Kings Bowl Lava Field, Snake River Plain

Journal of Volcanology and Geothermal Research 351 (2018) 89–104

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Journal of Volcanology and Geothermal Research

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Phreatic explosions during basaltic ﬁssure eruptions: Kings Bowl lava ﬁeld, Snake River Plain, USA

Scott S. Hughes a,⁎, Shannon E. Kobs Nawotniak a, Derek W.G. Sears b,c, Christian Borg a, William Brent Garry d, Eric H. Christiansen e, Christopher W. Haberle f, Darlene S.S. Lim b, Jennifer L. Heldmann b a Department of Geosciences, Idaho State University, 921 South 8th Avenue, Stop 8072, Pocatello, ID 83209, United States b NASA Ames Research Center, Mountain View, CA 94035, United States c Bay Area Environmental Research Institute, Petaluma, CA 94952, United States d NASA Goddard Space Flight Center, Geology, Geophysics and Geochemistry Lab, Greenbelt, MD 20771, United States e Department of Geological Sciences, Brigham Young University, Provo, UT 84602, United States f Mars Space Flight Facility, Arizona State University, Tempe, AZ 85287, United States article info abstract

Article history: Physical and compositional measurements are made at the ~7 km-long (~2200 years B.P.) Kings Bowl basaltic Received 22 May 2017 fissure system and surrounding lava field in order to further understand the interaction of fissure-fed lavas Received in revised form 30 December 2017 with phreatic explosive events. These assessments are intended to elucidate the cause and potential for hazards Accepted 2 January 2018 associated with phreatic phases that occur during basaltic fissure eruptions. In the present paper we focus on a Available online 04 January 2018 general understanding of the geological history of the site. We utilize geospatial analysis of lava surfaces, litho- fl Keywords: logic and geochemical signatures of lava ows and explosively ejected blocks, and surveys via ground observa- Phreatic explosions tion and remote sensing. Pit craters Lithologic and geochemical signatures readily distinguish between Kings Bowl and underlying pre-Kings Bowl Fissure eruptions lava flows, both of which comprise phreatic ejecta from the Kings Bowl fissure. These basalt types, as well as Lava lake neighboring lava flows from the contemporaneous Wapi lava field and the older Inferno Chasm vent and outflow channel, fall compositionally within the framework of eastern Snake River Plain olivine tholeiites. Total volume of lava in the Kings Bowl field is estimated to be ~0.0125 km3, compared to a previous estimate of 0.005 km3.The main (central) lava lake lost a total of ~0.0018 km3 of magma by either drain-back into the fissure system or breakout flows from breached levees. Phreatic explosions along the Kings Bowl fissure system occurred after magma supply was cut off, leading to fissure evacuation, and were triggered by magma withdrawal. The fissure system produced multiple phreatic explosions and the main pit is accompanied by others that occur as subordinate pits and linear blast corridors along the fissure. The drop in magma supply and the concomitant influx of groundwater were necessary processes that led to the formation of Kings Bowl and other pits along the fissure. A conceptual model is presented that has relevance to the broader range of low-volume, monogenetic basaltic fissure eruptions on Earth, the Moon and other planetary bodies. © 2018 Elsevier B.V. All rights reserved.

1. Introduction such as Kamoamoa, Hawai'i in 2011 (Orr et al., 2012) to several tens of km such as Laki Craters, Iceland in 1783 (Thordarson and Self, Phreatic deposits are rarely preserved among fissure eruptions that 1993), and may extend for 100 s of km during flood basalt eruptions produce lava outflow lobes, but may play a more important role than (White and McKenzie, 1989). what is currently recognized. Because phreatic explosions often create Often occurring within larger volcanic fields on the flanks of large documented hazards in non-fissure-fed eruptions, such as the Kīlauea shields, along rift zones, or within calderas, individual fissure eruptions caldera eruption in 1924, phreatic activity could enhance the hazards present opportunities to examine processes related to dike injection posed by even small fissure eruptions. Basaltic fissure vent eruptions, and magma supply, both of which may be dependent on tectonic influ- often related to lateral dike propagation, are ubiquitous in both oceanic ences. Regardless of tectonic setting, nearly all basaltic fissure eruptions and continental settings. Historic fissure lengths range from a few km produce spatter ramparts and small cones, self-impounded lava ponds, individual lava outflow lobes, lava coatings on fissure walls, and lava fi ⁎ Corresponding author. drain-back into the ssure system during short-lived eruptive cycles. E-mail address: [email protected] (S.S. Hughes). However, few involve a strictly phreatic explosive phase. Kings Bowl

(a.k.a. Crystal Ice Cave) lava field on the eastern Snake River Plain of to the water table was unique at Kings Bowl during the eruption, or a Idaho, USA (Fig. 1) offers a unique environment to examine features unique mechanism was a significant factor in ESRP and other late- that may be associated with similar volcanic fissures. stage phreatic explosions at basaltic fissure vents. The lava field, including a partially drained central lava lake, encom- This paper presents the physical, geospatial, and geochemical attri- passes the namesake phreatic explosion pit within a relatively small, butes in order to develop a quantified conceptual model of the eruptive ~7 km-long, monogenetic eruptive fissure system. The Kings Bowl fis- sequence. It provides a baseline for future work that outlines myriad an- sure system, accentuated by the main central pit, a less-accessible ex- alog features on the Moon and Mars, as well as near-Earth asteroids, to plosion pit near the north end of the fissure, and numerous elucidate potential volcanic mechanisms that can be interpreted from subordinate explosion pits, provides evidence for such variation in remotely-sensed datasets. In this context, we consider Kings Bowl to eruptive style over a short time-frame (e.g. King, 1977; Greeley and be an analog for lunar rille and graben formation, Floor-Fractured Cra- Schultz, 1977; Kuntz et al., 1992; Hughes et al., 1999). The culminating ters (FFCs), volcanic constructs along eruptive and non-eruptive fis- phreatic explosions at Kings Bowl, a rare occurrence on the ESRP, raise sures, and ejecta deposits from explosive processes on the Moon, the question of why evidence for phreatic explosions during the waning Mars, and perhaps on near-Earth asteroids. Companion studies (Sears stages of fissure eruptions, as opposed to early vulcanian phreatic and et al., 2014, 2015; Kobs Nawotniak et al., 2014, 2016) focus on energy syn-eruptive phreatomagmatic blasts, is scarce. requirements of ejecta blocks and the explosive parameters of the phre- This study aims to determine the mechanisms involved in the culmi- atic steam blasts that occurred during the Kings Bowl eruptive cycles. nating phreatic explosions and how they are associated with effusive eruption and emplacement of fissure-fed lava flows. We determine 1.1. Geologic setting the origin of ejecta blocks blasted from the fissure and the sequence of events that occurred during the purportedly monogenetic eruption. Situated along the Great Rift of the eastern Snake River Plain (ESRP), Our primary hypothesis is that phreatic steam generation required Kings Bowl is one of several basaltic fields in the region (Fig. 1), includ- magma withdrawal from the fissure system, which provided residual ing Wapi, Hells Half Acre, and Craters of the Moon, that erupted within heat for explosive interaction with groundwater. This paper outlines the last ~5000 years. Radiocarbon ages for these features provided by the important mechanisms that produced the culminating explosions, Kuntz et al. (1992, 2007) in years B.P. are: Kings Bowl = 2220 ± 100; following the effusive emplacements of several lava flows, and evalu- Wapi = 2270 ± 50; Hells Half Acre = 5200 ± 150; and the Blue Dragon ates the nature and importance of lava drain-back from the lava lake flow at Craters of the Moon (a polygenetic system active since as a factor in hydro-volcanic explosions. ~ 15 ky ago) = 2076 ± 75. Other recent lava flows at Craters of the Our analysis of the eruptive processes at Kings Bowl builds on exten- Moon have essentially the same ~2100 years B.P. age (Kuntz et al., sive previous work in order to decipher the eruptive mechanisms and 2007), indicating several contemporaneous eruptions along the north- understand the history of the fissure system. Data collected using ern and southern segments of the Great Rift. A series of older (late Pleis- field- and lab-based methods enable a wide–scale (mm to km) charac- tocene) aligned eruptive vents, previously referred to as the Inferno terization of the Kings Bowl region. Specifically, ground investigations, Chasm rift zone (Greeley et al., 1977), make up a topographic ridge remote sensing imagery, geochemical analyses, petrographic analyses, ~3 km east of Kings Bowl that roughly parallels the Great Rift. Wapi ejecta distribution, differential GPS (dGPS) topographic profiles and lava field is thus considered to lie directly on the Great Rift and is con- high-resolution aerial (UAV-borne) scans of key geologic features are temporaneous with Kings Bowl; whereas, Inferno Chasm is significantly used to develop the conceptual model. We also examined possible older (based on loess and vegetation cover) and is considered to belong mechanisms that might explain why phreatic deposits are scarce within to a separate, but older volcanic rift zone (Greeley and Schultz, 1977; basaltic fissure eruptions. Simple explanations include: ejecta deposits Greeley et al., 1977). Lava from the Inferno Chasm eruption likely from explosive eruptions were buried by subsequent lavas, the depth flowed into the topographically lower Kings Bowl region, thus

