Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

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Journal of Volcanology and Geothermal Research

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Keanakākoʻi Tephra produced by 300 years of explosive eruptions following collapse of Kīlauea's caldera in about 1500 CE

Donald A. Swanson a,⁎, Timothy R. Rose b, Richard S. Fiske b, John P. McGeehin c a U. S. Geological Survey, Hawaiian Volcano Observatory, PO Box 51, Hawaii National Park, HI 96718, USA b Department of Mineral Sciences, Museum of Natural History, Smithsonian Institution, PO Box 37012, NMNH MRC-119, Washington, DC 20013, USA c U.S Geological Survey, 12201 Sunrise Valley Drive, MS 926A, Reston, VA 20192, USA article info abstract

Article history: The KeanakākoʻiTephraatKīlauea Volcano has previously been interpreted by some as the product of a caldera- Received 22 March 2011 forming eruption in 1790 CE. Our study, however, finds stratigraphic and 14C evidence that the tephra instead Accepted 17 November 2011 results from numerous eruptions throughout a 300-year period between about 1500 and 1800. The stratigraphic Available online 28 November 2011 evidence includes: (1) as many as six pure lithic ash beds interleaved in sand dunes made of earlier Keanakākoʻi vitric ash, (2) three lava flows from Kīlauea and Mauna Loa interbedded with the tephra, (3) buried syneruptive Keywords: cultural structures, (4) numerous intraformational water-cut gullies, and (5) abundant organic layers rich in Kīlauea 14 Keanakākoʻi Tephra charcoal within the tephra section. Interpretation of 97 new accelerator mass spectrometry (AMS) Cages Hawaiian caldera and 4 previous conventional ages suggests that explosive eruptions began in 1470–1510 CE, and that explosive Kīlauea explosive eruption activity continued episodically until the early 1800s, probably with two periods of quiescence lasting several de- 14C age cades. Kīlauea's caldera, rather than forming in 1790, predates the first eruption of the Keanakākoʻi and collapsed in 1470–1510, immediately following, and perhaps causing, the end of the 60-year-long, 4–6km3 ʻAilāʻau eruption from the east side of Kīlauea's summit area. The caldera was several hundred meters deep when the Keanakākoʻi began erupting, consistent with oral tradition, and probably had a volume of 4–6km3.The caldera formed by collapse, but no eruption of lava coincided with its formation. A large volume of magma may have quickly drained from the summit reservoir and intruded into the east rift zone, perhaps in response to a major south-flank slip event, leading to summit collapse. Alternatively, magma may have slowly drained from the reservoir during the prolonged ʻAilāʻau eruption, causing episodic collapses before the final, largest downdrop took place. Two prolonged periods of episodic explosive eruptions are known at Kīlauea, the KeanakākoʻiandtheUwēkahuna Tephra (Fiske et al., 2009), and both occurred when a deep caldera existed, probably with a floor at or below the water table, and external water could readily interact with the magmatic system. The next period of intense explosive activity will probably have to await the drastic deepening of the present caldera (or Halemaʻumaʻu Crater) or the formation of a new caldera. Published by Elsevier B.V.

1. Introduction reported as about 80 (Ellis, 1825), approaching 400 (Desha, 2000, p. 279), almost 800 (Desha, 2000, p. 280), and 5405 (Douglas, 1834). Geologic and radiometric evidence presented here shows that the caldera and tephra are not linked as closely as was once assumed. The caldera formed nonexplosively in about 1470–1510 CE. Explosive erup- “Kilauea…has not always been the tame creature of today.” tions began shortly thereafter and continued intermittently for the (Hitchcock, 1909, p. 167) next 300 years to produce the KeanakākoʻiTephra.Thestratigraphic relation of the oldest tephra to the caldera faults, the 14C age of that The caldera atop Kīlauea Volcano, and the conspicuous tephra tephra, and the physical characteristics of reticulite near the base of blanket that surrounds it, have been linked temporally and genetically the tephra provide the evidence that constrains the age and depth by volcanologists for the past several decades. Both were commonly of collapse. interpreted to have formed in November 1790 (Fornander, 1996, p. 241; Cahill, 1999, p. 69), when an explosive eruption killed a large 2. Definition and general characteristics of Keanakākoʻi Tephra number of people (Swanson and Christiansen, 1973), variously The surface tephra deposit surrounding Kīlauea's caldera is formally ā ʻ ⁎ Corresponding author. Tel.: +1 808 967 8863; fax: +1 808 967 8890. named the Keanak ko i Ash Member of the Puna Basalt (Easton, 1987). E-mail address: [email protected] (D.A. Swanson). The Keanakākoʻi contains prominent deposits coarser than ash, however,

0377-0273/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2011.11.009 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 9 so we modify its formal name to the KeanakākoʻiTephraMember,or Keanakākoʻi consists of well-bedded vitric ash (Fig. 1), generally less Keanakākoʻi for short. vesicular than pumice, which contains several percent of wallrock Various workers have defined the Keanakākoʻi somewhat differ- lithic clasts in all grain sizes. In general, the lithic content is nearly ently. This paper follows the Decker and Christiansen (1984) and constant from bed to bed, but it increases in the upper part of the Easton (1987) definition that includes a bed of reticulite below, and vitric section. Within the vitric section (Fig. 1) is a single bed of of pumice (the “golden pumice” of Sharp et al., 1987) above, the rest crystal-rich scoria less than 50 cm thick, layer 6 of McPhie et al. of the section, which consists of well-bedded vitric and lithic ash, scoria, (1990), that may be the product of a relatively dry explosive eruption, and lithic lapilli and block deposits. These deposits were mostly formed with some involvement of groundwater. Layer 6 is an important by tephra falls, but some result from surges and other pyroclastic marker bed, as shown below. Above a major erosional unconformity density currents (PDCs; Fig. 1). are lithic fall and PDC deposits, including definite surges, ranging in McPhie et al. (1990) give the most detailed field description of the grain size from ash to blocks tens of centimeters in diameter. Ballistic Keanakākoʻi, dividing it into 16 subunits termed layers, and we mostly blocks dot the ground surface, some reaching more than 2 m in diam- follow their terminology. They focused attention on the products of ex- eter (Swanson et al., 2010). The “golden pumice” caps the lithic part plosive eruptions rather than of lava fountains and so did not describe of the section along the western and southwestern sides of the the upper pumice and lower reticulite layers. A gray to pink lithic ash caldera. underlies the reticulite in most places northwest, north, and northeast Near the caldera, the thickness of the Keanakākoʻi section ranges of the caldera (Fig. 1; Stone, 1926, p. 29; Finch, 1942). Though nowhere from about 11 m to only a few tens of centimeters, reflecting the pre- more than a few centimeters thick, this ash nonetheless indicates that vailing NE wind direction, the inferred locations of vents within the explosive activity (possibly a surge and accompanying fall) began caldera, and the height of the caldera wall. Most deposits are primary, a little earlier than the reticulite, and we see no reason to exclude resulting from pyroclastic falls and PDCs, but some deposits were it from the Keanakākoʻi. Consideration of 14C ages later in the paper reworked in places by water and wind, and erosional unconformities supports this decision. are common. In fact, erosion during Keanakākoʻi time removed We agree with past workers that the Keanakākoʻi is the product of several meters of section along master drainages. phreatomagmatic eruptions (McPhie et al., 1990; Mastin, 1997; The only pyroclastic deposits overlying the Keanakākoʻi are lithic Mastin et al., 2004), except for the reticulite and “golden pumice,” ejecta of the 1924 phreatic eruptions from Halemaʻumaʻu Crater which are clearly fallout from high lava fountains. The reticulite oc- (Jaggar and Finch, 1924; Decker and Christiansen, 1984) and several curs completely around the caldera and is an important marker thin or local deposits of vitric ash and Pele's hair from fissure erup- unit. On the west, north, and east sides of the caldera, it is sandwiched tions, mainly in 1959, 1971, and 1974 (Neal and Lockwood, 2002). between two lithic units, the underlying gray to pink ash and the overlying pink Housing Area block and ash fall (described below; equivalent to layers 0 and 5 of McPhie et al., 1990).The bulk of the 3. Previous interpretations of age

Interpretations of the age of the Keanakākoʻi have a long and Lava flows and tephra from complicated history. Dana (1890, p. 43– 45) more or less assumed 19th and 20th centuries that its entire thickness was produced by the 1790 eruption, as Ca. 1820 Golden pumice did Brigham (1909, p. 36),butHitchcock (1909, p. 167–169) clearly Lithic blocks to ash, some with accretionary 1790 lapilli. Falls and PDCs (layers11-16). Capped had reservations. Sidney Powers (1916) divided the section into a by ballistic deposits younger, blocky 1790 deposit and one or more older, finer ash de- posits on the basis of a large erosional unconformity (Fig. 1)and Ca. 1670-1700 Lithic-vitric ash, thickly bedded, possible buried soils. Finch (1925, 1942), Stone (1926), Stearns and some ballistic blocks (layers 8-10) Clark (1930),andWentworth (1938) agreed that deposits of several Lithic-bearing vitric ash, thinly bedded pre-1790 explosive eruptions dominated the Keanakākoʻi, finding (layer7?), capped by incipient soil carbonaceous material and soils as well as other physical differences Ca. 1650 Layer 6 scoria fall below the erosional break at the base of their 1790 deposits. Finch Two lava (1925) even reported molds of trees rooted in ash near the base of flows from the tephra section and buried by younger ash. Finally, Howard Lithic-bearing vitric ash, Mauna Loa thinly bedded. Few erupted Powers (1948) interpreted several “humus” layers and small unconfor- pumice lapilli. Falls and during this mities to suggest at least 9 phreatic and 11 magmatic explosive erup- fine surges (layers 1-4) time period tions older than the 1790 deposit.

