University of South Carolina
Scholar Commons
Faculty Publications
Earth and Ocean Sciences, Department of
12-10-1998
Distribution of Isolated Volcanoes on the Flanks of
the East Pacific Rise, 15.3°-20°S
Scott M. White
University of South Carolina - Columbia, [email protected]
Ken C. Macdonald
University of California - Santa Barbara
Daniel S. Scheirer
Brown University
Marie-Helene Cormier
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Publication Info
Published in Journal of Geophysical Research, Volume 103, Issue B12, 1998, pages 30371-30384.
White, S. M., Macdonald, K. C., Scheirer, D. S., & Cormier, M. (1998). Distribution of isolated volcanoes on the flanks of the East
Pacific Rise, 15.3°-20°S. Journal of Geophysical Research, 103 (B12), 30371-30384.
© Journal of Geophysical Research 1998, American Geophysical Union
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JOURNAL
Distribution
OF GEOPHYSICAL
of isolated
RESEARCH,
volcanoes
VOL. 103, NO. B12, PAGES 30,371-30,384, DECEMBER
on the flanks
10, 1998
of the East Pacific
Rise, 15.3øS-20øS
Scott M. White
and Ken C. Macdonald
Departmentof GeologicalSciencesand Marine ScienceInstitute,Universityof California,SantaBarbara
Daniel
S. Scheirer
Departmentof GeologicalSciences,Brown University,Providence,RhodeIsland
Marie-Hel•ne
Cormier
Lamont-DohertyEarth Observatoryof ColumbiaUniversity,Palisades,New York
Abstract. Volcanic constructions,not associatedwith seamount(or volcano) chains, are
abundanton the flanksof the EastPacific Rise (EPR) but are rare alongthe axial high. The
distributionof isolatedvolcanoes,basedon multibeambathymetricmaps,is approximately
symmetricaboutthe EPR axis. This symmetrycontrastswith the asymmetriesin the distribution
of volcano chains (more abundanton the west flank), the seafloor subsidencerates (slower on the
westflank), and the distributionof plate-motion-parallelgravity lineaments(more prominenton
the west flank). Most of the isolatedvolcanoescompletetheir growth within -14 km of the axis
on crustyoungerthan 0.2 Ma, while seamountchain volcanoescontinueto be active on older
crust. Volcanic edificeswithin 6 km of the ridge axis are primarily foundadjacentto axial
discontinuities,suggestinga more sporadicmagmasupplyand strongerlithosphereable to
supportvolcanicconstructions
nearaxial discontinuities.The volume of isolatednear-axis
volcanoescorrelateswith ridge axis cross-sectional
area,suggestinga link betweenthe magma
budgetof the ridge andthe eruptionof near-axisvolcanoes.Within the studyarea,off-axis
volcanic edifices cover at least 6% of the seafloor and contribute more than 0.2% to the volume of
the crust. The inferredwidth of the zone whereisolatedvolcanoesinitially form increaseswith
spreadingrate for the Mid-Atlantic Ridge (<4 km), northernEPR (<20 km), and southernEPR
(<28 km), so that isolatedvolcanoesform primarily on lithosphereyoungerthan 0.2 Ma (< 4-6 km
brittle thickness),independentof spreadingrate. This suggestssomeform of lithosphericcontrol
on the eruptionof isolatedoff-axis volcanoesdue to brittle thickness,increasednormal stresses
acrosscracksimpedingdike injection,or thermal stresseswithin the newly forming lithosphere.
1. Introduction
At slow to intermediatespreadingrates, volcanic edifices are
built on the ridge axis itself [e.g., Smith and Cann, 1992;
Off-axis volcanoes created near fast-spreadingmid-ocean
Kleinrock et al., 1994; Magde and Smith, 1995; Smith et al.,
ridgesprovideinsightaboutthe processes
occurringin the early
1995]. In contrast, the only previous study of near-axis
stagesof oceaniclithosphereformation. The distributionof the
volcanoes
along a fast spreading ridge, EPR at 9ø-10øN,
chainsof large volcanoes( > 200 m high ) hasbeendocumented
concluded that small isolated volcanoes form outside the
alongthe southernEastPacific Rise (EPR) in a variety of studies
[e.g., Batiza, 1982; Smith and Jordan, 1988; Bemis and Smith, neovolcaniczone,5-10 km from the ridgeaxis,on crustalagesof
1993; Scheirer and Macdonald, 1995; Shen et al., 1995; Scheirer
0.1-0.2 Ma [Alexanderand Macdonald, 1996].
The EPR from 15øS to 20øS has been the focus of numerous
et al., 1996b; Rappaportet al., 1997]. Smaller volcanoeshave
not been well documentedpreviously,as their size makesthem studies,including bathymetric mapping, seismic surveys,the
difficult to detectin abyssalhill terrainwithouthigh-resolution, MELT experiment, petrologic investigations, and deepfull-coverage multibeam bathymetry which has only recently submergencevehicle operationson the ridge axis (Figure 1)
become available [Scheirer et al., 1996a,c]. Also, recent
advances in understanding the structure of abyssal hills
[Macdonald et al., 1996] make it possibleto distinguishsmall
volcanicconstructionsfrom the seafloorfabric of abyssalhills.
Copyright1998by theAmericanGeophysical
Union.
Papernumber98JB02791.
0148-0227/98/98JB-02791
$09.00
[Forsyth
etal., 1991•;
MELTSeismic
Team,1998;Scheirer
etal.,
1998]. This studyfocuseson near-axisvolcanoes,catalogingall
the topographichighs with at least 50 m relief and a quasicircular base, on crust out to magnetic anomaly 2 (-1.9 Ma)
along520 km of the EPR (Figure2). We mapthe distributionof
near-axisisolatedvolcanoesalong and acrossthe ridge axis and
thenplacetheseobservations
in the contextof geneticmodelsfor
off-axis volcanismnear fast-spreadingridges. Volcanoesunder
200 m high havenot beenincludedin previousstudiesof the offaxis volcanismalongthe southernEPR (SEPR), and many of the
30,371
WHITE
30,372
ET AL.' ISOLATED
VOLCANOES
NEAR SEPR
Figure 1. Shadedrelief map,from a 1 km grid illuminatedfrom northwest,of the bathymetriccoveragefor the
flanks of the southernEast Pacific Rise. The studyarea is outlined,and the generallocationof Rano Rahi
Seamount
Fieldis labeled.Insetshowstheoceanicplatesof theeasternPacificwith thestudyareashaded.
volcanoeseastof the ridge were mappedin 1996 [Scheireret al.,
1998] after the initial study of the Rano Rahi SeamountField
[Scheireret al., 1996b].