Fig. 1. Regional map of study area based on Landsat imagery (Google Earth®) depicting the position of Kings Bowl fissure and lava field (inset) along the Great Rift, and its spatial association with Wapi lava field, Inferno Chasm, and other recent basaltic lava flows on the eastern Snake River Plain. Yellow outline indicates lava flows included in the Craters of the Moon National Monument and Preserve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 91 underlying both Kings Bowl and possibly Wapi lava fields. This scenario Samples of lava flows or ballistic ejecta blocks that are completely de- fits well with plains volcanism characterized by coalescent and overlap- void of secondary material are thus scarce, but not totally absent. ping low shields (Greeley, 1977, 1982). Reworked dust in most cases is easily removed from sawn pieces used Pleistocene-Holocene basaltic low-shield volcanism on the ESRP, for thin-section microscopy and from comminuted pieces prepared for typified by overlap and coalescence of their encompassing lava fields, geochemical analysis. essentially begins as eruptive fissures oriented in response to the regional stress field. Although most ESRP fissure systems are oriented per- 2. Research methods pendicular the regional extension of the Basin and Range, their orientations vary with local stress conditions. Dike-induced surface de- Interpretations are derived from the investigation of major compo- formation evident at Kings Bowl, like many other volcanic fissure sys- nents of the Kings Bowl lava field, including eruptive fissure segments, tems, reflect the dimensions and depths of feeder dikes, reservoir lava field and lava lake formation, and blocks ejected by phreatic volumes, and constraints on eruption dynamics (e.g., Pollard et al., steam explosions. Attention focused specifically on self-impoundment 1983; Mastin and Pollard, 1988; Rubin and Pollard, 1988; Holmes et lava lake levees, the lake surface and lava outflow lobes that spilled al., 2008; Head and Wilson, 2016). Individual fissure systems on the from the lake, lava drain-back, slabby lava mounds on the lava lake sur- ESRP, comprising sub-parallel vent alignments, dike-fed eruptive fis- face, phreatic explosive ejecta blocks, and eruptive vs. non-eruptive fissures, extension cracks and other features related to shield growth sure margins. Lesser attention was paid to the eruptive details of spatter have been attributed to exposed volcanic rift zones (Kuntz, 1992; ramparts and lava squeeze-ups on the central lava lake, which are being Kuntz et al., 1992; Kuntz et al., 2002) or inferred rift zones buried be- investigated separately in companion papers. neath younger volcanic deposits (Hughes et al., 2002a). More recent Field work entailed visual inspection and the evaluation of key vol- studies of the spatial arrangement of vents on the ESRP (Wetmore et canic features, high-resolution UAV imagery, sampling for lithology al., 2009) demonstrate that eruptions may not always be strictly focused and geochemistry, and topographic measurements via differential into volcanic rift zones (the northern Great Rift being a notable GPS. Data collected by these techniques were mainly used to either con- exception). firm previously hypothesized emplacement mechanism(s), or re-define The relatively minor eruption of Kings Bowl likely represents an in- and characterize those features that were more complex than originally cipient, albeit aborted, growth stage of monogenetic low shields on the inferred. Ground inspections involved qualitative and, in some cases, ESRP (King, 1977; Greeley and Schultz, 1977; Greeley, 1982; Kuntz et quantitative dimensional measurements and geologic analysis corrobo- al., 1992; Hughes et al., 1999), and is thus an essential analog to plains rated by digital field photography and remote imagery. Features were volcanism on other planetary bodies. Previous research at Kings Bowl observed with respect to stratigraphic position and cross-cutting rela- portrays a short-lived eruption of basaltic magma (Kuntz et al., 1992) tions in order to evaluate eruptive mechanisms and construct a viable that produced myriad volcanic features, including spatter ramparts sequence of events to be incorporated into the overall model. and cones, feeder dikes, extension cracks, flow impoundment levees, Basalt samples from locations over various parts of the lava field (Fig. ponded flow lobes, squeeze-ups, slabby lava mounds, lava drain-back, 2B) were obtained from Kings Bowl erupted lava, ejecta blocks phreatic blast pits, fissure wall collapse, and ejecta blankets of blocks emplaced during phreatic explosions, walls of the eruptive fissure, and and ash (King, 1977; Greeley and King, 1977; Kuntz et al., 1992). outcrops in the main pit crater. Brief field descriptions and geospatial These detailed studies further show that the Kings Bowl system repre- coordinates of sample locations are provided in Supplemental Table 1. sents a series of approximately twelve en echelon eruptive fissure seg- Samples collected from the main pit crater included representatives of ments interspersed with non-eruptive fissure segments, and Kings Bowl lavas and the underlying pre-Kings Bowl lava from accessi- concomitant parallel extension cracks on either side of the fissure sys- ble outcrops down to ~25 m depth in order to determine total thickness tem separated by N1km(Fig. 2). of Kings Bowl lava at the fissure. Hand specimens were retained for All field investigations conducted thus far (e.g Greeley and Schultz, macroscopic descriptions; thin-sections of Kings Bowl and pre-Kings 1977; Greeley et al., 1977; King, 1977; Kuntz et al., 1992; Hughes et Bowl basalt were prepared (Spectrum Petrographics®) for lithologic al., 1999), including this study, indicate that the main pit crater resulted and textural analysis using standard petrographic microscope from phreatic explosions near the end of lava lake growth. Steam blasts techniques. ballistically ejected non-juvenile basaltic blocks up to ~2 m diameter Samples were analyzed by x-ray fluorescence spectrometry (XRF) in that became strewn over ~200 m distance onto the lava surface (Fig. the Department of Geological Sciences at Brigham Young University for 2). Most ejecta blocks appear on the west side of the fissure system; major and 22 trace element concentrations (Table 2). Analytes included whereas smaller blocks on the east side of the fissure were concealed major elements reported as oxides: SiO2, TiO2,Al2O3, total Fe as FeO, in a layer of fine tephra due to prevailing W – E winds, and larger blocks MnO, MgO, CaO, Na2O, K2O, and P2O5; and trace elements Ba, Ce, Cl, were returned to the pit by human activities prior to the site being Cr, Cu, F, Ga, La, Nb, Nd, Ni, Pb, Rb, Sc, Sm, Sr, Th, U, V, Y, Zn, and Zr. Geo- protected. Some blocks penetrated thin, 10–20 cm thick congealed chemical data from Inferno Chasm and Wapi (Fig. 1) were also included lake crust where lava below the surface was still in a molten and some- in this study for comparison to Kings Bowl and pre-Kings Bowl lava. what pressurized state, thus allowing lava squeeze-ups in the form of These comparisons were made to distinguish Kings Bowl lava from ~0.2–2 m diameter round protrusions (a.k.a. “mushroom caps”) over Wapi, Inferno Chasm and pre-KB lava flows, and to potentially identify the impact points. Other more typical elongate squeeze-ups related to the source of ejecta blocks. cracking and vertical displacement of the congealed crust sporadically Geospatial analysis was used to determine the original dimensions appear on the lava lake surface. The density and size of the impact-gen- and volume of the lava lake, compare the maximum volume of the erated squeeze-ups vary considerably within a relatively small area of lava lake relative to the volume of the entire Kings Bowl lava field, and the lava lake (Figs. 2Band3), with the highest concentrations and larger constrain the total drain-back volume. This required precise elevations features located near the pit. of the original lava impoundment levees (Fig. 2) and the mounds of Kings Bowl lava is easily recognized in the field and in Landsat imag- shelly lava on the lake surface (Fig. 3) believed to represent the original ery, with a fresh, nearly black appearance typical of Holocene basaltic high stand of the lake surface. That belief required verification that the lavas on the ESRP that have not been covered with wind-blown lava mounds all rest at the same elevation as the levees, and comprise (loess) or lacustrine deposits. However, over the past two millennia congealed lava lake layers with shelly crust rather than spatter or since the eruption, light-colored wind-blown dust and minor caliche other forms of juvenile tephra from parasitic vents on the lake surface. have infiltrated vesicles in some areas, which is apparent when the oth- Relative elevations of mounds (~3 m above surrounding lava lake) erwise fresh-looking exterior is broken to expose interior lithology. and levees were visually checked in the field using a Brunton hand- 92 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104

Fig. 2. The Kings Bowl fissure system. (A) Landsat (Google Earth®, grayscale) image of the Kings Bowl (KB) lava field illustrating eruptive segments along the ~7 km-long en-echelon fissure system, central lava lake, lava outflow lobes, and remnant spatter cones (South Grotto and Creons Cave). The main pit crater (KB), KB North pit crater, and subordinate pit craters along the eruptive fissure are all determined to be phreatic explosion pits. Non-juvenile blocks partially cover the lava field mainly on the west side of the fissure system; whereas, ash fallout was mainly deposited on the east side due to prevailing W-E wind direction. (B) Schematic map shows superposition boundaries of younger & older (y/o) flow lobes where known, differential GPS transects, and the locations of lava flow samples (excluding ejecta blocks and pit wall samples) collected for geochemical and petrographic analyses. Sample names are the same as those in Supplemental Table 1 and Table 2, without the “KB” prefix. Dashed outline shows detailed area depicted in Fig. 3; lava field outline shown in light gray; darker gray region illustrates the main lava lake confined by self-impounding lava levees; dashed outline of light gray region inside lava lake shows the extent of “mushroom cap” lava squeeze-ups on the lake surface. S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 93

