JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 95, NO. B8, PAGES 12,689-12,696, AUGUST
10, 1990
Axial Seamount' An Active Ridge Axis Volcano
on the Central Juan De Fuca Ridge
H. PAUL JOHNSON
Schoolof Oceanography,
Universityof Washington,
Seattle
ROBERT W. EMBLEY
NOAA, PacificMarine Environmental
Laboratory,HatfieldMarine ScienceCenter
Newport, Oregon
INTRODUCTION
Juande FucaRidgecanbe dividedinto six 50- to 100-km-long
AxialSeamount
(some
of themanuscripts
in thisspecialsegments,
withtheAxialSegment
beingthethirdsegment
section
referto theedificewiththemoreprecise
nameof northof theBlanco
Fracture
Zone(Figure
2).
"AxialVolcano"),
alarge
ridgeaxisvolcano,
islocated
onthe AxialSeamount
represents
a largemass
excess
associated
central
segment
oftheJuan
deFuca
Ridge
approximately
250 witha "hotspot"-influenced
spreading
center.Theterm
nauticalmileswestof the Washington/Oregon/British
"hotspot"
is usedin quotations
heresincegeochemical
Columbia
coast.Currently
bothvolcanically
andhydro-evidence
(Desonie
andDuncan,
thisissue;
Rhodes
etal.,this
thermally
active,
Axialliesdirectly
attheintersection
of the issue)
argues
thatAxialis nota geochemical
hotspot
in the
Cobb-Eickelberg
Seamount
Chain
andtheJuan
deFuca
Ridgetraditional
Hawaiian
orIcelandic
sense.
Examination
of the
(Figure
1). Thevolcanic
activity
associated
withtheseamount
older,
moredistal
portion
oftheCobb-Eickelberg
(C-E)chain
formation
strongly
interacts
with,andis affected
by, the (Figure
1) showsthatit is a relatively
thick"band"
of
normalseafloorspreading
processes
at the intersection.
seamounts,
ratherthan a singlebathymetric
lineation.
Because
of thisunique
geologic
setting,
itsproximity
towestRiddihough
etal. [1983]andDavisand Karsten[1986]
coastportsandoceanographic
institutions,
andits shallowobserved
thatalmost
all of theseamounts
formedat theC-E
depth,Axial hasbecome
the focusof a largenumber
of hotspot
andsmaller
seamounts
foundin proximity
to theJuan
scientific
investigations
overthepastdecade.
deFucaRidgenorthandsouth
of Axiallie onthePacificplate
AxialSeamount
became
anearlytarget
forstudy
using
the to thewest,ratherthanonthesmaller
JuandeFucaplateto
newly developed
swath-mapping
tools of Sea Beam,theeast[Riddihough
etal., 1983;DavisandKarsten,
1986].
SeaMARCI andII, andGLORIA.Duringtheentiredecade Thegeneral
setting
ofAxialSeamount
intheeastern
Pacific,
ofthe1980s,
geologists
andgeophysicists
fromawidevarietyandtheoverallmorphology
of theedifice,
canbe seenin
of institutions,
including
theU.S.Geological
Survey,
NOAA, Figures
1 and2, andin thecolorPlates
A andB ofthisissue.
the PacificGeoscience
Centre,aswell as the Universities
of To thesouth,structures
of theVanceSegment
overlapwiththe
Washington,
Victoria,
andBritish
Columbia,
andOregon
StateSouth
RiftZoneofAxialandinteract
withtheseamount
inthe
University,
conducted
extensive
surveys
anddetailed
sampling
SEquadrant.
Thiszoneofinteraction
between
theVance
and
programs
of boththesummit
andflanksof thevolcano.SinceAxial segments
is quiteextensive,
with at least30 km of
1984,theNOAAVENTS
Program
hasdevoted
amajor
surfaceoverlap.
Thefarthest
northward
projection
ofspreading
from
shipandsubmersible
effortto high-density
studies
of the theVanceSegment
is a zoneof intense
fissuring
(observed
in
summit
areaanditshydrothermal
systems.
Theshallow
1500- SeaMARCI andII data)ontheouterflankof thevolcano
m summit
depthof theseamount
hasallowed
access
with [Johnson
andHolmes,
1989;Appelgate,
thisissue].