2. GeologicalFramework of the Study Area:
Overview
of the EPR From
15 ø to 20øS
Located between the Easter Microplate and the Garrett
Fracture Zond, the EPR within the study area is an ultra-fast
spreadingridge (Figure 1). Since 1 Ma, the Pacific plate has
been spreading slightly faster than the Nazca plate north of
17ø15'S, at a half-rate of 75 mm/yr versus 65 mm/yr. The
asymmetryreversesjust southof 17ø15'S,so that the NazcaPlate
is spreadingeastwardat a half-rate of 80 mm/yr, roughly 20
mm/yr faster than the Pacific plate half-rate [Cormier et al.,
1996; D. Wilson, personalcommunication,1997]. Bathymetric
mappingrevealsthat the seaflooron the Pacific plate subsidesat
a slowerrate thanpredictedby the globalaverageof lithospheric
cooling with crustal age, implying hotter mantle and thinner
lithosphereon the west flank [Cochran, 1986; Scheirer et al.,
1998].
Sevensmall overlappingspreadingcenters(OSCs) are located
within the studyareaat approximately15ø55'S,16ø30'S,17ø56'S,
18ø22'S,18ø37'S,19ø05'S,and 19ø50'S[Lonsdale,1989; Scheirer
et al., 1996a](Figure2). TheseOSCs are all left-steppingoffsets
whose propagation history explains most of the spreading
asymmetry [Cormier and Macdonald, 1994; Cormier et al.,
1996]. The 15ø55'SOSC hasremainedrelativelystationary,the
16ø30'SOSC hasmigratednorthward,and the otherOSCs have
been rapidly migrating south [Cormier et al., 1996].
The
presenceof a long-lived axial magma chamber(AMC) has been
inferred from the continuity of a midcrustalseismicreflector
underthe ridgethroughmostof thisregion,exceptnearOSCs
[Detrick et al., 1993; Kent et al., 1994; Mutter et al., 1995].
Petrologicevidencefor fractionation-controlled
compositional
variations
alongthispartof theEPR alsoimpliestheexistence
of
a long-livedAMC [Sintonet al., 1991]. Axial lavassampled
in
the study area also show spatialvariationsin geochemistry
corresponding
to thestructural
segmentation
of theridge[Sinton
et al., 1991' Mahoneyet al., 1994].
Ridge axis morphologyappearsto be an indicatorof magma
budgeton fast spreadingcenters[Macdonaldand Fox, 1988;
Scheirerand Macdonald,1993]. The ridge axisdepthis shallow
(< 2700 m deep) and relatively uniform (--2600-2800 m) from
15ø to 19øS, then deepens rapidly approaching the large
overlapping
spreading
centerat 20.7øS[ScheirerandMacdonald,
1993]. The cross-sectional
areaof the axial high increasesfrom a
small,triangularhigh at near 15øSto a large,domalhighwith one
of thelargestcross-sectional
areasof anymid-ocean
ridgeat 17ø18øS,suggesting
a correspondingly
highmagmabudget,andthen
decreasesto a small, triangularhigh towardthe southernend of
the studyarea [Scheirer and Macdonald, 1993]. The off-axis
thicknessof seismiclayer 2A weakly correlateswith ridge crosssectionalarea at 17ø-18øS [Carbotte et al., 1997]. However, this
correlation is less certain for the EPR 13ø-21øS [Hooft et al.,
ß1997]. The numerous seamount chains in the Rano Rahi
Seamount Field, at 16ø-19øS,indicate unusually high off-axis
magmasupplyin the studyarea[Scheireret al., 1996b].
.
3. Methods
and Definitions
This study cataloguesall conical edifices on crust younger
than about 1.5-2
Ma on the southern East Pacific
Rise from
15.3øSto 20øSusing the full-coveragemultibeambathymetric
WHITE ET AL.' ISOLATED VOLCANOES NEAR SEPR
.114 ø
-113 ø
30,373
-112 ø
•11a
.15ø []
-15'
Axis
I
I
I
"•'
!-:
;•,i.::i::;?'
-19'
-19'
.........
VOLCANOES
ol
VOLCANO
CHAIN
I
I
.....
vo•c,.o..,.os,c,
I
::ß; .........................................
'2OøI
•
•I
-114'
:.'•'
•
•
•
-'113'"
•
•
•
-•l
-20"
-'I'12'
Figure2. Shaded-relief
mapof 1kmgridofbat.
hymetry
ofthestudy
area(illuminated
fromNW)withtheridge
axis,0.2Ma,and1.0Ma isochrons
drawnassolidlines.Circles
showisolated
near-axis
volcanoes
withstars
over
volcanoes
within6 kmof theridgeaxis.Symbols
arescaled
according
totheapproximate
basal
diameter
of the
edifice,
butsmaller
edifices
areshown
withsymbol
sizelarger
thantheiractual
diameter
atthismapscale.Straight
lines are drawn over volcano chains.