Fig. 3. Detail of surficial features (from Fig. 2) in the central lava lake near Kings Bowl main pit. Dashed lines show differential GPS profile transects over the lava lake (L-1 and L-2); solid lines are profiles over lava mounds (A–D). held pocket transit; however, dGPS topographic profiles of selected mounds (Figs. 3 and 4) were used to confirm visual assessments and provide precise elevations. Topographic profiles over lava mounds, the main lava lake, and other parts of the lava field enabled quantification of precise changes in elevation over distances up to several hundred meters. Differential GPS profiles measured throughout the lava field (Figs. 2Band3) comprised ~18,350 data points at 1–2 cm elevation precision. Data were collected using Topcon HiPer II® and Trimble R8 GNSS® dGPS stadia-rovers with base stations established at elevated positions along the fissure. Two differential GPS profiles (L-1 and L-2) were measured WNW – ESE across the main lava lake, over flow levees and outflow lobes on the west side of the fissure (Fig. 3). Measurements were obtained at a variable time-controlled spacing of ~0.5–2 m for profile L-1, and at a fixed spacing (distance-controlled) of 0.5 m for profile L-2, over a total distance of 1472 m. Three ~W–Eprofiles were obtained across lava flow lobes representing the southern (S-1), central (S-2), and northern (N-1) parts of the lava field (Fig. 2B). Measurements of these three profiles were obtained at an average spacing of 0.177 m (distance-controlled) over a total distance of 2936 m. Four shorter profiles (A–D) over lava lake mounds (Fig. 3) were used to verify uniformity in elevation above the lake surface, determine the lava lake high level, and provide cross-reference control points with one of the lake profiles. The dGPS transects were measured across seven selected lava mounds for a total transect distance of 289.2 m, with one of the mounds measured by two perpendicular transects (A Fig. 4. Ground images of Kings Bowl main features. (A) Blocks ejected by phreatic and B). Data point spacing, determined manually, averaged ~1.37 m explosions litter the lava lake surface ~100 m from the fissure. Lava mounds fi representing the high level of the original lake surface are shown in the distance. Yellow over lava lake surfaces, but reduced signi cantly between positions on fi – eld book in foreground is 19 cm long. (B) Large ejecta blocks up to ~2 m accumulated each mound (~0.15 0.20 m spacing). with nearly full coverage of the lava lake surface ~5 m from the rim of the main Kings Dimensions of the entire lava field and central lava lake were deter- Bowl pit crater. Numbered lava mounds on the horizon lie along topographic transect C mined by analyzing georeferenced topography and 2-D imagery using in Fig. 3. (C) Image looking south into the main 30 m-deep Kings Bowl pit illustrates the Esri ®ArcGIS 9.1. Lava field boundaries, extension cracks, spatter now-deflated layers of KB lava overlying older pre-KB lava. Irregular walls in this and other pit craters along the fissure system are likely related to collapse following magma ramparts, pits, and other relevant features are clearly defined on remote evacuation, and may also reflect removal by steam blasts that plucked fragments from imagery. Georeferenced topographic maps and DEM digital topography walls during phreatic explosions. See Fig. 5 for camera positions of images A—C. (For based on U.S. Geological Survey 1:24,000 scale sheets (DRGs) were interpretation of the references to color in this figure legend, the reader is referred to overlain with georeferenced aerial imagery of the Kings Bowl region the web version of this article.) (Google Earth©). After confirming the match-up of lava field boundaries between topographic sheets and imagery, polygonal outlines of the lava field were constructed and merged in ArcGIS to calculate total surface along much of the fissure system, not just around the main Kings area of the entire lava field (including small outlying patches at the Bowl pit and the “Kings Bowl north” pit depicted in Fig. 2A. Phreatic north and south ends of the fissure) and the central lava lake. pits appear in several forms: roughly circular, lozenge-shaped, and elongate. Some of the pits coalesced into single continuous blast vents easily 3. Results visualized on the ground and in unmanned aerial vehicle imagery (Fig. 5). Subordinate pits nominally appear to range in size from ~5 to Field investigations confirmed many previous interpretations of the 15 m across at their greatest widths; however, ejecta blocks strewn ad- eruptive sequence, but presented new questions regarding pit forma- jacent to many of the narrower fissure margins suggest that pits may be tion. Multiple explosion pits and their associated ejecta blocks appear as narrow as ~3 m. 94 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104

~1.5 m lower layer and a ~3.5 m thick upper layer separated by the thin layer of fine tephra. Differential GPS mapping of ejecta blocks with N20 cm diameters along a 150-m nearly continuous phreatic fissure segment (Fig. 5A) immediately north of the main pit (Borg et al., 2016) revealed at least six discontinuous ejecta fields that overlap one another. Although several distinct subordinate pits are visibly distinct, GPS analysis suggests that several others coalesced into a single elongate phreatic blast corridor. The narrowest pits typically have steep fissure walls that incurred min- imal excavation by blasts, and are open down to variable depths, some as deep as 20 m. Wider pits are steep-sided at the top; and each one ta- pers gradually inward to form a parabolic cross-section down to ~8– 10 m where debris has collected at the narrowest zone.

3.1. Lithology and geochemistry

Lithologic and geochemical data enable an assessment of petrologic variability within the Kings Bowl system and a comparison to neighboring Wapi and Inferno Chasm ESRP tholeiites. These data are mainly used to distinguish Kings Bowl lava from pre-Kings Bowl lava as a means to evaluate the origin of ejecta blocks on the lava lake surface. Relatively dense porphyritic samples of pre-Kings Bowl lava were easily distin- Fig. 5. Composite aerial image of Kings Bowl main pit crater and immediate surroundings guished from subphyric, shelly pahoehoe of Kings Bowl lava. fl obtained during unmanned aerial vehicle (UAV) yover with high-resolution camera. Hand specimens of fresh lava and microscopic analyses of represen- Insets A—D illustrate details of lava surface: (A) margin of open fissure that experienced phreatic blasts of ejecta blocks along an extended pit crater; (B) small pit craters and tative thin-sections indicate that Kings Bowl lava is generally remnant spatter covering fissure at north end of Kings Bowl main pit crater; (C) lava hypocrystalline at or near flow surfaces. Typical lithology is dark gray lake surface ~100 m from pit crater rim illustrates smooth lake surface impacted by to black, with 2–4% plagioclase laths (1–2mm)and1–2% olivine ejecta blocks, some of which penetrated the surface to yield molten squeeze-ups; (D) (~1 mm) in a fine (0.01–1 mm) hyaloophitic groundmass dominated margin of Kings Bowl pit crater with large-size ejecta blocks near collapsed rim. Small by plagioclase microlites. Pre-Kings Bowl lava, exposed as fresh out- black arrows indicate camera positions and view directions of images in Fig. 4. Original UAV image processed by M. Downs and J. Busto as composite by Agisoft Photoscan®, crops in the main pit, is generally med-light gray, holocrystalline, taken by a Canon PowerShot® SX260 HS camera, 4000 × 3000 resolution with 1.5494 diktytaxitic with 5–10% plagioclase (1–2mm)and1–2% olivine um nominal pixel size. Flying altitude = 57.0 m, no. images = 218, ground resolution of (~1 mm) in a medium-grained (~0.4–1.0 mm) felty to ophitic ground- 2 original = 0.0168594 m/pix, coverage = 0.106692 km . mass. Although textures and vesicularity vary within each flow, and both flows are porphyritic with similar proportions of plagioclase and olivine, their respective lithologies can be distinguished in the fresh sur- Remnants of spatter cones and ramparts at South Grotto, Creons faces of phreatic explosion ejecta blocks.

Cave and a few other scattered locations along the fissure (Fig. 2A) re- Overall systematic trends of TiO2 vs. MgO, P2O5 vs. FeO, Sr vs. CaO veal distinct eruptive phases. These relict features and the relatively and Cr vs. Ni (Fig. 6) are compared to geochemical variations (e.g., low volume of erupted lava reflect a fairly short-lived, albeit complex Hughes et al., 2017 and unpublished in-house data) among samples col- eruptive sequence. The segmented pattern of flow lobes along the fis- lected from neighboring systems, the low shields of Wapi lava field and sure (Fig. 2) inhibits the determination of which part of the fissure Inferno Chasm. Geochemically, the data plotted in variation diagrams erupted first; however, the fissure segment immediately south of the (Fig. 6) exhibit groupings and trends that delineate each lava type. main central lobe was clearly emplaced early in the eruption (Greeley Nine ejecta block samples (not shown in Fig. 2B) were designated sep- et al., 1977) or at least prior to the eruption of the main central lobe. Ev- arately to avoid their inclusion in the delineation of each geochemical idence for an early localized eruptive stage appears in remote sensing field (dashed outlines in Fig. 6). Samples KBP14–01 through −07, col- imagery (Fig. 2A) as older, degraded lava surface. Field investigations in- lected at various levels in the main pit below Kings Bowl lava, geochem- dicate the lava surface is covered by a thin veneer of fine tephra that has ically plot into a separate group, which we used to delineate pre-Kings been reduced to soil. Relative superpositions of overlapping flow lobes, Bowl lava that is distinct from any of the other lava types. Two ejecta where known (y/o designations in Fig. 2B), also indicate this lava sur- block samples were also determined to lie within the geochemical face to be older than surrounding flow lobes. field of pre-Kings Bowl lava. Pit samples collected higher in the se- The earlier Kings Bowl eruptive phase is also recognized in the walls quence from the walls, KBP14–08 through −10, and the uppermost of the main pit crater, where a compound sequence of shelly to dense surface sample, KBP14–11, were included in the delineated geochemical pāhoehoe layers exhibit a definite depositional break separated by in- fields of Kings Bowl lava. tercalated tephra. The ~10–30 cm thick tephra layer, which is exposed Major and trace element analyses (Table 2) reveal significant geo- only along a few meters in the pit wall, is significantly degraded to red- chemical complexity during the eruption, especially two important fea- dish soil, and likely represents a minor pyroclastic interval during the tures related to the fissure system. First, the compositional range of short-lived Kings Bowl eruption. Alternatively, the reddish layer may Kings Bowl erupted lava is commensurate with other Pleistocene-Holo- represent baked soil; however, the deposit appears only locally and cene olivine tholeiites on the ESRP (e.g., Hughes et al., 2002b), in partic- other soils exposed in the pit below Kings Bowl lava are light-colored, ular those in the vicinity (Miller and Hughes, 2009; Staires, 2008). The not baked. It is crudely layered (where it has not been subject to biotur- ranges (in wt%) of major oxides in Kings Bowl lava, including MgO bation), and contains a few ~0.2–0.8 cm spherules, which we interpret (8.5–10.2), TiO2 (2.3–2.6), total Fe as FeO (12.3–13.4), CaO (9.7–10.8) to be either accretionary or fragmental lapilli. The thickness of Kings and P2O5 (0.52–0.61) are notable, especially for a relatively small Bowl lava at this location in the pit is 5.1 m (Fig. 4C) where it overlies eruptive volume, yet not unusual for ESRP tholeiites. Overall ranges in flat topography, and reaches a local maximum of 6.1 m where early some compatible and incompatible trace elements (in ppm) are also no- lava filled a small depression (sample KBP14–08). Thus two sequences table, with ~20% or more variation (e.g., Ba = 253–304, Cr =336–400, of spatter-fed lava comprise the total thickness of ~5.1 m (Fig. 2C): a and Ni = 119–166 ppm), while other trace elements are more uniform S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 95

