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HomeMy WebLinkAboutComm. No. 2019-12 John Olsen Testimony 8.6.19F Comm. No. 2019-12 •, Cycles of explbsive and effusive eruptions at Kilauea Volcano,, Hawaii Donald A. Swanson', Timothy R. Rose2, Adonara E. Mucek3, Michael O. Garcia3, Richard S. Fiske2, and Larry G. Mastin4 'U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718, USA 'Department of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, D.C. 20013, USA 3Department of Geology and Geophysics, SOEST (School of Ocean and Earth Science and Technology), University of Hawai'i, Honolulu, Hawai'i 96822, USA 'U.S. Geological Suivey, Cascades Volcano Observatory, Vancouver, Washington 98683, USA ABSTRACT The subaerial eruptive activity at Kilauea Volcano (Haivai`i) for the past 2500 yr cAm be divided into 3 dominantly effusive and 2 domi- nantly explosive periods, each lasting several centuries. The prevail- ing; style of eruption for 60%, of this time was explosive, manifested by repeated phreatic and phreatomagmatic activity in a deep summit caldera. During dominantly explosive periods, the magma supply rate to the shallow storage volume beneath the summit dropped to only a few percent of that during mainly effusive periods. The frequency and duration of cTplosive activity are contrary to the popular impression that Kilauea is almost unceasingly effusive. Explosive activity appar- ently correlates Kith the presence of a caldera intersecting the water table. The'decrease in magma supply rate may result in caldera col- lapse, because erupted or intruded magma is not replaced. Glasses -with unusually high MgO, TiOs, and KBO compositions occur only in explosive tephra (and one related lava flow) and are consistent with disruption of the shallow reservoir complex during caldera formation. Kilauea is a complex, modulated system in which melting rate, supply rate, conduit stability (in both mande and crust), reservoir geometry, water table, and many other factors interact with one another. The hdaards associated with explosive activity at Klauea's summit would have major impact on local society if a future dominantly explosive period were to Iast several cenhn-ics. The association of lowered magma supply, caldera formation, and explosive activity might char- acterize other basaltic volcanoes, but has not been recognized. INTRODUCTION Kilauea (Hawai'i) is an iconic effusive volcano, known for its lava flows and high fountains. Approximately 17% (250 km') of the volca- no's subaerial narks has been resurfaced by lava flows in the past ?QO yr (Fig. 1). and the ongoing Pu'u '0'6 eruption on the east rift zone, nearly continuous since 1983, covered >125 km= with >4 km' of lava by 2014. Our analysis of Klauea's past 2500 yr shows, however, that explo- sive eruptions were dominant for periods lasting several centuries, not just brief diversions at an otherwise effusive volcano. We find that Kilauea has been in a dominantly explosive mode –60% of the past 2500 yr. The ef- fusive style of the p ;tit 200 yr is, from that perspective, misleading. For this paper we distinguish lava fountains, which at Kilauea in- variably feed lava flows and contain only juvenile components, from ex- plosive eruptions, which do not feed. lava flows and have at least some lithic components. In this usage. most of Kilauea'S explosive eruptions are phreatomagmatic or phreatic, though some may be products of overpres- surized may=ntatic gats. Kilauea had many explosive eruptions older than chose discussed here (Easton, 1987), but details are lacking. We deal with only the past 2500 yr. for which many ages and stratigraphic. controls are available, and examine only periods lasting centuries, not short-term events of sev- eral years or less. TWO DONIINANTLY EXPLOSIVE, PERIODS Recent studies indicate two long periods of time during which ex- plosive activity dominated ,the summit region and adjacent south slope of Kilauea (Fig. 2A). More than 140'calendar-calibrated "C ages (Ta- Sites of dated flows 155.25•W 1 0 1500-180Q CE PACIFIC • 1000-1500 CE o P, OCEAN • 200 BCE -1000 CE V • >200 BCE P J P 9.5"N of oe P 1P tcilometers KN Ages of map units 19x5' W Post -1800 CE 10 15th century cE PACIFIC OCEAN = pOre-01000('% Mad rnd from .r to arrcr 155.25° W W". 