Breakouts in a cooling-limited rhyolite lava flow The origin and evolution of breakouts in a cooling-limited rhyolite lava flow Nathan Magnall1,†, Mike R. James1, Hugh Tuffen1, Charlotte Vye-Brown2, C. Ian Schipper3, Jonathan M. Castro4, and Ashley Gerard Davies5 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 2The Lyell Centre, British Geological Survey, Edinburgh EH14 4AP, UK 3School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand 4Institute of Geosciences, University of Mainz, Becherweg 21, Mainz D-55099, Germany 5Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA ABSTRACT and inflation. The extended nature of some sessment, as well as for interpreting prehistoric breakouts highlights the role of lava supply deposits, improving our understanding of silicic Understanding lava flow processes is im- under a stationary crust, a process ubiqui- lava emplacement processes may be more press- portant for interpreting existing lavas and tous in inflating basalt lava flows that reflects ing than previously considered. for hazard assessments. Although substantial the presence of thermally preferential path- Lava flows initially advance in the direction of progress has been made for basaltic lavas ways. Textural analyses of the Cordón Caulle steepest gradient and their preliminary paths are our understanding of silicic lava flows has breakouts also emphasize the importance of thus relatively easy to forecast. However, after seen limited recent advance. In particular, late-stage volatile exsolution and vesicula- the lava flow front advance ceases, secondary the formation of lava flow breakouts, which tion within the lava flow. Although breakouts lava flows, or breakouts, can form if effusion represent a characteristic process in cooling- occur across the compositional spectrum of continues. Throughout this study we consider a limited basaltic lavas, but has not been de- lava flows, the greater magma viscosity is breakout to be a new, morphologically distinct scribed in established models of rhyolite em- likely to make late-stage vesiculation much region of lava flow advance, formed from the placement. Using data from the 2011–2012 more important for breakout development core of an otherwise stopped or slowed portion rhyolite eruption of Puyehue-Cordón Caulle, in silicic lavas than in basalts. Such late-stage of the lava flow. Such breakouts extend or widen Chile, we develop the first conceptual frame- vesiculation has direct implications for haz- the inundated area and uncertainties in where work to classify breakout types in silicic ards previously recognized from silicic lava they will occur pose a challenge for forecasting lavas, and to describe the processes involved flows, enhancing the likelihood of flow front lava inundation hazard. In basaltic lava flows, in their progressive growth, inflation, and collapse, and explosive decompression of the continued supply of mobile lava to a stalled morphological change. By integrating multi- lava core. flow front leads to inflation, and breakouts oc- scale satellite, field, and textural data from cur through rupturing of the surface crust (e.g., Cordón Caulle, we interpret breakout forma- INTRODUCTION Walker, 1971; Kilburn and Lopes, 1988; Blake tion to be driven by a combination of pres- and Bruno, 2000; Applegarth et al., 2010b). sure increase (from local vesiculation in the Studies of hazards from silicic volcanic Flow lengthening can be substantial when such lava flow core, as well as from continued sup- eruptions usually focus on explosive activity breakouts are long-lived and fed by thermally ply via extended thermally preferential path- partly because effusive silicic eruptions are rare preferential pathways that mature into lava ways) and a weakening of the surface crust events. However, the emplacement of rhyolite tubes (e.g., Calvari and Pinkerton, 1998). How- through lateral spreading and fracturing. lava flows has been previously associated with ever, breakouts in silicic lava flows have only Small breakouts, potentially resulting more hazards such as explosions from their surface been identified from a collapsed portion of a from local vesiculation than from continued (Jensen, 1993; Castro et al., 2002), lava flow dacite lava flow margin on Santiaguito volcano, magma supply, show a domed morphology, front collapse (Fink and Manley, 1987; Baum Guatemala (Harris and Flynn, 2002; Harris developing into petaloid as inflation increas- et al., 1989), and the potential generation of et al., 2004). Consequently, little is known about ingly fractures the surface crust. Continued pyroclastic density currents (Fink and Kieffer, breakouts in silicic lavas. Here, we present the growth and fracturing results in a rubbly 1993). Of six such effusive eruptions in the first conceptual model for their formation and morphology, with the most inflated break- 20th and 21st century (Katsui and Katz, 1967; consider the processes