Fig. 6. Chemical variation diagrams of TiO2 vs. MgO, P2O5 vs. FeO, Sr vs. CaO and Cr vs. Ni for Kings Bowl (KB) lava sampled on the surface and near the top of the main KB pit, pre-Kings Bowl (pre-KB) lava collected from the main KB pit, and ejecta blocks. Dashed outlines of clustered points illustrate overall variations (sans outliers) within each group. Three pre-KB lava samples were collected from KB lava surfaces, one near the north margin of the field and two others from near the eruptive fissure just north of Creons Cave. Plots of unpublished data from Inferno Chasm and two regions in the Wapi lava field are shown for comparison.

(e.g., Cu = 45–51, V = 282–314, Sr = 244–263, Zn = 101–117, and collected from the Kings Bowl lava field lead to the possibility of Zr = 196–220 ppm). Spatial trends in geochemical variability of magma recycling from an older reservoir and a high potential for Kings Bowl lava are not recognized in either the pit sequence magma mixing in the fissure system. A few “outliers” (a pre-Kings (KBP14–08 to 11) or in surface samples. Bowl sample and an ejecta block in Fig. 6) may represent mixed The second geochemical feature related to the fissure system is that magma, or otherwise involve such complex magmatic processes. separate trends are apparent for the Kings Bowl lava and the older pre- Kings Bowl lava flow lobes exposed in the main pit (Fig. 6). Geochemical 3.2. Geospatial analysis discrimination of pre-Kings Bowl flows (MgO = 6.2–7.9 wt%; TiO2 = 1.9–2.6 wt%) from Kings Bowl lava is readily apparent, and confirms The calculated areal extent of the entire lava field, measured using not only the lower contact of Kings Bowl lava, but also that ejecta blocks ArcGIS, is 3.24 km2, which is essentially equivalent to the 3.3 km2 area were derived from both sources at depths reaching at least down into previously reported by Kuntz et al. (1992). We recalculated the area pre-Kings Bowl lava flows (Fig. 4C). Also, three surface samples, as polygonal shapes, based on field and image interpretation, to provide KB15–005 collected from a seemingly fresh outcrop near the northern data for calculating volume. The determination of total amount of margin of the Kings Bowl lava field and KB15–109 & -110, collected magma erupted, including that which was lost during the deflation of from proximal lava along the fissure immediately north of Creons the main lava lake, perhaps during multiple episodes of lake filling Cave (Fig. 2B), were lithologically and geochemically confirmed to be and draining, and that which remains in the lava field after drain-back pre-Kings Bowl lava collected from the main Kings Bowl pit crater. and outflow, is somewhat more problematic. Geochemical trends in Fig. 6 illustrate the complexities among The previously reported volume estimate for the Kings Bowl field neighboring lava fields (e.g., Hughes et al., 2017) and within the lava (0.005 km3; Kuntz et al., 1992) suggests an average lava thickness of types in this study. Both Wapi and Inferno Chasm lavas, located south ~1.5 m based on the reported area of 3.3 km2. The areal extent of the and east of Kings Bowl, respectively, are geochemically distinct from lava field measured herein by GIS mapping (3.24 km2) provides only a Kings Bowl lava and from each other. Inferno Chasm plots within or slight improvement in precision of the calculated area. However, the in- close to the fields of pre-Kings Bowl lava in Fig. 6. In some plots (e.g., clusion of dGPS profiles (Fig. 2B) allow for significant refinement of the

P2O5 vs. FeO, Sr vs. CaO) Inferno Chasm is geochemically indistinguish- lava thickness, and therefore volume, both at present and during the able from pre-Kings Bowl lavas sampled in the main Kings Bowl pit; high stand. however, any relation between the two is not confirmed with other Topographic profiles measured over the lava lake (Fig. 7)constrain chemical data. Moreover, the three samples of pre-Kings Bowl lava original high lake level, elevation change during deflation, and volume 96 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104

Fig. 7. Lava surface profiles (W–E) of differential GPS transects illustrated in Fig. 2B. Each profile point distance was calculated as distance along a center-line to avoid irregular, non-linear tracking. Steep drop-offs in elevation at the margins of N-1 and S-1 profiles represent relatively small, undrained lava ponds. See text for explanation of pre-KB surface elevations, shown as dashed lines projected beneath each profile. loss due to deflation. Profiles also enable further constraints in elevation taken as the average over the entire 3.24 km2 lava field, yields a total variations along the fissure system and flow surface topography of volume of 0.0125 km3, 2.5 times previous volume estimates. Although ponded lobes, terraces and small tumuli. Although point-to-point spac- this value is significantly higher than that previously reported, it likely ing in these five dGPS profiles provide a measure of total transect dis- reflects the variable thicknesses encountered from fissure to margin, tance, each point distance was geometrically recalculated along a and we regard this to be a maximum value. Thicknesses compiled in transect centerline to eliminate off-line wandering and yield the over- Table 1 range from essentially zero up to ~8.5 m. For these assessments, land distance from the starting point. mounds were excluded from lake profiles (L-1 and L-2) to avoid misrep- Estimates of average lava thickness are based on surrounding eleva- resentation of the current elevation of the lake. The L-1 and L-2 profiles tions projected as the pre-existing land surface beneath each profile an- indicate average thicknesses of 4.0 and 4.1 m, respectively, very close to chored by terrain beyond Kings Bowl lava field as well as the contact the overall average. Average lava thicknesses for both the north and depth identified in the main pit (Fig. 7). Because there is no way to de- south profiles (N-1 = 4.6 m, S-1 = 4.5 m) are higher than the overall termine the actual pre-Kings Bowl surface, the pre-existing surfaces average due to the accumulation of stagnated lava ponds that produced were assumed to be relatively smooth (i.e., straight lines beneath each steep-sided platforms of lava. The lower average thickness for the main profile) although they were most likely similar to the slightly irregular lava field (S-2) profile is due to flow confinement on the west by higher topography surrounding the lava field. The base elevation of Kings ground and outflow to the east over relatively flat ground. Bowl lava at the main pit (1483 m) is also used as the base level for The estimate of the lava lake volume lost during lake deflation takes both lake profiles (L-1 and L-2), which cross the fissure nearby. The into account that the decrease in volume was due to both lava outflow base of the main lava field profile (S-2) at the fissure is conservatively through breached levees and drain-back into the fissure near the main estimated at 1485 m because the profile indicates ~2 m higher elevation pit. Other losses, such as lava being channeled into a new lobe north overall compared to the lake profiles. This estimate is consistent with or south of the main central lobe are not considered. The maximum vol- the gently north-sloping regional topography. A third consideration is ume is directly related to the amount of vertical deflation, represented that the pre-KB elevation along profile S-2 where it passes close to the by the difference in elevation between the existing lava lake surface flow margin at a lobe embayment (Fig. 2B) could be determined di- and that determined by the average elevation (1492.2 m) of the rectly. This was necessary for the relatively long S-2 profile, which has mounds and levees in dGPS profiles (Fig. 8). The GIS-mapped area of a dramatic difference in marginal elevation from W to E and a low the central main lobe, the largest segment of the lava field which point ~100 m from the E margin. Profile S-2 (Fig. 7) therefore required the projection of a non-linear, segmented pre-Kings Bowl topographic Table 1 surface with slope inflection points. Kings Bowl lava thickness (m) estimated from dGPS profiles. Lava thicknesses were calculated for each dGPS point by subtracting Profile Dist. (m) W. margin Fissure E. Margin Min. Max. Ave. the pre-KB surface elevation, interpolated between known (measured L-1 694 0.19 1.96 N/A 0.19 5.86 4.01 or estimated) elevations, from the actual dGPS elevation. The average a fi L-2 615 0.15 2.52 N/A 0.15 6.88 4.13 thickness for each pro le is the average of calculated thicknesses for N-1 560 0.12 6.69 0.23 0.12 7.09 4.56 all data points. This method, albeit based on a simplified pre-existing to- S-1 492 0.15 5.49 0.12 0.01 8.22 4.45 pography, indicates a range in average thickness of 3.33–4.56 m be- S-2 1730 0.10 8.60 0.12 0.01 8.60 3.33 neath dGPS profiles, and an overall distance-weighted average Total 4091 Distance-weighted average= 3.87 thickness of the existing lava field of 3.87 m (Table 1). That value, if a Lava mounds excluded from thickness measurements. S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 97