30,374
WHITE ET AL.: ISOLATED VOLCANOES NEAR SEPR
data recently obtainedin this region [Scheirer et al., 1996a,c]
(Figure 2). Using topographicmapsderivedfrom 200 m grids
and contouredat 10 m intervals,we pickedvolcanoeshavinga
minimum of five roughly circular closed contours above the
surrounding
seafloor(Figure3). We chosea minimumheightof
50 m based on the roughly 10 m vertical resolution of the
SeaBeam 2000 multibeam system, the dominant source of
bathymetrydata in the studyarea,to mitigatethe possibilityof
picking small multibeam artifactsas volcanoes. Closed-contour
artifactsof up to 30 m can appearnear the edgesof the -10 km
wide swaths[Kleinrock et al., 1992; $cheirer et al., 1996a]. As a
result, volcanic edifices under 50 m high are not part of this
regional study, althoughthere are some tiny volcanic mounds,
lessthan20 m highand 100 m in radius,on the axial high [ White
et al., 1997; R. M. Haymon,unpublisheddata, Sojourn02R/V
Melville, 1996].
For each volcano, the geographicalpositionof the center of
the edifice,the lengthof baseparallelandperpendicular
to ridge
strike, the length of summitparallel and perpendicularto ridge
strike,andthe heightweremeasured
directlyfrom the maps. The
volumeof eachedificeis calculatedby approximatingits shape
as a truncatedright-ellipticalcone. Volcanoesare assumedto be
-112' 05'
nearly circular in plan view, whereasfaulted horstsare elongate
parallelto the ridge, even where drapedby lava flows. Edifices
with an aspectratio >2 elongatewith respectto the trend of the
ridge axis are likely to be fault blocksratherthan volcanoesand
were omitted.
From thiscompilationof all volcanicedificesof 50 m or taller,
we define a volcano chain as a group of volcanoes
simultaneously
satisfyingtwo criteria(Figure4). First,a volcano
chain must consist of three or more edifices with colinear summit
areas. Second,the distancebetweenthe edgeof the baseof two
adjacentvolcanoesin a chainmustbe lessthanthe basaldiameter
of the larger volcano of the pair. Classifying a volcano as
belongingto a chainimpliessomeconnectionamongthe edifices,
so this proximity rule was derived empirically to fit the
previouslyidentifiedRano Rahi volcanochains[Scheireret al.,
1996b]but preventwidely separatedbut colinearvolcanoesfrom
being classified as part of the same volcano chain. The low
height cutoff for the volcanoes catalogued in this study
introduces a higher spatial density of off-axis volcanoes,
comparedto previousvolcano distributionstudiesin regionsof
abundant volcano chains [Scheirer and Macdonald, 1995;
Scheirer et al., 1996b]. This high spatial density of volcanoes
-112' 00'
-16 ø 00'
-16' 00'
-16
ø05'
,½•,(,•ii,
-16'
05'
..
;• -- ,,.--•
....
-112' 05'
-3450
-112' 00'
-3350
-3250
-3150
meters
Figure3a. Mapof-•410krn2on-•1.5MaNazcacrust.Typicalisolated
near-axis
volcanoes,
atleast50minheight
and roughlycircularin plan view, are outlinedin bold. Contourintervalis 10 m on a 200 m grid. Someof these
small volcanoesappearto be alignedalongthe abyssalhills, althoughthey do not form a volcanochainas defined
in the text. Severalof thesevolcanoesarebuilt uponplatforms,presumablyproducedby off-axiseruptions.
WHITE ET AL.: ISOLATED VOLCANOESNEAR SEPR
-113'10'
-113'05'
-15' 50'
-15'55' e'
...
-113' lO'
-3•x)o
-esso
-•oo
meters
Figure3b. SameasFigure3a,except
for-320 km2of-0.2 Ma Pacificcrust.
Figure 4. A typicaleastflank volcanochainin 3-D persepective,
verticalexaggeration
of 5X, 100 m contours.
Volcanochainsconsist
of severalcolinearedifices,typicallysittingatopa plateau-like
base.Thelargestvolcanois
1400m highwitha -15 km2 base.
30,375
30,376
WHITE ET AL.: ISOLATED VOLCANOES NEAR SEPR
meanssomevolcanoeswill be apparentlyalignedalong any Magde and Smith, 1995; Smith et al., 1995]. Volcanoes 50 m
randomline; so, thesecriteriaare essentialto definingwhich highformimmediatelyoff the SEPRaxialhigh,ascloseas 1 km
volcanoescomprise chains independentof the size of the from the ridge axis, but volcanoesfoundwithin 6 km of the axis
volcanoes or the orientation of the chain.
are mostly adjacent to OSCs. Only 3 of 15 volcanoesfound
Isolatedvolcanoes
are definedin this studyas topographic within 6 km of the axis are farther than 15 km from the nearest
highswith a minimumheightof 50 m, with an aspectratio_<2, OSC (Figure2). Beyond6 km from the ridgeaxis,volcanoesare
and not identified as a chain-volcano based on the criteria
explainedabove(Figure3).
Althoughnearlycompletebathymetryis availablewithin the
scatteredthroughoutthe studyarea.
This study catalogued1070 volcanoes(includingthose in
chains)with heightsof at least50 m on -113,000k m2 of
survey area, the abundance and volume of isolated volcanoes
seafloor.Thesevolcanoescover6% of the seafloorin the study
werenormalized
to valuesper 1000km2, excluding
the area
area. A total of 680 isolated near-axis volcanoeswere identified,
coveredby seamountchains.This minimizes effects of burial of
and555 of thesevolcanoeswerelessthan100 m high. We found
isolatedvolcanoes
by lava flows andmasswastingassociated
with seamountchains. To determinethe age of the crusta
volcanoresidesupon,themagnetic
isochron
picksin the study
areaweregriddedusinga minimumcurvaturespline[Smithand
Wessel,
1990]to obtaina gridof thecrustalagewithinthestudy
only four isolated near-axis seamounts(volcanoestaller than
1000 m), all locatedon the Nazcaplatefrom 17øto 18øSbeyond
25 km from the axis (Figure 5a). Theseseamounts
contributea
disproportionateamountto the total volume of the isolatednearaxisvolcanism(Figure 5b). Becausethe isolatedvolcanoeshave
suchan unevensize distribution,the medianvolumerepresents
the typical size of the volcanoes better than the mean. The
medianvolumeis usedfor this reasonin all subsequent
analyses
of isolatedvolcanosizeandgrowthpatterns.
areaincorporating
all knownmagneticisochrons[Cormieret al.,
1996; D. Wilson, personalcommunication,1997]. The seafloor
ageat thelocationof eachvolcanowastheninterpolated
fromthe
crustalage grid.