Keanakako‘i Tephra With the development of the base surge concept in the late Housing Area blockf all Early 1500s and ash (layers 0 and 5) 1960s, interpretations swung back to assigning all, or almost all, of ā ʻ Ca. 1500 Reticulite the Keanak ko i to the 1790 eruption. Deposits interpreted to have Gray-pink lithic ash been produced by surges were noted in the youngest lithic deposits Ca. 1410- ‘Aila‘au shield and flow 1470 CE (Swanson and Christiansen, 1973), which all previous workers had assigned to the Keanakākoʻi. Thereafter, the entire tephra section Ca. 1000- Observatory shield 1400 CE was reinterpreted to be of 1790 vintage; most of its internal uncon- Ca. 450- Kulanaokuaiki Tephra formities were ascribed to surge erosion, and the overall upward 1000 CE change in componentry of the deposits—from vitric low in the section to almost completely lithic in the upper part of the section—was Fig. 1. Simplified stratigraphic column showing units mentioned in text, not to scale. At base of Keanakākoʻi, gray-pink lithic ash and reticulite rest directly on one of the interpreted as the general evolution of a single eruption (Christiansen, following named units, depending on geographic location: ʻAilāʻau shield and flow 1979; Malin et al., 1983; Decker and Christiansen, 1984; McPhie et al., east and southeast of caldera, Observatory shield (or distal flows from shield) around 1990; Mastin, 1997; Neal and Lockwood, 2002). Uncertainty remained, other sides of caldera, and Kulanaokuaiki Tephra in local areas north, west, and south however, as to whether the eruption lasted only a few days or as of caldera. Lava flows interbedded with Keanakākoʻi are indicated in italics. Numbers long as several months in 1790, primarily because of buried erosion in parentheses indicate layers in McPhie et al. (1990). Only largest erosional uncon- formity is shown; smaller ones occur throughout section, especially separating layers surfaces and different dispersal directions of single layers (McPhie 7and8.AlldatesinCE. et al., 1990). 10 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

Thus the interpretations came full circle in 100 years. Late 20th- 4.2. Interbedded lava flows century workers once again agreed with Dana's late 19th-century view that most or all of the Keanakākoʻi was produced in 1790. Three lava flows are interbedded with the Keanakākoʻi, one erupted We reach a different conclusion, mostly on the basis of new evi- from Kīlauea and two from Mauna Loa. The Kīlauea pāhoehoe flow dence. We confirm the early and mid-20th-century view that only the issued from an arcute fissure with associated spatter rampart along relatively thin lithic deposits above a major erosional unconformity— the southern margin of the caldera (Jaggar, 1926; Stearns and Clark, and probably only a part of those—belong to the 1790 event, and we 1930, p. 150; Finch, 1942). This flow, unit 1790f of Neal and Lockwood find that the rest of the Keanakākoʻi was produced during 300 years (2002), spread down a broad, shallow drainage and several smaller, of sporadic explosive eruptions following the formation of the caldera. vertically walled gullies eroded into vitric ash containing accretionary The rest of this paper discusses the evidence for these interpretations lapilli (Figs. 3, 4). It underlies lithic lapilli and ash in erosional contact and how they bear on the timing and mechanism of caldera collapse. with the vitric tephra. McPhie et al. (1990) place it between layers 10 and 11. 4. Field evidence for breaks in deposition of the Keanakākoʻi One Mauna Loa flow, not previously reported as interbedded in the Keanakākoʻi, occurs on Kapāpala Ranch near ʻŌhaikea (Fig. 3,OH), None of the physical evidence presented below requires long periods 6 km west of Halemaʻumaʻu. Here, the Kīpuka Maunaʻiu pāhoehoe of quiet. The evidence is so abundant and varied, however, that it pro- flow rests on 10–15 cm of fine ash in the Keanakākoʻi, some bearing vides strong circumstantial evidence against all or even most of the accretionary lapilli, and underlies several centimeters of similar ash, Keanakākoʻi being of 1790 age and the product of one eruption. also containing accretionary lapilli. The base of the Keanakākoʻiat this location rests on the older Kulanaokuaiki Tephra (Fiske et al., 4.1. Ash beds in sand dunes 2009). At a nearby locality lacking the lava flow, a 49-cm-thick section of Keanakākoʻi overlies Kulanaokuaiki and contains accretionary lapilli At least 6 thin beds of light gray, very fine-grained, dominantly ash near its top and bottom and an organic-rich brown weathered lithic ash are interbedded with olive-colored, windblown, vitric sand zone 16 cm below the top; the lava flow may have erupted during in numerous dunes 10–15 km southwest of the summit (Fig. 2). The the period of weathering. Charcoal along the base of the flow has ash beds, typically 2–3 cm thick, are separated from one another by been dated and is discussed in a later section. 20 cm to more than 1 m of cross-bedded sand, derived by wind erosion Sidney Powers (1916, p. 232, 238) indicated that another Mauna of medium- and coarse-grained vitric ash in the KeanakākoʻiTephra Loa lava flow, the young Keʻāmoku ʻ aʻā flow (unit m4y of Neal and (Fig. 1,layers1–4). The fine lithic ash in each bed is not mixed with Lockwood, 2002), overlies “the largest part” of the KeanakākoʻiTephra the coarser, vitric dune sand, and the youngest ash contains numerous at an unspecified locality northwest of the caldera. Howard Powers unbroken accretionary lapilli. Less than 10% of juvenile material (1948, p. 284) reported that the reticulite unit low in the Keanakākoʻi (pumice, dense black glass) is included in the otherwise lithic ash. underlies the flow, again without giving a locality. Subsequent workers These features suggest that the ash beds are primary deposits, not failed to find the reticulite under the flow but did identify younger reworked, although the beds are discontinuous across an outcrop Keanakākoʻiashontop(Neal and Lockwood, 2002). From this they in- and truncated by vitric sand, probably owing to subsequent dune ferred that the Keʻāmoku flow predated the entire Keanakākoʻiand activity such as continues today at some dunes. The evidence indicates interpreted the reticulite lapilli to have trickled out of sight between that each ash bed is a separate fall deposit, probably corresponding to therubbleblocksontheʻaʻā flow top. eruptions that created some or all of layers 11–15 of McPhie et al. To resolve this disagreement, we searched for reticulite along the (1990).Itisdifficult to account for the primary lithic fall deposits within poorly exposed base of the flow. One place near Kīpuka Puaulu (Fig. 3, sand dunes made of coarser, reworked Keanakākoʻi vitric ash if all of the KP), 4 km northwest of Halemaūmaʻu, revealed a 5–6-cm-thick pocket Keanakākoʻi had been erupted in only a few days to weeks. of reticulite and an overlying 0–2-cm-thick bed of gray to olive vitric ash (equivalent to layers 1–3ofMcPhie et al., 1990) unequivocally sandwiched between the rubbly base of the Keʻāmoku and the un- derlying pāhoehoe flow. The reticulite rests directly on the pāhoehoe, and the gray to olive vitric ash overlies and filters into the loose reticu- lite deposit. This discovery confirms the early observations of Sidney and Howard Powers, although “the largest part” of the Keanakākoʻiis probably younger than the flow. The young Keʻāmoku flow, dated at another location, has a 14C age consistent with its stratigraphic position (see later section).

4.3. Stone structures built during Keanakākoʻi time

Remnants of several small stone structures made by humans occur several kilometers south of Halemaʻumaʻu; locations are not given here to protect the sites. With archaeologists from Hawaiʻi Volcanoes National Park, the best-preserved structure was excavated in 1998 and found to have been built on a faintly orange ash bed resting on scoria of layer 6 (Fig. 1; McPhie et al., 1990), a prominent marker bed discussed in more detail below. All younger ash fell onto the structure, probably starting with layer 12. The orange color of this particular ash, not reported by McPhie et al. (1990), is widespread Fig. 2. Six beds of fine, gray, lithic ash, each with marker tag, in dune made of darker in this area and is unrelated to fumaroles or other local sources of vitric sand 11 km south–southwest of center of Halemaʻumaʻu(Fig. 3, SD). Many hot gas. It probably developed on layer 7 (Fig. 1) during a period of dunes in this area, and others west of the map area of Fig. 3, contain several fine- weathering when soil began to form and people felt they could return grained lithic ash beds truncated by windblown vitric sand in the dune. Third ash bed from bottom contains abundant unbroken accretionary lapilli. Beds are 2–3cm safely to the area. By comparison, the surface of the nearby 1959 thick. Kīlauea Iki fall deposit is showing faint oxidation in places, some D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 11

Fig. 3. Map showing features mentioned in text and locations of all dated charcoal samples listed by number in Table 1, col. 1. Some locations have two or more samples from different beds containing charcoal. Note how all charcoal locations are in areas that are vegetated today (shaded areas on map); vegetated areas during Keanakākoʻi time were apparently similar. Features mentioned in text: AS, summit of ʻAilāʻau shield; HM, Halemaʻumaʻu; HVO, Hawaiian Volcano Observatory; KC, KeanakākoʻiCrater;KF,Kīlauea lava flow interbedded in Keana- kākoʻi; KI, Kīlauea Iki tephra deposit; KK, Kīpuka Kī;KMC,Kīlauea Military Camp; KP, Kīpuka Puaulu; MLE, Mauna Loa Estates; OH, ʻŌhaikea; SD, Sand dunes with ash layers; SW, Sand Wash; TM, Tree Molds; VV, Volcano Village. Base map from USGS 7 ½-minute quadrangles Kīlauea Crater, Kaʻū Desert, Volcano, and Makaopuhi Crater. Small-scale map shows location of larger map and the 5 volcanoes on the Island of Hawaiʻi.