2�J6 , Figure 1. Locations of all 14C -dated lava flow samples on Kuauea Volcano (Hawai'f), color coded by time periods discussed in text. Data are available in Table DRi (see footnote 1). Two sample loca- tions north of Kilauea Caldera (KC) with ages older than 200 BCE are shown beyond the limit of Kilauea, because the dated flows are in the subsurface overlain by tephra. Map colors indicate gene.. A ..ges of lava flows compiled from map units of Wolfe and V ris (1996), assuming that their unit poo is entirely younger than iJO0 CE. All flows younger than 1800 CE were recorded during or shortly after the eruption. The 151h century'Aili'au flow field is shown separately to emphasize its large size; samples along Its margin date the flow field (Clague et al., 1999). Other flow fields: PO—Pu'u 'O'6 flow field (1983–presentL which has enlarged somewhat from depiction of Wolfe and Morris (1996) used here; MU—Wuna Ulu flow field (1969- 74); KN—Kipuka Nene flow field (200-300 BCE). bles DR2 and DR3 in the GSA Data Repository'; Stuiver and Reimer. 1993; Reimer et al., 2004) define the length -of each explosive period, as . interpreted in Fiske et al. (2009) and Swanson et al. (2012a). The Uwekahuna Ash contains deposits of explosive eruptions be- tween ca. 200 BCE and 1000 CE (Fiske et al., 2009). Only three lava flows have been, found interbedded with the Uwekahuna Ash. Two are south of Kilauea Caldera; a third is interleaved with tephra low on the caldera wall and may correlate paleomagneticadly with one of the other flows (Fiske et al.; 2009). The Keanakako'i Tephra Member was produced between ca. 1500 and 1800 CE (Swanson et al.. 201.2a). Only one lava flow was erupted at Kilauea's summit during that time, from the outermost ring. fault bounding the south caldera. Thus, for 2 periods of time lasting 1200 yr and 300 yr, 10auea's summit, normally the site of frequent lava . flows (Holcomb, 1987; Neal and Lockwood. 2003), had little effusive ac - 'GSAIData Repository item 2014233, Tables DRI-DR3 and Figure DRi, showing all "C ages of lava flows and tephra and their calendar -calibrated ages, and Tables DR4 and DR5, presenting chemical data, is available online at www .geosociety.org/pubs/ft'-)014.htm, or off request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. rGEOLOGY, July 2014: v. 42; no. 7: p. 631-634; Data Repository item 2014233 I doi:10.1130/G35701 1 1 Published online 22 May 2014 (V2014 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. 631 Figure 2. A: Histogram Volume (km') showing number (No.) N 21 00,L of tephra ages, prepared m a from Tables DR2 and DR3 0o (see footnote 1). B: Histo- 0.3 gram showing number of Explosive different dated lava flows 11 per century in each erup- 260 tive period. Numbers indf- o tate how many different Z flows were dated per pe- 5.5 riod. From 1800 CE, only Effusive flows outside the caldera are counted, because in- ZLC tracaldera flows are not E recognizable for earlier =i periods. Analytical data j and original figure from which histogram was pre- pared are in Table DRi -sco 0 500 '000 1500 2000 and Figure DR11; entire Calendar year (BCE negative) calendar ranges were used except where constrained by stratigraphy. C: Histogram of esti- mated volume (not corrected for pore space) erupted subaerially dur- ing each eruptive period. Gray is dominantly effusive period. Black is dominantly explosive period. UA—Uwekahuna Ash; KT—Keanakako'i Tephra Member. tivity. Instead, almost all summit eruptions were explosive: sporadic, vio- lent, and brief. In contrast. most eruptions during the intervening periods were frequent, effusive, and sometimes lasted for decades. This suggests a cyclicity in summit activity. shifting from mostly effusive to mostly ex- plosive and back again. DISTRIBUTION OF LAVA FLOW AGES Did the rest of the volcano, beyond the extent of marker ash beds for the two dominantly explosive periods, behave similarly? To address this question, we compiled and calendar -calibrated all 93 known r'C ages of 10auca lava flows younger tJhan ca. 500 BCE (only a few ages are older than that -.Table DR 1.1. Figure 2B and Figure DR1 show the distribution of the calendar -calibrated flow ages with time for the past 2500 yr, with the two periods of major tephra production indicated. Lava flow ages cluster in periods between explosive eruptions. This is best shown for the past 1000 yr. Numerous ages plot in the 100G-1500 CE time interval, when the present summit shield was under construction (Holcomb, 1987; Neal and Lockwood. 