In contrast to isolated volcanoes, volcano chains have a much
4. GeneralDescriptionof the VolcanoPopulation
Near
the SEPR
greaterproportionof large volcanoes. Over half the 390 chain
volcanoescountedin this studyare taller than 100 m and 85% of
the seamounts(>1000 m high) in the area occur in volcano
chains.
No conical edifices taller than 50 m exist at the summit of the
The seamount chains of the Rano Rahi Seamount Field
all have more than one volcanic edifice over 500 m high
axial high alongthe entirelengthof the EPR [Macdonaldet al.,
[Scheirer et al., 1996b]. However, we identified a few new
1992; Scheirer
chainson the east flank that are composedentirely of volcanic
edificesunder500 m high.
Eruptionsfrom all off-axis volcanoeson crust <2 Ma have
increasedthe volumeof the crustby 0.2%, assuminga uniform6
and
Macdonald,
1993; Alexander
and
Macdonald,1996], in strikingcontrastto the abundantvolcanic
edifices
in the neovolcanic
zone of slow or intermediate
spreadingcenters[Smith and Cann, 1992; Kleinrocket al., 1994;
1200
-'::;---:i---:i::::?i::::i-:-:i:::
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
....
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r---T--*-r'---r----r----r----r
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r ....
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....•.... '•.... ;-- -;.... {.... {----4
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800"
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,
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,
,
• ' ' -J J -4•-- x_ _-L__._L..
L- -L L _ L_ _L_
,
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1__,'_.._:._.1_.[__21_1 .1.
,
,
,
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,
,
,
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,
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: ....,,.....,
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....!....
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!•
! i ': ':•
-"'i-"i'-?-"i-'!--!',*• • ? !'
!!!!:::'..'•.::•i!!i::'•!::i!:::':!Z!!::
:'"; :._•:4:.' .'-•:-•:.4:"•-':;•'
400-
•s :*•,:,,z
....
-•- -'
' ' •*• •t;
.... ---,4• -r...m.-
• a.
•.• .........
_-
_
':::i:*i
•-.::•' -i•::. •1•.'-•"•
....
:
km east of axis
....
':....
:---,
km east of axis
Figure5. Scattergraphs
of (a) heightof individual
edifices
versus
distance
fromtheaxisand(b) semilog
volumes
of
eachedificeversusdistancefromtheaxis. Negativedistancefromtheridgecorresponds
to west(Pacific)flank.
Mostof theisolatednear-axis
volcanoes
aresmalledifices,onlya fewof whichhavebeencatalogued
in previous
studies.
WHITE ET AL.' ISOLATED VOLCANOES NEAR SEPR
30,377
to
of 100m3/s.At leastsomeof thesevolcanoes
probably
formin a
the crust is a lower limit because lava flows that do not create a
seriesof eruptionspunctuatedby pausesor intrusive activity
[Lonsdale, 1983]. While submarineeruptionshave never been
observed,subaerialcindercones,spattercones,littoral cones,and
km thick crust.
This estimate of off-axis
volcanic
contribution
volcanic edifice at least 50 m high are not included in these
values. Many of the larger volcanoes,especially the volcano
chains,are built uponplatforms,up to a few tensof metershigh,
presumablymade up of basalticflows relatedto the edificesthat
sit atop them (Figures 3 and 4). These platforms were not
includedin the our volume or area estimatesbecausethey failed
to meet the closed-contouror aspect-ratiocriteria for volcanoes.
However,
days[e.g., Thorarinsson,
1969;Holcomb,1987;Lockwoodet al.,
1987]. Formation of similar-sized submarine volcanoes may
occuron a generallysimilartimescale,makinga periodof 2-10
daysa reasonable
order-of-magnitude
estimate.
a conservative estimate of their areal extent would
doublethe amountof seafloorcoveredby flows from the isolated
volcanoes, and significantly increase our estimate of the
contribution
vent craters have been observed to form in a matter of hours to
from near-axis volcanoes to the volume of the crust.
Of the 555 volcanoes50-100 m in height,9 have craterslarge
enoughto show up on 200 m gridded bathymetrymaps, 85 are
5. Distribution
of Isolated
Volcanoes
Acrossand Along the Axis
Owing to asymmetricspreadingin the study area, seafloor
crustalagedoesnot correspond
directlyto thedistanceoff-axisin
the studyarea(Figure2). Thereforewe binnedthe populationof
small off-axis volcanoesform entirely by the eruption of basalt isolated near-axis volcanoesin 0.1 Myr crustal age intervals,
with pillow morphology(ratherthanby mixed intrusive/extrusive overlappingby 0.05 Myr, runningthe lengthof the studyarea,
incorporatingthe local spreadingasymmetries[Cormier et al.,
activity), we can estimatea minimum time of formation of 2-10
days of continuouseruptiongiven a typical lava flux of 10-100
1996]. When plottedin 0.1 Myr binsas a functionof crustalage,
m3/sforpillowsto form[GreggandFink,1995]. Thelargest the numbersof isolatedvolcanoesreachtheir long-termaverages
isolated
volcanoes
withvolumes
exceeding
30km3 wouldtakeat by 0.2 Myr (Figure 6a). Also, the overall abundanceof isolated
least10 yearsof continuous
eruptionto form at a lava flow rate near-axisvolcanoesis symmetricabout the ridge, in contrastto
irregularor "hat-shaped"
[Smithet al., 1995;Head et al., 1996],
and the remainingare domeshaped(Figure 3b). Assumingthese
a
15
10
0
(1.0)
(0.5)
WEST
0.0
0.5
crustal age (Ma)
1.0
EAST
b0.15
0.1
o.o5
o
(1.0)
WEST
(0.5)
0.0
crustal
age(Ma)
0.5
•.0
EAST
Figure6. (a) Numberof volcanicedificesfoundon 1000km2 of seafloornot coveredby volcanochains.