50 years after the eruption. Perhaps several decades of weathering They are clustered along, but not confined to, the three erosion sur- were needed to produce the weak oxidation of layer 7. faces recognized and clearly described by McPhie et al. (1990): The other structures have not been excavated, but two are so dis- below layer 6, above layer 7, and above layer 10. These gullies were rupted by faulting that it is possible to see beneath them and confirm probably eroded by running water. that each was built at the same stratigraphic level as the excavated Most of the gullies intersect present-day master drainages at high structure. angles, though today they hang on the sides well above the incised modern drainages. Excellent examples are visible along the south- 4.4. Water-cut gullies and preserved master drainages west side of Sand Wash (Figs. 3, 7, 8). The gullies were probably rills draining into master streams that have not changed location (ex- Narrow, steep-sided (locally undercut) channels partly filled by cept for bank erosion) since that time. In other words, today's drain- water-worked detritus and capped by primary tephra are fairly com- age system was largely developed at the time such gullies were cut. mon in the Keanakākoʻi in the southern part of the caldera (Figs. 5, 6). This implies sufficient time between explosive eruptions for gullies Many are small and easily overlooked, but several are as deep as 1 m. to be eroded. 12 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

fi Fig. 4. Pāhoehoe flow interbedded in Keanakākoʻi Tephra 3 km south–southwest of Fig. 6. Vertical wall of gully eroded into vitric ash and lled with lithic lapilli and ash, ʻ ʻ center of Halemaʻumaʻu. Ash beds in distance rest on flow, those near shovel are including two accretionary lapilli ash beds, 2 km southwest of center of Halema uma u. under it.

Keanakākoʻi was erupted. Sand Wash is one of the main drainages Some of the gullies have been modified into U-shapes, possibly by in the southern part of the caldera and may follow an arcuate caldera lithic PDCs. Fig. 4ainMcPhie et al. (1990) shows an example of this fault buried by tephra. process. In some places, remnants of vertical walls, probably cut by water, are preserved at the bottom of the otherwise scoured and filled 4.5. Other erosion surfaces gully (Fig. 7). The axes of other gully fills capped by primary tephra parallel Many small-scale erosion surfaces occur within the Keanakākoʻi. today's master drainages and occur as remnants against the steep Some are probably surge-related, for they occur in discontinuous sides of those streams. Such relations are well exposed just west of ash beds with antidune structures and large-wavelength, undulatory Sand Wash (Fig. 9) and are shown in Fig. 4binMcPhie et al. (1990). upper surfaces. Good examples are in the vitric part of the section in They testify to periods of master stream erosion, partial filling, drap- Sand Wash, probably equivalent to layers 1–4ofMcPhie et al. (1990). ing by later tephra, and renewed stream cutting. Many of the small-scale erosion surfaces are not clearly caused by A prominent erosional unconformity in Sand Wash (Fig. 3) appar- surges, however, and may well record minor water and wind ero- ently separates layers 10 and 11. At least the upper part of the lithic sion—not unexpected in the rainy environment of the caldera. Such tephra above the unconformity was erupted in 1790. The unconfor- small features probably have little time significance. mity extends nearly to the modern floor of the wash and has relief of about 6 m (Fig. 10). The filled paleorills mentioned above mostly 4.6. Organic-rich layers and soil occur along the unconformity (Fig. 7, 8). These relations indicate that Sand Wash was cut before the lithic tephra was deposited. On the wet, heavily vegetated north side of the caldera, Stone Such relief seems far too great to ascribe to PDC erosion. Moreover, (1926, p. 34) found at least four black, carbonaceous layers, each the orientation of the wash, nearly concentric to the caldera and at about 1 cm thick, within the Keanakākoʻi, some containing “remains high angles to PDCs originating at the summit, makes it an unlikely and impressions of ferns and other plants.” Finch (1925) noted candidate for deep PDC erosion. This unconformity is striking physical molds of small trees and branches completely within the tephra dur- evidence of a hiatus in deposition before the upper lithic part of the ing excavation at the current site of the national park's visitor center,

Fig. 7. Filled U-shaped gully with nearly vertical steps on lower left side, interpreted as water-cut rill draining into Sand Wash (Fig. 3, SW) that was modified by one or more pyroclastic density currents. Well-bedded vitric and mixed vitric–lithic ash and fine la- Fig. 5. Small water-cut gully with steep sides eroded through layer 6 scoria and over- pilli (layers 1–4) are below unconformity, and generally coarser lithic ash to blocks lying lithic-bearing vitric ash into vitric ash that underlies layer 6. Gully filled with (layers 11–16) fill gully. Photograph is from floor of Sand Wash. Height of exposure, dominantly lithic lapilli and ash About 2 km southeast of center of Halemaʻumaʻu. 6m. D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 13

seven” levels of revegetation in the Keanakākoʻi, the lower two of which are at stratigraphic positions recognized by our work. Those early workers were present during a period of road and building construction on the wet, north side of the caldera and ob- served many exposures unavailable to us today in the protected confines of the national park. Most modern tephra studies have concentrated on the drier, barren south and southwest sides of the caldera, where organic-rich layers would not be expected and indeed have not been found (Fig. 3). We found charcoal and other carbonaceous layers exposed in pits and along cracks at many places east and southeast of the caldera, and in road cuts and building excavations in Volcano Village (Fig. 3) north of the caldera. One cut along Kalanikoa Road in the village, and sever- al pits east of the caldera, expose at least three such layers, and two are common (Fig. 11). In some places, the ash directly under an organic-rich layer is slightly oxidized and apparently reflects the early stage in develop- ment of a soil in which plants were growing (Fig. 11). The current surface of the 1959 Kīlauea Iki tephra deposit (Fig. 3, KI) has a similar, vaguely orange hue in the wet forest. Several decades may be a rea- sonable time for soils to start forming in this temperate, wet environ- ment lacking much seasonality. One level of common charcoal occurrence is at the base of the Keanakākoʻi section, associated with the gray lithic ash (Fig. 1). Another is in and overlying the reticulite above the gray ash. Still other charcoal comes from a pink, lithic block and lapilli fall on the northeast side of the – Fig. 8. Narrow, subvertical slot or pocket (arrows) eroded into bedded vitric lithic ash, caldera (layer 5 of McPhie et al., 1990). Another charcoal-bearing level filled by lithic ash, and overlain by coarser lithic ash and lapilli in vertical exposure just west of Sand Wash. Such erosional pockets are common at the bottom of modern is just above layers 6 (and 7, if present). Still another occurs higher in gullies cut into fine ash in this area. the section, along the erosional break between layers 10 and 11. Finally, large pieces of charcoal have been found in the youngest lithic block deposit at Kīlauea Military Camp (Fig. 3) in the northern part of the and Stearns and Clark (1930, p. 151) reported that T.A. Jaggar had caldera. seen six “vegetable” layers in the same hole. The organic-rich layers A challenge in correlating specific charcoal-rich layers with the and trees in growth position within the Keanakākoʻi indicate times stratigraphy of McPhie et al. (1990) is that the layers occur in a thickly during which either plants returned following tephra falls or litter ac- forested, poorly exposed area that was also vegetated during Kea- cumulated on the ground surface from plants that survived an erup- nakākoʻi time, whereas the stratigraphy was mostly defined in well tion. Finally, Howard Powers (1948) interpreted “not less than exposed areas with little vegetation then and now (Fig. 3). The

Fig. 9. Beds of lithic ash and lapilli (layers 11–16) unconformably overlying vitric ash and draping into, and partly filling, an old drainage. The old drainage parallels the modern drainage. Lithic stratigraphy in the fill is the same as that capping the ridge at left, including a primary bed of accretionary lapilli ash 3–5 cm thick. This shows that the old drainage was in about the same location as today's, and that it had relief of several meters. Later erosion beveled off the fill in the channel, forming a terrace. About 2.2 km south–southeast of center of Halemaʻumaʻu. 14 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

Fig. 10. View looking southeast down Sand Wash, showing erosional unconformity across layers 1–4 and possibly layer 7, with about 6 m of maximum relief. Unconformity devel- oped before main lithic deposits (layers 11–16) were erupted. Erosional surface dips into modern wash, evidence that paleodrainage was in similar location as today's. About 2 km southwest of center of Halemaʻumaʻu, SW in Fig. 3. distribution of most of the individual stratigraphic units is strongly The charcoal-rich layers indicate breaks in tephra deposition long controlled by the prevailing northeast trade wind and the low height enough for vegetation to return or for litter to form distinct layers. of the south and southwest caldera rim. Thus, many stratigraphic They obviously minimize the possibility that the Keanakākoʻi was en- units defined in the composite section are simply not found where tirely erupted in 1790 or even within a few years at any time. They the organic layers occur, in today's forest east and north of the caldera. also provide a means to quantify the age of various parts of the sec- Until more detailed work has been completed, the precise stratigraphic tion through 14C dating. position of charcoal layers at some sites relative to the McPhie et al. (1990) stratigraphy will remain somewhat uncertain. 5. 14C ages of Keanakākoʻi tephra

Since 1998, 97 new 14C ages have been obtained on Keanakākoʻi tephra and related deposits (Table 1; Fig. 3). These ages have proven crucial for quantifying the age relations within the tephra and between the tephra and its underlying deposits. A large number of analyses is necessary to define the age relations, owing to the youth of the de- posits and the consequent large uncertainties in calibrated calendar ages (Guilderson et al., 2005). Four conventional ages from Rubin et al. (1987, Table 10.1, lab numbers W3871, W4006, W4919, and W5106) help to define the ages of the two Mauna Loa flows interbedded in the Keanakākoʻi. Most of the charcoal formed from small twigs, leaves, and roots; this minimizes the chance of “old wood” contamination. A few pieces of charcoal, however, came from larger pieces of wood, whose 14C age could significantly predate the time of fire (Wilmshurst et al., 2011).