involved. outs developing into a cleft-split morphology, Reynolds et al., 1980; Fierstein and Hildreth, Previous studies of silicic lavas have inter- reminiscent of tumulus inflation structures 1992; Singer et al., 2008; Bernstein et al., 2013; preted their emplacement in terms of the slow seen in basalts. These distinct morphologi- Tuffen et al., 2013), three occurred at Puyehue- spreading of crystal-poor domes, e.g., for Holo- cal classes result from the evolving relative Cordón Caulle, southern Chile (in 1921–1922, cene rhyolite lava flows in the western USA contributions of continued breakout advance 1960, and 2011–2012; Katsui and Katz, 1967; (Fink, 1980a, 1980b, 1993; DeGroat-Nelson Lara et al., 2004; Singer et al., 2008; Tuffen et al., 2001) and older rhyolites in Australia †n.magnall@ lancaster .ac.uk et al., 2013). Thus, for contemporary hazard as- (Dadd, 1992; Smith and Houston, 1995) and GSA Bulletin; January/February 2019; v. 131; no. 1/2; p. 137–154; https://doi .org /10 .1130 /B31931 .1 ; 13 figures; 2 tables; Data Repository item 2018238 ; published online 15 August 2018. Geological Society of America Bulletin, v. 131, no. 1/2 137 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/4604454/137.pdf by British Geological Survey user on 01 February 2019 Magnall et al. New Zealand (Dadd, 1992; Stevenson et al., features such as pumice diapirs (Fink, 1980b; processes in a cooling-limited rhyolite lava flow. 1994a, 1994b). As a lava flow advances, cooled Fink and Manley, 1987) have been previously Here, we use satellite and field observations to surface fragments cascade down the lava flow observed in rhyolite lava flows, breakout pro- understand the processes occurring prior to, and front and are overridden by the core in a tank- cesses have not been discussed for rhyolite lava during, breakout formation. Microtextural char- tread-style motion typical of block and ‘a’ā lava flows. Nevertheless, aspects of these processes acteristics from thin sections and synchrotron flows (Fink, 1983; Harris et al., 2004). Rhyolite could be linked; pumice diapirs are inferred to computed tomography help constrain crystalli- flows form a cooled surface crust that buckles be caused by vesiculation of the flow core and zation and vesiculation histories, allowing us to into ogives as the lava flow front slows (Fink, buoyant rise to the surface, and vesiculation create a conceptual model for breakout forma- 1980a; Castro and Cashman, 1999). The crust may also be important in breakout formation. tion in a silicic lava flow. insulates the lava flow core, potentially main- Advances in our understanding of the em- taining core mobility long after effusion ceases placement of silicic lava flows have been ham- THE 2011–2012 ERUPTION OF (Manley, 1992; Farquharson et al., 2015). Just pered by their infrequency. The 2011–2012 PUYEHUE-CORDÓN CAULLE as for basalts, the crust can therefore exert a rhyolite eruption of Puyehue-Cordón Caulle controlling influence on lava flow advance and in Chile (69.8–70.1 wt% SiO2; Castro et al., Puyehue-Cordón Caulle is a basaltic to rhyo- emplacement (Magnall et al., 2017), making 2013) enabled some of the first detailed scien- lite volcanic complex located in the southern the formation of breakouts a possibility. Al- tific observations of emplacement processes. volcanic zone of the Chilean Andes (Fig. 1). though a lobate lava flow front (Bonnichsen and This lava flow produced numerous breakouts The 2011–2012 eruption began on 4 June Kauffman, 1987; Manley, 1996) and late-stage and provides a new resource to assess breakout 2011, producing a 15-km-high Plinian ash col- A W70˚ W60˚ S30˚ Northern N Chile Argentina branch S30˚ Puyehue-Cordón Caulle Northeast breakout S40˚ Puerto Montt S40˚ Figure 1. Lava flow outlines of the 2011–2012 Cordón Caulle Vent lava flow, southern Chile derived from Landsat 5 and Earth Ob- S50˚ serving (EO-1) Advanced Land Imager (ALI) satellite images S50˚ (NASA, 2011c, 2011b, 2011a, 2011e, 2011d, 2012, 2013), data W80˚ W70˚ W60˚ W50˚ available from the U.S. Geologi- cal Survey. (A) Position of the 26/Jun/2011 lava flow front through time and Southern 31/Jul/2011 the timing of breakout forma- branch 09/Oct/2011 tion. (B) Landsat 5 image of the 04/Nov/2011 initial lava effusion. The red line 23/Dec/2011 trending E-W is likely a satura- 26/Jan/2012 tion effect/anomaly. (C) EO-1 1 km 13/Jan/2013 ALI image of the initial break- out formation. (D) EO-1 ALI B CD image showing continued break- out formation. A number of separated lava flow outlines can be found in Data Repository file 1 (see footnote 1). Breakouts 26/Jun/2011 1 km 04/Nov/2011 26/Jan/2012 138 Geological Society of America Bulletin, v. 131, no. 1/2 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/4604454/137.pdf by British Geological Survey user on 01 February 2019 Breakouts in a cooling-limited rhyolite lava flow umn (Castro et al., 2013), followed by coupled imagery was also used.