Article

Volume 11, Number 8

31 August 2010

Q08024, doi:10.1029/2010GC003058

ISSN: 1525‐2027

Paving the seafloor: Volcanic emplacement processesduring the 2005–2006 eruptions at the fast spreadingEast Pacific Rise, 9°50′N

A. T. FundisDepartment of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville,Florida 32611, USA (afundis@ufl.edu)

S. A. Soule and D. J. FornariGeology and Geophysics Department, Woods Hole Oceanographic Institution, Woods Hole,Massachusetts 02453, USA

M. R. PerfitDepartment of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville,Florida 32611, USA

[1] The 2005–2006 eruptions near 9°50′N at the East Pacific Rise (EPR) marked the first observed repeateruption at a mid‐ocean ridge and provided a unique opportunity to deduce the emplacement dynamics ofsubmarine lava flows. Since these new flows were documented in April 2006, a total of 40 deep‐towedimaging surveys have been conducted with the Woods Hole Oceanographic Institution’s (WHOI) TowCamsystem. More than 60,000 digital color images and high‐resolution bathymetric profiles of the 2005–2006flows from the TowCam surveys were analyzed for lava flow morphology and for the presence of kipukas,collapse features, faults and fissures. We use these data to quantify the spatial distributions of lava flowsurface morphologies and to investigate how they relate to the physical characteristics of the ridge crest,such as seafloor slope, and inferred dynamics of flow emplacement. We conclude that lava effusion ratewas the dominant factor controlling the observed morphological variations in the 2005–2006 flows. Wealso show that effusion rates were higher than in previously studied eruptions at this site and varied system-atically along the length of the eruptive fissure. This is the first well‐documented study in which variations inseafloor lava morphology can be directly related to a well documented ridge‐crest eruption where effusionrate varied significantly.

Components: 10,500 words, 13 figures.

Keywords: fast spreading mid‐ocean ridge; EPR; 2005–2006 eruption; lava morphology; volcanic collapse; kipuka.

Index Terms: 8416 Volcanology: Mid‐oceanic ridge processes (1032).

Received 26 January 2010; Revised 28 June 2010; Accepted 8 July 2010; Published 31 August 2010.

Fundis, A. T., S. A. Soule, D. J. Fornari, and M. R. Perfit (2010), Paving the seafloor: Volcanic emplacement processes duringthe 2005–2006 eruptions at the fast spreading East Pacific Rise, 9°50′N, Geochem. Geophys. Geosyst., 11, Q08024,doi:10.1029/2010GC003058.

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1. Introduction

[2] The products of submarine volcanic eruptionsat mid‐ocean ridges (MORs) cover >60% of theplanet’s crust, yet, our understanding of volcanicprocesses in these settings has been limited by ourinability to directly observe active eruptions. Dur-ing the course of the past few decades of high‐resolution seafloor mapping, only a few seafloorvolcanic eruptions have been well documented[Embley et al., 1990, 1995; Chadwick et al., 1991,1995; Haymon et al., 1993; Rubin et al., 1994;Gregg et al., 1996; Sinton et al., 2002], and in mostcases there are gaps in our knowledge of both thesize and continuity of individual lava flows thatcomprise the eruptive episodes. Volcanic deposi-tion at MOR axes greatly impacts the permeabilitystructure of the upper oceanic crust, and thushydrothermal systems. It is therefore importantto understand the mechanisms controlling lavaemplacement and deposition at MORs. This studyexamines the flow surface morphology and intra-flow features of the erupted products of submarineeruptions that occurred over ∼6 months in mid‐2005 to early 2006 [Rubin et al., 2006, 2008;Tolstoy et al., 2006] in order to place constraints onthe mechanisms of lava emplacement and deposi-tion at a fast spreading MOR.

[3] In 2006, an eruption centered near the magma-tically robust 9°50′N region of the EPR was dis-covered and the flow area fully mapped [Tolstoyet al., 2006; Soule et al., 2007] (Figure 1). Micro-seismicity data, Po‐disequilibrium dating, and con-tinuous time series temperature records collected atnearby hydrothermal vents indicate that the eruptionoccurred as a series of pulses between mid‐2005 toJanuary 2006 [Tolstoy et al., 2006; Cowen et al.,2007; Soule et al., 2007; Rubin et al., 2006, 2008;Fornari et al., 2010]. This discovery and subsequentinvestigations of the geological, hydrothermal andbiological impacts of the eruptions provide the firstdocumented evidence for a repeat eruption at aMOR site, in agreement with predictions of decadalrecurrence intervals at this fast spreading ridge[Haymon et al., 1993; Perfit and Chadwick, 1998].

[4] The parameters that control submarine lavasurface morphology differ from those that controlsubaerial flows. In submarine environments, lavatemperature and crystal content remain relativelyconstant due to the rapid formation of a thick,insulating crust [Gregg et al., 1998; Fornari et al.,2004] and thus, do not change the rheology of thelava and the subsequent manner in which the lava

surface deforms [e.g., Soule et al., 2004]. Experi-ments using wax as an analog to lava have shownthat surface morphology is dependent on the time-scales of cooling and advection [Fink and Griffiths,1992; Gregg and Fink, 2000; Sakimoto and Gregg,2001]. Due to the significant thermal gradientbetween lava and seawater, cooling rates of sub-marine lavas with differing surface morphologiesshould be relatively similar. Thus, prior to this study,the dominant parameter believed to control surfacemorphology was the timescale of advection, whichis a proxy for the local volume flux influenced byeruption rate and pre‐existing topography [e.g.,Gregg and Fink, 2000; Sakimoto and Gregg, 2001].

[5] Here we use seafloor photography and high‐resolution bathymetric profiles (1‐m vertical pre-cision) from deep‐towed camera surveys and Alvindive imagery to produce a geologic map of thevolcanic features associated with this most recenteruption at the EPR near 9°50′N. Our analysis ofthe distribution of lava morphologies emplacedduring 2005–2006 shows that lava effusion ratewas the dominant factor controlling the observedmorphological variations in the 2005–2006 flows.We also show that the effusion rates were higherthan in previous eruptions (i.e., 1991–1992 [Haymonet al., 1993; Rubin et al., 1994; Gregg et al.,1996]) at this site and were variable along thelength of the eruptive fissure.