5.1. Techniques

Charcoal was the only material dated. It was hand picked from field samples dried for several hours at low temperature within 2 days of collection. The samples selected for radiocarbon dating were converted to pure carbon in the form of graphite at the USGS Radiocarbon Laboratory in Reston, Virginia. Radiocarbon dating of graphite was done by accelerator mass spectrometry (AMS) at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Liver- more National Laboratory (CAMS/LLNL) in Livermore, California or the Fig. 11. Two layers rich in charcoal within Keanakākoʻi Tephra at junction of Chain of NSF-Arizona AMS facility at the University of Arizona in Tucson. Craters Road and Crater Rim Drive, 3.2 km east of center of Halemaʻumaʻu. Lower char- Raw sample material was chemically treated to remove contami- coal layer (LC; Table 1, no. 59), near base of scale (11 cm long), caps fine-medium gray nant carbon using successive acid and alkali washes: 1 M HCl (2 h at ash overlying reticulite. Just above end of scale is coarse scoria of layer 6. Upper char- 60 °C), 0.1 M NaOH (overnight at 60 °C), 1 M HCl (2 h at 60 °C). The coal layer (UC; Table 1, no. 78) caps red-brown, incipient soil on top of medium- grained, vitric and mixed vitric–lithic ash overlying layer 6. Above upper charcoal is neutralized sample was dried in an oven at 50 °C. Each sample was bed of coarse lithic ash and fine lapilli, the oldest lithic unit here. then combusted in air-free, sealed 6-mm Vycor tubes with CuO and D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 15

Table 1 Reported 14C ages, with 1 sigma error, referenced to stratigraphic unit, and figure in which calibrated age is plotted. Latitude and longitude referred to Old Hawaiian datum. WW, AMS age; W, conventional age.

No. Lab no. Field no. N Lat W Long 14C age BP Unit No. in Fig. 12

1 WW5901 S06-53Z 19.43973 155.23837 495±45 Reticulite on ʻAilāʻau or other young flow 6 2 WW5891 S02-23 19.44754 155.27539 305±35 Do. 11 3 WW5892 S03-2-1 19.43553 155.24218 370±35 Do. 12 4 WW2997 S0-25B 19.39438 155.24795 490±40 Do. 7 5 WW2048 9-11-98-1B 19.42940 155.24178 490±50 Do. 8 6 WW2292 10-22-98-2A 19.40293 155.23499 314±30 Do. 10 7 WW2249 10-27-98-2A 19.39658 155.25361 380±50 Do. 13 8 WW2999 S0-27B 19.38608 155.24692 490±40 Do. 14 9 WW63791 S07-40 19.44702 155.23773 305±30 Do. 9 10 WW75782 S09-2 19.37492 155.24582 330±25 Do. 15 11 WW2245 10-14-98-2 19.44425 155.31096 390±40 Reticulite on Kulanaokuaiki Tephra 17 12 WW5405 S03-1B 19.43609 155.24148 680±40 Do. 5 13 WW5403 S02-21A 19.44432 155.25185 390±40 Do. 19 14 WW3905 F01-1-4 C 19.47188 155.26202 358±41 Do. 16 15 WW2246 10-16-98-1 19.46133 155.24859 360±40 Do. 18 16 WW2255 S9-28-6 C 19.46528 155.25694 550±50 Do. 4 17 WW3540 F01-1-4B 19.47188 155.26202 390±50 Basal ash, under reticulite and on Kulanaokuaiki 2 18 WW3904 F01-58-2 19.47841 155.26865 678±37 Do. 1 19 WW2049 9-11-98-2A 19.42651 155.25218 210±40 Basal ash, under reticulite and on ʻAilāʻau flow 3 20 WW3899 S01-51 19.43152 155.25732 558±38 Housing Area fall, on reticulite 20 21 WW3900 S01-60 19.43185 155.23837 342±44 Do. 21 22 WW5900 S06-53 C 19.43973 155.23837 345±35 Do. 22 23 WW3902 S01-13A 19.41920 155.24522 239±37 Do. 23 24 WW5409 S04-211 19.39042 155.22758 190±35 Layer 6 within Keanakākoʻi44 25 WW2618 S9-49-1 C 19.34683 155.23294 230±45 Do. 45 26 WW5902 S06-68 C 19.33373 155.20897 220±35 Do. 46 27 WW4097 F02-20-2 C 19.35209 155.21133 260±40 Do. 47 28 WW2929 F0-15-2 C 19.32396 155.23868 300±40 Do. 48 29 WW3538 S7-103-12A 19.35209 155.21133 320±50 Do. 49 30 WW5895 S05-75A-C 19.38057 155.18736 215±35 Layer 6 on ʻAilāʻau flow 50 31 WW3091 S0-28 C 19.38264 155.22222 230±40 Do. 51 32 WW3000 F0-11-2 19.34083 155.22715 550±40 Layer 6 resting on Kulanaokuaiki Tephra 36 33 WW2882 S0-7-2 19.32751 155.25546 510±40 Do. 37 34 WW5893 S04-118B 19.35209 155.21133 395±35 Do. 38 35 WW2440 S9-49-5 C 19.34683 155.23294 320±50 Do. 39 36 WW2922 F0-14-1 19.32733 155.22285 340±50 Do. 40 37 WW2970 S0-9-1 19.34700 155.26504 320±40 Do. 41 38 WW2957 S0-8-2A-X 19.32281 155.25257 420±40 Do. 42 39 WW2956 S0-8-2X 19.32281 155.25257 330±40 Do. 43 40 W3871 None 19.44626 155.31895 230±60 Young Keʻāmoku flow in Kīpuka Kī 28 41 W4919 None 19.44626 155.31895 350±70 Do. 29 42 WW63773 S07-1 C 19.44626 155.31895 380±40 Do. 30 43 W5106 None 19.41611 155.34972 300±80 Kīpuka Maunaʻiu flow near ʻŌhaikea 31 44 W4006 None 19.41222 155.34250 290±70 Do. 32 45 WW3541 F01-37-1c 19.41190 155.34185 340±40 Do. 33 46 WW3891 S01-6A 19.41216 155.34085 254±44 Keanakākoʻi, 44 cm above Kulanaokuaiki, ʻŌhaikea 34 47 WW3895 S01-43B 19.41296 155.34141 238±42 Keanakākoʻi on reworked(?) Kulanaokuaiki, ʻŌhaikea 35 48 WW3901 S01-61 19.43185 155.25645 298±37 Unconformity between vitric and lithic sections 91 49 WW3898 S01-48 19.43152 155.25732 255±42 Do. 92 50 WW3897 S01-46 19.43152 155.25732 103±40 Base of possible lithic surge 97 51 WW2053 8-17-98-2 19.43699 155.27795 110±50 Lithic surge 94 cm above Housing Area 96 52 WW2052 8-17-98-1 19.43699 155.27795 230±50 Do. 95 53 WW75775 S07-66IJ 19.43710 155.27702 95±25 Growth position 63 cm above Housing Area 94 54 WW63804 S07-60H-I 19.43852 155.27560 60±30 Local base of lithic section 93 55 WW2845 F0-30 C1 19.32012 155.29554 190±40 Lithic ash above accretionary lapilli ash 98 56 WW3542 S01-7 19.41248 155.34065 200±40 Do. 100 57 WW2966 S0-5-1 C 19.30974 155.30347 250±40 Do. 99 58 WW2855 S0-12 C1 19.39437 155.24873 330±50 Ash between reticulite and layer 6 (no Housing Area) 24 59 WW3903 S01-21 19.38880 155.25375 303±37 Do. 25 60 WW2847 S9-74A 19.40821 155.25584 260±40 Do. 26 61 WW2954 S0-20 19.38919 155.24880 280±40 Do. 27 62 WW2616 S9-37D 19.45172 155.25613 395±45 Ash above reticulite and below lithic part of section 63 63 WW5404 S02-21B 19.38919 155.25185 410±40 Do. 59 64 WW2620 S9-65 19.42940 155.24178 425±60 Do. 60 65 WW2047 9-11-98-1A 19.42940 155.24178 350±60 Do. 61 66 WW2617 S9-38 C 19.42801 155.22352 320±45 Do. 52 67 WW2619 S9-59A 19.42415 155.24538 305±55 Do. 57 68 WW5411 S05-67 G 19.42035 155.25240 270±35 Do. 58 69 WW2054 9-11-98-2B 19.42651 155.25218 240±50 Do. 56 70 WW2248 10-22-98-3 19.40293 155.23499 260±40 Do. 55 71 WW3890 S01-4 19.42940 155.24178 237±46 Do. 62 72 WW5407 S04-25 19.42581 155.22604 180±40 Do. 64 73 WW5406 S04-1 19.44793 155.28560 125±40 Do. 65

(continued on next page) 16 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

Table 1 (continued) No. Lab no. Field no. N Lat W Long 14C age BP Unit No. in Fig. 12

74 WW2614 S9-36B 19.41743 155.23701 215±50 Do. (above WW2613) 54 75 WW2613 S9-36A 19.41743 155.23701 195±50 Do. (above Housing Area) 53 76 WW2996 S0-25A 19.39438 155.24795 400±45 Ash above layer 6 and below lithic part of section 67 77 WW2998 S0-27A 19.38608 155.24692 240±40 Do. 68 78 WW2250 10-27-98-2B 19.39698 155.25362 240±50 Do. 71 79 WW2851 S9-74B 19.40821 155.25584 200±50 Do. 74 80 WW2856 S0-12 C2 19.39437 155.24873 160±50 Do. 72 81 WW63786 S07-13 19.34444 155.22778 125±40 Do. 86 82 WW3090 S0-28B 19.38264 155.22222 70±40 Do. 78 83 WW3092 S0-35A 19.36935 155.22485 20±50 Do. 79 84 WW5897 S05-246B 19.37431 155.20807 220±35 Do. 80 85 WW2251 11-10-98-1 19.37192 155.25958 190±50 Do. 81 86 WW2252 11-10-98-2 19.37192 155.25958 160±40 Do. 82 87 WW5410 S04-212 19.39698 155.23155 235±35 Do. 70 88 WW3093 S0-35B 19.36935 155.22485 130±40 Do. 85 89 WW2844 F0-12-2 19.33511 155.22458 170±50 Do. 76 90 WW2253 S7-103-12 C 19.35209 155.21133 210±50 Do. 77 91 WW2293 10-22-98-2B 19.40251 155.23480 134±26 Do. 73 92 WW2294 10-22-98-2 C 19.40251 155.23480 242±33 Do. (19 cm above WW2293) 69 93 WW5408 S04-208B 19.38658 155.22510 160±40 Do. 75 94 WW5898 S06-15B 19.31021 155.21820 150±35 Do. 83 95 WW5896 S05-186AB 19.32747 155.22246 85±40 Do. 84 96 WW2247 10-16-98-3-2 19.44505 155.32843 550±50 Keanakākoʻi(?) on Kulanaokuaiki Tephra, Kīpuka Kī Not plotted 97 WW1932 S8-8 C 19.42801 155.22352 140±45 Lower part of lithic section resting on ʻAilāʻau flow 90 98 WW2968 S0-5XC 19.30974 155.30347 270±40 Keanakākoʻi vitric ash on Kulanaokuaiki Tephra 66 99 WW2254 S9-1 C 19.31642 155.29975 180±50 Vitric ash under lithic ash 87 100 WW2846 F0-30 C2 19.32012 155.29554 200±50 Do. 88 101 WW2967 S0-5-2 C 19.30974 155.23294 60±40 Do. 89

1δ13C=−26.07‰; 2δ13C=−25.13; 3δ13C=−23.92‰; 4δ13C=−26.47‰; 5δ13C=−26.01‰; 6δ13C=−26.36‰.