Histogrambinsare 0.1 Myr wide andoverlapby 0.05 Myr. (b) The medianvolumeof volcanicedificesin the same
bins usedpreviouslyfor volcanoabundance.Vertical barsshowthe rangefrom the 25th to 75th percentilesof
edifice volumes,illustratingboth the wide scatterand the skewnessin the volume distributionof isolatedvolcanoes.
The medianvolumeshowsno significantchanges,suggesting
isolatedvolcanoestypicallyform in <0.1 Myr, then
do not continue to erow.
30,378
WHITE ET AL.: ISOLATED VOLCANOES NEAR SEPR
the extremely asymmetric distribution of the volcano chains
(Figure 2). Althoughthe dataare noisy,a steadymedianvolume
is reachedin the first bin from the ridge axis (Figure 6b). The
observation
that
the
median
volume
of
isolated
near-axis
volcanoes remains constant with distance from the axis indicates
that typical isolatedvolcanoeserupt mostof their volume within
the time spanof one agebin, lessthan0.1 Myr (Figure 6b). That
the timespanof the edifice building stageis not resolvablewith
0.1 Myr bins is not surprisingbasedon our inferenceof a time
spanof days to weeks for the main eruptive activity of typical
isolated volcanoes.
Severalcomplicationscan obscurethe distributionof isolated
volcanoeswith respectto crustalage. Seamountchainsand their
debris apronsmay overprint the isolated volcanoes[Scheirer et
al., 1996b],as dealtwith previously.Also, largelyuninvestigated
basaltic sheet flows and normal faulting may contribute to an
apparentdecreasein abundanceof volcanoesbeyondtheir initial
zoneof formationby masswastingand burial. Finally, sediment
accumulation could bury volcanic edifices, but this is not a
significantfactor in our study area. Sedimentationrates of 3-6
m/Myr are typical of recent estimatesfor this area [Marchig et
al., 1986; Lyle et al., 1987; Dekov and Kuptsov, 1992]. This
could lead to undersamplingof smalleredifices at the outer edge
of the studyarea, where 10 m of sedimentscould be deposited.
Forcrustyounger
dian1 Ma, thedeposition
of lessthan6 m of
sedimentis not likely to changethe overall distributionof >50 m
high edifices. No systematic correction to the across-axis
Abundance
of IsolatedVolcanoes
(~40
kin.
centered
on
the
axiS)
15 min. x 0.25 Ma crustalage bins
distribution data was made for increasing sedimentthickness
becausethe < 5 m uncertaintyis beyond the resolutionof the
multibeam bathymetrydata.
Along the axis, there is a transitionat 19øSfrom abundant
seamount chains to the north to an area without any volcano
chainsto the southwhereonly small isolatedvolcanoesare found
(Figure 2). The ridge axis depth increases, and the crosssectional area, a proxy for axial magma budget, decreases
significantly near this transition [Scheirer and Macdonald,
1993](Figure7). A similar correlationbetweengreaterseamount
volcanismand larger cross-sectionalarea of the axial high was
also found for the EPR 8ø-18øN [Scheirer and Macdonald, 1995]
and for the EPR near the Wilkes Fracture Zone [Keeley et al.,
1995]. Isolatednear-axisvolcanoesoccur everywherealong the
axison bothsidesof the ridge, 15ø-20øS,andthe magmasupply
to boththe ridgeaxis andthe isolatednear-axisvolcanoesmay be
related.
To investigatealong-axis correlationsbetween the isolated
volcanoesand ridge-axis morphology, we used quarter-degree
latitude bins (28 km) extending20 km to both sidesof the axis
(crust <0.25 Ma). These bins sampleenoughvolcanoesper bin
(>5) to be statisticallymeaningfulbut encompassa shortperiod
of time so that we may assumethe cross-sectionalarea of the
ridge has not changedsignificantly[Scheirerand Macdonald,
1993]. The total volume of isolated volcanoes correlates well
with the averageaxial cross-sectional
area(Figure8a). However,
fewer isolatedvolcanoesper unit area are found where the axial
-' 20
Isolated Volcanoes
'VolcanoChains
15
Abundance
ofChainVolcanoes
15rain
x0•4Macrustal
age
bins
(-60 kin. centered on the axis)
IO
-20'
Figure 7. (top) Total numberof volcanoesin quarter-degree
latitudebins,40 km wide, and centeredon the axis.
Isolatedvolcanoabundances
are normalizedfor the areacoveredby seamountchainsin eachbin. (middle) Crosssectionalareaof the ridgeaxis from Scheirerand Macdonald(1993).(bottom)Map view of the axis showingthe
corridor
usedto correlate
to theaxis.Volcano
chains
arefoundonlywherethecross-sectional
area>4 km2, but
isolatedvolcanoes
arefoundalongtheentireridgeandincreasein numberfrom 19øto 20øS,whereseamount
chains
are absent.
WHITE ET AL.' ISOLATED VOLCANOES NEAR SEPR
It
reflector below the axis [Detrick et al., t 993] is not related to the
10o
....................
:..,
..............{............ {................
{...........
g
30,379
total volume of nearbyisolatedvolcanoes. Cross-sectionalarea
of the ridge axis probablyaveragesthe magmabudgetover >0.1
Myr, whereasthe depth to the AMC reflector may vary over a
....
10- :::::2::::::::::::::::.-::
::•::
::::.-:
:::::::::
:2::::.-:
::•::
:•:
:2:::
:::::::::::::::::•::
:.-:.--.:
::::-.:
::::-::::
2:::•::
:.':
:::
;:::.-:
:::::::.-:
:::
::•::
:::2:"::
2:::::::::::.-•
•::2::::•::•:•:•*,::•:•:2•::•:•:2:::::::•:•:•:•:•::::2:•:•:•::•:•:•::::::•:2•:•
".'•1;::::•::
shorter time [Scheirer and Macdonald, 1993; Carbotte et al.,
1997; Hooft et al., 1997]. Using a 40 km wide corridorcentered
on the ridge axis averagesthe off-axis volcanismover 0.25 Myr.