Pisces
submersibles
(bothCanadian
andRussian)
aswellas Interaction
oftheAxialSegment
withNorthern
Symmetrical
Alvin.Thisextensive
sea-going
efforthasmade
Axialoneof (alsonamed
Cobb)Segment
to thenorthis morecomplex.
themostintensely
studied
andbestcharacterized
areasof Bothmagnetics
andmorphology
datashowthattheAxial
seafloorin theworld.
Segmentforms a "bentspreading
center"connection
zone
betweenthe two segments,an unusualintrasegmentconnection
GEOLOGICAL
SETTING
region
thatis apparently
nonoverlapping
at thepresent
[Johnsonand Holmes, 1989]. An elevated saddle of basement
Axial Seamountis locatedastridethe centralsegmentof the rock connectsthe west flank of Axial with the easternmost
Juande FucaRidge,anintermediate
spreading-rate
ridgewith flank of Brown Bear Seamount(PlateA), the next oldest
a total openingvelocity of 6 cm/yr. The Juande Fuca Ridge seamount in the C-E chain.
The Son-of-Brown Bear
is bounded
on the southby theBlancoFractureZoneandon Seamount,lying directlyeast of Axial Volcano,and three
the northby a triple junctionformedby the ridge, the left- smallervolcanos
just to thenorthandsouth(PlateA) arefour
lateral Nootka fault and the SoyancoFracture Zone [Atwater, members of the rare class of volcanos that formed near the
1970;Hyndmanet al., 1979]. Like the EastPacificRise,the ridgeaxisandendedup on theJuande Fucaplate,ratherthan
the Pacific plate.
Copyright 1990 by the AmericanGeophysicalUnion.
Paper number 90JB01037.
0148-0227/90/90JB-01037502.00
STRUCTuRAL COMPONENTS OF THE VOLCANO
Structurally,the edificeof Axial Volcanocanbe interpreted
to represent a complex interplay of three usually distinct
12,689
12,690
JOHNSONAND EMBLEY.' •NTRODUCTION
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t40 ø
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ISLAND
EICKELBERG
COBB
PACIFIC
PLATE
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JUAN
FUCA
DE
PLATE
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km
,•
500
BROWN
I
at
U.S.A.
BEAR
42øN
a::
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RDA
PLATE
..
40ø
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!
Fig. 1. Seamountsand spreadingcentersin the northeastPacific, adaptedfrom Riddihoughet at. [1983]. Thick hachured
lines are subductionzones. Note that the Cobb-Eickelberg Chain has a finite width and actually consistsof a band of
seamounts
rather than a linear
chain.
geologicalprocesses:seafloorspreading,seamountformation, and movement of the water column above Axial; and (4)
andHawaiian-style
rifting. Theelongatestructure
of theentire specificexperimentsand studiesthat focus on the active
edifice (PlateA) appearsto representthe distributionof an hydrothermal
systems.
"excess"of magmaticactivity along the entire Axial Segment.
The elongateNorth and SouthRifts showevidenceof "rifting"
or, more correctly phrased,longitudinal growth due to alongstrike, near-surface magma transport. Finally, the domeshapedsummit and caldera argue for seamountlikeformation
processes,including eruption from (and subsequentcollapse
into) localized magma reservoirs.
Axial as a Volcano
The large-scale context of Axial is presentedin the papers
by Desonie and Duncan and by Rhodes et al. Desonie and
Duncanexaminethe age andchemistryof the Cobb-Eickelberg
(C-E) seamounts,using dredgesamplescollectedmostlyfrom
the distal members of the chain, and find a westward age
The summit of Axial, with its well-defined, three-sided
progressionaway from the spreadingcenter, consistentwith
caldera
(Figure
3 andPlate
B) andassociated
activethegeneral
assumption
thatAxialistheyoungest
member
of
hydrothermal
systems,
hasprovided
thefocus
ofmany
ofthe theprogressively
older(tothewest)
seamount
chain.The
studies
inthisvolume.