Ag at 900 °C to produce CO2. The CO2 for each sample was cryogeni- with certainty these two possibilities. This means that every charcoal- cally isolated from volatile gases and H2O in a high-vacuum manifold. tephra pair must be evaluated carefully; context and consistency Finally, sample CO2 was reduced to 1 mg carbon as graphite precipi- among multiple samples are important, and every relation is inter- tated on 63 mesh Fe powder in the presence of hydrogen at 575 °C pretative, not obvious. (Vogel et al., 1984). The resultant graphite was pressed into an alumi- In the following sections, several charcoal ages are interpreted as num target and shipped to one of the AMS labs for dating. either the products of fires predating the KeanakākoʻiorofKeanakākoʻi- Radiocarbon ages were determined with an assumed δ13Cof age fire burning old wood. Vegetation grew on the older Kulanaokuaiki −25‰ using the Libby half life of 5568 years. The δ13C value was Tephra, a good substrate for plant growth, particularly given the 500– measured for 6 samples (Table 1) at the University of California at 600-year interval since the latest Kulanaokuaiki eruption (in about Davis (UC-D) Stable Isotope Laboratory. Ages were reported with 1 900–1000 CE; Fiske et al., 2009). We tried, wherever possible, to avoid sigma error. Calibrations to calendar years were made using the this problem by dating deposits resting on lava flows emplaced only a CALIB radiocarbon calibration software (Stuiver and Reimer, 1993) few years to a few decades before the start of the Keanakākoʻi erup- in conjunction with the IntCal04 calibration datasets (Reimer et al., tions, using the map of Neal and Lockwood (2002) as a guide. 2004). Calendar calibrations were output and plotted with 2 sigma The following sections present the ages in relation to observed error, and the following discussion uses these 2 sigma calendar stratigraphic relations and outline the reasons for accepting or reject- dates. The program OxCal version 4 (Bronk Ramsey, 2009) was used ing specific ages as meaningful to Keanakākoʻi history. Ages from sites to combine multiple radiocarbon ages and to estimate calendar that are best defined stratigraphically are examined first, followed by dates of deposits in known stratigraphic sequence. those less certain.

5.2. What started the fires? 5.3. Lower part of Keanakākoʻi

Charcoal associated with a tephra-fall deposit can have several Charcoal derived from small leaves, roots, and stems occurs at the origins. When erupted, basaltic tephra is hot, about 1165 °C at Kīlauea, base of, in, and along the top of the reticulite (Fig. 1) at many places in and falling lapilli can start fires away from vents. For example, pumice the summit area (Fig. 3, nos. 1–16). The mingling of charcoal with the lapilli from a 300-m-high lava fountain at Mauna Ulu (Kīlauea) in reticulite, and the continuation of the internal charcoal with that both October 1969 set fire to several trees 2.5 km from the vent (Swanson directly below and above the reticulite, suggest that fires were burn- et al., 1979). Radiant heat from high fountains caused dry grass and ing as the reticulite fell. They were likely started by the reticulite it- small shrubs to burst into flame on exposed slopes 700 m away self, given the widespread occurrence of the charcoal. None of the (Swanson et al., 1971). charcoal comes from the centers of large pieces of wood; this helps Another syneruption origin of charcoal is by fire generated near minimize the chance of “old” charcoal. the vent by spatter, hot blocks, or small flows and moves downwind The 10 most robust dated sites are those where the reticulite just before, during, or just after tephra is deposited. Fire can also be rests directly on pāhoehoe (Table 1,nos.2–10; Fig. 12,nos.7–15) or started by lightning during the eruption. on a 0.1–2-cm-thick gray lithic ash—the oldest Keanakākoʻi Tephra— In addition, fires unrelated to an eruption could have been started deposited directly on the pāhoehoe (Table 1,no.1;Fig. 12, no. 6). by natural causes or human activity some time before or after the The underlying flows range in calendar age from about 1470 CE tephra fall. (youngest ʻAilāʻau flow of Clague et al., 1999) to about 1350 CE (some Charcoal made by syneruption fire records the age of the tephra; of the youngest flows from the Observatory shield, the edifice that other charcoal generally does not. We know of no way to distinguish existed before its summit collapsed to form today's caldera; Neal and D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 17

Fig. 12. Summary plot of calibrated 14C ages used in this paper, arranged in stratigraphic order using our best interpretation. Different colors correspond to possible calendar ages of specific stratigraphic units or closely constrained units. The two lava flows (Kīpuka Maunaʻiu and young Keʻāmoku) are interpreted to be older than layer 6; this interpretation lacks stratigraphic control but is supported by the sequence analysis shown in Fig. 13. Vertical line indicates 1790, year when Keanakākoʻi was previously thought to have been erupted. Sample numbers are cross-correlated in Table 1 with the original reported ages. One age (Table 1, no. 96) is not plotted, because it is from charcoal poorly constrained stratigra- phically and clearly older than Keanakākoʻi.

