Cycles of explosive and effusive eruptions at Kı¯lauea Volcano, Hawai‘i Donald A. Swanson1, Timothy R. Rose2, Adonara E. Mucek3, Michael O. Garcia3, Richard S. Fiske2, and Larry G. Mastin4 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718, USA 2Department 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 4U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98683, USA ABSTRACT Sites of dated flows 155.25o W 155o W The subaerial eruptive activity at Kīlauea Volcano (Hawai‘i) for 1500-1800 CE PACIFIC the past 2500 yr can be divided into 3 dominantly effusive and 2 domi- 1000-1500 CE A OCEAN O nantly explosive periods, each lasting several centuries. The prevail- 200 BCE-1000 CE >200 BCE L ing style of eruption for 60% of this time was explosive, manifested UNA A by repeated phreatic and phreatomagmatic activity in a deep summit 19.5o N F M caldera. During dominantly explosive periods, the magma supply rate O to the shallow storage volume beneath the summit dropped to only a ZONE LOPE few percent of that during mainly effusive periods. The frequency and S KC `Ailā`au flow RIFT field EAST duration of explosive activity are contrary to the popular impression PO ZONE that Kīlauea is almost unceasingly effusive. Explosive activity appar- MU 0 10 RIFT ently correlates with the presence of a caldera intersecting the water Kilometers table. The decrease in magma supply rate may result in caldera col- KN Ages of map units lapse, because erupted or intruded magma is not replaced. Glasses o 19.25 N with unusually high MgO, TiO , and K O compositions occur only in OUTHWEST post-1800 CE 2 2 S 15th century explosive tephra (and one related lava fl ow) and are consistent with 1000-1500 CE disruption of the shallow reservoir complex during caldera formation. PACIFIC OCEAN pre-1000(?) CE Modified from Wolfe and Kīlauea is a complex, modulated system in which melting rate, supply 155.25o W Morris (1996) rate, conduit stability (in both mantle and crust), reservoir geometry, water table, and many other factors interact with one another. The Figure 1. Locations of all 14C-dated lava fl ow samples on Kīlauea Volcano (Hawai‘i), color coded by time periods discussed in text. hazards associated with explosive activity at Kīlauea’s summit would Data are available in Table DR1 (see footnote 1). Two sample loca- have major impact on local society if a future dominantly explosive tions north of Kīlauea Caldera (KC) with ages older than 200 BCE are period were to last several centuries. The association of lowered shown beyond the limit of Kīlauea, because the dated fl ows are in magma supply, caldera formation, and explosive activity might char- the subsurface overlain by tephra. Map colors indicate general ages acterize other basaltic volcanoes, but has not been recognized. of lava fl ows compiled from map units of Wolfe and Morris (1996), assuming that their unit p4o is entirely younger than 1000 CE. All fl ows younger than 1800 CE were recorded during or shortly after INTRODUCTION the eruption. The 15th century ‘Ailā‘au fl ow fi eld is shown separately Kīlauea (Hawai‘i) is an iconic effusive volcano, known for its lava to emphasize its large size; samples along its margin date the fl ow fl ows and high fountains. Approximately 17% (250 km2) of the volca- fi eld (Clague et al., 1999). Other fl ow fi elds: PO—Pu‘u ‘Ō‘ō fl ow fi eld no’s subaerial fl anks has been resurfaced by lava fl ows in the past 200 yr (1983–present), which has enlarged somewhat from depiction of Wolfe and Morris (1996) used here; MU—Mauna Ulu fl ow fi eld (1969– (Fig. 1), and the ongoing Pu‘u ‘Ō‘ō eruption on the east rift zone, nearly 74); KN—Kīpuka Nēnē fl ow fi eld (200–300 BCE). continuous since 1983, covered >125 km2 with >4 km3 of lava by 2014. Our analysis of Kīlauea’s past 2500 yr shows, however, that explo- sive eruptions were dominant for periods lasting several centuries, not just bles DR2 and DR3 in the GSA Data Repository1; Stuiver and Reimer, brief diversions at an otherwise effusive volcano. We fi nd that Kīlauea has 1993; Reimer et al., 2004) defi ne the length of each explosive period, as been in a dominantly explosive mode ~60% of the past 2500 yr. The ef- interpreted in Fiske et al. (2009) and Swanson et al. (2012a). fusive style of the past 200 yr is, from that perspective, misleading. The Uwēkahuna Ash contains deposits of explosive eruptions be- For this paper we distinguish lava fountains, which at Kīlauea in- tween ca. 200 BCE and 1000 