Floored
withveryrecent
lavaHows,same
study
also
shows
aneastward
decrease
inagedifference
some
ofwhich
areyoung
enough
toonlap
actively
venting
between
theseamounts
and
thecrust
onwhich
they
are
formed,
hydrothermal deposits, this caldera is a potential natural
arguing that the spreading center and the "mantle source"
laboratory. Because of the shallow 1500-m depth of the
responsible for the seamountshave been approaching each
caldera
floor,high-temperature
hydrothermal
systems
canother
astheJuan
deFuca
migrates
toward
thefixedC-E
undergo
"phase
separation,"
a
term
used
by
water
chemists
"hotspot."
Desonie
and
Duncan,
however,
find
that
there
isnot
when they are discussing"boiling" of the hydrothermalHuid
a clear, mantle plume signatureto the geochemistryof the
(Butterfield
etal.,thisissue).
Theinfluence
ofthis"phase
surface
rocks
ofthese
outer
seamounts
andcallupon
mixing
separation" on hydrothermal systems is profound and is a
between a weak mantle plume and normal spreading-center
theme
thatrunsthrough
several
of themanuscripts
in thismagmas
toexplain
theobserved
compositions.
special
section.
Rhodes
etal.,ontheother
hand,
concentrating
onsamples
from the summit and flanks of Axial volcano itself, find no
THISISSUE
The manuscriptsin this issuecan be divided into four broad
categories: (1) the large view studiesthat look at Axial as an
active submarine volcano; (2) the more focused summit
geologicalandgeophysicalstudies;(3) studiesof thechemistry
evidence
at all of anymantleplumeinfluence
in the
geochemistryof the rocks. These authorsargue that although
the geochemicaldata (as well as the morphologicalevidence)
appearto favor a very activeandextensivemagmaticplumbing
systemfor Axial Seamountandonethatis separateanddistinct
JOHNSON
ANDEMBLEY:INTRODUCTION
t30 ø
12,691
122 ø
126 ø
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...
...
ß
..
52 ø
ß
52 ø
.
.ß.
ß
ß
ß
ß
o
ß
ß.Van.
couver
ß
SFZ
•eattle
48 ø
48 ø
ß
ß
ß
"NORTH
•
oo
NSR CCobb)
AMERICAN
•" AXIA
L
••.
JUAN
DE
PLATE
FUCA
PLATE
44 ø
44 ø
PACIFIC
PLATE
ß
o
(,•
40 ø
40 ø
130ø
126ø
122ø
Fig.2. Schematic
diagram
oftheridgesegments
of theJuandeFucaRidge,inthenortheast
Pacificß
Thicklinesindicate
thespreading
centersegments;
thinner
linesindicate
fracture
zonesandthelocation
of thecontinental
marginßThe
followingabbreviations
are used: SFZ,Sovanco
Fracture
Zone;NSR,NorthernSymmetrical
Ridge;AXIAL,Axial
Seamount
Segment;
VA, VanceSegment(or segment
B in the olderliterature).
fromtherestof theJuandeFucaRidgevolcanics,
thevolcano withincrustformedduringthenormallymagnetized
Brunhes
represents
thesurfaceexpression
of a thermal,ratherthata epic,theedificedoesnotproduce
a simplepositiveanomaly
geochemical,
anomaly
in themantleß
thatcanbe simplyrelatedto thebathymetry.
Bothinversion
Potentialfieldmeasurements
overAxial Seamount,
including of the anomalydata andforwardmodelingcoupledwith rock
seasurface
magnetics
andbothseafloor
andseasurface
gravity magnetics
measurements
showthattheseamount
contains
large
data,haveprovided
someinsightintotheinternalstructure
of zonesof relativelylow magnetization
associated
with the
thevolcanoß
TiveyandJohnson,
usingthecloselyspaced
sea summitregionß While the interpretation
is inherently
surface
magnetics
dataacquired
duringmanyyearsof cruisesnonunique,
the anomalies
are bestexplainedby a thinned
in the area,showthateventhoughtheseamount
lies entirely (<700 m) sourcelayerunderlyingthesummitof thevolcano.
