Journal of Volcanology and Geothermal Research 351 (2018) 89–104 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores 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 subordi- nate 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 https://doi.org/10.1016/j.jvolgeores.2018.01.001 0377-0273/© 2018 Elsevier B.V. All rights reserved. 90 S.S. Hughes et al. / Journal of Volcanology and Geothermal Research 351 (2018) 89–104 (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.