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10-10-2000 Basaltic Lava Domes, Lava Lakes, and Volcanic Segmentation on the Southern East Pacific Rise Scott M. White University of South Carolina - Columbia, [email protected]

Ken C. Macdonald University of California - Santa Barbara

Rachel M. Haymon University of California - Santa Barbara

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Publication Info Published in Journal of Geophysical Research, Volume 105, Issue B10, 2000, pages 23519-23536. White, S. M., Macdonald, K. C., & Haymon, R. M. (2000). Basaltic lava domes, lava lakes, and volcanic segmentationon the southern East Pacific Rise. Journal of Geophysical Research, 105 (B10), 23519-23536. © Journal of Geophysical Research 2000, American Geophysical Union

This Article is brought to you by the Earth, Ocean and Environment, School of the at Scholar Commons. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105,NO. B10, PAGES 23,519-23,536,OCTOBER 10, 2000

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

ScottM. White, Ken C. Macdonald,and RachelM. Haymon Departmentof GeologicalSciences and Marine ScienceInstitute, University of California, SantaBarbara

Abstract. Meter-scaleDSL-120 sonarmapping and coregisteredArgo II photographic observationsreveal changesin eruptivestyle that closelyfollow the third-orderstructural segmentationof the ridge axis on the southernEast Pacific Rise, 17ø11'-18ø37'S. Near segmentends we observeabundant basaltic lava domeswhich average20 rn in heightand 200 rn in basaldiameter and havepillow lava as the dominantlava morphology.The ubiquityof pillow lava suggestslow effusionrate eruptions.The abundanceof lava domes suggeststhat the fissureeruptions were of sufficientduration to focusand producea line of volcanicedifices. Near segmentcenters we observefewer but larger lava domes, voluminousdrained and collapsedlava lakes,and smoothlobate and sheetlava flows with very little pillow lava. The abundanceof sheetflows suggeststhat high effusionrate eruptionsare common. Fewer lava domesand large lava lakes suggestthat fissureeruptions do not focusto point sources.This patternwas observedon eight third-orderridge segmentssuggesting that a fundamentalvolcanic segmentation of the ridge occurson this scale.The third-ordersegment boundaries also correlatewith local maxima in the seismic axial magmachamber reflector depth throughout the studyarea and decreasedacross-axis width of the regionof seismiclayer 2A thickeningalong the one segmentwhere sufficient cross-axisseismic lines exist. The geochemicallydefined magmatic segment boundaries in the studyarea matchthe locationsof our volcanicsegment boundaries, although rock samplingdensity is not adequateto constrainthe variationacross all the third-ordervolcanic segmentsthat we identify. Theseobservations suggest that variationin the processesof crustalaccretion along axis occursat a lengthscale of tensof kilometerson superfast spreading(> 140 km/Myr full rate) mid-oceanridges.

1. Introduction intersectionsof neighboring segmentswhich have slightly different strikes (deviations from axial linearity (Devals)). The spreadingaxis of the mid-oceanridge is interruptedat Theseare the smallest RADs that are easily detectable using many lengthscales by morphologicridge axis discontinuities hull-mounted multibeam echosounders. Off-axis discordant (RADs) that divide the ridge into structural segments. zonesassociated with thesediscontinuities are either very Individual structuralsegments often have a geochemistryor smallor undetectable,so it is inferredthat the intervening tectonichistory distinctive from their neighbors[see Batiza, segmentsare short-lived (103-105 years) [Macdonald et al., 1996; Macdonald, 1998, and references therein]. As seafloor 1988]. Thesethird-order segments are typicallyten to a few mapsare madeat higherlevels of resolutionand with more tensof kilometerslong. An evenfiner scaleof segmentation complete spatial coverage,progressively finer scales of (fourth-order),defined by even smaller RADs, can be segmentationbecome apparent. Classificationof segments resolvedwith near-bottom sensors or fine-scalerock sampling based on segmentlength, RAD offset size, geochemical [e.g., Langmuiret al., 1986; Macdonaldet al., 1988]. variations,and longevity of segmentsand their bounding Many lines of evidencepoint to a segmentedvolcanic RADs permit the recognition of similar orders of plumbingsystem beneath fast spreading ridges, but detailsof segmentationon different ridges [Langmuir et al., 1986; the segmentationare unknown owing to the lack of Macdonaldet al., 1988, 1991]. Yet the originsof thesescales informationabout the eruptivehistory of the ridge. Seismic of ridge segmentationremain elusive. investigationsalong the southernEast PacificRise (SEPR) In this paper,we focus on a level of segmentationthat showthat a phase-reversedreflector, apparently produced at operateson a smallerscale than the segmentsdefined by the roof of the Axial MagmaChamber (AMC), is present transform faults (first-order), or by large overlapping undermost of the ridge,except at OSCs[Detrick et al., 1993]. spreadingcenters (OSCs) and propagatingrifts (second-order, Even where the AMC reflector is continuous, the AMC itself see Table 1). The discontinuitieswhich define this finer scale may be segmentedinto discretemelt pocketsseparated by of segmentationare typicallyvery small OSCs (offsetsless zones of crystalline mush [Singh et al., 1998]. Seismic than -1 km), local topographiclows or saddlepoints, and refractionstudies suggest that the crust is thicker and forms over a narrower zone around ridge offsets than near Copyright2000 by theAmerican Geophysical Union. midsegmenton superfastand fast spreading ridges [Barth and Mutter, 1996: Hooft et al., 1997; Carbotte et al., 1997; Papernumber 2000JB900248. Canaleset al., 1998]. Geochemicalanalyses of closely 0148-0227/00/2000J B 900248 $09.00 spaced (<10 kin) rock samplesshow variations in lava

23,519 23,520 WHITE ET AL.' SEPR VOLCANIC SEGMENTATION

Table 1. Characteristicsof SegmentationUpdated From Macdonald et al. [1991] Segments Order I Order 2 Order 3 Order 4 Segmentlength, km 600 _+300 140_+ 90 20 _+10 • 7 _+5 •

Segmentlongevity, > 5 x 106 0.5- 5 x 10a - 104 - 105 < 10q a years Discontinuities Order 1 Order 2 Order 3 Order 4 Type transform,large overlappingspreading OverlappingSpreading DevALs, offsetsof axial propagatingrills centers(oblique shear Centers,DevALs, summitcaldera or fissure zones,rift valleyjogs) SaddlePoints • systemsa(intravolcano (intervolcanogaps) gaps) Offset, km > 30 km 1-30 km 0 - 1 km:' < 0.2 km" Depthanomaly 300-600m 100-300m 30-100m • 0-30 m:'

Off-axis trace fracture zone V shapeddiscordant zone faint or none none a Changesbased on detailedobservations of SEPR compositioncorrelating with second-or third-orderstructural 2. Overview of the Structure of the EPR, segmentson the EPR [Langnzuiret al., 1986; Sintonet al., 17ø11'-18ø37'S 1991' Reynolds eta!., 1992]. Merging our fine-scale observationsof eruptivestyles on the SEPR with the insights The SEPR is currently the fastest spreadingmid-ocean provided by previous seismic and geochemicalstudies ridge [DeMets et al., 1990], openingat rates(-140 mm/yr) permitsus to developa new model for the significanceof close to the fastest documentedrate in the geologic record third-order ridge segmentsas individual volcanic systems [Wilson, 1996]. Accordingly, the SEPR is probably alongthe superfastspreading ridge axis. producingthe most continuousrecord of mid-oceanridge Publishedlava morphologydata from mappinoof fast to magmatismand volcanism. The spreadingcenter throughout superfastspreading ridges indicate that tile lava morphology tile surveyarea, 17ø11'-18ø37'S,is a local topographichigh, is dominatedby sheetflows generatedby fissureeruptions underlainby a low-densityregion [Scheirer et al., 1998]. The [Renardeta!., 1985' Genreeta!., 1986' Hay, urn et al., 1993' topographicaxial high of the SEPR in the survey area is Auzeude et al., 1996: Fornari et al., 19981. However, the flanked by low abyssalhills and by numeroussmall, near- vast majorityof this mappinois fi'om locally detailedbut axis, isolated volcanoes and volcano chains [White et al., spatially limited submersibleobservations concentrated 1998; Scheirer eta!., 1996b]. The survey area has an overall toward tile middle of magmatically robust ridge segments. morphologythat is typical of fast and superfastspreading From these observationsthe belief arose that fast spreading ocean ridges [Lonsdale, 1989; Macdonald et al., 1992; ridgesmainly erupt sheetflows and very little pillow lava Cochran et al., 1993: Scheirer et al., 1996a; Se,¾wre et al., [Bonatti and Harris(m, 1988' Perfit aim Chad•'ic'k, 1998]. 1997] (Figure l ). One exceptionto these spatiallylimited observationsis an Our surveyof the SEPR, 17ø11'-18ø37'S,contains the Argo surveyof the northernEPR (9ø09'-9ø54'N)that covered representativemorphologies of fast and superfastspreading 83 km of tile ridge crest (includingall but the ends of a ridges:the axial high may be characterizedby a smoothdome, second-order ridge segment) and found lobate flows half-trou•,h broad-shallow trou,,h or deep-narrow trough associatedwith large collapsedlava lakes [Hay, urn et al., (Figure2). From 17øil' to 17ø56'Sthe ridgeaxis lies along