12,692
JOHNSONAND EMBLEY: INTRODUCTION
04'
02'
46000 '
58'
56'
54'
52'
45050 '
08'
06'
04'
02'
130000 '
58'
56'
54'
Fig. 3. Detailed bathymetrymap of Axial summitand caldera,from the NOAA Sea Beamcoveragein the area. Note the
three-sidedcaldera,with an orientationthatis obliqueto all othertectonictrendsin the region. Depthsarecalculatedusing
a watervelocityof 1500m/s. Primarynavigationwasby LORAN-C,but wasshiftedto GlobalPositioning
Systembase.
Contour
interval
is 10 m.
The necessarythinning could be causedby the presenceof a Rift Zone, describedby Tivey and Johnson,is also consistent
shallow, subcalderamagma chamber, residual heat in a with this interpretation.
transportpipe from previouseruptions,or a zone of extensive
The processof rifting over the SouthRift Zone is discussed
subsurface
alteration.
at lengthin the paperby Appelgate. Usinghigh-resolution
Hildebrandet al., usingbothseasurfacegravityanomalies SeaMARC I data, Appelgate shows that the surficial
anddatafroma newseafloorgravimeter,
reportevidencefor morphology
of the SouthRift Zone systematically
changes
a shallow,low-densityregionunderlyingthe summitcaldera, from linearvolcanicridgesnearthe summitto smallcratered
alsosuggesting
a partiallymoltenmagmachamberat depth conesat the distalend. Furthermore,
he showsthat ridgewithin the volcano. Away from the summit,the gravitydata parallelfaultsalongtheSouthRift arethesources
for themost
of Hildebrand
et al. suggest
thattheNorthRiftZonerepresentsrecentlavaflowsin thisregionandthattheexposed
faultsare
a tongue of high-density material at shallow depths, an likely to serve as shallow conduits for the horizontal
interpretationthat is consistentwith a zone of cooleddike (southward)
transport
of magma.Finally,usingprocessed
side
intrusionsassociated
with Hawaiian-style
rifting. A narrow, scandata in the SouthernRift Zone,Appelgateidentifiesa
extremely
high-amplitude
magnetic
anomaly
overtheNorth verylarge(48km2 in area;estimated
1.8km3 in volume)lava
JOHNSONAND EMBLEY: INTRODUCTION
12,693
fieldeastof therift zone,which,hespeculates,
mayhavebeen (Figure3). Although
theplumes
showed
distinct
3HeandMn
a major contributorin the formationof the summitcaldera.
signatures,there was no detectablethermal signatureover the
seamount, an observation attributed by Lupton to the high
3He/heatratiosandlow salinitiesof the fluidsbeingvented
Summit Geology
within
the caldera.
The summit of Axial Seamountrepresentsa well-studied
Thermal anomalies were, however, found by Baker et al.,
region of recent eruptive volcanics in the submarine
who focus on the results of hydrographic and chemical
environmentand containsevidence of a variety of geological
samplingof the water columndirectly over the summit. Here,
processesthat are currently active. Embley et al., using
they found temperature anomalies that were restricted to a
Sea Beam and Sea MARC I swathmaps,coupledwith towed200-m-thick layer above the caldera. The summit watercamera and submersible observations, describe the basic
temperatureanomaly over Axial is significanfiy smaller than
geologyof the summit area and the associatedcaldera;these anomalies associatedwith other hydrothermal fields on the
data allow the compilation of maps of the summit area
Juande Fuca Ridge, perhapsa surprisingresultconsideringthe
comparablein detail to thoseavailable for subaerialvolcanos.
tremendous amount of magmatic activity that must be
Embley et al. show that much of the postcalderavolcanismand associated with the formation of the volcano.
associatedhydrothermalventing appearsto be associatedwith
Feely et al. examine the chemistryof the particulateswithin
South Rift Zone activity. The orientation of the 3 x 8 km
the water column over the summit region and use the data to
summit caldera, with its roughly rectilinear sidesthat trend in
differentiate between distinct regions within the general
the N160 ø direction, oblique to all other trendsin the area,has
summitplume. Thesethreemajor regionsinclude(1) a single
been a major enigmain interpretationof the morphologyof the
buoyant plume, (2) an upper nonbuoyantplume, and (3) a
seamount. Embley et al. discuss several of the factors that
lower nonbuoyantplume. Sediment trap data reported by
may be responsiblefor the calderaand favor a model basedon
Feely et al. alsoconfirm that the integratedparticulatefallout
a mobile "overlappingspreadingcenter."