2. Background

[6] The northern EPR between the Clipperton(∼10°N) and Siqueiros (∼8°30′N) transform faultsis a fast spreading (5.5 cm/yr [Carbotte andMacdonald, 1992]) MOR and one of the beststudied segments of the global ridge system [e.g.,Lonsdale, 1977; Macdonald et al., 1984, 1992;Langmuir et al., 1986; Perfit et al., 1994; Fornariet al., 1998, 2004; Perfit and Chadwick, 1998;Carbotte et al., 2009; White et al., 2002, 2006;Soule et al., 2007; Escartín et al., 2007; Goss et al.,2010]. Over the past two decades, two documentederuptions have occurred along this segment of theEPR near the 9°50′N region, one in 1991–1992[Haymon et al., 1993; Rubin et al., 1994] and themost recent one in 2005–2006 [Tolstoy et al., 2006;Rubin et al., 2006; Cowen et al., 2007; Soule et al.,2007]. Using data from deep‐sea camera surveysand Alvin dive traverses, Soule et al. [2007] esti-mated that the 2005–2006 eruption was 4–5 timesmore voluminous than the rough estimate for the1991–1992 eruption [Gregg et al., 1996]. The

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2005–2006 eruptions were fed by eruptive fissureslocated within the axial summit trough (AST),which is the site of most of the primary eruptivefissures [Fornari et al., 1998, 2004] and knownhigh‐ and low‐temperature hydrothermal vents[e.g., Haymon et al., 1991, 1993; Von Damm et al.,1995; Shank et al., 1998]. A fissure system located∼700 m east of the EPR axis in the 9°53′–9°55′Nregion was also active during the 2005–2006 flows(Figure 1). As suggested by Soule et al. [2007], themapped flow boundary indicates that multiple flowlobes were produced along the ∼18 km eruptivefissure system between 9°46′N and 9°56′N, possi-bly reflecting discrete eruptive events [Rubin et al.,2006, 2008] during the 2005–2006 eruption. Theselobes coincide with pre‐existing, ∼10–50 m wideeast‐west trending lava channels that have beeninterpreted by Soule et al. [2005] to have servedas distribution pathways for axially erupted lavas.The largest of these lobes, between 9°50′N and9°52′N, coincides with the highest density of high‐temperature hydrothermal vents along the ridge[Fornari et al., 2004; Soule et al., 2007], and forthe purpose of this study it is referred to as the9°51′N lobe. Lava deposition associated with the2005–2006 flows occurred in four distinct regions: 1)the area referred to in this study as the 9°51′N lobe,considered to be the main flow unit, 2) flows pro-duced by smaller axial eruptions north of 9°53′Nthrough a discontinuous series of fissures and anaxially sourced flow that was directed north by aridge‐parallel fault, 3) off axis flows erupted fromfissures ∼700 m east of the axis near 9°54.5′N,and 4) the area between 9°49′N and 9°46′N wheresmall volumes of axially erupted flows, foundlargely in the AST have been identified [Goss et al.,2010] (Figure 1).

[7] In the past, our understanding of the size oferuptions and the relationship between flowmorphologies and eruptive processes along theMOR system have been fundamentally limited byour inability to unequivocally distinguish individ-ual eruptive units. The key difference between thisstudy and studies of seafloor characterization ofother MOR sites where individual eruptions havebeen documented [e.g., Embley et al., 1990;Chadwick et al., 1991; Sinton et al., 2002] is thatwe were able to conduct surveys shortly after theeruptions occurred to confidently distinguish (byabrupt changes in sediment cover and glassiness oflava surfaces) and comprehensively survey the areaaffected by the eruptions. We surveyed the 2005–2006 flows with digital seafloor imaging on mul-tiple cruises so that we could positively establish

Figure 1. Location and bathymetry of the East PacificRise 9°50′N [White et al., 2006]. Area covered by the2005–2006 eruptions is outlined in black (derived fromcamera tow and side scan imagery data) and the fourdistinct regions of the flow are defined [Soule et al.,2007]. Hydrothermal vents are marked by red dots.

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the boundaries of the new flows and collectdetailed bathymetric data and lava samples.

[8] We use results from previous studies, includingthose presented by Kurras et al. [2000] and Whiteet al. [2006] to help define the pre‐existing lavamorphology and topography in the study area, andto compare and contrast them to seafloor volcanicfeatures created during the 2005–2006 eruption.White et al. [2002] presented results from a regionalArgo I survey where they estimated the morphol-ogy between 9°08′N and 9°56′N was comprisedof approximately 25% pillow lava, 50% lobate lavaand 25% sheet lava. Kurras et al. [2000] usedimagery from Alvin and deep‐towed camera sur-veys to conduct a similar analysis over a smallerarea, between 9°49′N and 9°52′N, that falls withinthe extent of the 2005–2006 eruptions. They reportedthat the crest of the EPR in this region was pre-dominately covered by lobate (66%) and sheetflows (20%), consistent with the idea that fastspreading ridges typically produce high‐effusionrate eruptions [Perfit and Chadwick, 1998]. Addi-tionally, they reported that the 9°50′N area hadexperienced three types of volcanic emplacementprocesses including: (1) axial summit eruptions,(2) off‐axis transport of axially erupted lava throughchannelized surface flows, and (3) off‐axis eruptions.Engels et al. [2003] investigated the distributionand formation of lava collapse features observedon the EPR axis in the 9°37′N area and reportedthat collapse almost always occurred within lobateflows. Moreover, collapse pits <2 m in diameterwere evenly distributed out to ∼300 m from theAST, while collapse features >2 m in diameterwere concentrated within 100 m of the axis.