Fig. 8. Lava surface profiles over several lava mounds that accumulated on the lava lake (see Fig. 2B for transect locations). Numbered mounds in profile C are illustrated in the field image in Fig. 4B. Profiles were used to determine the maximum elevation of the lava lake (1492.2 m) and compared to the elevations of lake profiles shown in Fig. 7.

encompasses the lava lake, is 1.625 km2 (1.625 million m2). Using the phases. First, pre-Kings Bowl lava sampled north of Creons Cave and same rationale stated above for calculating the overall volume of the along the northern flow margins implies that older magma, stored in a lava field, the central main lobe has an estimated volume of magma reservoir, apparently mixed with Kings Bowl magma during 0.0067 km3. Similarly, the area of the levee-impounded lava lake is the eruption. Mixing of recycled pockets of magma with Kings Bowl 0.576 km2. Volume lost is estimated by first calculating point-by-point melt may be similar to the type of magma-reservoir processes depicted differences between the maximum lake level (1492.2 m) and current for recent Kilauea East Rift Zone eruptions (Thornber, 2003; Thornber et lake surface along each of the two lake profiles (L-1 and L-2) from the al., 2003). The series of en echelon eruptive fissures at Kings Bowl, fissure out to lake margin marked by levees at 360 m. rather than a singular fissure over one feeder dike, could be the surficial Although these calculations yielded an average elevation difference expression of commingling small magmatic pockets as well. Second, the of 3.13 m over only two profiles (excluding the mounds), we consider overlapping chemical trends for both the early and late phases of the this value to be at least a first-order approximation of the average Kings Bowl eruption suggest a continuum of geochemical evolution change in elevation due to deflation. The deflated volume, calculated during magma ascent and storage. Each of the separate trends may be as 0.0018 km3 (0.576 million m2 × 3.13 m = ~1.8 million m3) repre- attributed to combined petrologic processes related to different sents a maximum loss of approximately 21% of the central main lobe (small) batch sources, crystal fractionation and possible “auto-assimila- and approximately 12.6% of the total erupted Kings Bowl lava if deflation,” i.e., variable mixing of primary magma with evolved magma dur- tion was due only to drain back. However, a significant proportion of de- ing reservoir resupply. Another possible cause for separate trends may flation was likely related to levee breach and lava outflow as depicted in be hybridization, i.e., source modification by assimilating crust, mantle Fig. 3, so this estimate is considered to be much higher than actual vol- or plume-derived components within a single magma batch rather ume loss. Even considering lake deflation to be due only to drain-back than requiring multiple sources (Leeman, 1982; Hughes et al., 2002b; (i.e., no loss due to breached levees), the total volume of lava originally Hughes, 2005; Shervais et al., 2006; Miller and Hughes, 2009). erupted would be ~0.014 km3, i.e., the sum of existing lava plus deflated These observations at Kings Bowl, as well as geochemical differences volume (0.0125 + 0.0018 km3). Given that field investigations and re- between Kings Bowl lava and neighboring Inferno Chasm and Wapi mote sensing reveal significant outflow from breached levees, much of lavas indicate significant complexity in the Great Rift. The relationship the original lake volume (perhaps from multiple episodes of lake filling) between Kings Bowl and Wapi magmatic systems remains tenuous, at is most likely represented in outflow lobes. This implies that the total least in terms of geochemical variability. Wapi lava has two distinct volume of lava erupted at Kings Bowl was probably not much more chemical types, one that represents early flows comprising the SE lobe than the calculated existing volume of ~0.0125 km3. and another that represents younger lavas comprising the proximal flows surrounding the vent. The fact that the two types of lava, and per- 4. Discussion – the Kings Bowl volcanic system haps others yet to be sampled, occur in the Wapi lava field attests to complex geochemical variations along the southern part of the Great fi Eruptions at Kings Bowl occurred in multiple episodes over presum- Rift. For example, the eld of Wapi near-vent lava in the plot of TiO2 ably a short time, partially filling a central low-lying area and forming at vs. MgO, lies along the Kings Bowl trend. This suggests that Wapi may least one, perhaps compound, lava lake in the central main lobe and sev- ultimately represent a magmatic source connection with the Kings eral other stagnated lava ponds along the fissure system. Previous esti- Bowl lava; however, other chemical trends in Fig. 6 do not entirely sup- mates of lava field dimensions led to the suggestion that the eruptive port that suggestion. On the contrary, the Sr vs. CaO and Cr vs. Ni plots activity produced no more than ~0.005 km3 of lava covering ~3.3 km2 both indicate a closer association the Wapi SE lobe to Kings Bowl, but area and possibly lasted only a few hours (Kuntz, 1992; Kuntz et al., a much wider separation between geochemical signatures. 1992). Volcanism stopped as magma supply was cut off. This loss of magma supply may be related to the proximity of Kings Bowl to the 4.1. Lava field and central lava lake much larger contemporaneous eruption of the ~5.5 km3 Wapi lava field (325 km2 area), which is situated on the same volcanic rift zone, al- Volatile-driven lava fountaining eruptions along the Kings Bowl fis- beit with slightly offset eruptive fissures (Kuntz et al., 1992, 2002). sure system initially produced spatter-fed lava flows that spread out Although the details of magma generation and evolution are not part mostly perpendicular to the fissure before being channeled into adja- of this study, two observations are important to deciphering eruptive cent low lying areas. Tube- and channel-fed lava flowed over low-relief 98 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 topography defined by older basalt lava surfaces, and interlobate de- fissure were likely concentrated into localized vents such as those pressions containing loess and variably-sized fragments of brecci- that produced the main Kings Bowl pit, Creons Cave, South Grotto, ated scoria. Topography prior to Kings Bowl eruption, projected and a prominent pit in the northern segment that is designated from elevation contours around the Kings Bowl field from the Pillar “Kings Bowl north” (Fig. 1A).Remnantsofventsoccurasdeposits Butte 7.5 Minute U.S. Geological Survey topographic map, and con- of congealed spatter ramparts and partial cones, and in some seg- firmed by dGPS profiles, sloped gently NNW, ~1510 to 1495 m eleva- ments they completely bridge the open fissure. tion change over the fissure length (slope = ~0.002). At elevations Despite these complexities, the layered sequence in the main Kings lower than adjacent surrounding low shields, the regional topogra- Bowl pit clearly indicates that the central part of the fissure system, phy had only meager control on lava flow direction. Lava outflow that which produced the main lava lake and largest phreatic pit, was ac- lobes thus spread laterally on either side of the fissure, with several tive during the most recent part of the eruption. The final thickness of lobes stagnating into local ponds and self-impounded terraces lava in the central part of the lava field represents a deflated lava lake (King, 1977; Greeley et al., 1977). The maximum flow length is around the main pit, at least near the fissure. The lava lake filled in a about 1.1 km measured perpendicular to the fissure system (e.g. shallow basin, a topographically low region confined by earlier volcanic Fig. 2). Surface features are dominated by shelly pahoehoe and constructs related to the adjacent Inferno Chasm rift zone (Greeley et al., small depressions created by deflation and deformation of the rigid 1977). flow crust. Asignificant proportion of the original lake volume was lost by two Although the Kings Bowl eruption is presumed to be monoge- processes recognized in the field: (1) the lava lake breached levees and netic, both the localized tephra layer exposed in the main pit and flowed outward and (2) lava began to drain back down the fissure. The the segmented pattern of vents and flow lobes indicate significant latter process likely occurred when the degassed magma supply waned, complexity in eruptive processes. Some complexity is confirmed by and the magma level in the fissure dropped accordingly. Lava drain- the compositional variability depicted in geochemical data (Table back, exemplified by ropy pahoehoe and downward flowing lava 2; Fig. 6); however, spatial trends are not evident in geochemical lobes into the fissure, is exposed along some of the narrower fissure seg- data either from Kings Bowl surface samples or by comparison of ments, but not in the central pit. An important consideration is the lack geochemical data from the older flow lobe to other samples. Like no- of evidence revealing lava drain-back along the walls of wider explosion table historic fissure eruptions, the main conduits along the eruptive or collapse pits.

Table 2 Representative XRF analysesa of Kings Bowl and pre-Kings Bowl basalts.