1991]. Sheetflow eruptionsshould produce thin lava flows the crest of a (!•med axial hi•,h•, ß From the OSC at 17ø56 ' to with little to no relief, creating no volcanic constructions. tile OSC at 18ø09'S,tile ridge axis lies to the westof a 15-30 However, our extensive 1996 survey of the superfast m hi•,h west facin,, scarpthat has no east-facingconjugate spreadingSEPR providesnew evidencethat outside of (i.e., half graben).From 18ø()9' to 18ø22'Stile ridgeaxis sits limitedregions of collapsedlava lakes,dome-shaped mounds within abroad but shallow trough. From 18022' to 18ø37'S of pillow lavaare the mostcommon volcanic structure (Figure tile scarpsbounding tl•e trough are higher, and the floor of the 1). trough narrows. We present evidence that third-order RADs are the A bathymetric map of this area was made from geomorphicexpression of breaks in the volcanic plumbing SeaBeam2()()()data collecteel in 1996 with the ridge axis system of the superfast spreading SEPR. The spatial centered beneath the ship in order to minimize the beam distribution of volcanic structures,primarily the contrast footprinton tile ridge crest [Cor, tier eta!., 1996; Scheireret between small dome-shapedpillow mounds near third-order al., 19981. The resultingmap was nearly artifact-freewhen RADs and extensivecollapses of hollow lobate and sheetlava griddedat 100 m. We picked the RADs used in this study flows near segmentcenters, suggeststhat third-order RADs from this bathymetry(Table 2). Given the resolutionof the are sites of lower effusion rate eruptions than segment multibeam data, the locations of the RADs picked in this centers. The systematicvariation from high-to-low effusion studyare preciseto within 500 m, but the natureof the RADs rate eruptions on third-order segments suggeststhat the is suchthat they are bestdescribed as zonesof offset (Figure volcanic systemson superfastspreading ridges are organized lb). The area of our survey containstwo third-orderOSCs at this scaleof segmentation. that offset the ridge in a left stepof I km and have left a faint WHITE ET AL.' SEPR VOLCANIC SEGMENTATION 23,521

-17" 15' -17' 50'

trackDSL-120

axis

.:

-17' 25' t8 ø 00'

-174 35' S UR VEY AREA

-120' -100'

-17 ø 45'

Axial Depth Profile 1-182o, ....• ß ,-aF•r::.•-• t I..... •;•.m.• ...... '"'•' i • -• -2oLatitude -18 (;') -16 depth (kin) i .17ø55'! i

-18' 30' ....113' 15' -113 ø 10' -113" 25' -113" 15'

ao Figure 1. Bathymetryof the axial high of the southernEast Pacific Rise l¾oma SeaBeam2000survey in 1996 [Cormier et al., 1996; Scheireret al., 1998]. The data were griddedat 100 m then plottedwith a contourinterval of 50 m and false illuminated from the northwest. (a) Dashedthick line tbilows the ridge axis. Solid bold line is the DSL-120 track. The locationsof the topographicprofiles in Figure 2 are labeledfor reference. Inset map showsthe location of the survey area with respectto the major plates. Inset diagram showsthe minimum depth along the ridge axis of the East Pacific Rise. (b) Locationsof the lava domesplotted as open circles and the third-orderaxial discontinuitiesfrom Table 2, which are bracketed(to the left) to show the length of ridge apparentlydisturbed by the discontinuity. Open circles are not scaledto the size of the lava domes.Magmatic segmentsidentified by Sinton et al. [ 1991] on the basisof geochemicalvariation in - 10 km spaceddredge sampling are shownas I, J, and K.

trace of rapid southward propagation [Cormier and 3. Volcanic Structures and Eruptive Style Macdonald,1994]. In additionto the OSCs,our surveyarea 3.1. Data Collection has four left-steppingoffsets in the spreadingaxis without significantoverlap of topographichighs and two significant Although the SeaBeam2000system provides a very useful topographicsaddles accompanied by a local narrowingof the view of the regional bathymetry and ridge structure over topographichigh without identifiable offset of the spreading kilometersof seafloor,the ability to resolvevolcanic features axis (Table 2). All of the third-order RADs are the smallest that directly show eruptive style or the positionsof volcanic offsetsin the ridge axis we can resolvesolely on the basisof ventsis beyondthe capabilityof any currentlyavailable near- multibeam bathymetry,but they correlate with variationsin surfaceswath mappingsystem. To achieve the high level of the volcanic structuresobserved independently by our near- resolutionneeded to image fissures,volcanic vents, and lava bottom survey. flow fields, we used the near-bottom DSL-120 sonar system 23,522 WHITE ET AL.: SEPR VOLCANIC SEGMENTATION

-17 ø 50'

-17 ø 20'

-18' 00'

-17 ø 30'

-18' 10'

-17 • 40'

-18 ø 20'

-17 • 50'

I ß... -18' 30' -113 ø 15' -113 ø 10' -113 ø 15'

Figure 1. (continued)

and the Argo II photoacousticsled during Leg 2 of the track connectingthese areas [Haymort eta!., 1997]. These Sojournexpedition on R/V Meh'i!!e in 1996 [Haymortet al., data allowed us to make lava morphology maps over large 1997]. The DSL-120 side-scansonar operates at 120 kHz and areas of the seafloor. provides backscatterreflectivity imagery as well as phase The Argo II visual observationsof lava morphologywere bathymetry. We were able to image a 900-10()0 m wide side- In,,,,e•l in real time into a •eo•raphic information system scan reflectivity swath, and a 700-80(.)m wide bathymetric (GIS) database, but the observations were made at random swath, continuouslyalong 150 km of ridge crest (Figure 1). intervals,and usuallyafter crossinginto an area where a new Only one line was run using the DSL-120 sonar becausethe lava morphology was seen [Ha.ynton eta!., 1997]. To swath width was sufficient to cover the entire axial estimate the area covered by a given lava morphologyfi'om IleovolcalliC zo!le. the points logged along Argo II tracks, we followed the GIS The DSL-120 survey provided a map fi)r more detailed method for bufi'ering a line of navigation data into an investigationsusing Argo II, a remotely operated vehicle effective swath width describedby Wright et al. [1995]. Our providing data fi'om three video cameras,two still cameras, procedureimproves on this methodby using averageAt7;o 11 lmagenex 675 kHz profiling sonar, 200 kHz side-scansonar, altitude tk)r each line to calculate the average swath width conductivity-temperature-depth, transmissometer, and along that line from field of view of the cameras. We magnetometer[Haymort et al., 1997]. The Argo II has a -10 convertedeach loggedobservation of lava morphologyto an m visual swath •idth, and it is operated around-the-clock area within the visual swath width of Argo H by the with real-timeteedback. We obtaineddense visual coverage "Thiessen"Polygon method (implemented by Environmental of the axial neovolcaniczone on multipletracks fi'om 17ø15'- Systems Research Institute Arc/INFO© version 7.1). The 17ø40'S and 18ø23'-18ø29'S and coverage along a single principle of this method is that the entire area within the WHITE ET AL.' SEPR VOLCANIC SEGMENTATION 23,523

Cross-Axis Profiles 5X verhcalexaggerahon . of ridge crest are statistically averaged. Where detailed A • lkm • A' geologicalmaps of lava morphologyare requiredover smaller ,:•- i • •. • . -2700 areas, as in the case of some of the lava domes described

-2800 later, the aforementioned statistical methods were abandoned -2900 , , . I .... I .... I .... i .... i .... ! .... / in favor of reviewing videotapes to precisely locate lava -113.27 -113.26 -113.25 -113.24 -113.23 -113.22 -113.21 -113.2 morphologycontacts. B Longitude( ) B'