Lockwood, 2002). The lack of Kulanaokuaiki Tephra minimizes the the resolution, we combined the reticulite ages for four groups to chance of old charcoal, because vegetation returns more readily to evaluate a single “best” age for the deposit, using OxCal version 4 weathered ash than to bare lava flows at Kīlauea. (Bronk Ramsey, 2009). Group R10 includes the ten ages obtained on Ages from 6 localities where the reticulite rests on top of Kulanao- bare lava flows or thin ash (Table 1, nos. 1–10); group R11, the 11 kuaiki Tephra (Table 1, nos. 11–16) illustrate the possible pitfalls of youngest ages, which may form a coherent cluster (Table 1,nos.2–3, old charcoal. Four of the ages (Fig. 12, nos. 16–19) are consistent 6–11, 13–15); group R14, all ages except the two oldest (Table 1,nos. with that of the reticulite on bare lava flows, but two (Fig. 12, nos. 1–11, 13–15); and group R16, all 16 ages. 4–5) are older and suggest either previous fires on the vegetated The results (Table 2) show that the late 1400s and early 1500s is a Kulanaokuaiki surface or old wood. credible time for the reticulite to have been erupted. The late 1500s or Fig. 14 shows that all of the radiocarbon ages have a wide range of early 1600s is another possible range, though generally at lesser prob- possible calendar dates. This is because the calibration curve has a ability. Group R16 is of questionable validity, given the two old ages low slope from about 1450 to 1600 CE, leading to multiple solutions (Table 1, nos. 12 and 16) obtained for charcoal on the surface of the for a single 14C age (Reimer et al., 2004). In an attempt to improve Kulanaokuaiki Tephra. Group R11 is only marginally defendable, 18 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 because it specifically excludes 3 ages of only slightly older 14C age. is separated by 0.5–2 cm of vitric ash from, pre-Keanakākoʻilava On balance, we favor group R14, because it includes all ages except flows or the Kulanaokuaiki Tephra. the two oldest, and a generalized calendar age of 1500 CE for the reti- Charcoal is fairly abundant at the base of, or just overlying, the culite eruption, allowing for 2–3 decades on either side of that year. A scoria within several kilometers of the caldera (Fig. 3, nos. 24–39). later discussion, utilizing dated units in stratigraphic sequence, also Ages were obtained from 8 localities where the scoria rests either favors a generalized date of about 1500 and shows that the late on older Keanakākoʻi Tephra (Table 1, nos. 24–29) or on a bare flow 1500s–early 1600s period is unlikely. The ages clearly show that the surface (Table 1, nos. 30–31)—situations that should minimize the reticulite was not erupted in 1790. chances of old wood or previous fire. The 8 ages have a reasonably consistent calibrated range (Fig. 12, nos. 44–51) and were lumped to- 5.4. Units directly below and above the reticulite gether to yield a combined 14C age of 239±14 BP (Table 3). This is our preferred age for layer 6, yielding a rounded calendar age of about Charcoal occurs at the base of the 1–2-cm-thick, gray-pink lithic 1650 CE. ash underlying the reticulite (Fig. 1). Of three ages with Kulanao- In addition, charcoal was dated from 8 localities where layer 6 kuaiki underneath, one (Table 1, no. 17) has a geologically reasonable rests on the Kulanaokuaiki Tephra. Two of these ages (Table 1, nos. age similar to that of the reticulite (Fig. 12, no. 2), and one (Table 1, 32–33; Fig. 12, nos. 36–37) predate any Keanakākoʻi and probably re- no. 18) is too old and probably the product of an earlier fire or old cord either earlier fires or old wood. Six of the ages (Table 1 nos. wood. The third age (Table 1, no. 19), from charcoal at the contact 34–39; Fig. 12, nos. 38–43), however, are broadly consistent with of the gray-pink lithic ash and the ʻAilāʻau flow, is anomalously the general age pattern. These 6 ages were combined to yield an age young for an unknown reason. of 349±20 BP (Table 3), more than 100 years older than the com- A coarse, pink, lithic fall and PDC deposit occurs in a limited area bined age for layer 6 resting on older Keanakākoʻi or the ʻAilāʻau northeast of the caldera, where it rests directly on the reticulite flow. The reason for this difference is uncertain, although it is reason- (Fig. 1). This deposit, equivalent to layer 5 of McPhie et al. (1990) able that the older tephra substrate both hosted older wood and had and called the Housing Area block fall by us, forms an excellent strat- experienced pre-layer 6 fire. igraphic marker. Its finer-grained marginal facies, pink to tan lithic If this difference is ignored, the combined age for the 14 accepted ash (layer 0 of McPhie et al., 1990) a few centimeters thick, occurs individual ages is 288±11 BP (Table 3). Calibration of this age yields on all sides of the caldera except the south. two calendar ranges of similar probability at 2 sigma, about 1530–1560 The Housing Area block-fall deposit was hot, as its pink color indi- and 1630–1650. The early range is forced by the 6 ages on older ash; the cates, and produced charcoal that was dated at four localities. Three younger range is consistent with our preferred age of charcoal on ages (Table 1, nos. 21–23; Fig. 12, nos. 21–23) are consistent with younger substrate. that of the underlying reticulite, and we combined them to yield an age of 307±23 radiocarbon years BP (Table 3) that we use later in 5.6. Interbedded lava flows the sequence analysis. The fourth age (Table 1, no. 20) is too old by at least 50 years (Fig. 4, no. 20). This anomalous locality is on top of Both 14C-dated lava flows interbedded with the Keanakākoʻi were a young flow of the Observatory shield (the lava flows of Volcano vil- erupted from Mauna Loa and occur west of the caldera beyond the lage; unit k4ov of Neal and Lockwood, 2002) that predates the ʻAilāʻau limit of layer 6. Thus field relations cannot determine if the flows flow. Possibly an old tree was burned by the Housing Area block fall, are older or younger than layer 6. The Kīlauea flow is not dated but or some pre-existing charcoal was incorporated in the deposit. occurs along the erosional contact at the base of the lithic section (Fig. 1) and thus is younger than layer 6. 5.5. Layer 6 scoria fall (McPhie et al., 1990) The young Keʻāmoku flow overlies reticulite and thin vitric ash above the Housing Area deposit and underlies lithic ash and lapilli. A widespread, easily recognizable scoria fall blankets the area Charcoal collected beneath the flow in Kīpuka Kī (Fig. 3) was dated southeastward from Kīlauea's caldera to the seacoast, 20 km distant. by conventional methods and later rerun (Table 1 and Fig. 3, nos. This deposit is one of the most intriguing in the Keanakāko ʻi owing 40–41; W3871 and W4919, respectively, in Rubin et al., 1987, table to its distinctive componentry—crystal-rich scoria, dense glass, and 10.1). Later, an AMS age was obtained on different charcoal from significant numbers of wall-rock lithics—and its southeastward dis- the same locality (Table 1 and Fig. 3, no. 42). The three ages combine persal direction that implies column heights into the westerly to 338±31 BP (Table 3). winds of the jet stream, most likely 10–20 km (Swanson et al., The Kīpuka Maunaʻiu flow overlies Keanakākoʻi vitric ash and thus 2006). Near the caldera, the scoria is generally about 20–30 cm is younger than both the reticulite and Housing Area deposits. It un- thick (maximum about 50 cm) and overlies 30–650 cm of vitric ash derlies lithic ash capped by an accretionary lapilli ash of probable above the reticulite (Fig. 1). Farther than 4–5 km southeast of the 1790 age. Three ages are available for charcoal at the base of the caldera, the scoria is less than 10 cm thick and rests directly on, or flow. Two were previously determined by conventional methods (Table 1 and Fig. 3, no. 43–44; W5106 and W4006 in Rubin et al., Table 2 1987, table 10.1). An AMS age was later measured on a different sample Combined 14C ages for four groups of reticulite ages defined in text. Percentages in pa- (Table 1 and Fig. 3, no. 45). A combined age is 323±32 BP (Table 3), rentheses indicate probability that the calendar age is within the indicated range at the remarkably similar to that for the young Ke'āmoku flow. Each has a 2 or 3 sigma level. See text. calendar date in the 2-sigma range of 1470–1640; sequence analysis Combined Calendar age (CE) narrows this range to early 1500s-mid 1600s (Fig. 13). Group 14C age 95.4% (2 sigma) 99.7% (3 sigma) R10 373±12 1453–1515 (72.1%); 1447–1524 (73.4%); 5.7. Lithic part of the section (layers 11 –16 of McPhie et al., 1990) 1599–1619 (23.3%) 1572–1627 (26.3%) R11 350±11 1471–1524 (46.5%); 1458–1530 (48.3%); 1558–1632 (48.9%) 1551–1635 (51.4%) Most of the lithic deposits lie above the largest erosional unconfor- R14 371±10 1454–1515 (70.4%); 1448–1524 (72.1%); mity in the Keanakākoʻi(Figs.1,7,10). This part of the section is 1600–1619 (25.0%) 1573–1624 (27.6%) dominated by lithic PDCs, including surges, and fall deposits, many R16 395±10 1446–1488 (95.4%) 1442–1510 (96.8%); with a small juvenile component. The lithic beds include the deposits 1601–1614 (2.9%) of 1790 as well as probably older falls and PDCs, and a thin, possibly D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 19

Table 3 scouring into unconsolidated vitric ash. Strictly interpreted, the char- Combined radiocarbon ages and calendar years using only the marker units, our best coal ages suggest a surge or other PDC event in about 1660, though estimate for the age of those units except the young surge (see text). For layer 6, two the age could be much younger if the older of the two ages is on old alternatives shown in italics are also given but are not preferred. See text. wood. Stratigraphic unit Combined Calendar age (CE) Another age from this area is on charcoal at the base of a surge 14C age 95.4% 99.7% deposit high in the lithic section (Table 1, no. 50) and calibrates at – Young surge 171±36 1655–1706 (18.5%); 1647–1895 (80.8%) 2 sigma to the 1680 1760 time frame (Fig. 12, no. 97). 1720–1820 (47.9%) Four ages are for charcoal collected in temporary excavations at Layer 6 (best 8) 239±14 1646–1668 (71.6%); 1641–1670 (72.9%); Kīlauea Military Camp (KMC) in 1998 and 2007. Two of these come 1783–1797 (23.8%) 1780–1800 (25.5%) from small pieces of charcoal in a 13-cm-thick surge deposit 94 cm Layer 6 349±20 1463–1529 (41.6%); 1454–1640 (99.7%) above the top of the Housing Area deposit and 36 cm above the (on Kulanaokuaiki) 1544–1634 (53.8%) Layer 6 (all 14) 288±11 1525–1558 (49.4%); 1520–1592 (52.7%); base of the lithic section; its top is 10 cm below the top of the Keana- 1631–1650 (46.0%) 1622–1655 (47.0%) kākoʻi(Table 1, nos. 51–52; Fig. 12, nos. 95–96). The two ages yield a Kīpuka Maunai'u 323±32 1475–1645 (95.4%) 1451–1656 (99.7%) combined 171±36 BP for the young surge and are consistent with a ā – – Young Ke' moku 338±31 1471 1641 (95.4%) 1450 1648 (99.7%) date from the mid 1600s to 1790 (Table 3). The age of the young Housing Area 307±23 1494–1602 (72.6%); 1471–1655 (99.7%) 1616–1648 (22.8%) surge can be narrowed to the mid to late 1700s when the ages of Reticulite (best 14) 371±10 1454–1515 (70.4%); 1448–1524 (72.1%); vitric and lithic ash beds below it are considered later in the paper 1600–1619 (25.0%) 1573–1624 (27.6%) (Fig. 14). A third KMC age is from a small plant in growth position, rooted in a probable surge deposit and overlain by a coarse ash-fine lapilli fall younger ash-fall bed associated with emplacement of ballistic blocks deposit (Table 1, no. 53), 63 cm above the Housing Area deposit, and lapilli (Swanson et al., 2010). 18 cm above the base of the lithic section, and 32 cm below the bull- The erosional unconformity is muted or absent in areas of low re- dozed ground surface. It has a 2-sigma calendar date of 1688–1730 lief on the western, northern, and eastern sides of the caldera. Here, a (Fig. 12, no. 94; not plotted is the stratigraphically inconsistent normally graded lithic–vitric ash bed less than 5 cm thick, overlain by range of 1809–1926). accretionary lapilli ash of similar thickness, lies along the vitric–lithic The fourth age from KMC is for charcoal at the base of the nor- contact. This doublet, and in particular the normally graded mixed mally graded mixed ash bed, exposed in a temporary trench, 18 cm ash, serves as a useful marker bed in the field. above the top of the Housing Area deposit and 71 cm below the top of Charcoal was collected and dated from 7 proximal or medial local- the Keanakākoʻi(Table 1, no. 54; Fig. 12, no. 93). Its calibrated range ities in, or at the base of, the lithic section. All of the samples come at 2 sigma is 1694–1727 (24%; the other calendar ages are younger from sections containing the reticulite and Housing Area block fall than 1812). or ash, but none contains layer 6. Consequently, the only stratigraphic The distal part of the lithic section is represented by thin beds of constraint on the dated samples is imposed by units low in the Keana- ash, some with accretionary lapilli, as in the sand dunes described kākoʻi section (Fig. 1). However, where layer 6 is present near the above (Fig. 2). Charcoal in lithic ash above prominent accretionary caldera, several centimeters to 3.5 m of mixed vitric–lithic ash separate lapilli-rich beds was recovered and dated from three distal localities it from the lithic section. Barring complications, the age of charcoal from lacking any stratigraphic constraint other than that of the lithic unit the lithic section or the unconformity at its base should therefore be itself (Table 1 and Fig. 3, nos. 55–57). They calibrate to calendar younger than about 1650. dates that could record eruptions from the late 17th century to the Two ages are from charcoal along an unconformity between the early 19th century (Fig. 12, nos. 98–100). vitric and lithic parts of the section (Table 1, nos. 48–49; Fig. 12, nos. 91–92), exposed during excavations near the national park's en- 5.8. Parts of the section constrained by marker units trance station. The unconformity in this area is on nearly flat ground not subject to water erosion and was probably formed by PDCs Charcoal at four localities was collected from vitric ash several cen- timeters above the reticulite and below layer 6, with the Housing Area deposit missing. The four ages (Table 1,nos.58–61) have 2-sigma calen- dar ranges consistent with, though not unique to, their stratigraphic po- Young surge sition between the two marker beds (Fig. 12,nos.24–27). Fourteen ages (Table 1, nos. 62–75) come from vitric and mixed Layer 6 best 8 vitric–lithic ash in sections above the reticulite and below the lithic part of the section but lacking the Housing Area (with two excep- Ke‘amoku tions) and layer 6 marker units (Fig. 1). Each 2-sigma calendar range is broadly consistent with its stratigraphic position (Fig. 12,nos. Kïpuka Mauna‘iu 52–65). Five indicate ages no younger than the mid 1600s, three are 1790 no older than the mid 1600s, and the remaining 6 ages could fit Housing Area best 3 anywhere above the reticulite, though they are constrained strati- graphically to predate the lithic section. Reticulite best 14 Twenty ages were obtained above observed layer 6 and below the observed lithic part of the section. Nineteen of those ages (Table 1 and 1400 1500 1600 1700 1800 Fig. 3, nos. 77–95) allow, and most require, a 2-sigma calendar age Modelled date younger than that of layer 6, consistent with the stratigraphy (Fig. 12, nos. 68–86). The one inconsistent result (Table 1 and Fig. 3, Fig. 13. Distributions of ages for charcoal associated with 6 marker units, calculated by no. 76; Fig. 12, no. 67) has a youngest 2-sigma calendar age that is combining several ages for each unit (see text) and then placing them in sequence slightly older than that of layer 6. The ash containing this charcoal using OxCal v. 4.1.6 (Bronk Ramsay, 2009). Relative ages of units Kīpuka Mauna'iu, comes from 10 cm above the top of layer 6 and rests on a slightly Ke'āmoku, and layer 6 best 8 are not known, but plot shows that layer 6 is almost cer- tainly younger than the two lava flows. Bars show 68.2% and 95.4% probabilities. The red surface, possibly an incipient soil at the top of layer 7 of McPhie date 1790 is assumed to be the youngest possible date. et al. (1990). This age is marginally too old for an unknown reason. 20 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