Sample type Kings Bowl basalt Pre-Kings Bowl basalt

Lava ﬂow surface Ejecta blocks Early phase Surf. Pit samples Ejecta blocks

Sample KB14-002 KB15-001 KB15-003 KB14-121 KB14-214 KB15-903 KBP14 KB15-005 KBP14 KBP14 KB14-105b KB14-70 -08 -01 -07

Major oxides (wt. %), normalized to 100% volatile-free, total Fe as FeO

SiO2 46.5 46.5 46.5 46.0 46.3 46.5 46.2 48.2 48.4 48.5 48.7 49.2

TiO2 2.58 2.60 2.40 2.30 2.38 2.36 2.28 2.43 2.19 1.86 2.07 2.47

Al2O3 15.1 14.9 15.0 15.0 15.0 14.8 15.0 15.6 15.7 16.5 16.1 14.6

FeOT 13.0 13.3 12.9 12.7 12.8 13.0 12.7 12.0 11.9 11.0 11.3 12.5 MnO 0.20 0.22 0.21 0.20 0.20 0.21 0.20 0.20 0.20 0.18 0.19 0.21 MgO 8.51 8.49 9.28 10.1 9.80 9.59 10.0 6.30 6.88 7.71 7.02 6.35 CaO 10.7 10.7 10.5 10.3 10.2 10.4 10.3 11.8 11.3 11.1 11.2 11.1

Na2O 2.41 2.24 2.22 2.39 2.17 2.16 2.34 2.18 2.40 2.33 2.45 2.41

K2O 0.51 0.50 0.48 0.44 0.46 0.48 0.44 0.69 0.55 0.55 0.58 0.67

P2O5 0.61 0.61 0.57 0.55 0.56 0.57 0.55 0.55 0.47 0.40 0.44 0.52 bmg# 0.57 0.56 0.59 0.61 0.60 0.59 0.61 0.51 0.53 0.58 0.55 0.50 Orig. total 99.89 99.61 99.95 99.89 99.86 99.59 99.87 99.83 99.90 99.89 99.91 99.76

Trace elements (ppm) Ba 277 275 263 284 267 260 266 360 290 263 257 309 Ce 42 42 39 36 41 36 33 51 46 38 40 49 Cr 344 336 383 371 388 400 387 195 212 264 246 227 Cu 51 49 46 51 47 47 44 69 73 64 65 79 Ga 19 18 18 18 18 17 18 19 20 19 18 20 La 18 17 19 16 19 17 17 23 24 17 21 25 Nb 19 18 18 17 17 17 16 20 17 15 16 20 Nd 24 26 25 23 25 24 24 28 25 21 22 27 Ni 113 119 146 152 151 163 163 71 77 111 85 60 Pb345343344464 Rb 10 10 10 9 11 10 9 15 14 15 15 18 Sc 34 33 33 30 32 32 31 35 32 28 31 37 Sm556556565555 Sr 262 254 254 258 254 247 254 237 208 208 207 207 Th 0.1 1.0 1.5 0.1 b.d. 0.5 0.9 0.7 1.2 1.5 2.8 1.4 U b.d 0.5 0.6 0.3 b.d. 0.2 0.7 b.d b.d 0.2 0.8 0.4 V 314 311 299 276 283 290 282 298 273 225 259 311 Y 383735343433334137313442 Zn 112 112 112 101 108 108 103 110 100 86 94 109 Zr 223 220 208 199 205 200 196 259 224 189 208 257

a Analytical uncertainties at 2-sigma levels are as follows: major and minor elements ± 1–4%; trace elements ± 1–8%, except Pb, Sm & Th = ±10-15% and U = ±40–50%; b.d. = below detection limit. b mg# calculations based on FeO = 0.90 mole fraction of total Fe. S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 99

4.2. Lava mounds if confirmed, may be minor disintegrated fragments of the lava lake or spatter ramparts. Larger mounds of slabby or shelly lava lake crust ~3 m taller than the An important observation related to the phreatic phase is that virtu- smooth lava lake surface appear in a somewhat regular pattern on the ally all ejecta blocks, examined by hand-specimen, thin-section micros- lava lake surface (Figs. 3 and 4A and B). Initial field interpretations in- copy and geochemistry, are non-juvenile, i.e., they were ejected as solid cluded several possible features typically found on flow surfaces, e.g., blocks. Field evidence shows that the greatest mass proportion of phre- pressure ridges, tumuli or hornitos. Close inspection revealed little evi- atic ejecta is dense pre-Kings Bowl lava, with less abundant material dence in the form of breakout lobes, spatter, uplift tension cracks, or from the solidified portion of the Kings Bowl lava. Although geochemi- tilted layers that could be used to argue in favor of any of these common cal data (Fig. 6) indicate a greater number of Kings Bowl analyses in features, and it was concluded that they were formed by some other ejecta blocks, that relation is a likely artifact of random sampling. Field process. More likely, they are remnants of another self-impoundment inspections showed that the largest blocks near the pit were dense, lith- levee system within the confines of the more obvious lake boundaries ologic equivalents of distal lobes of pre-Kings Bowl lava, not highly ve- and impoundment levees. Their positions and uniform elevation that sicular lava or congealed spatter from the Kings Bowl eruption. is equivalent to the outer levee elevation argue in favor of this process, Preliminary work on ejecta physical properties (Kobs Nawotniak et which would indicate that the lava lake was subject to multiple episodes al., 2014, 2016; Sears et al., 2014, 2015) provides additional evidence of filling and possibly draining. Either piles of solid slabs (like “ice floes”) for pre-King Bowl lava dominating the population of ejecta blocks. Lith- were stacked up by lateral compression of solidified skin of the lake sur- ologic analyses, including vesicularities correlated to masses deter- face (in a manner similar to shuffling a deck of cards), or they were sim- mined by weighing blocks less than ~12 kg on a portable scale, ply regions of the lava lake that had significantly congealed before the indicate an average block density of 2790 kg/m3 for blocks ranging in lake drained and subsided in elevation. vesicularity from ~0% (dense, ~3000 kg/m3) up to ~25% (scoria, Both scenarios seem plausible, such that mounds represent piles ~2300 kg/m3). The vesicularities of 181 blocks (many that were too produced by compressive forces as the self-impoundment levees grew heavy to weigh) inspected along three transects from the main pit indi- at the margin of the active lava lake. Little evidence, such as disruption cated the following percentages of blocks on the surface: dense = 45, of flow surfaces in the form of contortions and shear zones, exists to sug- low = 25, medium = 14, high = 13, and scoria b3%. Measured block gest that the lava mounds have moved laterally. Some mounds display a sizes (geometric average of three dimensions) further show that most relatively continuous lava lake surface on their lower flanks, which ap- of the largest ones on the surface, ~0.8 to 1.5 m diameter, essentially pears to have draped over the sides or actually flowed around the all of which are high density, occur within 30 m of the pit rim, while mound during lake deflation, indicating that the mounds are close to even larger blocks remain in the pit. The largest ejected block size their original positions on the lava lake surface. In a few places large drops off to ~30 cm at 150 m distance from the rim. Moreover, north- ropy pāhoehoe surfaces between mounds indicate flow back towards ward along the west side of the fissure (Figs. 3 and 5A), blocks up to the fissure. ~60 cm occur as far as 330 m north of the main pit. We interpret the lava mounds to be formed in situ, or nearly so, with the possibility of some minor lateral compression being involved as a 4.4. Fissure eruption secondary process. Similar high-stand remnant mounds or lava plateaus are evident on the 1959 Kīlauea Iki lava lake on the Big Island of Hawai'i. Tectonic setting may play a role in Kings Bowl magma supply and The Kings Bowl mounds represent immobilized parts of the active lava eruption dynamics relevant to the unusual occurrence of the associated lake that were left standing as lava drained from beneath the congealed phreatic explosions. Many recently-active basaltic eruptive fissures are lava surface. Thus, we consider them, like the levees, to be constructed generally associated with a divergent tectonic regime, such as multiple of congealed lava representing the maximum height of the lava lake, fissure swarms aligned with the Mid-Atlantic Ridge in Iceland rather than being formed as parasitic vents (hornitos) or tumuli (Einarsson, 2008; Stefansson et al., 2008). Others occur in strictly (Hughes et al., 2014). within-plate mantle plume systems, such as the Southwest and East rift zones of Kīlauea volcano, Hawai'i (Clague and Dalrymple, 1987; Dvorak, 1992; Tilling and Dvorak, 1993), while still others have a 4.3. Phreatic blasts and ejecta blocks more complex association of tectonism and magmatism such as the Summit rift zone of Cerro Azul volcano in the Galápagos (Naumann Culminating phreatic steam blasts along the Kings Bowl fissure sys- and Geist, 2000). By comparison, the Great Rift and other aligned vent tem produced the 80 × 35 m main pit crater, the Kings Bowl north pit corridors of low shields on the ESRP are situated in the extensional (Fig. 1) and numerous subordinate pits that are easily recognized in ae- Basin and Range lithospheric regime that was previously impacted by rial imagery and ground surveys. Key features related to the phreatic the Yellowstone hotspot ~10 m.y. ago (e.g., Armstrong et al., 1975; phase, which modified fissure walls and conduits, were verified by Pierce and Morgan, 1992; Kuntz et al., 1992). Kings Bowl is therefore field investigation along the entire length of the Kings Bowl fissure sys- not associated with an active divergent tectonic boundary or within- tem. These include: (1) an obvious width of the eruptive fissure greater plate hotspot; however, it has many similarities to volcanic rift zones than ~2 m, i.e., the maximum apparent feeder dike width measured and associated basaltic fissure systems in other parts of the Basin and where segments are intact and walls are coated with juvenile lava; (2) Range province. The occurrence of numerous vents and the aforemen- irregular fissure walls that indicate collapse or other process of removal; tioned potential for commingling of geochemical types, basaltic (3) deposits of ejecta blocks along the margins; and (4) deposits of non- magmatism along the Great Rift apparently exhibits many of the same juvenile fine tephra on the east (downwind) side of the fissure system. complexities in magmatism as those in Hawai'i, Iceland, or the Juvenile spatter is observed along fissure margins as relics from the Galápagos, so the magma plumbing systems and fissure eruption dy- earliest fissure eruptions where ramparts have not fully collapsed; and namics are probably not much different. we observe no evidence of surface features that are typical of Magma reservoirs on the ESRP, as precursors to monogenetic phreatomagmatic eruptions such as tuff rings, palagonitic tuff, or shields, are generally depicted as magma batches stored within vertical surge deposits. Fine, wind-transported tephra (ash and small lapilli) de- chambers at lower-to-upper crustal levels (e.g., Kuntz, 1992; Holmes et posited east of both the main pit and the Kings Bowl north pit (Fig. 2A) al., 2008). Although Kings Bowl and other fissure-fed lavas on the ESRP appears to be non-juvenile; however, this observation is inconclusive as depict dike eruptions with concomitant extension features (Kuntz et al., these deposits have degraded significantly into soil and have been ex- 2002), magma supply potentially involves more complex networks of tensively reworked with eolian dust. Juvenile components of the ash, dikes and shallow sills such as those described in regions south of the 100 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104