•59'S 3.2. I,ava Lakes With Overflow Tubes and Channels -2800 Two areasof extensivecollapse in the volcanic carapaceat -2700-290O•, , , I• I ,,,I -11334 -113.33 -113.32 -113.31 -113.3 -11329 -11328 -113.27 the crest of the smoothly domed part of the axial high were Longitude ( ) C C' imaged at the meter-scaleusing the DSL-120 side-scansonar system,and one was subsequentlydensely surveyed with the -2700 Argo 11photoacoustic sled. These collapsesare similar to the axial summit collapse trough at 9ø26'-9ø56'N on the EPR -2900-2600 I' I ' ' ' ' I ; '' ' I " ' ' ' ; , i.... I' [Hay,•on et al., 1991: Fornari et al., 1998]. The -113.39 -113.38 -113.37 -113.36 -11335 -113.34 -113.33 Longitude ( ) northernmost collapse was discovered by the Cyana D expedition in 1984 [Re/taM et al., 1985] and named Aldo -2700 Lake by the Naudur Expedition in 1992 [Auzende et al., -2800 1996]. We refer to Aido Lake and the entire string of -2900 collapses, 17ø26'- 17ø29'S, as the "A!do Lake trough" -113.43 -113.42 -113.41 -113.4 -113.39 -113.38 -113.37 -113 36 (Figure 3). The southernstring of collapsesfrom 17ø42' to Longitude ( ) 17ø45'S was discovered during our 1996 DSL-120 sonar Figure2. Bathymetricprofiles plotted along lines of constantsurvey [tho'//•o/• et al., 1997]and ha• beennamed "Pisco latitudeacross the axial high from SeaBeam2000 data gridded trough" (Figure 4). at 100 m. Vertical exaggerationis 5'!. Arrows indicate the Both of these areas of extensivecollapses have a similar locationof the ridge axis. The lines below the profile indicate structure. The individual collapses are ---10-15 m deep the interred position of graben-boundingfaults. From top to bottom, the ridge axis lies (a) on the crest of a smooth dome, (Figure 5). They are elongateparallel to the spreadingaxis (b) in a half trough west of a scarp, (c) in a broad, shallow and never more than 100 m wide. Both areas of extensive trough, and (d) in a deep, narrow trough. These four axial collapsescontinue for •-7 km along axis but consistof many morphologiesrepresent the four most common morphologies individual elongate collapse pits that have an en echelon of fast-spreadingridges. arrangementin many places. Visual investigationof these siteswith Argo 11showed very broad, fiat lobate flows at the edgesof the collapses.Most of the lobatecrust surrounding polygonsurrounding each point (eachlogged observation in the collapsescovers cavernousvoids that can be seen by our case)is closerto that point than any other point defining lookingback underthe roof at the edgeof the collapse. Lava any other polygon. This method was chosen becauseit pillars or continuouswalls of coalescedpillars supportthe preservesall of the originalobservations of lava morphology overhangingroof. Glassy selvagescould be seen on the while providinga way to convertthe nominal point data into pillars and walls, and the floor of the collapseswas mainly nominal area data without creating overlapping areas. rubble or sheet flow. These observationssuggest that these Problemswith this point-to-areaconversion method arise in collapsesare drainedlava lakes[Ballard et al., 1979; Fornari determiningprecise locations of lava morphologycontacts, et al., ! 998]. particularly in places where observationswere logged Caved-in lava tubes and lava channels containing sheet infrequently. While such problems make this method flows were observedat the margins of the collapse troughs unsuitablefor creatingmeter-scale geologic maps, the mix of (Figure 3 and 4). None of the tubesor channelsextend lava morphologiesare accuratelyrepresented when kilometers beyondthe DSL-120 swathwidth. Lava flowing throughthe

Table2. Third-OrderRidge Axis Discontinuities, Location and Description Latitude Longitude Structure/Morphology Length, knl 17 ø I 1' S 113 ø 08.5' W startof survey,midsegment 17 ø 18.5' S 113 ø 10.6' W saddlepoint 14.7 17 ø30'S 113 ø 13.5'W left-steppingnonoverlapping offset 22 17 ø 35.75'S ll3 ø 15'W left-steppingnonoverlapping offset 11 17 ø 40' S 113 ø 15.75' W saddlepoint 8.1 17 ø56'S 113 ø 17.75' W left-steppingoverlapping spreading center 29.6 18 ø 02.5' S 113 ø 19' W left-steppingnonoverlapping offset 12.3 18 ø 09' S I13 ø 20.75' W left-steppingoverlapping offset at transitionbetween 12.4 half grabenand full grabenridge axis 18 ø 22' S 113 ø 22' W left-,qteppingoverlapping spreading center 24.6 18 ø 30.75' S 113 ø 24.7' W left-steppingnonoverlapping offset 18.5 18 ø 33.5' S 113 ø 25.25' W endof sur',.,,),i•Ol-tll of o',erlappingspreading center 23,524 WHITE ET AL.' SEPR VOLCANIC SEGMENTATION

1 KM

t

Figure 3. (left) Sidescanreflectance image from DSL-120 of Aldo Lake trough, and (right) geologic interpretation. Side- scanreflectance shows higher backscatterreflectivity as darker areas. The right panel shows a line drawing interpretationof the geologybased on DSL-120 bathymetrygridded at 5 m and Argo 11 video coverage. The major collapsesare outlined in solid black lines and representindividual lava lakes. The lava Figure 4. (left) Side-scanreflectance image and (right) lakes are arranged en echelon, implying they formed over en geologicinterpretation of Piscotrough. Darker areasin the echeloneruptive fissures. The areasof light gray fill are either side-scanhave higher backscatter reflectivity. The symbols lava sheet flows or collapsed lava tubes. Approximate usedin theleft panelare same as Figure 3, withthe exception locations of flow fronts and lava deltas at the ends of the tubes of two axiallava domes, shown in lightgray fill with"D" on are shown with a dashed line. the right panel. The domesare differentiatedfrom flow fronts by having a quasi-circularbase, a strong(dark) acoustic backscatterreflection on theirside nearest the sonarvehicle, a longestof these conduitswould travel a total distanceof <1 correspondingweak (light) reflection on their far side, and km, and the averageflowline lengthis only 200 m. All of the positiverelief of > 10m. Low-reliefflow fronts appear to abut thebase of thedomes. Because these domes are slightly off identified conduit systems actually end <500 m in axis,and apparentlypreexist the flowsemanating from the perpendiculardistance from the collapse. The DSL-120 collapse,either temporal variations in eruptionstyle on axis, or bathymetryshows that these large lava conduits are all no the eruptionof smalloff-axis dikes probably caused these more than 6-8 m deep (Figure 5). Their distal endsterminate domes to form. WHITE ET AL.: SEPR VOLCANIC SEGMENTATION 23,525

-113ø13.5 ' -113 13.0' We observethat volcaniccollapses, elongate parallel to the -17 ø 28' spreadingaxis, are the sourceof lava tubes and sheet flows and are the loci of hydrothermalflow [Hay, ton, et al., 1997: O'Neill, 1998]. We hypothesizethat these collapse features form directly over eruptive fissures,like troughs of similar di•nensionsfound on other ridges [Fornari et al., 1998: U.S. Geological Surver Juan de Ft•ca Study Group, 19861. Fissureeruptions in Iceland and Hawaii [e.g., Sigurdssona•td Sparks, 1978: Holco,tb, 1987: Lockwood et al., 19871 produce en echelon segmentssimilar in length and offset to the SEPR collapsedlava lakes. The Argo !I video revealsthat the marginsof the collapseshave sheetflow' lava morphology . . ) / , •:'/ , that appears to be generated fi'om a line source eruption similar to historical fissure eruptionsof basaltic volcanoes. The local lava morphologyof primarily smooth Iobates and -17' 28.5' sheet flows and the occurrenceof a relatively thin roof over large areas of void space adjacent to the collapsessuggest a rapid, high effusion rate eruption [Bonatti and Harrison, 1988; GriJ.fiths a•M Fin/,', 1992; Gregg and Fink, 1995; Gregg et al., 1996]. Hydrothermal venting issuing directly from ,"' ...' ...... ,:'"?, fissuresand brecciapiles on the floor of the collapseis a key observationto supportthe interpretationthat collapsesform directly over the eruptive fissures[O'Neill, 1998]. Eruptive fissurescan provideboth a pathwayfor fluid flow and a heat ,"!' .' • • I / • I I :zl/ ! i/ sourcethat is large enoughto power hydrothermalcirculation [Haymort et al., 1993: Wright et al., 1995; U.S. Geological SurveyJuan de Fttca StudyGroup, 1986].