5.9. Stratigraphically unconstrained or poorly constrained ages 100) are indistinguishable (Fig. 12, nos. 98 and 88). At another locality in the same area (Table 1 and Fig. 3, no. 98), charcoal along the contact Keanakākoʻi Tephra directly overlies Kulanaokuaiki Tephra across of the Keanakākoʻi vitric ash and the underlying weathered Kulanao- broad areas away from the summit where no marker bed occurs. In kuaiki has a calibrated age (Fig. 12, no. 66) most likely from the time be- such areas, the base of the Keanakākoʻiisdefined by a thin deposit of tween the reticulite and layer 6, once again possibly related to a fire fresh vitric ash resting on the weathered Kulanaokuaiki. One example associated with layer 6. is in Kīpuka Kī (Fig. 3, KK), 6 km northwest of the caldera, where glassy Two samples of charcoal from near or at the base of the lithic part Pele's tears and hair of the Keanakākoʻi rest on weathered ash; charcoal of the Keanakākoʻi were collected near, but not in the same section as, along the contact yields a pre-Keanakākoʻi 2-sigma calendar age of the Kīpuka Maunaʻiu flow at ʻ(Ōhaikea Fig. 3) southwest of the sum- 1299–1441 CE (Table 1 and Fig. 3, no. 96); this age is not plotted in mit. The samples may be at about the same stratigraphic level as Fig. 12. the flow, to judge from the local ash stratigraphy. The calibrated Charcoal was dated from the top of the vitric section under lithic ages (Table 1 and Fig. 3, no. 46–47; Fig. 12, nos. 34–35) range from ash at three distal localities (Table 1 and Fig. 3, no. 99–101). The vitric early 16th century to early 19th century; they confirm that the ash ash is only 1–2 cm thick and lies directly on Kulanaokuaiki Tephra. is Keanakākoʻi, not Kulanaokuaiki, but do not constrain the ages of Given the stratigraphic context, the 2-sigma calibrated ages (Fig. 12, any units within the Keanakākoʻi. nos. 87–89) suggest a fire in the mid-late 1600s to early 1700s, possibly Finally, charcoal from the lower half of a 9-cm-thick, coarse to fine associated with the fall of layer 6, the western margin of which is just lithic ash resting on the ʻAilāʻau flow in Mauna Loa Estates (Table 1 east of the charcoal sites. One pair of samples straddles the vitric–lithic and Fig. 3, no. 97) was calibrated as late 17th century to early 19th contact; the 2-sigma calibrated ages of the two (Table 1,nos.55and century (Fig. 12, no. 90). No marker bed occurs in the section, but

A Young surge

94

93 B Young surge 92

74 91 75 90 73 89 72

88 71

87 70 C Young surge 86 69 56 85 68 55 67 84 97 66 81 53 65 54

80 1790

1790 64 50 79 63 57

78 62 49 Housing Area 77 best 3 48

Layer 6 Reticulite best 14 Layer 6 best 8 best 8

1790 1600 1700 1800 1500 1600 1700 1800 1600 1700 1800 Modelled date, C.E.