2003); few are within die 1500-1800 CE period, when the Kea nakako'i tephra was deposited, and numerous lava flows have been observed to form since 1800 (1823 CE is the actual age of the oldest known post-Kcanakiko'i flow). The number of flow ages is small before 1000 CE, but there appears to be an increase in the period 200-500 BCE (Fig. 2A), from 0.8 flows/100 yr to 3.3 flows/100 yr, bracketing the 200 BCE -1000 CE age of the Uwekahuna Ash. Three factors could weaken this pattern. The spatial distribution of lava flow ages is uneven (Fig. 1). The lower east rift zone is underrepre- sented. For example, flows from Heiheiahulu (Holcomb, 1987) and the so-called 1790 flow (Moore and Trusdell, 1991), both ascribed to the 18i1 century, are not dated. Second, young flows cover old ones, so die temporal record becomes obscured with age. Few ages, however, plot in the 1200 -yr -long period of tephra production, in contrast to die many flow ages in the following, much briefer, 500 -yr -long period. Third, flows along die Puna Ridge, the 75 -km -long submarine ex- tension of the east rift zone, etre not dated well enough for our analysis (Smith ct al., 2002). Palagonite rind thicknesses suggest ages for dredged samples of 700-24.000 yr, mostly 2000-7000 yr (Clague et al., 1995). This suggests relatively little eruptive activity during the past 2500 yr. The large flows at the base of the ridge are not young, based on sediment cover (Clague et al., 1995). We discuss only the subaerial edifice in this paper, but suspect that our conclusions apply to the Puna Ridge. Acknowledging these caveats, we think that the clear pattern for the summit area holds for the entire subaerial edifice. We interpret th-_ sub- aerial volcano to have undergone alternating periods of mostly explosive and mostly effusive eruptions for the past 2500 yr. Successive periods con- stitute explosive -effusive cycles of varying duration. MAGMA SUPPLY DROPS DURING PERIODS OF MAINLY EXPLOSIVE ACTIVITY How do eruptive volumes and rates of magma supply compare be- tween the explosive and effusive periods? The volume of magma erupted during periods of dominantly explosive activity is far less than that dur- ing effusive periods (Fig. 2C), and the calculated magma supply rate is correspondingly lower, only 1%-2% of the effusive rate (Table 1). Our estimates of flow volumes (Table 1; Fig. 2C) are compromised by variable flow thickness and coverage by later flows. A simple comparison of tephra and flow thickness at the summit area, however, illustrates the disparity between effusive and explosive volumes. TABLE 1. ESTIMATED VOLUME OF LAVA ERUPTED ON SUBAERIAL KTLAUEA DURING DOMINANTLY EFFUSIVE AND DOMINANTLY EXPLOSIVE PERIODS Calendar age range Volume (km') Magma supply rate (km°/yr•) Dominant style 500r200 BCE >0.6 not calculated Effusive 200 BCE -1000 CE 0.3 2.5 x 10-` Explosive 1000-1500 CE 11 2.2 x 10-2 Effusive 1500-1800 CE 0.15 5 x 10-' Explosive 1600 -present 5.5 2.6 x 10-1 Effusive Note: Effusive volumes estimated using mapped areas of flows and areas projected beneath younger flows, as shown on geologic maps (Wolfe and Morns, 1996; Neal and Lockwood, 2003), assuming an age consistent with 14C data and unit label; thickness was estimated from field observations and topographic gradient. Explosive volumes were estimated from area and average thickness of juvenile tephra. Volumes were not adjusted for vesicularity (lava flows) or pore space (tephra). 'Average supply rate to ground surface for entire period. Not calculated for earli- est period, which began before 500 BCE and so is incomplete. The 140 -m -high wall of Kilauea Caldera is made almost entirely of flows erupted between )000 and 1500 CE (Neal and Lockwood. 