3.3. Axial Basaltic Lava Domes Small domed volcanicedifices form on the ridge axis Figure 5. DSL-120 side-scanphase bathymetryof the mainlyin areasaway from the collapsedlava lake complexes southernpart of the Aldo Lake trough contouredat 2 m and unassociated with lava channels. These axial volcanic intervals. Closed contours are labeled "L" for local lows and "H" for highs. Lava tubes,3-6 m deep,emanate from the 10- edificesare subtle topographic features which average 200 m 12 m deepcollapsed lava lake. At the downslopeend of the in diameterand 20 m in height. Theseaxial edificeswere tube, the flow front of the lava delta is 6-8 m high. The floors identifiedin boththe:DSL-120 side-scan backscatter imagery of the collapsedlava tubes nearly on level with the floor of the and bathymetryas local topographichighs with a quasi- lava lake collapses,and the nearlyuniform height of the walls ellipticalbase. Some coalesce into rows aligned with the ridge of the collapsedlake suggest that the entirestructure may have axis, as if eruptingfrom an axis-parallelfissure, but the formed in a single eruption. outlinesof discretestructures can still be traced(Figure 6). Axial edifices are also found as isolated features or in small clusters.We classifythe small domededifices on the SEPR ridge crest as "lava domes"because our observationsfit that in lava deltas -6-8 m high (Figure 5) that look like inflated geneticclassification, although they are similarto constructs tube-fed pahoehoeflows on subaerialvolcanoes [Honet al., genericallycalled "pillow mounds"[e.g., Perfit and 1994]. Chadwick, 1998]. We describebelow in detail how the SEPR The same style of eruptionsthat created Aldo Lake and axial lava domes fit the definition of lava domes as volcanic Pisco troughs may occur south of 17ø56'S, although the edificesbuilt by lavapiling up overa primaryvolcanic vent overall structural style of the ridge is different. No such [Williams,1932]. The differentiationof pillow lava domes collapsetroughs or associatedlava distributionsystems were fromother types of pillowmounds is warrantedat thisstage observedsouth of 17ø56'S, where the spreadingaxis runs sincesome studies imply that pillow moundsfound in certain througha grabenbisecting the axial high. However,evidence settingsmay be secondary,tube-fed or lava flow inflation of lava ponding and subsequentcollapse is documented featureswhich would havemuch different implications for visually in the Argo 1I survey as well as by the Naudur volcanicsystems and eruptionmechanics [Appelgate and Expedition dives [Auzende et al., 1996] in the areas near Embley,1992; Bo'an et al., 1994;Smith and Cann,1999]. 18ø16' and 18ø25'S. In theseareas, large vertical scarpsof Are the dome-shapedfeatures imaged by the DSL-120 the axial summit graben may dam the lava flows and cast sonartrue lava domes? Lava domesform directlyover acoustic shadows that make collapse troughs difficult to primaryvolcanic vents led directlyby dikes,not from rootless detect in DSL-120 side-scan records. When these two areas ventsfed by surfacelava flows. Distinguishingvolcanic southof 17ø56'Sare included,the centersof tour out of eight ventsfrom rootlessvents provides insight about how the axial of the third-order segmentsare characterizedby extensive magmadelivery system creates topography at the ridgeaxis. volcaniccollapse associated with drainedlava lakes. The presenceof a lava domeimplies that an underlying 23,526 WHITE ET AL.' SEPR VOLCANIC SEGMENTATION

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.. :...... • .•:•:}"-"•'-; :• t '

•;':::•":':"-•.•?;:"•'.•?."h/:.:, ' ,-"%•-,•":'" .;:':. :":• • 2 .,:'-.::.:,. -:.•,,•,..:,•...: •.•.::-,.•:...: ..... •..-;:;•;.-...... :.....-..::--• .:,. •..• dome .:-.':...:.:.?::•..••j!•::•* .....5•>, •...:';:'}• •.::...:;-:..? ::::•:-:•., :•-:-.:'½...... •..;::.:;::: .-z.•:•:•:l::. boundary ?::' '""--" :'•- :" '• :.•::?,-:"-•.:½'".;•.•:•:?•;; ...... } --, • ::".-'•'"•-.-.':;•:•.•:•i::. '"'•'

•/.•: • •.½.:•:&½..•:.,:•&:?--.• .....•_•,..• .•;:•.:.:.:.:.•.•...' ! .':..•:•-:-:,..:•....,•;.•.:-'-:.::/:-?-:-:i ..-:::..?.':.:..•...... -:• .-.•:•.':.•.• . I I ...... • •'•:•:'•::'•:.::•'-."•-.L.:..'"' ..'-•.-:.:•:'.:•..:'• •:•1•:;:'' ...... t .. ! •g•:..:-.••!:•.:.•,:: • •.• ...;:•::•:•-....::.½..:.•.•:;...... •. .ß ...::-::.--, ?--• '•:.•?f'"";'•':: .:•:&:.:'%•.... -•':::-T• •'-4.... *'"-:' • .:•:•.$ •:'•:':"'•..•,:-. ' ..••':'•:-"': •::•]. •.' •}::J•'::"' •:.:•:.::s•. •'.... .&•':,.•. -%•...•:• •:':•::•'-•:-•:' ...... :':•i¾ .. :": ß •" ,::.•.'":':: •.. • •' "D•'•:J:"...... •.:. .... ß-:L.•½•ff •'::'•4•?'.• •." :.•:...... •::.....-:-:.'-:¾:•:..?ß

.•:.;''.•;:' • r:.: :.:-- •..... •/•. ..,•.:.;• -..;'•...!:• •-••: .:•:.• :' . • •.•-- :.)L.':•. :• : -:½ •.•..•(':.:..r',,;:•*':: • •::'. 17 ø 53.0' ..17" 53.0'

..•: :½..::::•:;• ...... • .. . f. •.:...:;•--:..;•..:-..:....,t.•.....-:-•.•.

..':•;'-..•?:.•:-:':.. & • '"':•::••;•; ' '.•:L•.'-'.•' '* '•:'.•':'•'•-••...4:•'•. '";-*..•.•.•:...:.3::•' '...':.: •i,;.•:-• •...::•,:•!•'•.•½-;.} -.-•.:•:• •*' ..'•'f'".'.• - . .•.:...•...• •. :...... •.:.• ...... :• ...... -•.. •...

: :5m

0.2 km

Figure 6. An exampleof axial lava domesfound at the end of a third-orderridge segment.Left panelshows the DSL-120 side-scanreflectance image fi'om data gridded at 2 m. Darker areas in the side-scan have higher backscatterreflectivity. On the right panel,the DSL-120 phasebathymetry, gridded at 5 m, hasbeen contoured at 5 m intervals. Black outlines show the digitized boundariesof the domes. Large fissuresare continuousthrough several domes. primaryvent eruptedrelatively short, slow moving lava flows. that the axial lava domesformed over primaryvolcanic vents. In contrast, rootless volcanic constructions, such as tumuli Further evidence that axial volcanic mounds are not rootless and hornitos, form in relatively long lava flows with well- conesis thathydrothermal sulfide chimneys commonly grow organized lava tube systems that require steady eruptions near fissuresor collapseson the mounds[O'Neill, 1998] lasting several days or longer to develop [e.g., Cah,ari and (Figure7). This levelof hydrothermalactivity requires a Pinkerton, 1998; Kes•thelyiand Self, 1998; Hon et al., 1994;. significantsustained heat source, such as an intrusion creating Walker, 1991]. aneruption at a volcanicvent. A singlelava flow, even long- The visual observations made with Argo II help to duration inflation of a flow field, would not be able to establishthat the axial lava domes imaged by the DSL-120 provideenough heat to powerseveral hydrothermal chimneys. sonarform over primaryvolcanic vents. Review of the video All of thisevidence suggests that axial lava domes described from Argo !I reveals that the domes are made mainly of hereform by eruptionsfrom primaryvolcanic vents. pillow lava, with minor lobatelava flows primarily found near Althoughthe precedingevidence suggests that the axial the summitsof someof the larger domes(Figure 7). Tumuli lavadomes are not secondary volcanic constructions ted from form exclusivelyin inflated lava fields, but sincepillow lava surfacelava flows, seafloormapping alone cannot detect is not generally associatedwith extensive lava fields on the subsurfaceconduits, such as preexistinglava tubes. Lava SEPR and since the axial lava domesdo not appearto be part suppliedalong strikefrom the segmentcenter through of any extensiveflow field [Haymon et al., 1997], we infer reoccupiedlava tubeswould appear to havemany of the WHITE ET AL.: SEPR VOLCANIC SEGMENTATION 23,527