3. Methods

3.1. Deep‐Towed Camera Surveys

[9] A total of 40 deep‐towed imaging surveys wereconducted in the study area during 2006–2007using the Woods Hole Oceanographic Institution’sTowCam system [Fornari, 2003]. Approximately60,000 digital color images were collected.Bathymetric profiles (1‐m vertical precision) werecompiled using a 100 kHz altimeter and depthacquired using the TowCam’s SeaBird SBE25CTD. The TowCam was equipped with a colordigital camera (3.5 megapixel) for 30 of the towsand a fiber optic camera (4 megapixel) was usedduring two cruises (AT15–6 and AT15–27) for10 tows. TowCam’s speed over the bottom wastypically ≤0.5 knots. Towing altitude was normally

5–7 m above the seafloor, producing images thateach cover an area of ∼4.5 m × 6 m of seafloorwhen the color digital camerawas in use and∼3.5m×3.5 m when the fiber optic system was utilized.Images were acquired at 10–15 s intervals, pro-ducing 0–20% of overlap between each successiveimage. During 18 surveys, the TowCam was navi-gated using a bottom‐moored long baseline (LBL)transponder network [Hunt et al., 1974;Milne, 1983;Soule et al., 2007] that located the images to within5–8 m [Haymon et al., 1991; Fornari et al., 1998].During 22 of the surveys that were conductedoutside the LBL network area, the TowCam wasnavigated using layback calculations that locatedthe images to within ∼100 m [Soule et al., 2007](Figure 2). TowCam image data were supplementedby down‐looking video imagery collected duringpost‐eruption Alvin dives. Dives 4204 and 4296traversed areas south of the off‐axis flow near 9°53′Nand dive 4205 traversed the southern extent of theeruption near 9°46′N.

3.2. Image Analysis

[10] Each TowCam image was classified for rela-tive age and lava flow morphology. Boundaries ofthe 2005–2006 flows were easily identified by thelack of sediment on the flow surfaces and thepresence of fresh glassy lava surfaces; these criteriaprovided the basis for relative age estimates (2005–2006 versus older flows). Lava flow surfaces ineach of the images were classified into one of fourmorphological categories: pillow flow, lobate flow,sheet flow, and hackly flow, consistent with thedescriptions used by Ballard and van Andel [1977],Ballard et al. [1982] and Kurras et al. [2000](Figures 3a–3d). In instances where multiplemorphologies were present in an image, the dom-inant morphology by area was recorded. Threemorphologic types of sheet flow (lineated, ropy andhackly) were observed and all are indicative of highflow rates at the time of emplacement [Gregg andFink, 1995; Gregg et al., 1996; Chadwick et al.,1999]. Hackly flows were distinguished fromother sheet flows because initial observations of the2005–2006 flow morphologies from the TowCamobservations and Alvin dives indicated that thiseruption produced a higher percentage of hacklyflows than previously existed in the region, espe-cially within the 9°51′N lobe. The discriminationof hackly flows from lineated and ropy sheetflows is used here to help refine the emplacementmechanisms involved in erupting the 2005–2006flows.

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[11] Images were also classified for the presenceand abundance of collapse pits, kipukas (defined inthis study as an area of exposed flow older than the2005–2006 flow and completely surrounded by the2005–2006 flow), faults, and fissures (Figures 3e–3h). Collapse pits were classified into three sizecategories using the terminology and size classesdescribed by Engels et al. [2003]: lobate blisters(<2 m diameter), skylights (∼2–10 m diameter),and lava pond collapse (>10 m diameter). Kipukaswere only recorded if they were identifiable in<3 image frames (ranging from <1 m to ∼6 m indiameter). Potentially larger kipukas that are noteasily distinguished from true gaps in lava depo-sition were not included in our study. Faults andfissures were rare within the imaging surveys.

3.3. Analytical Methods

[12] Of ∼60,000 images collected, about half(27,205 images) imaged the 2005–2006 flows. Dueto areas of extensive collapse in the new flows, itwas not possible to classify flow morphologies in∼10% of these images (2,104 images). Assumingeach TowCam image covered ∼23 m2 of seaflooron average, the camera surveys covered ∼0.58 km2

(25,101 images). Images where morphologic andtectonic structures were classified represent lessthan 4% of the flow surface area. The surveys,however, evenly cover the extent of the 2005–2006flows along and across axis (Figure 2), and we thusconsider our analyses to be representative of theentire 2005–2006 eruption.

[13] Observations from the towed camera surveyswere entered into a database and subsequentlylinked to time‐stamped navigation and near‐bottomprofiles generated from the CTD depth and altitudedata. These georeferenced observations were thencompiled and used to plot distributions of the variouslava flow morphologies and volcanic structuresobserved in the photographs (Figures 4 and 5).

4. Results

4.1. Abundance of Lava Morphologies

[14] Lobate flows are the dominant lava morphol-ogy type observed in the 2005–2006 flows,accounting for 52% (13,106 images) of the surveyarea. Sheet flows comprise 31% (7807 images) ofthe observed morphologies, hackly flows accountfor 15% (3636 images) and pillow flows accountfor only 2% (552 images) of the survey area. Rel-ative to the other three morphologic types, lobate

Figure 2. Tracklines showing area of the 2005–2006eruptions surveyed with TowCam. Blue lines representnavigational precision of ∼5 m, red lines represent∼100‐m precision and the green lines represent >100‐mprecision (see text for discussion).

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flows are ubiquitous over the surveyed area but arethe dominant morphology in the northern andsouthern regions of the flow away from of the 9°51′Nlobe (Figure 6). Sheet flows are the dominant flowtype flooring the AST and within flow channelsproduced by the 2005–2006 eruption outside of theAST. Over 60% of pillow flows in the surveyedarea are located within 100 m of the mapped flowboundary; 87% of pillows in the imagery arelocated within 200 m of the flow boundary(Figure 7). Relative to the rest of the flow, pillowsare more abundant at the northern and southern-most distal ends of the 2005–2006 flows (Figure 6).