from the summithydrothermalsystemsis not as large as some
The currenttectonicactivity within the calderais graphically
of the other vent fields from other segments(i.e., Endearour
demonstratedin this volume by Fox, who used a precision
Segment)on the Juande Fuca Ridge.
pressuresensorto infer movementof the caldera floor over a
period of a year. Data from this recorder showed an abrupt
15-cmdownward
movement
of thecaldera
flooroveraperiod Hydrothermal
VentFields
of
several weeks during this period.
Simultaneous
hydrographic
measurements
showed
thepresence
of a new Although
low-level
hydrothermal
activity
anddeposits
are
water temperatureanomaly and a change in bottom current found in severalareasof the caldera(shownby Embley et al.),
velocities
onthecaldera
floor
occurring
atthesame
timeas most
ofthestudies
in thisissue
focus
ontheASHES
vent
theproposed
vertical
movement.
Foxattributes
this
correlation
field,which
contains
theonlyhigh-temperature
vents
inthe
toa period
ofactive
caldera
deflation
andarelated
thermal
caldera.
Most
ofthese
investigations
were
supported
bythe
plume
withassociated
cold
water
entrainment.
NOAA
VENTS
Program.
The
site
is
ideal
for
shortor
longterm experiments, being located on the flat, unobstructed
calderafloor, away from the calderawall. Hammonddescribes
WaterColumn
Studies
theventfieldin detail,drawing
fromtheresults
of Alvinand
A structureon the seafloor the size of Axial Seamountcan Pisces IV dives and deep-tow cameraphotographs. There is
presenta formidablebarrierto thefreecirculationof seawater a varietyof hydrothermal
depositsin the ASHES field, and
in the region. Positionedat the intersectionof the C-E becauseof the intensityof interestin the region,the deposits
Seamount Chain, the bathymetric expressionof Axial are well-mappedand sampledfor chemistry,geology,and
intensifieswater circulationpatternsin the region. Cannon biology. Hammondproposesthat the largestfield, the ASHES
and Pashinskidiscussa seriesof year-longmooredcurrent systemnear the southeast
comerof the caldera,is controlled
meter measurementsmade both on the summit of the volcano both by the faulting associatedwith the wall formation and by
andonthebathymetric
saddlethatconnects
Axial withBrown the permeabilityof the lobatepillow flows that makeup the
Bear Seamount(Plate A). The authorsinterpretthesedata to calderafloor in this region.
show that there is substantiallong-term averagedsouthward
Butterfield et al. presentthe detailedchemicaldata from the
flow alongthe entireeastsideof the seamount.West of Axial, ASHES ventfield, whichis the first well-documentedexample
in the saddleseparatingit from Brown Bear Seamount,lies a of a phase-separated
deep-seahydrothermalsystem.Using the
zone of convergence between southward and northward extensivecollection of water samplestaken from the ASHES
flowing currents. After converging,thesetwo currentsseem field in 1986, 1987, and 1988, they describethe geochemical
to mergeandflow westwardawayfrom the ridge. In addition effectsof boiling on a hydrothermalsystem. They observea
to being importantin their own fight, thesecurrentsystems widerangeof fluid compositions
from thefield, includingboth
also determinethe ultimate fate of the hydrothermaleffluent gas-rich and gas-poor compositionsand both high and low
that issuesfrom the summit of the active volcano.
chlorinity values. Butterfield et al., however,proposethat the
The hydrothermalvent systemson the summit of Axial entirerangeof observedcompositions
canbe explainedby the
Volcano make a detectableheat and chemical signal in the processof boiling and subsequentphase segregationof a
overlying water column. Lupron describesthe data from hydrothermalfluid with a single original composition. Fox
hydrographicsurveysover andnearthevolcano,takenbetween describesthephysicalseparationof the brine andvaporphases
1980and1987,whichshowcomplex
patterns
of•He-andMn- thatresultfroma "boiling"
ventsystem
andpresents
a relative
rich plumeswithin the water column. Theseplumesrepresent permeability model within the fluid conduits as a possible
regionalconcentrations,
as well as local effects,over both the mechanismto produce that separation. Litfie et al. discuss
summit and a local topographiclow known as Helium Basin both the theoretical"noise"producedby hydrothermalsystems
12,694
JOHNSONAND EMBLEY: INTRODUCTION
in generalandactualsoundmeasurements
of noisegenerated Although, as statedearlier, Axial Volcano is already a
by vents at the ASHES field.