It would be interestingto see if there is a correlation between
areas of enhancedoff-axis volcanism and areas along the axis
that are underlain by axial magma chambers with a high
............................ 't............................. ? ............................ •,
•
, ......... 'f ............................ • .............................
....................... • .................... • ...................... ?...........................
•" '•'•"-4' ...........• ...................
............................
?.............................
'..............................
'i------•=&----q
........
'..'
............................
• ............................. percentageof melt. No correlationis evidentbasedon the small
.....................
•..........................
•..........................
-:
.........
i......
!"'i;" I f-•-&4
: ............
:................ area studiednear 14øS [Singh et al., 1997]. We suspectthat the
.......................
i.....................
'":
.....................
' ......................
•.....................
•.....................
timescaleof changefor the axial magmalens is shortcompared
to the -0.2 Myr time interval for the eruption of near-axis
......
;";Z
;;;
;;;
;;Z
:7"7.•""X'7'7;;
;;'2
Z::
;;.•;;
7';.....
::';"':::';:
::•;;'Z";;';;
;:;'::':;'";':
;Z;.';:
:::";
Z"'..•.7•:
;Z•"'"••"
..........................
-,
:
.....................
,, ......................
4
,
:
.....................
-• ...............
-* ..................
ß
:
............................
............................
............................
............................
............................
............................
volcanoesbut await further seismicanalysis.
0
1
2
3
4
5
6
avg.axialarea(kin 2)
6. Height Distribution of Near-Axis Volcanoes
To comparethe distributionof volcanoesin this studywith the
characteristic seamount heights and areal densities of other
studies, volcano height distribution parametersare calculated
using
maximumlikelihoodregression[Smithand Jordan, 1988].
,
An
exponential
distributionis fit to the isolatedvolcanoheight
•0..............................
i.............................
,•.q ..................
i...........................................................
i............................
distribution,binned in 10 m height intervalsfrom 50 to 350 m
,
,
with at least five volcanoes per bin. This study predicts a
'
characteristicheightof 28 m with an areal densityof 31 edifices
F&-I
'
,
•
•
•
i•
,
,
per1000km2forthepopulation
of smallisolated
volcanoes
anda
.............................
............................................
,..............., •A•A •
•
,
,
0
0
2
i
i
3
4
•A•
•A•
•A•
a•. a•ialar• (kin
Figure 8. Correlation of isolated near-axis volcanoesto axial
mothology in 15' along-axisby 40 km wide axis-centeredbins.
(a) Cumulative edifice volume and (b) abundanceof volcanoes
with respectto the average axial cross-sectionalarea with 2sigmae•or barsaboutthe meanarea. Increasingvolumeof nearaxis volcanismseemsto correlatewith increasingaxial crosssectional area, and increasingnumbersof near-axis volcanoes
seemto co.elate with decreasingaxial area.
characteristicheightof 106 m with an areal densityof 6 edifices
per1000km2 forvolcano
chains,
including
thesmaller
volcano
chains. For seamounts>200 m high in the Rano Rahi Seamount
Field, the maximum likelihood regression predicts a
characteristic height of 295 m and areal distribution of 9.5
edificesper 1000km2 [Scheirer
et al., 1996b]. The distinct
distributionssuggestthat isolatedvolcanoesand volcanochains
comprise two separatepopulations(Figure 10 and Table 1).
Comparing their height distribution parameters, isolated
volcanoeseruptingoff-axison fast spreadingridgesand the axial
volcanic conesat slow spreadingridges also form two distinct
populations(Table 1).
7. Isolated
Volcano
Formation
and
Lithospheric Thickness
One of the controversies regarding submarine volcano
formation is whether melt supply or lithosphericpenetrability
controlsoff-axis volcanism[e.g., Vogt, 1974; Batiza, 1981, 1982;
cross-sectional
area is highest(Figure 8b). Thus a relatively
narrow ridge is associatedwith many small isolatedvolcanoes, Fornari and Luckman, 1987]. The width of the zone of isolated
while fatter ridge segmentshave a smallernumberof larger volcano formation may be relevant to this problem. We found
volcanoeswith a greatertotal volumecontribution. In the areas that the zone of initial formation of isolated volcanoes on the
with a low volumeof isolatednear-axisvolcanoesbut a large southernEPR extendsout to 0.2 Ma cruston either flank (Figure
axial cross-section
(Figure 8a), volcanochainsare the dominant 6a). Along a segmentof the northern EPR, Alexander and
expressionof off-axis volcanism.
Macdonald [1996] concludedthe zone of formation for small offIn agreementwith these correlations,volcanoesfrom 19ø to
axis volcanoesis 10-20 km wide, also correspondingto a crustal
20øSaremoreabundant
(10 compared
to 7 per 1000km2) and age of 0.1-0.2 Ma. On the Mid-Atlantic Ridge (MAR), most
generally
smaller
(median
volumeof 0.015km3 compared
to axial volcanoes occur in a zone with a total width of about 4 km,
0.026km3) thanfor thestudyareaasa whole.However,
the again correspondingto a crustal age of roughly 0.2 Ma [Smith
width of the isolated volcano formation
zone from 19 ø to 20øS
remainsthe sameas for the entirearea(Figures9a and9b).
In contrast to the observed correlation
between axial cross-
and Cann, 1992]. The common 0.2 Ma limit on the width of
isolated volcano formation suggestslithospheric penetrability
controls where new isolated volcanoes
form.
sectionalarea and volcano volume, ridge depth and volcano
volume show a weaker correlation. This may result from the
small variation in axial depth along most of the study area
Neither within our study area nor on the northern EPR
[Alexanderand Macdonald, 1996] was there any demonstrable
associationbetween the abyssalhill normal faults and off-axis
[Scheirer
volcanism.
and Macdonald,
19931.