4.2. Morphological Transitions

[15] By examining the transitions between the dif-ferent surface morphologies with respect to theAST, we can further constrain the genetic rela-tionship between the various surface flow morphol-ogies. There are two main types of morphologicaltransitions seen within the 2005–2006 flows: 1)gradational due to changing flow conditions, and 2)superposition of separate flows. The gradationaltransitions are observed between sheet and lobateflows, sheet and hackly flows, lobate and pillowflows, and lobate and hackly flows (Figures 8a–8c).Transitions between sheet and lobate lavas vary withrespect to the length scales over which they occur.Typically, they occur over a few meters as flat sheetflows develop a more lobed surface. In other cases,where lobate flow appears to have transitioned intosheet flow, the transitions are more abrupt. The sheet

flow in these cases appears to have flowed out fromunderneath the cooled lobate surface and is charac-terized by lineations that likely formed from beingraked against the partially cooled upper surface ofthe lobate lava [Chadwick et al., 1999].

[16] Transitions between sheet and hackly flowsalso vary with respect to the length scales overwhich they occur. In many cases, sheet flowstransitioned to a jumbled flow where the sheetsurface is characterized by small surface folds asthe partially cooled and more brittle surface deformsdue to continued flow of the underlying moltenlava. These jumbled flows commonly transitionedinto hackly flows as the folded surface of the flowcontinues to deform as it cools, ultimately breakingthe glassy surface crust. The gradation from sheetto hackly flows in these cases typically occur overdistances of 1–10 m. Sudden transitions betweensheet and hackly flows, with no intermediatejumbled surface morphology, were also observed,and may reflect superposition of lavas erupted atdifferent times.

[17] Transitions between lobate and pillow lavasoccur over lengths <1 m, tend to occur towardthe flow front termini, and are characterized by oneto a few pillows branching out of well‐definedlobate flows. Lobate flows near these transitionsare more decorated and have distinct lobes that areminimally coalesced. The final type of gradationaltransition we observed is between lobate andhackly flows. These are similar to the abrupt tran-sitions between sheet and hackly flows, where the

Figure 3. Images showing examples of classification scheme used for images of the 2005–2006 flows. The fourmorphology types: (a) pillow, (b) lobate, (c) sheet, and (d) hackly. The three collapse types: (e) lobate blisters, (f)skylight collapse, and (g) lava pond collapse. (h) An example of a kipuka, an area of exposed flow older than the2005–2006 flows and completely surrounded by the 2005–2006 flows. Horizontal scale across the bottom of eachphotograph is ∼3–5 m.

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flow surface abruptly changes character to a mor-phology representative of a higher rate of shear.These transitions tend to occur at the margins oflava channels, similar to results described by Souleet al. [2005].

[18] There are only 5 images from the TowCamsurveys where potential superposition contactswere observed within the 2005–2006 flows. Inthese images, the morphologies do not appear to betransitioning from one type to another; rather itappears that a flow of one morphological typepartially covered a flow of another flow morphol-ogy. For example, Figure 8d shows a lobate flowthat appears to have flowed over a lineated sheetflow, both of which have been identified as 2005–2006 flows using the criteria we established. The

spatially limited extent of such contacts suggestthat it is either difficult to identify intraflow con-tacts or that such contacts do not commonly existwithin the 2005–2006 eruptions.

[19] We used TowCam survey segments that wererun perpendicular to the AST and that surveyedknown lava channels to interpret the morphologicalevolution of a given lava flow as it moved awayfrom an eruptive source, and to test whether thevariations in distribution we see between the vari-ous morphologic transitions occurred along indi-vidual flow pathways with increasing distance fromthe eruptive fissure. Although individual flows areoccasionally characterized by lobate flows at theflow front rather than pillow lavas, the generaltrend is that lava morphologies associated with

Figure 4. Maps showing the distribution of the four main types of morphology of the 2005–2006 eruptions surveyedwith the TowCam. Each point represents an analyzed image and the presence of that specific morphology type.

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higher flow rates (i.e., sheet, hackly and flat‐lyinglobate flows) occurred near the eruptive source andtransition into morphologies that are typicallyassociated with lower flow rates toward the terminiof the flow. TowCam survey AT15–6:CT06 (seeFigure S2), conducted perpendicular to the axis at∼9°51.5′N, provides an example of this transition.1

The images from this camera tow show that thelava emplaced closest to the axis is a mixture ofsheet and hackly flows which have no clear con-tacts, but rather which transition from one to theother over sub‐meter length scales. Further from

the axis, the sheet and hackly flows transition intodominantly lobate flows, and eventually into pillowflows at the termini of the flow front. Similarrelations and transitions are seen throughout thesurveys (see auxiliary material for examples).

4.3. Distribution of Volcanic Structures

4.3.1. Volcanic Collapse Pits

[20] Collapse pits are common features of subma-rine lava flows at fast and superfast spreadingMORs. They form from flow inflation and post‐eruption drainback of lava into eruptive fissures ordown flow through lava conduits as supply from

Figure 5. Maps showing the distribution of the three size classes of collapse and kipukas that formed in the 2005–2006 EPR flows. Lobate blister collapse is collapse <2 m in diameter, skylight collapse is ∼2–10 m in diameter andlava pond collapse is >10 m in diameter. Kipukas range from ∼1–6 m in diameter.

1Auxiliary materials are available in the HTML. doi:10.1029/2010GC003058.

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the vent diminishes [Fornari et al., 1998], or froma pressure gradient created along the upper crust ofthe flow due to the cooling of entrapped seawater‐derived vapor [Engels et al., 2003; Perfit et al.,2003; Soule et al., 2006]. As described in previ-ous studies, these features are depressions where 2–10 cm thick surface crusts have collapsed, reveal-ing cavities beneath [e.g., Ballard et al., 1979;Sinton et al., 2002; Engels et al., 2003; Perfit et al.,2003; Fornari et al., 2004]. In this study, collapsepits cover 19.2% (4817 images) of the total sur-veyed area.