.-113"15' -1 I3ø15' -113ø14.5'

Active Hydrothermal Vent

-17' 35.0' transitionalpillows -17 ø 350' øpillow-iobate • •1obate *•o0•

0•0

øooooo• o o

-17 • 35.5' -17 ø 355'

0.2 km 0.2 km -17" 36.0' -17 ø 36.0' -113ø15' -113014.5' -1 !3ø!5 '

Figure 7. Sidescanreflectance image (left panel) and lava morphologymap (right panel) of an area where good coverageof severaldomes was obtainedwith Argo H. Darker areasin the sidescanhave higher backscatter reflectivity. We revieweddown-looking color video fi'om severalclose-spaced A/58o II lines, focusingon the classificationof lavamorphology, to makethe lava morphology map. The lavaflow morphologyconsists mainly of pillows(bulbous to quasi-spherical,breadcrust cracking, striations, and glassy buds) with sometransitional pillow- lobateflows (somewhatbulbous, some breadcrust cracking, and few buds),and occasionallobate flows (slightly bulbousto flat, no breadcrustcracking, no buds). Lobateflows were observed at the summitof severaldomes. The sidesof thedomes are almostexclusively pillow lava;no sheetlava flows were observed. qualitiesof primaryvolcanic vents. However,a simple roughly10 m. Along the SEPR, topographicgradients are calculationof the pressuregradient in a lavatube along the generallymuch less than 10 m/km, thus making it improbable crestof thegently sloping SEPR suggests that gravity-driven that axial volcanicmounds are built 20 m high over secondary flow throughlava tubes is unlikelyto createaxial lava domes vents. On the Mid-Atlantic Ridge the steepertopographic nearsegment ends. Assuminggravity-driven Poiseulle flow gradients'might allow volcanic structures like thelava do'mes of lavain a cylindricaltube, the elevation drop E d requiredto seen on the SEPR to form over secondaryvents [Smith and producea flow at the end of the tubeis givenby Ed = Cann, 1999]. 8pQD/rcR4(p/-p,)g [Turcotw an,d Schubert,1982]. Axial pillow domesare distinguishablefrom lava shields, Reasonablevalues for this equation area lavaviscosity p=10 -• thoughboth are primaryvolcanic edifices formed by small, Pas [Head et al., 1996]; lava density p/=2500 kg/m -•[Head et nonexplosiveeruptions. Lava shieldsare built from lava al., 1996];water density p,,=1000 kg/m 3: typicalvolume flux flows that reach tar from their vent, resultingin a very low throughthe tube Q=5 m3/s (as reported for subaerial flows by slope profile and an extensiveapron of lava flows Kauahikaua et al. [1998]); a lava tube radius R=3 m surroundingthe shield [Walker, 2000; Rossi, 1996]. In (matchingthe sizeof the largestcollapsed lava tube seen in contrast.the SEPR axial pillow domesappear to coverfairly DSL120 recordson the SEPR), and gravitationalacceleration small areas and are not the source of extensive lava flows. on Earth g=9.8 m/s2. The minimumelevation drop The ratio of edifice height to basal radius can be used to theoreticallyrequired to form a pillow flow withoutany discriminatebetween a lava dome composedof short flows heightat a distanceD of I km fromthe segmentcenter is that constructa high edificewith a smallbasal radius and a 23,528 WHITE ET AL.' SEPR VOLCANIC SEGMENTATION

8O 120 OSC';boundedsegments= 70 i ...... •.. • lOO

E 80 o

-- 60 o (A) ß'• 40 E

: 2o

• ß 3%:-. SegmentEnd SegmentCenter

70

60 0 lOO 2oo 300 radius (meters) E 50 o Figure 8. Plot of the heightand radiusfor eachof the domes picked in this study. The radiusof a dome was calculatedas (B) o 40 its basal peri•neterdivided by 2•r for all of the axial lava domesimaged with >50% DSL-12()bathymetric coverage. E Error bars show the ability of our DSL-12() side-scan = 30 bathymetry to measurethe hei,'•htto within 4m and a 10% error on our ability to measureradii. The lines show the least squareslinear .'egressionbest fit line to height:radiusratios 20 from Icelandiclava shields(h=0.()38r)[Rossi, 1996]. The stars indicate the sizes of documented small basaltic lava domes, Segment End Segment Center labeledas fi)llows:LH, estimatedsize of Low Headon King GeorgeIsland in Antarctica[S, wllie et al., 1998]; KEJ, 1978 80 dome formed on Kick'era-Jenny[Devine r•/•! Sig/t/'dxso/•, 1995]: B, Bogosloferuption of 1927 in the Aleutian Islands [Mille/' et •!., 1998]; MC, MarneIon Central dome formed in 70 i 4thordersegments 1791 on Reunion Island that subsequentlycollapsed

[Nelt//tan/•va/! Pathrag,1951]. Theseexamples are all of the ß 60 :. small basaltic lava domes known to us, and all lie near the E o centerof the field defined by the southernEast Pacific Rise : -- 50 (SEPR) axial domes. o (c)

. .a 40 lava shield composedof long flows that constructa low E edificewith largerbasal radius. The height:radiusratio of the 3o SEPR axial lava domes is closer to that of other basaltic lava domesthan that of basalticlava shields(Figure 8). This 20 suggeststhat SEPRaxial lava domes,although composed of , basalt,generally have much shorter lava flows and possibly Segment End Segment Center more endogenic growth than the low-relief lava shields on Figure 9. Histogramsof the cumulativenumber of axial lava land. domesfor eachof threepossible orders of segmentationin the Dome-formingbasaltic eruptions are uncommonon land surveyarea. Bin intervalis normalizedto represent20% of wherethe growth of lavaflow crustis too slowto impedethe the distancefi'om segmentend to midsegmentin orderto expansionof the flow nearthe vent. Quenchingof lavain the adjustfor segmentsof varyinglength. (a) When only submarineenvironment could create a thick crustquickly overlappingspreading centers (OSCs) are considered,a enoughand with sufficient strength to inhibitthe spreading of bimodal trend to the histogramdistribution of axial lava lava when the effusion rate is low. This would favor the domesimplies a finer orderof segmentationbisects most of formationof basalticlava domesduring low effusionrate the OSC-boundedsegments. (b) The decreasingtrend from segment end to the center of the number of axial volcanic submarine eruptions. Basaltic lava domes in a forearc moundsis observedonly when all of thethird-order segments extensionalsetting were studiedafter being exposedon an are considered. (c) On the very fine scale fourth-order islandand were interpretedto haveformed by endogenous segmentsidentified from the DSL-120 maps,no systematic growthin a submarineenvironment ISraellie et al., 1998]. distribution of lava domes is observed. WHITE ET AL.' SEPRVOLCANIC SEGMENTATION 23,529