The depth of the AMC
The only obvious linear alignment
of volcanoes was
30,380
WHITE ET AL.' ISOLATEDVOLCANOESNEAR SEPR
a
20
15-
10d
5-
0
0.0
(0.5)
(1.0)
WEST
b
0.5
crustal age (Ma)
1.0
EAST
19ø.20øS
0.075
0.05
0.025
o
(1.0)
(0.5)
WEST
0.0
crustal
age(Ma)
0.5
1.0
EAST
Figure9. (a) Abundance
and(b) medianvolumeof isolatedvolcanoes
from 19øSto 20øS(binnedandnormalized
asin Figure6), a regionof full bathymetric
coverage
andnovolcanochainsor largevolcanicedifices.Thissmaller
regionshowsthesamegeneraltrendsasthewholestudyareabuthasmorevolcanicedificesanda smallermedian
edifice volume.
found perpendicularto the trend of the seafloor fault fabric
(Figure 2), suggestingthat preexistinglines of weaknessalong
faults play a secondaryrole in determiningthe near-axisvolcano
distribution. Stressesgeneratedby the magma source are the
most probableprimary control on the distributionof isolated
near-axis
volcanoes.
Perhapsthe depth to the brittle-ductile transition in oceanic
lithospherecontrolsthe width of the zone of initiationof isolated
volcanoes.
Once the brittle lid reaches a sufficient
thickness to
prevent dikes from either initiating or erupting, new volcanoes
stop forming. The ambientstressesat the dike tip and thermal
diffusivity at the dike walls principallylimit the ability of magma
to propagate upward through the brittle lid and erupt [Rubin,
1995]. In simple terms, the magma pressuremust overcomethe
elastic stiffnessof the overlying rock and the local compressive
stressperpendicularto the fractureplane to initiate a fracturefor
propagationof a dike [Secorand Pollard, 1975]. Presumably,a
magma body of constant size will have greater difficulty
initiating a crack at the baseof thicker brittle lithospherebecause
Even whenoneof thesesmallmagmalensesproducesa dike,
thermalstresses
generated
by underplating
new, hot materialto
the baseof coolerlithosphere
wouldcauserelativecompression
at thetopof theplate,asthebaseof theplatecoolsandcontracts
fasterthan the older lithosphereaboveit [Bratt et al., 1985;
Wessel,1992]. This relativecompressional
stressat the top of
the platemay increasein magnitude
with lithospheric
agefor
rapidlythickeninglithosphere
(< 100Ma) andimpedetheascent
of dikesthrougholderlithosphere.It is worthnotingthatmany
volcanochainson the EPR alsoappearto initiatewithin0.2 Myr
of theridgeaxis,althoughtheyexperience
mostof theirvolume
increase farther off-axis [Scheirer and Macdonald, 1995;
Scheirer et al., 1996b]. Local heating of the lithosphere
associatedwith seamountchainsmay decreasethe usualrate of
lithosphericthickening,producinga meltchanneleffectthatmay
allow continuedvolcanicactivityfartheroff-axisnearpreviously
active eruptivecenters.
To estimate the depth to the brittle-ductile transition at
different spreadingrates, we use a simple one-dimensional
the mean normal stress across dike-induced cracks increases with
thermalmodelof lithosphericcoolingthatincreasesthe depthof
depth. On the basisof the small size of mostisolatedvolcanoes, the isotherm with square root of crustal age from the axis
we speculatethat they originatefrom rathersmall magmabodies [Turcotteand Schubert,1982]. The simplemodel was modified
to includeinformationaboutthe axial thermal structureby adding
incapableof penetratinga thick brittle lid.
WHITE ET AL.: ISOLATED VOLCANOES NEAR SEPR
30,381
100
g
10
RanoRahiSeamount
Field
•
g
•'•
1
4_
•
•
-
(Scheireret al, 1996c)
"-..
t•
-
• X••
I •'
Volcano
Chain•
o•th•
', '"
northern
EastPacific
Rise
'•..
(Scheirer
and
Macdon•d,
1995)
", Volcano.Chains
(this
study)
•-•
•
•I •• volcanoes
isolated
n•ar-axis
(4his
study)"..
0.01 II
0
,
....
,t ,
,
I
I
, ',
....
,
ß 'I
5•
ß '•
....
.
ß 'I
1000
1500
....
....
'-.."
2000
height(m)
Figure10. Maximum
likelihood
regression
predictions
of numbers
of volcanoes
per1000km• based
on
cumulative
volcano
heightdist•bution.T•anglesshowisolated
volcanoes
withsolidlinefit to datain heightrange
of 50-350m. Circlesarevolcanochainscat•oguedin thisstudywith dashedline fit to datain 50-600m height
range.Thedist•bution
fortheRanoRahiSeamount
Fieldcontains
a kigherconcentration
of largevolcanoes
but
usesa heightrangeof 200-1200m andp•ma•ly westfla• coverage.
the depth to the low-velocityzone calculatedby Purdy et al.
[1992], whichdisplacesthe isothermsdownward. Althoughwe
acknowledgethis modelis simplistic,it givesa straightforward
approximationof the thicknessof the brittle lithosphere.The
model suggeststhat a brittle lid thicknessof-4-6 km (or the
mean normal stressesacrosspotential cracks at the base of
lithosphereof thisthickness)is sufficientto blockthe formation
of new isolatedvolcanoesat all threespreadingrates(Figure 11).
Table 1. SeafloorVolcanoHei[[ht Distribution
StudyArea
HeightRange,
rn
[•,km-1
In comparison,
the thermalmodelof Wilsonet al. [1988] that
includescoolingat the baseandthe top of a convectingmagma
chamberfor the fast spreadingEPR placesthe brittle-ductile
transitionat a depthof roughly3 km belowthesurfaceof 0.2 Ma
lithosphere.If thethermalmodelof PhippsMorganand Chen
[1993] is used at thesethree spreadingrates,the thicknessof
brittlelithosphere
becomes
roughly8 km in thezoneof isolated
volcanoes formation. So, while the exact thickness is model-
CharacteristicVo,(1000
km2)
-1
Source
Height•rn
15.3ø-20øS EPR
S. Pacific
50-350
50-210
Isolated Volcano
35.9+0.2
15.9_+0.6
"superswell"
95øW Galapagos
50-315
34.5+2.5
Areas
30
62
31.1+.1
33+2
Bemis and Smith
30
370-•_30
Kleinrock
This study
[1993]
et al.