[21] The smallest collapse pits, lobate blisters,account for 17.8% of the overall collapse. Largercollapse pits, skylights, represent 27% of theoverall collapse. Engels et al. [2003] found thatlobate blisters were relatively evenly distributedwithin ∼300 m from the AST, with only a slightlyhigher frequency near the ridge axis. They also

reported that the abundances of skylight and lavapond collapse peak dramatically within ∼100 mfrom the AST margin. The results we present hereare similar since the distribution of both lobateblisters and skylight collapse generally decreasewith increasing distance from the AST. However,in this study, the distribution of lobate blisters inaddition to skylight collapse peaks within ∼100 mof the AST (Figure 9). While there is significantdifference in the distribution of collapse across theAST, there is only a slight difference in the dis-tribution of collapse pits along the AST. In thesouthern extent of the main flow unit between9°49′ and 9°48′N, there is an increase in the percentcollapse observed relative to the total number ofobservations (Figure 6).

[22] Lava pond collapse imaged in this study cov-ers 9.8% (2659 images) of the survey area. Theprimary mechanism responsible for creating this

Figure 6. Observations of flow morphology and presence of collapse binned by 500 m of latitude. The percentage ofthe 2005–2006 flows surface area covered by the surveys was calculated by assuming each photograph covered anarea of ∼23 m2.

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aerially extensive collapse is post‐eruption drain-back of lava into primary eruptive fissures [Fornariet al., 1998] at the site of eruption. Therefore, asexpected, lava pond collapse imaged in this study isalmost completely concentrated within the ASTwhere erupted lava is constrained and accumulateswithin the walls of the trough (Figure 5).

[23] The great majority of collapse features (∼90%)occur in lobate flows and the rest in sheet flows,which is consistent with results reported by Engelset al. [2003]. It is apparent that these networks ofinterconnected collapse areas play a role in thetransport of lava during eruptions [e.g., Haymon

et al., 1993; Schouten et al., 2002; Fornari et al.,1998, 2004]. We see evidence that subsurfacepathways were exploited by 2005–2006 flows,with 2005–2006 lava imaged at the base of somecollapse pits located in older flows (Figure 10a).We also observe isolated fragments of older lavacrust lying on the surface of the 2005–2006 flows,that was presumably pushed up by lava flowingwithin these subsurface networks (Figure 10b).

[24] It has been shown that vapor formed by lava‐seawater interactions [Perfit et al., 2003; Chadwick,2003; Engels et al., 2003; Soule et al., 2006] isimportant in the development of collapse features.

Figure 7. (a) Map showing the location of 100 m bins relative to the 2005–2006 flows boundary [Soule et al., 2007].(b) >60% of pillow flows are within 100 m of the mapped flow boundary and >80% occur within 200 m, indicatingthat pillows formed from slow effusion rate and decreased lava supply at the distal flow margins. (c) Kipukas are alsocorrelated with proximity to the flow boundary.

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We find features such as lava drips on the under-sides of lava crusts throughout the 2005–2006flows, supporting the involvement of seawatervapor within the active flow. Vapor pressure mayalso play a role in expelling older lava crusts ontothe surface of the new flows.

4.3.2. Kipukas

[25] Kipukas <6 m in diameter were seen in 4.3%(1170 images) of the survey images. These areas ofolder lavas exposed through the new flows indicatethat the flow is ∼1–2 m thick [Soule et al., 2007];kipukas are associated with lobate forms in 88% ofthe observations, pillow forms in 9%, sheet flowsin 3%, and hackly flows in <1% of the observations.These small kipukas are typically individual lobesof sheet flow or lobate flows that divert and wraparound areas of older flow. Kipukas commonly occurwhere there is no clear topographic barrier, or wherethere is an increase in seafloor roughness often whenpillows or inflated lobate lavas characterize thepre‐eruption surface. Although, kipukas are foundthroughout the survey area, they are mostly con-centrated in the northern and southern regions of theflow area, following similar trends to the distribution

of lobate and pillow flows. Additionally, an increasein the number of kipukas is correlated with prox-imity to flow boundaries (Figure 7).

4.3.3. Faults and Fissures

[26] Only one fault north of 9°52′N, against whichthe 2005–2006 flows dammed [Soule et al., 2007],and 14 fissures were seen in the survey images. Theimaged fissures are associated with the AST orwith the off‐axis fissures north of 9°52′N. Escartínet al. [2007] noted that volcanism at MORs, andthe fast spreading EPR specifically, efficientlyburies faults within 2 km of the axis. Thus, the lackof faulting we see within the 2005–2006 flows,which is predominately within 2 km of the axis, isconsistent with what we would expect for recentlyrepaved seafloor, and consistent with studies of thesame area prior to the eruption [Fornari et al.,2004; Escartín et al., 2007].

5. Discussion

[27] In this study, we use lava surface morphologyas an indicator of relative effusion and flow ratesfor the 2005–2006 flows. Previous studies have

Figure 8. Examples of transitions between different morphologies of the 2005–2006 EPR flows. (a) A sheet/lobatetransitional morphology abruptly transitioning into a lineated sheet flow (∼6 m × 4.5 m). (b) A sheet flow abruptlytransitioning into hackly flow (∼4.8 × 3.6 m). (c) Pillow flow developing toward a flow front (∼4.8 m × 3.6 m). (d) Apotential example of superposition where a lobate flow appears to have flowed over a lineated sheet flow (∼7m × 5.3 m).

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suggested a variety of intrinsic and extrinsic para-meters, in addition to effusion rate, that may influ-ence the development of distinct flow morphologiesincluding: lava rheology (i.e., composition, tem-perature, and crystallinity) and the pre‐existingslope and terrain [e.g., Bonatti and Harrison,1988; Rowland and Walker, 1990; Griffiths andFink, 1992; Gregg and Fink, 1995; Gregg et al.,1996]. We show below that the pre‐existing slopeand terrain did not significantly affect the mor-phology of the 2005–2006 flows. Additionally,petrographic and geochemical analyses of volca-nic glass samples suggest that the lava rheologyremained relatively constant throughout the courseof the eruption. Samples from the 2005–2006 flowsare relatively homogeneous compositionally andare mostly aphyric (�5% plagioclase and olivine)regardless of morphologic type [Soule et al., 2007;Goss et al., 2010]. Thus local flow rates, primarilycontrolled by lava effusion rates, remain the keycontrols on surface morphologies developed during

the 2005–2006 eruptions. Using the empirical rela-tionship between flow rate and morphology, weconclude that the 2005–2006 event erupted at highervolumetric rates than those estimated for the 1991–1992 eruption [Gregg et al., 1996]. Additionally,flow rates were highest and lava output was mostvoluminous within the 9°51′N flow lobe, furthersuggesting that this was the locus of the diking eventthat fed the main phase of the eruption.