3.4. Axial Lava Domes Mark the Ends 6x104 of Third-Order Segments 3rd order segments A clear correlationthat emergedfrom the Sojourn2 survey 5x104 data was the large number of axial domes near the ends of segments. To quantify this systematically,we digitized the outline of the baseof each dome and calculatedits positionas a point at the centroid of its base. The distance from the E 4xl centtold of the dome to the nearest segment boundary was determined by straight line distance between the centtold E point and the segmentendpoint. Separatecalculations were 'oo 3xl 04 o made for the OSC-bounded segments,for all of the third- order segmentsidentified in multibeam bathymetrymaps, and fi)r the fourth-ordersegments identified in the DSL-120 sonar eß 2xl 04 data. Becausethe segmentshave different lengths,we used a normalized distance along the segment. The distance from E dome to segmentend was divided by half of the total length 1x10 4 of the segment to get the normalized distance. This normalization method was employed for the total edifice distribution, the total base coverage, and the average basal area of the axial domes. The resultswere graphedusing bins 0 segment end representing20% of the distance from the segment end to segment center segment center. Figure 10. The mean basal area of the axial lava domes When normalizedto the OSC-bounded segmentsonly, the distribution of axial lava domes shows a bimodal distribution showsa generaldecrease away from third-order segment ends. Bin interval is normalized to include 20% of the third-order (Figure 9a). The abundanceof domes increasesat both the segmentlength (same as Figure9). centersand the ends of segments. The explanationfor this is that other RADs exist near the centers of some of the OSC- bounded segments When all third-order seomentsin the survey area are used (Table 2), the abundanceof axial lava producepillow lava flows and that effusion ratesan order of domesdecreases from segmentend to segmentcenter (Figure magnitudehigher can produce sheet lava flows [Griffiths and 9b). Dome size, representedby average basal area of the Fink, 1992;Gregg and Fink, 1995]. mounds in each bin, shows the opposite relationship, Becausethe distributionof lava domesand collapse decreasingnear third-orderRADs (Figure 10). troughsalong the ridge suggeststhat effusion rate varies A finer scale of discontinuity(fourth-order) v•a• identified systematicallyover the third-ordersegments, we quantified within the DSL-120 records, where offsets of <200 m can be the along-axisdistribution of lava morphologyas an detected [Ha)'tnon et al, 1997]. When the distribution of independentindicator of changesin volcaniceffusion rate. either the number of domes or their average basal area is The highly variablenature of lava morphologyrequires normalized to these very short segments,no patternsemerge almostcomplete visual coverage for accuratemapping, and (Figure 9c). Thus the formation of axial lava domesdoes not statisticalmethods are the most efficient way of dealingwith appearto be controlledby processesoperating at the smallest thelarge quantity of rawdata this generates. Dense coverage scaleof segmentation. of the ridgecrest that we obtainedwith Argo II from 17015' to 17ø40'Sprovides an idealdata set which encompasses the 3.5. Lava Morphology threenorthernmost third-order segments of our studyarea [Haymort et al., 1997]. By comparing submarine lava flow morphology and The real-time observationsfrom Argo I1 of lava volcanic structuresto featurescreated by subaerialvolcanism, morphologywere assigned to oneof threelava morphology inferences can be made about eruption dynamics on mid- categories: (1) pillow, (2) lobate, and (3) sheet. The ocean ridges [e.g., Ballard et a/.,1979]. Low effusion rate, discontinuousobservations were expandedinto areas,where slowly advancingpahoehoe flows becomepillow flows when eacharea is moreproximai to theobservation "point" location they enter the water [Jones, 1968; Moore, 1975]. The flat, thanany other observation, and is clippedby theactual Argo often iineated or autobrecciated, surfaces of submarine sheet 1I camerafield of view. The lavamorphology was binned at lava flows are usually interpretedas indicatorsof high lava 20% of the distancefrom segmentend to segmentcenter, effusion rate [e.g., Ballard et a/.,1979; Gregg and Fink, following the methoddescribed for the axial lava domes. The 1995]. Sheet flows are observed mainly in just two totalarea covered by eachof thethree lava morphologies was environmentsin our Argo 1I survey:(1) floors and marginsof dividedby thetotal area of Argo I1 coverageto normalizethe collapsed lava lakes, and (2) floors and levees of lava dataset to 100%of thecoverage area within each bin. channels(Figures 3 and 4). These observationsare consistent When the datafrom the threenorthernmost segments of with the idea that submarinesheet flows forin by rapidly our surveyare stackedand binnedat 20% of the segment flowing (higheffusion rate) lava, analogousto subaeriallava length,the lava morphologyexhibits a regionalpattern pondsand channels[Wolfe, 1988; Petersonet al., 1994]. The consistent with the observed distribution of volcanic inferencesfrom subaerial analoguesare supportedby structureson the ridgeaxis (Figure !l). At segmentends the laboratorystudies showing that low effusionrate eruptions most common lava morphologyis pillow lava, and the 23,530 WHITE ET AL.: SEPR VOLCANIC SEGMENTATION

EffusionRate LOW HIGH 3O% 2O% Sheet Flows 10% O% 8O%

60%

Lobate Flows 40%

20%

0%

60%

40% Pillow Flows 2O%

0%

Total Number of Pillow Domes

Volcanic Segmentation SegmentEnd SegmentCenter

Idealized Half Segment .,.•.•ili?iiiii?ß "' Lobate Lava E Ridge Axis Topography and ...... ::'•'•':' Flows Lakeso Major Volcanic Structures Pillow Lava Domes v

-5-10 km

Figure11. Thelava morphology forthe three northernmost third-order segments of this survey where dense Argo II coverageis availablefollows the pattern expected fi'om the observations of pillow lava domes using DSL-120 sonar.To obtainthe percentage of each lava morphology, the total area covered by thelava morphology was statisticallydetermined, asdescribed in thetext, then divided by thetotal area visually imaged in eachbin interval by 1996Argo II survey.Bin intervalis normalizedto include2()(,• of thethird-order segment length (same as Figure9), andbins are mirrored around the segment center. The major volcanic structures seen on the ridge crest (drawnschematically on bottom panel) reflect changes in thedominant lava morphology observed. Using lava morphologyasan indicator of effusionrate suggests that effusion rate drops off systematicallytoward the ends of third-order(volcanic) ridge segments.