remarkablywell-studiedpiece of seafloor,the recentvolcanic
The natural distributionof hydrothermalfauna, and the eruptions, the concurrent hydrothermal activity, and the
impactof geologicalcontrolson thisdistribution,is described proximityof theseamountto NorthAmericanwestcoastports
by Arquit. The major impactof submersible
samplingon the virtuallyguaranteethatAxial Seamountwill continueto be an
hydrothermal
faunais discussed
by Tunnicliffe,whopointsout intenselyinteresting
naturallaboratory
for a longtimeto come.
that vent animalsare very sensitiveto disruption,in spite of
REFERENCES
living in the extremely harsh environment of a highAtwater, T., Implication of plate tectonicsfor the Cenozoictectonic
evolution of western North America, Geol. Soc. Am. Bull., 81,
temperature vent.
CONCLUSIONS
3513-3536, 1970.
Davis, E. E., and J. L. Karsten,On the causeof the asymmetric
distributionof seamounts
aboutthe Juande FucaRidge: RidgeAlthough detailed conclusionsare not appropriatefor an
crestmigrationover a heterogeneous
asthenosphere,
Earth Planet.
introductorymanuscript,it is difficult not to make somefinal
remarksregarding the current statusof our understandingof
Sci. Left., 79, 385-396, 1986.
Hyndman,R. D., R. P. Riddihough,and R. Herzer, The Nootka fault
the remarkablegeologicalfeature representedby Axial.
zone--A new plate boundaryoff westernCanada,Geophys.J. R.
Clearly, Axial Seamountis an active underseavolcano, with
Astron. Soc., 58, 667--683, 1979.
recentlavaflows,activehydrothermal
systems,
anda calderaJohnson,
H.P.,andM. L. Holmes,
Evolution
inPlateTectonics:
the
thatmaybepresently
undergoing
inflation/deflation
cycles. vol.
Juan
deFuca
Ridge,
inTheEastern
Pacific
Ocean
andHawaii,
N, Decade of North American Geology, edited by E. L.
Thegravity
andmagnetics
dataargue
foreither
a shallow Winterer,
D.M.Hussong,
and
R.W.Decker,
pp.73-92,
Geological
magmachamberunderlyingthe summitcaldera,or a warm
Society
of America,Boulder,
Colo.,1989.
and/oralteredinterior.The vigorous
hydrothermal
activity Riddihough,
R. P.,M. W. Beck,R. L. Chase,
E. E. Davis,
R. D.
associated
withthecaldera,
while(surprisingly)
notof the Eastern
Hyndman,
S.H.Johnson,
and
G.C.Rogers,
inGeodynamics
ofthe
PacificRegion,Caribbeanand ScotiaArcs,Geodyn.Ser.,
same
scale
asthelarge
fields
found
ontheEndearour
Segmentvol.9,edited
byR.Cabre,
pp.5-23,
AGU,
Washington,
D.C.,
1983.
andExplorerRidgeto the north,still havea majorimpacton
the regionalwaterchemistry,biology,andperhapshydrology.
The detailedstudiesof theASHES vent field have,for the first
R.W. Embley,NOAA,PacificMarineEnvironmental
Laboratory,
time,documented
the chemistry
of a phase-separationHatfield
Marine
Science
Center,
Newport,
OR97365.
H. P. Johnson,Schoolof Oceanography,
Universityof Washington,
dominated
hydrothermal
system.
Presently
onlyobserved
on Seatfie,
WA98195.
Axial Seamount,this processhas a major impact on the
chemistryand geology of the vent field and will no doubt be
seenon manyof the shallowsubmarine
volcanos
of the
(Received
February
10,1990;
westernPacific in the future.
accepted
February
10, 1990.)