5.1. Influence of Pre‐existing Slopeand Terrain on Surface Morphology

[28] The EPR at ∼9°50′N has been studied for∼20 years by near‐bottom imaging [e.g., Haymonet al., 1991; Kurras et al., 2000], providing asubstantial volume of data that can be used toassess changes caused by seafloor eruptions, and toinfer the processes responsible for creating the lavaflow surface morphologies. To assess whether pre‐eruptive surfaces influenced the extent and mor-

Figure 9. Map showing the location of 100 m bins relative to the AST. The distributions of collapse <2 m in diam-eter (lobate blisters) and collapse ∼2–10 m in diameter (skylight collapse) are similar relative to distance from theAST. Collapse features are primarily concentrated within 100 m of the AST.

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phology of the 2005–2006 eruptions, informationcollected from the recent TowCam surveys wasaugmented by four studies of the area prior to the2005–2006 eruptions. These included: (1) camera‐tow surveys of the seafloor in 1997 and 1999[Kurras et al., 2000], (2) bathymetric data collectedby WHOI’s Autonomous Benthic Explorer (ABE)in 2000 [Fornari et al., 2004], (3) bathymetric datacollected by the 30 kHz EM300 multibeam systeminNovember of 2005 [White et al., 2006], and (4) sidescan sonar data collected in 2001 [Fornari et al.,2001, 2004; Schouten et al., 2002; White et al.,2002; Escartín et al., 2007].

[29] Kurras et al. [2000] analyzed camera‐towsurveys collected along and across the axis of theEPR prior to the 2005–2006 eruptions. Four ofthese deep‐towed camera survey lines traversedareas that were subsequently covered by the 2005–2006 flows, and were conducted in areas in closeproximity to post‐eruption survey lines. The pre‐eruption surveys were reanalyzed using the classi-fication schemewe adopted, and used in conjunctionwith pre‐eruption side scan sonar data [Fornari et al.,2004] to investigate whether the morphology ofthe pre‐existing terrain correlated with or possiblyinfluenced the morphology produced during therecent eruption. EM300 multibeam bathymetry data[White et al., 2006] and ABE Imagenex 120 kHzscanning altimeter bathymetry data collected in

2000 by ABE [Fornari et al., 2004] were used todetermine the pre‐existing slope of the terrain priorto the 2005–2006 flows [e.g., Gregg et al., 1996].The EM300 survey covered the entirety of the areasubsequently covered by 2005–2006 flows; how-ever, it is gridded at a lower resolution than the ABEsurvey, which covered a smaller area at 9°50′N. Thelocal pre‐existing slope surrounding each imagelocation was derived from the 30 m × 50 m griddedEM300 depth measurements and 5 m × 5 m griddedABE depthmeasurements by calculating the averagegradient between the grid cell and its four nearestneighbors at each image position.

5.1.1. Pre‐Existing Slope Analysis

[30] We derived slopes of ∼0°–11° from theEM300 bathymetry and ∼0°–19° from the ABEbathymetry within the area of the new flows.Analysis of lava morphology as a function oftopographic slope derived from both gridded depthdeterminations indicates that the distribution ofslopes for each of the morphologies, with a possi-ble exception in the distribution of pillows, corre-lates with the overall distribution of slopes in thesurvey area (Figure 11). These results contrast withobservations from the Puna Ridge [Gregg andSmith, 2003], where a strong partitioning of mor-phology as a function of slope is observed

Figure 10. Evidence that subsurface pathways were utilized by the 2005–2006 EPR flows. (a) 2005–2006 pillowlava imaged at the base of collapse in older lobate terrain (∼6 m × 4.5 m). (b) Older lava crust broken and lyingon the surface of a 2005–2006 flow that was presumably pushed up by lava flowing within subsurface networks(∼4.8 m × 3.6 m).

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(although the range of slopes is much larger).Relative to the overall slope distribution, pillows inthis study trend toward slightly higher pre‐existingslopes calculated from both the EM300 and ABEbathymetry data (Figure 12). However, we notethat few pillows were identified in these surveysrelative to other flow morphologies (2%) and weare therefore cautious in interpreting this as evi-dence that pillows form on higher slopes. Addi-tionally, some pillow flows were observed justoutside of the AST and on the floor of the ASTwhere slope is essentially flat. The slope in the dataappears to be higher in these areas due to thegridding method of the bathymetric data, whichcould account for the small peak of high‐slope pil-lows. As a further check on whether slope stronglycorrelates with seafloor morphological variations,we analyzed lava morphology as a function of topo-graphic slope derived from near‐bottom TowCamdepth profiles (auxiliary material). Unlike griddedbathymetry data, which may underestimate the actualslope of steeply dipping faults or flow fronts dueto their small aerial extent relative to grid cells,TowCam bathymetry profiles provide a more faith-ful representation of seafloor slope. In addition,these profiles take into account topography gener-ated by the 2005–2006 eruption, although in mostcases we do not see significant differences betweenpre‐ and post‐eruption bathymetry [Ferrini et al.,2009], which further confirms the relatively thin(<1–2 m) flow thickness. Regardless, these resultssupport our conclusion of very limited to no cor-relation between topography encountered by the2005–2006 flows and the resulting lava morphology.

