Comm. No. 2019-12 John Olsen Testimony 8.6.19

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.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. REFERENCES CITED' Bakcr, S., and Amehmg, F., 2012, Top-down inflation and deflation at the summit of Kilauea Volcano, Hawai'i observed with inSAR: Journal of Geophysical Research, v. 117, B 12406, doi:10.1029/201IJB0f)9123. Chen, C. -Y., Frev, F.A., Rhodes, J.M., and Easton, R.M., 1996, Temporal geo- chemical evolution of Kilauea Volcano: Comparison of Hilina and Puna Basalt, in Basu, A., and Hart, S., eds., Earth processes: Reading the isotopic 633