2003), when the Observatory shield was built (Holcomb, 1987); its total thick- ness is more, because the base of the shield is covered by caldera fill. In contrast, the maximum exposed thickness of the Keanakako'i tephra is only -11 m (McPhie et al., 1990; Swanson et al., 2012x). The flow thick- ness is several meters thick 5 km southwest (downwind) of the summit, and the tephra is only several centimeters. A similar comparison can be made for the Uwekahuna Ash on Kilauea's south flank. It is at most a few tens of centimeters thick, thin- ning to only a few centimeters at the coastline (Fiske et al.. 2009), but the overlying and underlying flows are each at least several meters thick. The striking difference in erupted volume between periods domi- nated by effusive and explosive activity must reflect a major disruption to the supply system that lasts for centuries. The disruption could take place anywhere between the point of melt accumulation in the mantle and the shallow storage system beneath Krlauea's summit. Perhaps increased magma supply to Mauna Loa volcano causes a drop in supply to Kilauea (Gonneimann et al., 2012). Once magma enters the Kilauea plume, it could be diverted away from the shallow reservoir, perhaps as intrusions into the crust or lower shield (Lin et al., 2014). A subhorizontal mantle pathway of magma transport at -30 km depth, interpreted by Wright and Klein (2006; see also Wolfe et al., 2003), might be a zone within which magma could stall or. -be divertedil Magma probably did not bypass the summit reservoir system and immediately erupt on the Puna Ridge, be - 632 www.gsapubs.org I July 2014 1 GEOLOGY .p cause lava compositions on the ridge are highly fractionated, reflecting shallow storage (Clagrrue et al., 1995). Whatever the cause, a major change in the dynamics of Kilauea's magma supply system is needed to explain the effusive -explosive cyclic behavior. RELATION OF CALDERA TO CYCLES Both explosive periods occurred when a deep caldera indented Kilauea's summit. The Powers caldera existed during due time of depo- sition of the Uwekahuna Ash (Powers, 1948: Holcomb, 1987) and only began filling ca. 1000 CE, as estimated from the age of the oldest flow overlying the Uwekahuna Ash south of the caldera (Fiske et al., 2009). The modem caldera formed ca. 1500 CE, when the Kcanakako'i explo- sive period started, and began to fill ca. 1500 CE (Swanson et al.. 2012x). The phreatomagmatic and phreatic nature of most of the explosive erup- tions suggest~ that the two calderas were often deep, at or below the water table (Mastin, 1997; Mastin et al.. 2004). Today the water table is —615 m below the highest point on the caldera rim, —490 m below the caldera floor (Keller et al., 1979). We think it likely that parts of the floor were at least that deep during the dominantly explosive periods. The water table is unlikely to have been higher during the past few hundred years, to judge from the presence of fresh basalt above, and altered basalt below, today's water tabic (Hurwitz et al., 2002, 2003). -The cause of caldera formation at Kilauea is uncertain. If the caldera collapsed because a large volume of magma was rapidly erupted or in- truded into the east rift zone, as conventionally thought (Holcomb et al., 1988), then why did the average magma supph? rate drop by almost two orders of magnitude and stay low for centuries? No relatively shallow, top-down process in the edifice is likely to cause such a long-term change in behavior. Recognition of eruption cycles at Klauca raises a new possibility. Perhaps caldera formation results from reduction in magma supply to the shallow storage reservoir, a term we use for a configuration of multiple magma storage volumes (Fiske and Kinoshita, 1969; Dawson et al., 2004; Baker and Amelung. 2012: Poland et al., 2014). If magma supply dropped significaritly, eruptions or intrusions might deplete the shallow reservoir, eventually leading to collapse of the overlying edifice. This bottom-up model seems best suited to explain the linked formation of the caldera and the ensuing decline in magma supply. Before each explosive period, an effusive eruption took place that could have depleted the storage reservoir if magma supply were low. Ex- trusion of the 4-6 km' -Ail.`au flow field (Fig. 1) during an —60 yr period (Clague et al., 1999) immediately preceded collapse of the modem calde- ra. The Kpuka NEnE flow field (Fig. 1; Holcomb, 1987) covers more than 130 km' (extrapolating back to its summit source) on Kilauea's south flank and immediately underlies the Uwekahuna Ash. Much of its volume could have flowed into the ocean, given its coastline width of —7 km (Fig. 1). We estimate that at least 0.5 km -'remains on land, and the total volume erupted could be several times larger. An alternative bottom-up model is that an increase in mantle melt- ing rate supplied the large flow fields preceding caldera formation. The increase could have depleted a relatively large volume of its melt compo- nents, so that little