ESRP (e.g., Richardson et al., 2015; Muirhead et al., 2016). Geochemical width of ~8 m and a total volume of ~2.0 km3 (assuming all magma and geophysical models (Leeman, 1982; Kuntz, 1992; Leeman et al., stored in a single vertical reservoir without shallow sills), with concom- 2009) suggest that primary basaltic magmas in the ESRP are generated itant shallow crustal extension ranging from 0.64 to 4.50 m along a from lithosphere and either erupt after minor fractionation or stagnate, 14 km segment of extension cracks. Rift zone width, measured as the mix and hybridize in mid-crustal realms (e.g., Bradshaw, 2012). Major separation of Kings Bowl extension crack sets, varied from 1205 to and trace element geochemistry of regional ESRP basalts has further re- 1860 m and was used to calculate the range in depth to dike top at vealed magma diversity that is likely related to magma generation in 800–530 m (inset in Fig. 9). The average depth of the calculated dike small separate batches and by variable mixing of primary and evolved top at the Kings Bowl fissure, 633 m, was determined to be significantly compositions (Hughes et al., 2002a, 2002b; Hughes, 2005). Such pro- less than the averages for two other non-eruptive segments of the Great cesses imply extended processes of magma evolution involving multi- rift; and the magma reservoir was shown to be anchored at 23–31 km ple layered sills (e.g., Shervais et al., 2006; Miller and Hughes, 2009), depth (Holmes et al., 2008). These dimensions are appropriate for a which yield a variety of magma types capable of being injected laterally mid-crustal source region that could involve differentiating sills within or upward into vertical reservoirs. the vertical column. Kinematic analysis of the Kings Bowl fissure system by Holmes et al. (2008) provides a quantitative approach to Kings Bowl magma dynam- 4.5. Kings Bowl eruption dynamics ics and dike dimensions based on field measurements of extension cracks aligned on either side of the eruptive fissures, and on buoyancy We evaluate the Kings Bowl eruption dynamics based on the as- equilibrium and boundary element models. Extension crack sets that sumption that magma volumes and depths to feeder dike tops are crit- bound eruptive fissures, grabens, or even non-eruptive fissures without ical to the interpretation of the Kings Bowl fissure system, as well as a volcanic vent represent the accommodation of crustal extension oth- similar fissure-generated systems. Eruption dynamics dictate that erwise being accommodated by the vertical magma reservoir below magma will seek a level of neutral buoyancy where lithostatic pressure the eruptive fissure. We incorporate this complex scenario (Fig. 9) nearly equilibrates with the pressure at the tip of the magma column, into the eruptive history and the mechanisms of Kings Bowl pit forma- i.e., the dike reservoir top (Pollard et al., 1983; Mastin and Pollard, tion. Their analysis suggests that the subsurface feeder dike(s) at Kings 1988; Rubin and Pollard, 1988; Gudmundsson, 2003; Holmes et al., Bowl, i.e., vertical component of the magma reservoir, had a maximum 2008; Head and Wilson, 2016). At that level, buoyancy-driven forces are exacerbated by volatile exsolution (Wilson et al., 2011), leading to magma expansion and lower density that enhances buoyancy, and to vapor-phase over-pressurization that forces magma into lower lithostatic pressure regimes (upward). Holmes et al. (2008) estimate a depth of ~700 m for volatile exsolution during the Kings Bowl eruption. Once the gas-driven forces subside, or magma is withdrawn from the fissure system, magma level in the fissure is allowed to drop back to a deeper level, perhaps as deep as the original dike top. Without con- tinual deformation, i.e., without initial inflation followed by deflation of local crust during an eruption, a rigid crust such as the ESRP will likely allow a dramatic drop in lithostatic pressure when magma supply is cut off (Kuntz, 1992). The eruptive fissure(s) would either remain vacated or potentially filled by collapsing fissure walls or down-dropped crustal blocks. Current water table depth at Kings Bowl is 245 m (King, 1977), so magma withdrawal to the dike top level (below 530 m) would also allow groundwater infiltration resulting in phreatic explosions. Squeeze-ups resulting from ballistically ejected blocks that impacted or penetrated the lava lake surface indicate that the lake was still molten and pressurized, at least in some areas (Fig. 2B), by a mechanism that inhibited pressure drop. Internal magma pressure could be maintained by one or more combined mechanisms, including, but not necessarily limited to: continued magma influx from the eruptive fissure, blockage of any lateral movement including drain-back, or back-flow of lava beneath the lake's crust where the squeeze-up areas were down slope from the lake's margins (an essentially “artesian” condition). Some of the larger blocks close to the pit evidently did not puncture the lake surface. This may indicate that either different areas of the lava lake had cooled and congealed while other areas were still partially molten, or that more than one explosive episode was responsible. In the latter scenario, some blocks might have been ejected during an earlier phase that occurred while the lake was still partially molten, which was followed by a later, stronger episode that ejected the largest blocks. Phreatic blasts of essentially non-juvenile ejecta along the Kings fi fi Fig. 9. Schematic model of an eruptive segment of the Kings Bowl fissure system. Model Bowl ssure system imply that molten lava was either not a signi cant illustrates the geometry of feeder dikes in the Basin and Range stress regime, which component of the ejecta or that it possibly comprised fine lapilli and ash ascend to levels of neutral buoyancy, inducing deformation that results in extension deposited as fallout east of the fissure. From this we make two overall cracks on either side of the fissure. Rift zone width, defined by crack sets, is proportional deductions: (1) molten lava drain-back from the lava lake could only to the depth to the dike top. Gas-driven eruptions ensue from zones where dikes reach comprise a minor proportion of the ballistic ejecta, and probably could lower pressures appropriate to allow volatile exsolution and expansion that enhances fi magma ascent (adapted from Kuntz, 1992 and Hughes et al., 1999; inset adapted from not have played a signi cant role in providing the thermal energy re- Mastin and Pollard, 1988; Rubin, 1992;andHolmes et al., 2008). sponsible for steam production; and (2) the fissure was likely open, S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 101 devoid of molten lava during the phreatic phase. For this to occur lava (1) Lava outflow from eruptions along the fissure system fills in local drain-back from the lake would have to initially stall in order for the depressions and a shallow central basin, producing spatter lake to maintain enough pressure to allow squeeze-ups. Withdrawal ramparts and cones, inflationary lava lobes, small stagnated of magma from the fissure would need to be relatively much faster to lava ponds, and a self-leveed central lava lake that filled in two allow groundwater to enter the hot, open fissure. or more stages. Apparently, during the phreatic phase, lava drain-back from the lake (2) Magma supply is cut off, resulting in fissure evacuation while had not proceeded sufficiently to cause a drop in pressure just below the lava lake remains pressurized. Lake deflation stalls when drain- congealed lake surface. Yet the conditions for groundwater influx most back and outflow are initially inhibited, possibly by obstructions likely required a drop in magma pressure against the fissure walls. Rem- in the fissure due to congealed spatter or dikes, or by solidified nants of spatter cones and sagging remnants of the lava lake surface are levees and other parts of the lake. Wall support within the fissure intact across some segments of the fissure where the fissure has been is essentially lost, allowing collapse of older basalt and the over- vacated below. These remnants imply that the lava lake would have lying lava carapace. been held intact, at least for a while in some areas, as lava drained (3) Groundwater flows into the open fissure and reacts with the in- from the fissure and allowed groundwater influx. The resulting steam tense heat in deeper regions (likely near the top of the feeder blasts removed much of the solid fissure cover as ejecta blocks, which dike). The ensuing phreatic explosions eject non-juvenile ash enabled the lake to flow back towards the fissure. Regardless, the lava and blocks. Tephra is blasted in ballistic trajectories with suffi- lake did not experience significant drain-back into the fissure during cient energy to leave variable-size blocks on the surrounding sur- the phreatic phase(s); and, quite likely, as evidenced by drain back fea- face. This episode, beginning while the lava lake is still tures exposed in several places along the fissure, lava flowed back into pressurized (at least partially), possibly continues intermittently the fissure subsequent to the violent phreatic explosions. Final collapse through the final stages of pit formation. Although the largest of the walls of the main central pit, where large clasts accumulated after blocks, up to 2 m diameter, land within a few meters of the the eruption, may have removed evidence of drain-back. A model for vent, smaller blocks (up to ~20 cm) are thrown as far as 200 m the latest magmatic phase and subsequent phreatic blowouts of the from the vent, while finer ash is blown downwind and deposited Kings Bowl system can be summarized as follows based on the preced- on the east side of the fissure. ing discussion points. (4) Final draining of the lava lake, either through breached levees or into the fissure causes it to deflate to a lower elevation, leaving 5. Conceptual model slabby lava mounds and lake levees as a record of the high lake level. Explosion pits continue to collapse to their final form, per- Multiple early-stage fire-fountain eruptions led to spatter-fed lava haps with concomitant final phreatic blasts. flows along the King Bowl fissure system and the build-up of spatter ramparts and small cones. Continued eruptions contributed to filling a shallow basin with lava up to ~10 m thick as indicated by the elevations of self-impoundment lava levees and remnant mounds of original lake 5.1. Implications for basaltic fissure eruptions crust. These features represent a higher level of the lava lake prior to deflation to its current level. Magma supply during the growth of the cen- The Kings Bowl conceptual model is highly relevant to the broader tral lava lake (and other smaller ponds) was high enough to maintain a range of low-volume, monogenetic basaltic fissure eruptions, which pressurized interior and cause breakouts of lava through levees that are common in volcanic fields globally. Two important attributes of contributed to outflow lobes. Magma supply was cut off, perhaps due the Kings Bowl eruption have significant implications not only for basal- to lateral flow of magma feeding the nearby contemporaneous Wapi tic fissures, but also for volcanology in general. (1) The eruption was a eruption. Rapid loss of magma supply led to the collapse of fissure relatively low-volume, short-lived episode contemporaneous with a walls enabling groundwater influx that triggered multiple steam blasts more extensive low shield eruption at Wapi, but spatially and chemi- along the fissure and at least 500 m along the central portion near the cally distinct from the more voluminous vent system. (2) When main pit. magma supply was withdrawn, it culminated in a series of phreatic ex- Initial blasts occurred while lake pressure was still high enough in plosions that blasted out non-juvenile clasts. Except for a relatively places to produce squeeze-ups when the congealed crust was pene- meager amount of fine ash and lapilli (some early stage accumulation trated by ejecta blocks. Lava drain-back from the lake is clearly evident of accretionary lapilli recognized in a thin, localized intercalated ash along narrow fissure segments, but it was initially inhibited in the area layer) blown downwind, field lithologic evidence suggests that the of squeeze-ups, most likely by blockage due to solidified parts of the tephra was essentially fragments of solid material devoid of primary lake, or congealed spatter or dikes. Spatter ramparts and cone remnants scoria. Phreatic blasts were “mostly dry” in the sense that there was are preserved north and south of the main pit, and other places along no muddy or pasty commingling of magma and water. the fissure; and dikes frozen in place are observed at the north end of The first attribute is appropriate to many fissure eruptions, espe- the main pit (Greeley et al., 1977; Hughes et al., 1999). The Kings cially those along radial or circumferential volcanic rift zones that Bowl pit crater exemplifies processes of a small, but highly energetic occur on the flanks of large shield volcanoes. These often occur where eruption that ejected blocks up to 2 m strewn over 200 m onto the the lava plumbing system has dramatic differences in elevation along lava lake surface. The lava lake subsided as much as 5 m as the molten its length. Preservation of spatter ramparts and remnants of the fissure interior either flowed outward through breached levees or drained itself certainly requires a low-volume or waning eruption, where back into the fissure system. Smaller pits with adjacent block deposits magma supply is cut off before these features can be obliterated by a indicate that this sequence, albeit without the growth of a substantial subsequent low shield-building phase. The global presence of numerous lava lake, occurred at numerous places along the fissure system. The remnant eruptive fissures and spatter ramparts within major volcanic current state of the fissure is an open rift, with groundwater levels systems attests to this attribute as a common occurrence. below the accumulated solid debris in most of the fissure, and is season- A well-documented example is the Kamoamoa fissure eruption in ally exposed in the deeper parts of the main pit (~30 m depth). March 2011 on Kīlauea's East Rift Zone, which lasted only a few days. Our conceptual model of the Kings Bowl eruptive sequence and pit The breakout occurred during the ongoing 1983 – present eruption of crater formation (Fig. 10) schematically summarizes our interpretation nearby Pu'u 'O'o after the main crater floor collapsed and magma of the events that occurred over what was possibly a short time period, dropped 115 meters (Orr et al., 2012). Similar to previous fissure erup- perhaps no longer than several hours or days. The main events include: tions of Kīlauea's East Rift Zone, the Kamoamoa event diverted lava 102 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104