5.1.2. Pre‐Existing Morphology Analysis

[31] The four TowCam surveys used in this studyconducted prior to the 2005–2006 flows wereanalyzed in areas where they overlapped themapped 2005–2006 flows boundary (Figure 13).These morphology observations were compared tomorphologies of the 2005–2006 flows locatedwithin 100m of the pre‐eruption surveys. The 100mbuffer accounts for navigational uncertainties in thelayback calculations. In the 1997 surveys, lobatelavas were ubiquitous over the area surveyed andaccounted for 74% of the observed morphologies.Hackly lavas comprised 11% of the morphologiesobserved, sheet lava was less abundant comprisingonly 8% of the morphologies in the surveyed area,and 7% of the lava was pillows. Areas that werepredominately lobate in the 2005–2006 main flowlobe prior to the recent eruptions were overprintedby sheet and hackly flows during the 2005–2006eruptions. The post‐eruption surveys indicate that,within the 100 m buffer zone of the pre‐eruptionsurveys, the abundance of lobate, sheet, and hacklymorphologies changed significantly after the 2005–2006 eruptions. This is the first well‐documentedstudy in which variations in seafloor lava mor-phology can be directly related to known eruptionepisodes where effusion rate varied significantly. Inrecent surveys within the 100 m buffer zone, lobatelava was much less abundant than in the pre‐eruption surveys, accounting for only 34% of themorphologies observed. Sheet flows are the domi-nant morphology type observed, accounting for65% of the morphologies within the study area,34% of which are hackly and 31% lineated or ropey

Figure 11. Lava morphology plotted as a function of pre‐existing slope as calculated from the bathymetric surveysconducted with the EM300 multibeam system and ABE. The local pre‐existing slope surrounding each image locationwas derived from the 30m× 50m gridded EM300 depthmeasurements and 5m× 5mgriddedABE depthmeasurementsby calculating the average gradient of the four nearest grid cells at each image position to account for uncertainties innavigation. All four morphologies can be found over the entire range of pre‐existing slopes, indicating pre‐existing slopewas not a dominant controlling factor of the 2005–2006 EPR eruption surface morphology.

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sheet. Only 1% of the morphologies observed arepillow. The results of this comparison indicate thatthe pre‐eruption morphology was not a dominantcontrolling factor or predictor of the morphology ofthe 2005–2006 eruption.

[32] The lack of correlation between pre‐eruptionseafloor slope and the morphology produced sug-gests that local flow rate is likely controlled byeruption rate. The comparison of the pre‐ and post‐

eruption morphologies shows a substantial increasein the presence of sheet and hackly flows as well asthe presence of many new lava channels over alarge portion of the 2005–2006 lava flows. Com-parison between the pre‐eruption near‐bottom sidescan in the 9°28′–55′N EPR crestal area [Fornariet al., 2004; Escartín et al., 2007] and post‐eruptionside scan over the 2005–2006 flows [Soule et al.,2006] confirms that prior to the 2005–2006 erup-tion, this area was characterized by lobate flowswith few lava channels, which changed dramati-cally with the emplacement of the 2005–2006flows that were dominated by lava channels. Thisfurther supports the conclusion that the 2005–2006eruptions produced lava at a higher effusion ratethan what is typical for this portion of the EPR[Soule et al., 2007].

5.2. Distribution of Morphology Along Axis

[33] The volume of lava erupted during 2005–2006at the EPR axis varied substantially along the∼18 km long eruptive fissure system. In somelocations, lava flows reached <0.5 km from theAST, while in other locations flow lobes extend asfar as ∼3 km from the AST. The most voluminousoutput of the 2005–2006 eruptions occurred withinthe 9°51′N lobe, coincident with the highest densityof active high‐temperature hydrothermal venting[Fornari et al., 2004] and the highest MgO lavacompositions within the eruption area [Goss et al.,2010]. Relative to the rest of the newly emplacedflows, the 9°51′N lobe contains the highest per-centage of high‐flow rate morphologies (∼70%sheet and hackly flows) and is the locus of wherelava was transported farthest from the AST. The9°51′N area is also the inferred site of the primarydiking event that fed the 2005–2006 eruptionsbased on seismic evidence [Tolstoy et al., 2006].However, this region is not the only place thatexperienced higher eruption rates than those pro-duced by the 1991–1992 eruptions. While the9°51′N lobe represents the most voluminous andgreatest abundance of high‐effusion flows, we seean increase in sheet and hackly flows withinalmost all of the 2005–2006 flow lobes relative topre‐eruption abundances. We conclude from theseobservations that the entire length of the axialeruption experienced higher eruption rates thanprevious studied eruptions, regardless of variationsin the volume of material erupted along individualfissures.

[34] The relative distributions of the four mor-phology types along axis further indicates that the

Figure 12. The distributions of the four main morphol-ogies of the 2005–2006 eruptions correlate with theglobal distribution of slopes in the survey area, with aslight exception in the distribution of pillow lavas athigher slopes. (a) The relative frequency of each mor-phology type as a function of the maximum pre‐existingslope calculated from the EM300 data [White et al.,2006]. (b) The delta frequency, relative to the overallslope distribution, plotted as a function of the maximumpre‐existing slope shows there is a shift in slope rangeswhere pillows exceed the average distribution of slopestoward slightly higher slope values.

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Figure 13

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most recent eruption was focused along fissureswithin the AST at 9°51′N. Although all morphol-ogies are observed along the length of the 2005–2006 main flow unit, the relative abundance ofsheet and hackly flows is greatest in the centralportion of the flow and especially around the 9°51′Nlobe, whereas the relative abundances of lobate andpillow flows increase with along‐axis distance fromthis region (Figure 6). The relative abundance ofhigh‐flow rate morphologies appears to correlatewith the volume of material erupted, with thegreatest flow rates and flow lengths at the center ofthe eruptive fissures (near 9°51′N) and the lowestflow rates and flow lengths at the northern andsouthern ends of the eruptive fissures (near 9°55′Nand 9°47′N, respectively, Figure 4).