[1994]
24ø-30øN MAR
50-650
17.2_+0.5
60
195_+9
57ø-62øN
50-300
14.3_+0.5
70
310+_20
Smith and Cann
[1992]
[1995]
Reykjanes
Seamount
15.3ø-20øS EPR
15ø-19øS EPR
Magde and Smith
50-600
300-1200
Chain Areas
9.4_+0.4
110
3.4_+0.1
290
6.•.1
9.5_+0.4
This study
Scheirer
et al.
[1996]
8ø- 17øN EPR
200-800
4.3_+0.2
230
1.9_+0.2
Scheirer
and
Macdonald
[1995]
27ø-29øS EPR
Fitting;' -- ;'o
200-1000
308
2.7_+0.15
Rappaportet al.
[1997]
30,382
WHITE ET AL.' ISOLATED VOLCANOES NEAR SEPR
2
10
0
5
half-width
10
of the zone of formation
15
20
of isolated volcanoes
[Macdonald et al., 1992]; 11ø45'N [Perram and Macdonald,
1990]; and 12ø54'N [Antrim et al., 1988]. Constructional
volcanoesmay form at abandonedridge tips as melt pockets
become isolated by the "self-decapitation"of the propagating
ridge as spreadingstopson the abandonedsegment[Macdonald
et al., 1987]. Geochemistryof basaltsamplesfrom OSCs along
the southernEPR showsthat ridge offsets tend to erupt more
fractionatedmagma[Sintonet al., 1991]. Samplingof the small
volcanicconesnear OSCs would be the bestway to confirm their
origin,leadingto betterunderstanding
of the subcrustalprocesses
at ridge discontinuities.At present,suchsamplinghasnot been
doneat the resolutionnecessaryto addressthesequestions.
(km from ridge axis)
Figure 11.
Comparison of calculated depth to the 600øC
isotherm to off-axis distance for the maximum half-width of the
zone of volcano formation for the isolated near-axis volcanoes on
9. Conclusions
The distribution
of isolated off-axis volcanoes on the ultrafast
spreadingEPR, 15.3ø-20øS,
suggests
thatridgevolcanismat fast
spreadingratesis not confinedto the axis. While the abundant
volcanochainswestof the axis imply that an asymmetricoff-axis
magmasupplycreatesmost seamountsand large volcanoes,nearaxis isolatedvolcanoesare equallyabundanton both sidesof the
ridge. The medianvolumeof isolatedvolcanoesremainsnearly
constantas a function of distancefrom the ridge, indicating the
majorityof isolatedvolcanoesdo not grow significantlybeyond
0.2 Ma crustal age. The total volume of isolated volcanoes
appearsto increasewhere the ridge has a larger cross-sectional
specific,the influenceof lithosphericthicknesson dike-induced areaand hencehighermagmasupply. The inferredwidth of the
cracking of the brittle lid may limit how far off axis isolated zone of volcanoformationincreaseswith spreadingrate for nearaxis volcanism on the MAR, northern EPR, and southern EPR,
volcanoesform at all spreadingrates.
such that isolated volcanoesform only on lithosphereless than
0.2 Myr old in all cases,suggestinga lithosphericcontrol on
8. Hypothesesfor the Origin of Isolated Near-Axis productionof isolatedvolcanoesnearridges.
the southernEast Pacific Rise (SEPR) (this study),the northern
East Pacific Rise (NEPR) [Alexanderand Macdonald, 1996] and
the Mid-Atlantic Ridge (MAR) [Smith and Cann, 1992].
Assuming the brittle-ductile transition occurs at the 600øC
isotherm,new isolatedvolcanoesappearto form when the brittle
lid is lessthan 4-6 km thick (shadedregion). The meannormal
stressacrossany cracksat a depth of 4-6 km at the base of the
lithospherewould be -0.12-0.18 MPa.
Volcanoes (and Suggestionsfor Future Work)
On the basisof the spatialdistributionof volcanoes,formation
of isolatedvolcanoesappearsto be a steadystateprocessrelated
to axial magmaticupwelling,whereasvolcanochainsseemmore
consistentwith formationby sporadiceruptionof lavas derived
from mantleheterogeneities
as envisionedby Wilson[ 1992] and
Davis and Karsten [ 1986]. To explainthe distributionof isolated
near-axis volcanoes, low volume eruptions must occur on an
almoststeadystatebasisin the near-axisregion. We speculate
that a possiblerecordof the magmaticprocesses
creatingisolated
near-axis volcanoesmay be preserved as late stage wehrlitic
intrusionsand uppermostankaramiticlavas in ophiolites[Juteau
et al., 1988; C. A. Hopsonand J. Martinson,Wehrlitic magma
series and ultramafic transition zone, Point Sal remnant of the
Acknowledgments. We thankeveryoneinvolvedin makingthe highresolutionbathymetricmapsof the southernEast Pacific Rise that made
this work possible,andthe captains,crew,and watchstanders
on the Rapa
2 and 4, Gloria 2 and 3, and Sojourn I cruises. The unpublished
magnetic isochronpicks from Doug Wilson were essential. Deborah
Smith kindly provided the maximum likelihood regressionscript and
advice on its use. Earlier helpful reviews by R. Batiza and D. Naar
encouragedus to expandour data set,greatlyimprovingthe results. We
also thank J. M. Auzendeand K. Loudenfor their helpful reviews. We
appreciatethe longstanding
interestof C. Hopsonin our off-axismapping
of the EPR. This work was supportedby NSF OCE94-15632, OCE8911587, ONR N00014-94-10678,
and ONR N00014-93-10108.
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(ReceivedMarch 9, 1998;revisedJuly22, 1998;
acceptedAugust17, 1998.)