,volcanicislandshavesurprisinglyvibrantecosystems.Theseecosystemsevolveover
millionsofyears,alongwiththeislanditself.Lifeonvolcanicislandsstartswiththemostbasicbuildingblocks—autotrophic
bacteria.
“Therearebacteriathatcanmaketheirfoodfromjustthechemicalelementsandpartsoftheelementsthatarebeingemitted
fromvolcanoes,”Mandevillesays.“...Onceyouhaveamicroscopicecosystemestablished,youhaveenoughofafood
supplytosupportlargerlife.”
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Speciesfromnearbylandformsalsocontributetothedevelopingecosystem.Passingbirdsmaystoptonestonthenew
island,bringingseedsandsporesfromthemainlandorotherislands.Plantlifecanfloatthroughtheoceantoenduponthe
island’sshores.
Becausetheyevolveinsuchanisolatedenvironment,manyorganismsareconsideredendemicspecies—unlikeanyothers
intheworld.ThefinchesendemictotheGalapagosIslands,describedbynaturalistCharlesDarwininthe19thcentury,are
onefamousexampleofthis.ThesebirdsarefoundonlyintheisolatedGalapagosIslands.TheHawaiianIslands,evenmore
isolated,havealmost3,000endemicplantspeciesalone.
World’sYoungestIsland
Oneoftheworld’smostrecentlyformedvolcanicislandsispartoftheislandnationofTonga,intheSouthPacificOcean.
Tongaisanarchipelagoof170volcanicislands.ThenewlandmassformedinMarch2009asanexplosiveeruptionsent
steam,volcanicgases,andvolcanicashroughly800meters(2,625feet)intothesky,coveringtheuninhabitedislandof
HungaHa’apai—63kilometers(39miles)away—inblack,volcanicash.
Dayslater,asecond,smallereruptionfromaventbetweenHungaHa’apaiandthenewlandmasscombinedwithrockand
debrisfromtheinitialeruptiontofillthespacebetweenthetwo.Theresultwasasinglelandmassnearlydoubletheoriginal
sizeofHungaHa’apai.
ThoughHungaHa’apaipreviouslyhadrichplantandanimallife,theashfromtheeruptiondevastateditsecosystem.It’s
unclearwhetherthenewislandwillevolvetobeablesupportlargerlifeforms.
“Thewindandthewavesareconstantlytryingtoerodethatislandbackbelowsealevel.Theonlythingthat’sgoingto
outpacetheeffectsofthewaveandstormerosionisifthemagmasupplyproducesenoughlavaflowsandexplosive
depositstokeeppacewiththaterosion,”Mandevillesays.
Increasingtheheightoftheislandabovesealevel—throughlavaflowsanderuptions—iscriticaltoallowingbirdsfromnearby
islandsto“seedthenewislandwithlife,”hesays.
Intheyearssincetheeruption,theyoungislandhasmaintaineditselfabovesealevelbuthasnotgrownsignificantly.Itis
stillattachedtoHungaHa’apaiandisintheveryearlystagesofdevelopinganecosystem.Othersubmarinevolcanoesnear
Tongaremainactive,butnomajorexplosiveeruptionshavebeenreportedsince2009.
VOCABULARY
Term
PartofSpeech
Definition
archipelago
noun
agroupofcloselyscatteredislandsinalargebodyofwater.
autotroph
noun
organismthatcanproduceitsownfoodandnutrientsfromchemicalsinthe
atmosphere,usuallythroughphotosynthesisorchemosynthesis.
bacteria
pluralnoun
(singular:bacterium)single-celledorganismsfoundineveryecosystemonEarth.
Charles
Darwin
noun
(1809-1882)Britishnaturalist.
debris
noun
remainsofsomethingbrokenordestroyed;waste,orgarbage.
devastate
verb
todestroy.
ecosystem
noun
communityandinteractionsoflivingandnonlivingthingsinanarea.
emit
verb
togiveofforsendout.
endemic
adjective
nativetoaspecificgeographicspace.
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erode
verb
towearaway.
evolve
verb
todevelopnewcharacteristicsbasedonadaptationandnaturalselection.
finch
noun
small,commonbird.
initial
adjective
first.
island
noun
bodyoflandsurroundedbywater.
Krakatoa
noun
islandinIndonesia,siteofmajorvolcaniceruptionin1883.AlsocalledKrakatau.
landform
noun
specificnaturalfeatureontheEarth'ssurface.
magma
noun
molten,orpartiallymelted,rockbeneaththeEarth'ssurface.
mantle
noun
middlelayeroftheEarth,madeofmostlysolidrock.
microscopic
adjective
verysmall.
monitor
verb
toobserveandrecordbehaviorordata.
naturalist
noun
personwhostudiesthenaturalhistoryornaturaldevelopmentoforganismsandthe
environment.
passive
lavaflow
noun
non-violentvolcaniceruption,wherelavaoozesorseepsfromafissureorvent.
pressure
noun
forcepressedonanobjectbyanotherobjectorcondition,suchasgravity.
seafloor
noun
surfacelayerofthebottomoftheocean.
sealevel
noun
baselevelformeasuringelevations.Sealevelisdeterminedbymeasurementstaken
overa19-yearcycle.
seamount
noun
underwatermountain.
shelter
noun
structurethatprotectspeopleorotherorganismsfromweatherandotherdangers.
spore
noun
reproductiveunitofmanyorganisms,suchasplantsandbacteria,similartoaseed.
steam
noun
watervapor.
submarine
adjective
underwater.
tectonic
activity
noun
movementoftectonicplatesresultingingeologicactivitysuchasvolcaniceruptions
andearthquakes.
tectonic
plate
noun
large,moveablesegmentoftheEarth'scrust.
terrestrial
adjective
havingtodowiththeEarthordryland.
USGS
noun
(UnitedStatesGeologicalSurvey)primarysourceforscienceabouttheEarth,its
naturalandlivingresources,naturalhazards,andtheenvironment.
vent
noun
crackintheEarth'scrustthatspewshotgasesandmineral-richwater.
vibrant
adjective
lively.
volcanic
ash
noun
fragmentsoflavalessthan2millimetersacross.
volcanic
gas
noun
gassuchaswatervapororcarbondioxidethatisreleasedintotheatmospherebya
volcano.
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volcanic
island
noun
landformedbyavolcanorisingfromtheoceanfloor.
volcano
noun
anopeningintheEarth'scrust,throughwhichlava,ash,andgaseserupt,andalsothe
conebuiltbyeruptions.
volcanology
noun
thestudyofvolcanoes.Alsocalledvulcanology.
ForFurtherExploration
Articles&Profiles
NationalGeographicNews:UnderwaterVolcanoErupts,StainsSeas
Maps
NationalGeographicEducation:MapMakerInteractive—VolcanicEruptions
Websites
NationalGeographicEducation:Volcanoes
USGS:PrincipalTypesofVolcanoes
NationalGeographicEnvironment:Volcanoes
Funder
ThismaterialisbasedinpartuponworksupportedbytheNationalScienceFoundationunderGrant
No.DRL-1114251.Anyopinions,findings,andconclusionsorrecommendationsexpressedinthis
materialarethoseoftheauthoranddonotnecessarilyreflecttheviewsoftheNationalScience
Foundation.
©1996–2015NationalGeographicSociety.Allrightsreserved.
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