5.3. Distribution of Morphology AcrossAxis

[35] The formation of the four morphologies clas-sified in this study is dependent on flow rate. Thegeneral across‐axis trend that we observe in the2005–2006 flows is that axially erupted lavasformed sheet and flat‐lying lobate morphologiesnear the vent and less coalesced lobate and pillowmorphologies with greater distance from the axis.The majority of pillow flows imaged in the surveys(>80%) were found within 100 m of the flow ter-minus (regardless of distance), indicating that distalflow fronts were advancing at relatively slowerrates. As the flow surface morphologies likelyreflect the flow conditions at the time of solidifi-cation, this spatial characteristic in morphologiessuggests that flow rates were higher during theinitial stages of the eruptions and waned withincreasing time (i.e., distance traveled across theseafloor). The greater abundance of pillows at themargins of flow lobes should help us identifythe separate pulses that comprised the 2005–2006eruptions [e.g., Rubin et al., 2006, 2008], but thereis no substantial evidence of internal pillow frontsin the survey images. Flow fronts produced duringthe 2005–2006 eruptions, however, often terminateas lobate flows rather than pillow fronts, making itmore difficult to distinguish these separate pulsesfrom one another within the interior of the flow.

5.4. Distribution of Other VolcanicFeatures

[36] In addition to flow morphology, we examinedthe distribution of volcanic collapse, kipukas, faultsand fissures in the new flows. The results of ourexamination of volcanic collapse is very similar toprevious studies on the EPR [Engels et al., 2003],with the key feature of the distribution being aconcentration of collapse pits near the eruptivevents and a rapid decrease with distance downflow. It is useful to note that our accounting ofcollapse is conducted within a single eruption,whereas previous studies have looked at a portionof the EPR ridge crest that likely included theproducts of many eruptions, most of which resultedfrom lower effusion rates. The similarity in col-lapse distribution suggests that these features arenot particularly sensitive to eruption rate, anddepend more on the ability of the flow to inflateand deflate. Thus proximity to eruptive vents wheredrainback is common is the main controlling factor,as has been suggested by previous workers [Engelset al., 2003; Chadwick, 2003; Fornari et al., 2004].Likewise, the low abundance of faults and fissureswe observe within the new flow agrees with pre-vious interpretations that volcanism overprintssmall tectonic features formed within ∼1–2 km ofthe axis, other than those within the AST [e.g.,Escartín et al., 2007; Wright et al., 1995].

[37] Finally, we examined the presence of kipukas,which have not before been systematically mappedon mid‐ocean ridges. The presence of kipukasthroughout the 2005–2006 lava flows supports theidea that the flows were thin (1–2 m [Soule et al.,2007]). In addition, the greater abundance of kipukasat the distal end of the flows, where low‐flow ratemorphologies are more abundant, suggests thatkipukas may bemore likely to bemaintained duringperiods of slower flow advance.

6. Conclusions

[38] This study provides the most comprehensiveanalysis of imagery data over a well‐defined andmapped MOR eruption. From our analyses and

Figure 13. (a) Pre‐eruption camera surveys on the EPR crest [Kurras et al., 2000] were analyzed in areas where theyoverlapped with the mapped 2005–2006 flows. These surveys indicate that the pre‐existing terrain was largelycharacterized by lobate flows. (b) Morphology of the 2005–2006 EPR flows within the same region as the pre‐existing camera surveys. The black lines represent a 100 m buffer surrounding each pre‐eruption survey line toaccount for navigational uncertainties. The percentages of sheet and hackly flows are significantly higher in the2005–2006 EPR eruptions.

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interpretations of deep‐towed camera surveys,and high‐resolution bathymetric profiles, we drawthe following conclusions:

[39] 1) Surface morphology of the 2005–2006flows, and by inference flow rate, was not stronglyinfluenced by the pre‐existing slope or lava rheology,nor was pre‐eruption morphology a strong predictorof the morphology that developed during this erup-tion. We conclude that seafloor lava morphology isprimarily controlled by eruption rates at the vent andtheir variability along axis and with time.

[40] 2) The dynamics of the 2005–2006 eruptionswere substantially different than previous eruptionsalong this portion of the EPR. The increasedabundance of lava channels and high‐flow ratemorphologies suggest that the most recent eruptionwas characterized by higher effusion rates thanprevious eruptions in addition to being morevoluminous than the 1991–1992 EPR eruptions.This is the first well‐documented study in whichvariations in seafloor lava morphology can bedirectly related to known eruption episodes whereeffusion rate varied significantly.

[41] 3) The 2005–2006 eruption is characterized by52% lobate flows, 31% sheet flows, 14% hacklyflows and 2% pillow flows. Over 80% of the pillowmorphology observations occurred within 200 m ofthe flow boundary, indicating decreased flow ratesat the flow margins, presumably due to late‐stageadvance after supply from the vent had ceased. Inaddition, we find higher concentrations of low‐flow rate morphologies at the northern and south-ern extent of the fissure system where we also findsignificantly lower volumes of erupted material.

[42] 4) The greatest abundance of high‐flow ratemorphologies is found within the area covered bythe 9°51′N flow. This portion of the EPR crest alsoproduced the greatest volume of lava and creatednumerous lava channels, which served as distri-bution pathways that allowed the lava to travel asfar as ∼3 km off axis. The AST in this area alsohosts the majority of high‐temperature hydrother-mal vents along this second order segment and isthe location of the most primitive lava composi-tions throughout the flow. These observationsindicate that the 9°51′N area in the AST served asthe source of the 2005–2006 eruptions.

Acknowledgments

[43] We thank the officers and crew of the R/V Knorr, R/VNew Horizon, R/V Atlantis, and R/V Alvin shipboard opera-

tions groups, who all contributed to the success of our fieldstudies of the 2005–2006 East Pacific Rise eruptions. Thechief scientists of the cruises, J. Cowen, K. Von Damm, andT. Shank, generously made time available for TowCam sur-veys to be accomplished, and we thank the science parties ofthe cruises for their cooperation in collecting the TowCamdata. Marshall Swartz has been instrumental in the develop-ment and operation of TowCam and his contributions to thiswork, both in the field and on shore are gratefully acknowl-edged. We thank Ken Rubin, Adam Goss, and Dorsey Wanlessfor helpful discussions about this study as well as Scott Whiteand an anonymous reviewer for constructive and careful reviews.This work has been supported by NSF grants OCE‐0525863(Fornari and Soule), OCE‐0732366 (Soule), OCE‐0138088(Perfit), and the Woods Hole Oceanographic Institution VetlesenFoundation Funds (Fundis, Fornari and Soule).

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