magma was available to enter the volcano for several centuries. Both altematives are at odds with the ongoing eruption of Pu'u '0'6, during which the supply rate has generally been —0.12 km' yr'except for a temporary increase in 2003-2007 (Poland et al.. 2012). MgO-RICH AND FRACTIONATED VITRIC TEPHRA Small volumes of MgO-rich juvenile ash occur in both the Uwekahuna Ash and Keanakiko`i tephra. Microprobe glass analyses show that vitric ash with MgO to 1 I wt% occurs at several levels in the Kcanak5ko'i tephra (Martin et al.. 2004: Table DR4), and one thin vitric ash in the Uwekahuna Ash (Fiske et al.. 2009) contains MgO values of GEOLOGY I July 2014 1 Aww.gsapubs.org 12.5 wt% (Table•DR5A; Helz et al., 2014). Such high MgO contents in erupted melt etre unprecedented among summit lava flows at Kilauea; the highest published amount in glass is 1.0.2 wt9'o (Helz et al., 2014). The normal summit glass compositions (6-9 wt% MgO; Garcia et i:, 2003) indicate storage in the shallow reservoir and crystallization of mainly oliv- ine before eruption (Powers, 1955; Wright, 1971). We think it significant that the highest MgO melts were erupted dur- ing explosive, not effusive, periods. We interpret these high Mg0 values to record brief or no storage in a disrupted shallow reservoir not fully recov- ered following caldera collapse. Helz et al. (2014) reached a similar inter- pretation for the Kulanaokuaiki tephra and for the much olderPahalaAsh. The most highly fractionated compositions known at Kilauea's sum- mit also occur in vitric tephra. For example, in the Keanakako'i tephra, thin vitric ash just above layer 6 (McPhie et al., 1990. Swanson et al., 2012a) has only 4.0 wt% MgO and as much as 4.5 wt% TiO, and 1.0 wt% K 2 0 (Table DR4). We interpret such unusual compositions to reflect isolated storage and advanced fractionation of small pockets of magma undisturbed by fresh, mantle -derived magma during low magma supply. A marker bed in the Kulanaokuaiki tephra (unit 2 of Fiske et al., 20093 contains glass with unusually high values of TiO, (>3 wt%) and KO (>0.7 wt%) for its moderately low Mg0 value (6.7 wtO1a; Table DR5B). A temporally associated lava flow, one of two interbedded with the Kulana- okuaiki tephra south of the caldera, has a similar composition (Wolfe and Morris, 1996). This composition does not plot along the typical Kilauea fractionation trend during effusive periods, although similar compositions occur in lava flows earlier in Kilauea's history (Chen et al., 1996; Lipman et al., 2006). HAZARD INIPLICATIONS Previous studies indicate that hazards of explosive eruptions at Kilauea are substantial, including pyroclastic density currents as well as tephra falls and ballistic showers (Decker and Christiansen, 1984; McPhie et al., 1990; Dzurisin et al., 1995; Fiske et al., 2009; Swanson et al., 2012a, 2012b). Our work shows that the hazardous periods last much longer than previously thought. When the next dominantly explosive period begins, society may have to deal with centuries of repeated explosive activity at Kilauea's summit. CONCLUSIONS Recognition of' the explosive -effusive cycle raises far-reaching ques- tions about the d-vnamics of Kilauea volcano. Rather than erupting lava flows almost continuously, Kilauea is instead a more complex, modulated system in which melting rate, supply rate, conduit stability (in both mantle and crust), reservoir geometry, water table, and many other factors inter- act. The explosive -effusive cycle is the net result of this interaction. Future studies approaching Kilauea from a broad perspective are necessary to significantly advance our understanding of one of Earth's most studied volcanoes. Whether unrecognized explosive -effusive cycles occur on oth- er basaltic volcanoes is a topic for future research: ACKNOWLEDGMENTS We thank Dave Clague, Dan Dzurisin, Shaul Hurwitz, and Pete Lipman for insightful manuscript reviews. Tom Wright's comments on a preliminary version were provocative. Dave Sherrod supplied three unpublished 1°C ages. Discussions with Frank Trusdell were valuable. Robin Holcomb proposed eruptive cycles that differ from ours but stimulated our thinking. Garcia and Mucek were supported by National Science Foundation grant EAR -11 18741. This paper is School of Ocean and Earth Science and'rechnology (SOEST) contribution 9115. 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