volcanic structures are axial lava domes. Both indicate that each third-ordersegment physically behaves as a single low effusion rate eruptionsare mostcommon near the endsof volcanic system (as in the Icelandic usage [e.g., third-ordersegments. At segmentcenters, lobate lava flows Gud,ltmdsson, 1995]) with higher effusion rates above cover most of the area, and the characteristic volcanic centrally located vents and lower effusion rates at vents near structuresare large collapses. Furthermore,sheet lava flows segmentends. While third-orderRADs on slow-spreading are insignificantat segmentends but commonat the segment ridgesare gapsbetween large volcanic edifices on the floor of center. Surrounding A!do Lake trough, sheet flows are the median valley JMacdouald et al., 1991; Smith et al., extensive (> 50% of the total area surveyed).We conclude 19991, their analogues at fast spreading rates are that lava effusionrates are consistentlyhigher near the centers topographicallysubtle features such as smalloffsets (<1 km) of third-order segments. The regional changes in lava or saddlepoints in axial topography. morphology seen in the Argo !! survey support our interpretationthat the volcanic structuresseen with DSL-120 4.1. Axial l,ava Dome Formation reveal changesin the effusion rate of eruptionsover third- A lower boundon longevityof eruptiveactivity at order segments(Figure I I). individualaxial domes may be approximatedby measuring the volumeof the domeand dividingit by a theoretical 4. Discussionof Volcanic Segmentation volumetricflow rateappropriate for the lavamorphology on the mound. This calculation assumesthe entire edifice is Systematicchanges in volcanicmorphology along third- createdby continuousactivity from a pointsource vent. The ordersegments show that the eruption dynamics vary at this populationof axialdomes has an averagevolume of 4xl0 s m3 lengthscale (Figure l I). Becausechanges in theeruption and a maximumindividual volume of 8x!ff'm •. Mappingthe dynamicsreflect changes in the deliveryof magmato lava morphologieswith Argo 11 revealsthat the axial lava volcanicvents, the magma plumbing system also varies at this domesare primarilycovered by pillow basalt(Figure 7). scale.Hence we propose that along-axis changes in eruptiveVarious estimates can be obtainedfor the volumetricflow' rate stylecorresponding to third-ordersegmentation reveal that of pillowbasalt based on modelsand observations in different WHITE ET AL.' SEPR VOLCANIC SEGMENTATION 23,531 environments. Laboratory simulationsindicate that an -18•333 -18.167 -18 -17.833 -t7•:667 -17.5 47.333 -17.167 effusionrate fi'om 1-100 m3/seccould form pillow basalt [Gregga/td Fink, 19951. Observationsof flowingpillow basalt on tile coast of tile island of ttawai'i suggestthat an 2700 averagerate of 1 mX/smay be an appropriate approximation [Moore, 1975: Tribble, 19911. At an average-sixeddome 2000 xxhere4xl0 s m3 of basalterupt,,, at a rateoi' ! m•/s½c.,the whole edifice can be built in -4 days. Even for the largest axial lava dome found, a calculationfor the effusionrate of ! mX/sshows that it would take only 4 monthsof continuous eruptionto form. Theseresults indicate durations that are :•. 0•2 similarto the6 dayeruption of an -! km longfissure in the spreadingcenter in Afar [Allardet al., 19791. •:•0,3 Chains of axial lava domes that have been mapped here (Figures6 and7) andon the southernJuan de FucaRidge [Chad•'ickand E,•ble)', 1994] may form much like tile developmentof a "craterrow" alonga subaerialeruptive fissure [e.g., Thorarinsso/t,1969: Thorarin,s'xonand ,•0.60•7 Sigvaldaso/t,1972; Bjor•tsson et al., 1977:Sigttrdsson and Sparks,1978; Locktt'ood et al., 1987]. hi subaerialfissure eruptions,linear "curtain of fire"geysering becomes focused 3:5 ,,• 30 to a small number of volcanic centerswithin a few hours to daysdepending on the effusion rate and viscosity of thelava [Wylieet al., 1999]. Craterrows along fissure swarms in Z t5 Icelandoften form by lateraldike injection along tectonic rift 5 zones[Sigurdsson and Sparks,1978]. Duringtile 1978 0 Kraflaeruption, the rate of flow in thedike was calculated as -18.,:333 .•i'7.,333 •i7•. t67 decreasingwith increasingdistance from Kraflacaldera Latit:ude(decimaii degrees) [Sigurdsson,1987]. Thisis a consequenceof a more constrictedmagma conduit that reduces the magmaflow Figure12. Along-axiscomparison of ridge structure to the towardthe propagating dike tip [Hardee,1987; Rubin and locationof volcanic(third-order) ridge segments. First panel showsdepth of theridge axis, fi'om SeaBeam 2000 bathymetry Pollard,1987]. By analogy, we suggest that the reduction of IComtieret al., 1996],with _+10m error bars. Second panel overallmass flux at thepropagating end of a dikecreates the showstwo-way travel time to seismic layer 2A, and third panel along-axischange that we observe along third-order segments shows two-way travel time to axialmagma chamber (AMC) from sheet-lobatelava lakesnear segment centers to pillow reflector,both panels with _+0.003s error bars [Derrick et al., lava domesnear segment ends. 1993].Fourth panel is a histogramof theabundance of axial The1998 eruption of AxialVolcano on the Juan de Fuca pillowdomes binned at 2 minute(-3.7 km) interval. Ridgeafforded an unprecedented opportunity toobserve the Locationsof the volcanic(third-order) ridge discontinuity patternofsubmarine volcanism along the length of a laterallyzones are shown with a lightgray bar to showthe length of the intrudeddike. A lateraldike propagationevent down the ridgeapparently disturbed bythe discontinuity asin Figure lb. Segmentends correlate with lava domes and with AMC depth SouthRift of Axial Volcanofrom its calderawas tracked to somedegree, but there is no clearcorrelation with axial seismicallyfor 10days in 1998[Dziak and Fox, 1998]. The depthor layer2A thickness. eruptionassociated with this dike intrusion was mapped as a sheetflow in Axial caldera,at theproximal end of the dike, andpillow flow at the far end of the eruption [Embley etal., 1999].The eruption of theCleft segment of theJuan de Fuca segmentationof the EPR on thebasis of lavasderived from ridgein the 1980s also produced sheet flows associated with differentparental magmas [lxmg,zuir et al., 1986;Si/tto/t et collapsedlava lakes near the segment center and "pillow al., 1991;Reynolds et al., 1992]. Thesemagmatic segments mounds"at the segmentend without the anomalousmagma correspondalmost perfectly with the structural segmentation supplyassociated with Axial Volcano[Embley and of theridge axis, but sample spacing (average *-10 km) is too Chadwick,1994]. Theprogression of lava flow morphology wide to discern all of the fine-scale magmatic segments documentedalong intruding dikes on northeastPacific [Batiraanti Margolis, 1986; Langmuir et al., 1986;Sinton et spreadingcenters is the same pattern that we observe along al., 1991]. In our study area, four magmaticsegment third-ordersegments onthe SEPR. If lateraldike propagation boundaries were identifiedby differencesamong a suiteof is thecause of theobserved pattern in volcanicmorphology, it majorand trace elements in glass analyses from dredged rocks requiresfocusing the magma supply beneath the central parts [Sintonet al., 1991]. The magmaticsegments correspond to of ridgesegments to give the dikes a centerfrom which to third-orderRADs (Figure lb). However,the dredge samples propagate. were too widely spacedto be able to defineall of the morphologicthird-order segments that we observe.Work 4.2. Evidencefor Segmentationof theMagma Supply nowin progresson rock samples collected at a closerspacing Along-axisgeochemical variations in both the major and by wax coresand submersible dives hints that additional traceelement ratios highlights differences in the parental magmaticsegments may exist on the SEPR [Bergmanis et al. compositionof the magma supply, thus defining a magmatic 1999; Sinton et al., 1999]. 23,532 WHITE ET AL.' SEPR VOLCANIC SEGMENTATION

Sintonet al. |1991] pointedout that the distinctiveparental -•!":::• low magmacomposition defining each magmaticsegment on the SEPR requiresa discretemagma injection systemfor each -17 ø 16' segment. Seismic tomographyindicates that the magma supply to second-order ridge segments has a three- dimensionalorganization beginning far below the crust [Dunn and Toomey, 1997; MELT Seismic Team, 1998]. Closer to the seafloor, the magma supply may focus into finer-scale -17 ø 18' •-,..... Ridge;•Axi.s magmaticinjection centers. The breaks in continuity and local depth maxima of the Zone AMC closely match the location of third-order RADs, and