CE (Fiske et al., 2009). Only three lava variably feed lava fl ows and contain only juvenile components, from ex- fl ows have been found interbedded with the Uwēkahuna Ash. Two are plosive eruptions, which do not feed lava fl ows and have at least some south of Kīlauea Caldera; a third is interleaved with tephra low on the lithic components. In this usage, most of Kīlauea’s explosive eruptions are caldera wall and may correlate paleomagnetically with one of the other phreatomagmatic or phreatic, though some may be products of overpres- fl ows (Fiske et al., 2009). The Keanakāko‘i Tephra Member was produced surized magmatic gas. between ca. 1500 and 1800 CE (Swanson et al., 2012a). Only one lava Kīlauea had many explosive eruptions older than those discussed fl ow was erupted at Kīlauea’s summit during that time, from the outermost here (Easton, 1987), but details are lacking. We deal with only the past ring fault bounding the south caldera. Thus, for 2 periods of time lasting 2500 yr, for which many ages and stratigraphic controls are available, 1200 yr and 300 yr, Kīlauea’s summit, normally the site of frequent lava and examine only periods lasting centuries, not short-term events of sev- fl ows (Holcomb, 1987; Neal and Lockwood, 2003), had little effusive ac- eral years or less. TWO DOMINANTLY EXPLOSIVE PERIODS 1GSA Data Repository item 2014233, Tables DR1–DR3 and Figure DR1, showing all 14C ages of lava fl ows and tephra and their calendar-calibrated ages, Recent studies indicate two long periods of time during which ex- and Tables DR4 and DR5, presenting chemical data, is available online at www plosive activity dominated the summit region and adjacent south slope .geosociety.org/pubs/ft2014.htm, or on request from [email protected] or of Kīlauea (Fig. 2A). More than 140 calendar-calibrated 14C ages (Ta- Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, July 2014; v. 42; no. 7; p. 631–634; Data Repository item 2014233 | doi:10.1130/G35701.1 | Published online 22 May 2014 GEOLOGY© 2014 Geological | July Society2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 631 Figure 2. A: Histogram 100 A 97 (Clague et al., 1995). We discuss only the subaerial edifi ce in this paper, showing number (No.) but suspect that our conclusions apply to the Puna Ridge. of tephra ages, prepared 50 35 from Tables DR2 and DR3 9 Acknowledging these caveats, we think that the clear pattern for the No. ages of tephra 1 0 summit area holds for the entire subaerial edifi ce. We interpret the sub- (see footnote 1). B: Histo- B gram showing number of 12 28 aerial volcano to have undergone alternating periods of mostly explosive different dated lava fl ows 8 and mostly effusive eruptions for the past 2500 yr. Successive periods con- per century in each erup- stitute explosive-effusive cycles of varying duration. tive period. Numbers indi- 4 39 cate how many different 10 10 7 per century No. dated flows fl ows were dated per pe- 0 C MAGMA SUPPLY DROPS DURING PERIODS OF MAINLY riod. From 1800 CE, only 10 EXPLOSIVE ACTIVITY ) fl ows outside the caldera 3 How do eruptive volumes and rates of magma supply compare be- are counted, because in- 8 tween the explosive and effusive periods? The volume of magma erupted tracaldera fl ows are not 6 recognizable for earlier during periods of dominantly explosive activity is far less than that dur- periods. Analytical data 4 ing effusive periods (Fig. 2C), and the calculated magma supply rate is Volume (km and original fi gure from 2 correspondingly lower, only 1%–2% of the effusive rate (Table 1). Our which histogram was pre- UA KT estimates of fl ow volumes (Table 1; Fig. 2C) are compromised by variable pared are in Table DR1 0 -500 0500 1000 1500 2000 fl ow thickness and coverage by later fl ows. A simple comparison of tephra and Figure DR1; entire Calendar year (BCE negative) calendar ranges were and fl ow thickness at the summit area, however, illustrates the disparity used except where constrained by stratigraphy. C: Histogram of esti- between effusive and explosive volumes. 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—Uwēkahuna Ash; KT—Keanakāko‘i TABLE 1. ESTIMATED VOLUME OF LAVA ERUPTED ON SUBAERIAL KĪLAUEA Tephra Member. DURING DOMINANTLY EFFUSIVE AND DOMINANTLY EXPLOSIVE PERIODS Calendar age Volume Magma supply rate Dominant style range (km3) (km3/yr*) tivity. Instead, almost all summit eruptions were explosive: sporadic, vio- 500–200 BCE >0.6 not calculated Effusive lent, and brief.