Fig. 10. Conceptual model of Kings Bowl eruptive history. The entire series of events possibly occurred over several hours or days. See text for detailed description. supply, resulting in a hiatus in Pu'u 'O'o activity. The diversion was only juvenile component as fragmented scoria is incorporated into the blasts temporary, however, and thus produced a low-volume, short-lived (Wohletz and Sheridan, 1983; Francis and Oppenheimer, 2004,p.325– eruption. 327). Phreatomagmatic eruptions are typified by varying amounts of ju- The second attribute is apparently less prevalent in fissure eruptions, venile magma that is mixed with groundwater or shallow surface water although phreatic ultravulcanian explosions occur as precursor conduit- (Wohletz and Sheridan, 1983; Heiken and Wohletz, 1985; Carey et al., clearing eruptions of central vent composite cones. In contrast to the 2007), which in turn produces observed variability in juvenile vs. non- Kings Bowl phreatic explosions, the interaction of basaltic magma and juvenile components of the tephra. Well-studied examples are distrib- water during fissure eruptions more often results in explosive activity uted globally, including Diamond Head tuff ring and Koko Crater tuff that produces maars, tuff rings or tuff cones, which involve a significant cone in Hawai'i, USA; Hole-in-the-Ground maar and Fort Rock tuff S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 103 ring in Oregon, USA; Menan Buttes tuff cones in Idaho, USA; Crater elongate, i.e., those that represent close spacing of blasts along some Elegante and Cerro Colorado in Sonora, Mexico; Taal volcano, Luzon in parts of the fissure. Essentially all ejecta fragments were non-juvenile, the Philippines; Surtsey Island, in Iceland and many more (e.g., made up of previously solidified lava from the Kings Bowl fissure erup- Wohletz and Sheridan, 1983; Wood and Kienle, 1992). Extraterrestrial tion and at least one earlier basaltic sequence, the pre-Kings Bowl lava, examples on Mars also provide important implications of the availabil- that erupted from a nearby vent. ity of water during fissure eruptions (e.g., Keszthelyi et al., 2010; Brož The preservation of spatter ramparts, open chasms and other fea- and Hauber, 2013). tures related to basaltic fissure eruptions requires a drop-off in magma Phreatic explosions that are devoid of juvenile scoria thus require a supply, which aborts the potential construction of a low shield or lava mechanism that enables the fragmentation of existing solid rock by field that might bury or obliterate the fissure. Although the causes for steam without fresh magma being ejected. This apparently happened, magma withdrawal vary between volcanic fields, the dominant mecha- at least in part, during the complex 1886 eruption of Tarawera, New nism at Kings Bowl, was most likely related to contemporaneous erup- Zealand (Cole, 1970; Williams and McBirney, 1979, p. 258–260), but tion of the Wapi shield along other parts of the same fissure system at magma withdrawal was not recognized and the material ejected was the south end of the Great Rift. Geochemical evidence does not support entirely different. Perhaps a more appropriate event to compare the the notion that the erupted lavas for these systems were equivalent al- Kings Bowl eruption of large non-juvenile blocks to block-ejecting phre- though they most likely share a highly complex system of magmatic atic explosions in basaltic systems occurred during the 1924 deadly ex- reservoirs, including feeder dikes and shallow sills, along the rift. Two plosion of Halema'uma'u pit crater in Kīlauea caldera. An explosive processes were required for pit crater formation at Kings Bowl: (1) eruption had occurred before (~1790) and Kīlauea has erupted in simi- the drop in magma supply, which resulted in magma withdrawal and lar style since. Detailed analyses and summaries of the 1924 eruption fissure wall collapse, and (2) the concomitant influx of groundwater. (e.g., Jaggar and Finch, 1924; Finch, 1947; Macdonald, 1972, p. 242– These two attributes were necessary processes that led to the formation 245; Dvorak, 1992; HVO website https://hvo.wr.usgs.gov/kilauea/ of Kings Bowl and other pits by phreatic explosions along the fissure history/1924May18/) indicate that magma withdrawal initially down system. We suggest that basaltic fissure eruptions in general are more N120 m from the rim (and more as the eruption continued) led to the likely to include phreatic explosions when these two conditions are collapse of crater walls, influx of groundwater, and crater volume met. expansion. Supplementary data to this article can be found online at https://doi. Magma withdrawal at Halema'uma'u pit crater was coincident with org/10.1016/j.jvolgeores.2018.01.001. increased seismic activity along the East Rift Zone as magma migrated away from the summit region. The collapse allowed groundwater infiltration leading to the rapid production of steam in a series of phreatic Acknowledgements explosions. Labeled an ultravulcanian eruption, no juvenile material was involved, only accessory blocks and dust. Numerous blocks ranging The Kings Bowl fissure system, with the encompassing lava field, is up to a meter or more in size were blasted from the pit, including a 14- one of the primary targets of NASA project FINESSE (Field Investigations ton block, ejected from over 400 m depth that landed on the crater rim, to Enable Solar System Science and Exploration). Support for this pro- and an 8-ton block that landed nearly a km from the crater rim (Finch, ject was provided by FINESSE, NASA grant number NNX14AG35A to 1947). The proposed mechanics of the explosive eruption (e.g., Finch, Jennifer L. Heldmann, PI, through the Solar System Exploration Research 1947; Macdonald, 1972, p. 245) lean heavily towards the cause of the Virtual Institute (SSERVI). The authors thank all FINESSE team members explosions being repeated surges of groundwater into the conduit, for assistance in field work and data compilation. Special thanks are draining from surrounding rocks, as opposed to the build-up of pressure given to Hazel Sears, J.R. Skok and Rick Elphic for field data acquisition, due to vent plugging. These accounts, and others at Kīlauea, attest to the and to Mike Downs, Juan Busto and their communications team from capability of phreatic explosions to expel blocks similar to those at Kings the Kennedy Space Center for ongoing efforts to provide high-resolution Bowl with sufficient ballistic trajectories and serve as reminders of the UAV imagery and DTM production. Discussions with Chuck Conner and potential for explosive hazards posed by small fissure eruptions. Paul Wetmore during field outings, and two anonymous reviewers of an earlier version of the manuscript, were especially useful in formulating 6. Conclusions arguments presented here.

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