ß-. •. • .-:... abundanceof axial pillow lava domes within the study area (Figure 12). The only breaks in the AMC reflector occur -17 ø 20' .. beneath the two largest third-order OSCs in the study area where pillow lava domesare also found in abundance. Yet, even where there is no break in the AMC reflector, all local depth maxima of the AMC reflector are located within the zonesof third-orderRADs (Figure 12). The most significant -17 ø 22' local increases in the AMC reflector depth coincide with those RAD zones with the greatestnumbers of axial pillow domes, whereassmaller increasesin AMC depth seem to be associatedwith fewer pillow domes at the seafloor. These local maxima may indicate placeswhere the magmachamber -17 • 24' is segmented.For example, seismicimaging within a second- order segmentat 14ø-16øSon the EPR (north of our survey area) suggeststhat the apparentlycontinuous AMC actually containsseveral pocketsof pure melt separatedby crystalline mush zones [Singh et al., 1998]. -17 ø 26• Sonar and geological mapping, along- and across-axis seismicprofiles, and submersible-collectedrock samplesfrom Aldo Lake trough at 17ø27'Sto the pillow moundsin the zone of offset near 17ø30'S permit a rare opportunity to integrate Lake trough all of these data sets to examine the nature of a volcanic -17 ø 28' segmentin detail (Figure 13). Seismiclines acrossaxis show a deepening (from 1000 to 1210 m) and narrowing (from 1540 to 740 m) of the AMC from A!do Lake to the RAD [Hoe/? et al., 1997]. The width over which layer 2A thickens also reduces significantly (from 7.1 to 3.4 kin) [Carbotte et al., 1997]. Rock samplescollected on one side of the RAD at -17 ø 30' ,•::":'::::"•'-•:•:• ½ Ridge Axis 17ø30'S have different liquid lines of descent from those i:;•'.. Discontinuity collected just on the other side, suggestinga break in the Zone volcanic plumbing system [Bergmanis et ai., 19991. The combined evidence of a segmented magma supply from seismicand geochemicalmeasurements is consistentwith our -17" 32' hypothesis,based on the patternof volcahiclnorphology, that 5k; ' the SEPR is organizedinto volcanicsystems at the third-order segment scale. -113 ø 14' -113 ø 12' -I13 ø 10' 4.3. Significance of Volcanic Segmentation at Fast Figure 13. The distributionof axial lava domes at the Spreading Ridges northernmostthird-order ridge segment in our study area. SeaBeam2000 bathymetryof the axial high of the southern We proposethat a "volcanic segment"of a last spreading East Pacific Rise presentedas in Figure I but withoutcontour mid-ocean ridge is essentiallysynonymous with a "volcano" lines. Black lines outline the base of each axial lava dome in other settings where eruptions tend to form volcanic picked from the DSL-120 data, and the locationof the Aldo edifices. The broad definition of a volcano is a systemof Lake trough is shown in gray fill. Dashed lines show the volcanic vents tapping a common magma supply to build a width of the zone of layer 2A accumulationas determinedby volcanic structure [Sirekin and Siebert, 2000]. However, the Carbotteet al. [1997], and thick solid linesjust below show thin lithosphereat the SEPR has a low flexural rigidity that the width of the AMC [Hooft et al., 1997] at the locationsof preventsbuilding a large volcanicedifice on the axis [Buck et across-axisseismic lines. Aldo Lake trough, the inferred volcaniccenter of the segment,extends southward from the al., 1997]. Rather than being a constructionalvolcanic ridge, geographicalsegment center. Axial pillow lava domesoccur the axial high of a fast spreadingridge is thought to be an throughoutthe segmentand are most abundantaround the isostatic or flexural feature [Madsen et al., 1984; Eberle and segmentoffsets (shown within bracketsas in Figure lb). Forsyth, 1998]. Because the axial highs of the SEPR and WHITE ET AL.: SEPR VOLCANIC SEGMENTATION 23,533 other fast-spreadingridges are isostaticrises instead of being centerto -170 m at the ends of the segment,although these constructionalridges, the highestpoints on the ridge do not changes are within the uncertainty of the measurements mark the centers of individual volcanic constructions. For this [Carbotte et al., 1997]. Off axis, layer 2A increasesin reason,seismic layer 2A is consistentlythinnest along the axis thickessto -500 m; however,it reachesthis valuesanywhere of the ridge, but if the rise were a volcanic construction,it from <1 km to >3 km from the ridge axis [Carbotteet al., would be thickest at the axis [Carbotte and Macdonald, 1994; 1997: Hooft et al., 1997]. The averagetotal width over which Hooft et al., 1996]. Thus the elevatedtopography of the ridge layer2A thickensrapidly is -7 km at midsegment,but is only axis on fast spreadingridges cannot be directly comparedto -5 km at the segmentends [Carbotte et al., 1997] (Figure 13). constructionalvolcanic ridgeslike thosefound in the median The interpretationof layer 2A as volcanicpile is favoredby valley of slow spreading ridges [S,•ith et al., 1999]. The seismologistscurrently [Hooft et al., 1996: Carbotte et al., significanceof third-order segmentationis that it reveals the 1997], but altemative explanations for the low seismic discretevolcanic systemscomprising the ridge in the absence velocitiesare a metamorphicalteration or a crustalcracking of large volcanicconstructions. front [Spudich and Orcutt, 1980; McClain et al., 1985; Rohr In this context, what are fourth-order segments? The et al., 1988]. If layer 2A is the volcanicpile, our geological RADs defining fourth-order segmentboundaries are so fine mapping along this segmentsuggests an explanation;lava that near-bottom high-resolutionimages or very dense suites flows extend farther from the ridge axis near the segment of rock samples are usually required to notice them. As center, where Aido Lake is located. The lava conduits originally defined [Lang,•ttir et al., 1986: Macdonald et al., aroundthe Aldo Lake troughappear to transportlava as far as 1988], they are probably segmentsof eruptive axial fissures. -1 km away from the axis (Figure 3). At the segmentends, While third-ordersegments may remain distinct for hundreds axial lava domescontribute to thickeningthe volcaniclayer tothousands of years, they cannot persist for longerthan 1() -•- no farther than a few hundred meters from the axis. If this 10•s years, or elsethey would create an off-axisdiscordant same patternof eruptionsis sustained,then our observations zone. Even more ephemeral, fourth-order segments may are consistentwith a wider zone of seismic layer 2A persistas distinct segmentsfor just tens to hundredsof years, thickeningcaused by the accumulationof lava flows over a changing with each new crack propagation and eruptive wider neovolcanic zone near from the centers of volcanic episode (Table 1). This order of segmentation may be segments. If layer 2A is not the volcanic layer, then we significant on the timescales and length scales that most cannotexplain the correlationwe observe. The hypothesized directly affect along-axis variationsin hydrothermalactivity alternatives would require that fissure development or a [Hay, ton et al., 1991, 1993]. metamorphicalteration front extendsfarther from the ridge If third-order segments represent individual volcanic axis near midsegmentthan at segmentends. segmentson mid-oceanridges, why did the 1989 Argo survey Some near-ridgevolcanoes could have originatedas axial on the East Pacific Rise, 9ø09'-9ø54'N, ridge crest fail to lava domesthat continuedto grow as spreadingrafted them reveal the patterns of volcanism observed on the SEPR farther from the ridge axis. Pillow lava constructionshave [Hay, ton et al., 1991]'? Sonar coverage comparable to our been investigatedand sampledon the sidesof the axial high DSL-120 surveywas not available for the northernEPR at the on the northern EPR [Perfit et al., 1994]. The distribution of time of the Argo survey. A recently completed DSL-12() axial lava domes relative to third-ordersegmentation (more sonar survey indicates that the pattern of volcanic but smaller lava do•nesnear segmentends) is similar to the segmentationdescribed here applies to the EPR at 9ø-10øN distribution of isolated off-axis volcanoes relative to second- (D. Fornari, M. Perfit, M. Tolstoy, R. Haymon, D. Scheirer,P. ordersegmentation (more but smallervolcanoes near segment Johnson, G. Kurras, S. White, J. Getsiv, and shipboard ends) [White et al., 1998]. The similarity of these two scientificparty, Shipboarddata web site compiledduring R/V patterns is consistentwith a hypothesizedhierarchy of Meh'ille NEMO Expedition, Leg 2, http://128.128.21.37/, segmentationof the ridge axis magmasupply [Batiza, 1996: 2000). Basaltic lava domes are more abundant at the end of Macdonald, 1998] and suggeststhat more fundamentallevels third-order segment, while indicators of high effusion rates of magmaticsegmentation affect volcanicprocesses farther are more prevalentat the centerof third-ordersegments at 9ø- from the ridgeaxis. Furtherinvestigations will be necessary 10øN [White et al., 2000]. to find to what extent off-axis volcanismmay have evolved from the volcanicsegmentation of the ridgeaxis. 5. Speculations 6. Conclusion Two observationsinvite further comment. First, along- strike changesin the width of the zone over which seismic The volcanic morphology and structuresformed on the layer 2A thickens may correlate with volcanic segmentation SEPR vary systematicallyover third-ordersegments such that (.wider near midsegment). Second, the pattern of axial lava in/erred effusion rates are lower and lava flows are shorter at dome distributionon third-order segmentsis similar to the segmentends. Volcanic edificesin the form of small basaltic pattern of isolated, off-axis volcano distributionon second- lava domesare found on the axis of fast-spreadingridges in order segments(more but generally smaller at segmentends increasingabundance toward third-orderRADs. In contrast [White et al., 1998]). to the collapsedlava lakes that occur near the centersof some The best studiedthird-order segment in our surveyarea is of the third-ordersegments, the lava domesreflect a decrease the segmentfrom 17ø18.5' to 17ø30'S.This segmentcontains in overalleffusion rate towardsegment ends. The collapsed the Aldo Lake trough,which is locatedjust to the southof the lava lakes are associatedwith thin-crusted, hollow lobate lava geographic center of the segment (Figure 13). Seismic flows and sheet lava flows, and collapsesin the lobares reflection profiles acrossthis third-order segmentshow that suggestthat these flows have inflated several metersin areas seismic layer 2A increasesin thicknessfrom -140 m at the proximal to the eruptive vent. The general pattern of 23,534 WHITE ET AL.: SEPR VOLCANIC SEGMENTATION decreasingeffusion rate away from the centers of third-order Cochran, J. R., J. A. Goff, A. Malinverno, D. J. Fornari, C. Keeley, andX. Wang,Morphology of a "superfast"mid-ocean ridge crest segmentssuggests the volcanicprocesses at superfast and flanks: The East Pacific Rise, 70-9ø S, Mar. Geophys.Res., spreadingridges are fundamentally segmented at this scale. 15, 65-75, 1993. Cormlet, M.-H., and K. C. Macdonald,East Pacific Rise 18øS-19øS: Acknowledgments.Without the unflaggingsea-going support of Asymmetric spreadingand ridge reorientationby ultra-fast the DSL/WHOI engineers,we could not have completedsuch an migrationof ridgeaxis discontinuities, J. Geophys.Res., 99, 543- extensiveand detailedsurvey. We also thankCaptain Buck, the crew, 564, 1994. and the watchstandersaboard the R/V Meh,ille during Sojourn2 in Cormier, M.-H., D. S. Scheirer, K. C. Macdonald, R. M. Haymon, S. 1996. D. S. Scheirer and P. Sharfstein assistedin processingthe White,and Sojourn I ScienceParty, A geophysicalsurvey of the DSL-120 sonardata. E. E. E. 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