Fig. 14. Ages of charcoal associated with ash units stratigraphically between marker units. Numbers are those of Table 1, col. 1. Bars show 68.2% and 95.4% probabilities. A. Charcoal in vitric ash above layer 6 and capped by lithic ash denoted by “young surge.” Dotted lines show that several are similar to that of layer 6 and could be on charcoal formed during a fire started by the layer 6 eruption. B. Charcoal in vitric ash above reticulite (or Housing Area) marker units and below lithic ash. Dotted lines are from A. Some of the ages predate layer 6 and some are roughly coeval, consistent with stratigraphy. C. Lithic ash above layer 6 and below young surge. Three ages are only slightly younger than layer 6 but others are significantly younger, consistent with stratigraphic relations. D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 21 reticulite crops out nearby and was probably eroded away from the likewise implies eruptive quiet, perhaps lasting several decades. charcoal site before the dated lithic tephra was deposited. Clearly the 14C ages do not support previous interpretations that all Keanakākoʻi eruptions took place in 1790. The ages are about as 5.10. Ages in stratigraphic sequence consistent with the physical relations as can be expected, given the difficulty in calibrating such young ages to calendar years. Most The combined calibrated ages for the six dated marker units are of the “too old” ages likely document either old fires or old vegetation consistent with their stratigraphic position. Fig. 13 was constructed growing on the Kulanaokuaiki Tephra. One age— the young age for using the “Sequence” model in OxCal (Bronk Ramsey (2009), with a the gray lithic ash under the reticulite (Table 1, no. 19)—remains boundary condition of 1790 for the youngest eruption. The relative enigmatic. stratigraphic order among the two lava flows and layer 6 is not known, but all three are younger than the Housing Area and older 6. Age of Kīlauea's caldera than the Young Surge, and the combined ages make it very likely that layer 6 is younger than the two flows. The reticulite is unlikely Caldera subsidence has often been tied to the date of the Keanakā- to have been erupted in the early 1600s, a possibility permitted by koʻi Tephra (Decker and Christiansen, 1984; Holcomb, 1987). This the combined R14 age; taking into account younger tephra (Fig. 14B), paper shows that the Keanakākoʻi was erupted over a 300-year peri- the reticulite could not have been erupted so late. Fig. 13 shows clearly od. Where does the caldera fit into this time frame? This question that the Keanakākoʻi Tephra was erupted over an approximately 300- can be answered from key stratigraphic relations in the summit area. year period from about 1500 to 1790 CE. The outermost fault bounding the eastern margin of the caldera Charcoal ages for other tephra deposits can be constrained by their (Neal and Lockwood, 2002) cuts across the shield (Figs. 3, 16) built stratigraphic position between the marker units. For example, Fig. 14A during the ʻAilāʻau eruption, which lasted for about 60 years and shows 16 ages for vitric ash above layer 6 and below the lithic part of ended in about 1470 CE (Clague et al., 1999). The older Observatory the section, represented by the young surge. Many of the ages overlap eruptive center or shield (Holcomb, 1987), which collapsed to form with that for layer 6 and are for charcoal in or downwind of the layer most of the caldera, existed during the ʻAilāʻau eruption, because 6 dispersal. This raises the possibility that the large layer 6 scoria fall ʻAilāʻau flows advanced only a short distance west from their vent, generated fires that spread downwind, forming small charcoal frag- blocked by the Observatory edifice (Neal and Lockwood, 2002). ments that became incorporated in the immediately overlying ash. Thus the caldera postdates the ʻAilāʻau eruption and is younger than This idea is not testable but is appealing in that it minimizes the about 1470 CE. How much younger? number of fires needed to form the charcoal. The nature of the Keanakākoʻi reticulite helps answer this ques- Fig. 14B plots some ages for vitric ash without layer 6 in the sec- tion. Reticulite typically forms only in high lava fountains (consistent tion and shows that some of the ages almost certainly predate layer with very rapid ascent rates, according to Rust and Cashman, 2006) 6 and others are about the same age or younger. This is consistent and has seldom if ever been noted at Kīlauea from fountains less with the stratigraphic position of layer 6, interbedded with vitric than 300 m high, more often 400 m or more. In tephra deposits ash in the wall of the caldera (Fig. 1). from such high modern fountains (1959 Kīlauea Iki, 1969 Mauna Ages for lithic ash above layer 6 (Fig. 14C) show the expected Ulu, and 1983–86 Puʻu ʻŌʻō), reticulite is mostly found only several ki- range, ranging from only a little younger than layer 6 up to about lometers downwind from the vent. Near the vent, its place is taken by 1790. denser pumice and vitric scoria, products with much greater fall Fig. 12 shows all the calibrated ages arranged according to our velocities. preferred interpretation and constrained by their stratigraphic posi- The Keanakākoʻi reticulite is nearly pure, containing only small tion relative to the marker units. No sequence analysis was performed amounts of pumice and Pele's tears or achneliths (Fig. 17). It encircles on the entire data set, as was done for Figs. 13 and 14. Instead, just the the caldera, with changes in grain size and componentry (May et al., raw calibrated ages are shown; they generally decrease upsection and 2009) consistent with eruption from several vents within the caldera make our point with minimal statistics. Younger vitric units, and pos- (along a ring fracture?). Moreover, it is as thick as 65 cm in places sibly some of the older lithic units as well, have ages suggesting late along the western caldera rim and 20–30 cm along the northern 17th and 18th century eruptions. Most of the lithic unit, particularly and eastern rims. Thus the reticulite is almost certainly not a distal where it lies above the well defined erosional unconformity, was like- deposit several kilometers from the vent and must have had a source ly deposited in the 18th century. Explosive eruptions dominated by in the caldera. Why, then, is it so pure? lithic debris may have continued until at least February 12, 1794, This can be explained by eruption of high fountains on the floor of when Archibald Menzies reported “smoke, dust, and ashes arising a deep caldera—so deep that most of the pumice and denser pyro- from [the caldera]” some 20–25 km upwind of his location, though clasts were trapped within the caldera. The depth of the caldera was this has often been interpreted as a dust storm (Hitchcock, 1909, p. probably greater than 300 m and perhaps 200–300 m deeper. There 72, 161). The youngest eruption produced the golden pumice, depos- is no evidence of external water influencing the reticulite eruption, ited “possibly around 1820” (Sharp et al., 1987). so one limit on the depth of the caldera might be the water table, It is notable that the 14C ages of such young deposits can be placed today a little more than 600 m below the high point on the caldera in a reasonable stratigraphic order consistent with the physical stra- rim and about 500 m below the present caldera floor (Mastin, tigraphy. Many ages, however, were needed to construct this figure; 1997). However, one can envision scenarios whereby caldera collapse only a few would have been inadequate, given the wide uncertainty and eruption took place too rapidly for external water to play a role, in assigning calendar ages to such young 14C ages and the poorly con- even below the water table. Whatever the depth, this reasoning sug- strained nature of some samples. gests that the caldera was already deep when the reticulite was Our summary interpretation of the ages, in the context of the erupted. physical stratigraphy, is that the Keanakākoʻi eruptions began in The preceding discussion indicates that the caldera formed between about 1470–1510 CE and continued sporadically into the early 19th about 1470 and 1510 CE. Other evidence supports such an early caldera. century. They could have started a little earlier if the 1470 age for Small remnants of the two oldest deposits in the Keanakākoʻi, the gray the end of the ʻAilāʻau eruption is too young. Oxidized horizons, par- lithic ash and the reticulite, are plastered against the nearly vertical ticularly the one probably at the top of layer 7, may record periods walls of the caldera in several places, most notably just above the of quiet lasting several decades, enough time for soil to begin forming. northeastern floor of the modern caldera. Thin veneers of vitric ash The deep erosional unconformity at the base of the lithic section that correlate with deposits just above the reticulite cling to, and 22 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 occupy niches in, the vertical caldera wall 800 m west-southwest of KeanakākoʻiCrater(Figs. 3, 15). Such plastering relations indicate that the wall existed when this ash was erupted, pointing toward early formation of the caldera. In 1823, William Ellis, the first European visitor to the caldera, was told that lava erupted at the summit previously flowed over the sur- rounding countryside but that “for many kings' reigns past” it had been confined to what is today called the caldera (Ellis, 1825; Swanson, 2008). This agrees with the known eruptive history at Kīlauea's summit, where the most recent eruption outside the caldera was from the 15th century ʻAilāʻau shield (Figs. 3 (AS), 16; Neal and Lockwood, 2002; Clague et al., 1999). Thus the oral tradition is consistent with the geo- logic record in suggesting an old caldera, not one formed entirely in 1790. The evidence presented here suggests that the caldera was already deep when the Keanakākoʻi eruptions began. A depth of several hun- dred meters, required by the reticulite deposit, is far greater than the modern depth of about 120 m but comparable to the measured depth of about 400 m in 1825 (Byron, 1827, p. 184 [corrected by Mastin, 1997]) and the estimated depth of at least 540 m in 1823 (Mastin, 1997). Mastin (1997; Mastin et al., 2004) makes the point that the texture and volatile content of the vitric tephra (Fig. 1, layers 1–4) above the reticulite suggest that vents were near or at the depth of the water table, about 515 m below today's caldera floor and attain- able if the caldera were about 600 m deep. These observations and interpretations make it clear that a deep caldera existed in the early 19th century and probably in the early-mid 16th century, consistent with our independent interpretations. Finally, Hawaiian chants suggest that a sister of Pele, the volcano deity, dug the caldera so deeply soon after the ʻAilāʻau eruption that Fig. 15. Two different vitric ash deposits hang on south caldera wall 700 m west of there were warnings about water coming in and drowning the “fires Keanakākoʻi Crater. The upper is veneered onto vertical wall; the lower, more pumiceous, of Pele” (Emerson, 1915; Swanson, 2008). This oral tradition suggests is protected in niche in wall. Pocket knife for scale. a great initial depth of the caldera, perhaps close to or even intersect- ing the water table. It also suggests that a major collapse took place, Alternatively, a volume of 4–6km3 may have been intruded into but whether it was the final event in a series of incremental collapses one of Kīlauea's rift zones. In this scenario, the east rift zone or a single collapse is not clear from the chants. (Fig. 16)—much larger than the southwest rift zone—would have There could have been smaller, nested collapses of the caldera been the most likely site of intrusion, possibly enabled by a large floor between 1500 and the late 18th century; in fact, such collapses slip event that displaced the volcano's south flank many meters sea- are likely to have occurred as caldera fi lling took place episodically, ward and dilated the rift zone. A dike 40–60 km long, 5 km high, possibly including the large eruption in 1790. Nonetheless, the bound- and 20 m wide could accommodate such a volume. A dike 10 km ary faults created during the initial, main collapse persist today, and high, reaching down to the base of the volcano, would still require a the caldera was deep very early in its history. 10-m-wide dike, about an order of magnitude wider than most mea- sured dikes in Hawaiian shields (Walker, 1986). Such an event might account for much of the displacement in the Koaʻe fault system 7. What caused the caldera to form? (Fig. 18), which connects the east and southwest rift zones and forms part of the boundary of the mobile south flank (Duffield, The evidence just presented shows that the caldera formed just 1975; Fiske and Swanson, 1992) . More than 30 m of extension have after the ʻAilāʻau eruption ended and before the Keanakākoʻi eruptions accumulated across the 2.5-km-wide fault system since it was cov- began. It is possible that the collapse caused the long-lived eruption ered by a lava flow about 700 years ago. The extension probably to end. In any case, no explosive ejecta were projected over the rim, resulted from several large slip events, one of which could reasonably except for the thin gray-pink lithic ash at the base of the Keanakākoʻi. relate to caldera formation. Hawaiian chants suggest that rocks were “flying” as the caldera The volume of the ʻAilāʻau flow (Fig. 16) and shield, 4–6km3 formed (Swanson, 2008), but apparently these were confined to (Clague et al., 1999), is similar to that assumed for the early caldera. within the down-dropping caldera and could well have been rock Is that only a coincidence? Possibly its slow eruption, lasting for falls. about 60 years at a rate comparable to that of the current PuʻuʻŌʻō We estimate the caldera collapse volume to have been 4–6km3, eruption, gradually withdrew magma from the summit reservoir assuming a depth of 600 m, diameters of 3.5 km by 3 km (about the complex, eventually draining it to the point where its roof could no diameters of the modern summit depression), and various shapes of longer be supported. Probably more likely, caldera formation could collapse. This volume likely represents the amount of magma evacu- have been incremental, beginning during the eruption and ending ated from the summit reservoir. In keeping with most models for with a large collapse described by the oral tradition. caldera formation, this evacuation would have been rapid. It should be Another way to relate the large ʻAilāʻau eruption to caldera forma- asimplefield exercise to identify such a huge lava flow that gushed tion is to consider the mass of the lava flow and shield, about 1010 onto Kīlauea's surface only 500 years ago, but no such flow has been tonnes. Perhaps this mass, added over 60 years to the surface of the recognized anywhere on the volcano—including the Puna Ridge, the volcano, weakened it to the point where the edifice failed and the submarine extension of the east rift zone (Smith et al., 2002), though south flank (Fig. 16) slipped seaward, splitting open the east rift rapid sedimentation there might partly obscure such a flow. zone, draining the summit reservoir, and causing collapse as well as D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25 23

about 300 y, and the Uwēkahuna perhaps 1000–1200 y, ending in about 900–1000 CE. Both units formed when a caldera existed at Kīlauea's summit. This paper describes the Keanakākoʻi situation. The Uwēkahuna was erupted from the Powers caldera of Holcomb (1987), the forerunner of the modern caldera. The Uwēkahuna is present on the northwest wall of today's caldera below the Hawaiian Volcano Observatory (Fig. 3, HVO) and is described by Fiske et al. (2009) in two strati- graphic sections, the mid-Bluff and Bluff-base sections. The two sec- tions are about 100 m and 130 m, respectively, below the rim of the Powers caldera as defined at nearby Tree Molds (Fig. 3, TM; Fiske et al., 2009). These relations indicate that the Powers caldera was at least 130 m deep when the Uwēkahuna was erupted. The two sections are overlain by lava flows that built the Observatory shield (Holcomb, 1987) between about 1000 and 1400 CE and later collapsed to form the modern caldera. A caldera setting probably facilitates the potential for phreato- magmatic eruptions, as suggested for Kīlauea by Mastin (1997; Mastin et al., 2004). Both the Uwēkahuna and Keanakākoʻi record numerous

Fig. 16. Map showing locations of ʻAilāʻau flow and shield, east rift zone, south flank, phreatomagmatic eruptions, particularly early in their history, consis- and summit caldera of Kīlauea. The south flank of Kīlauea is mobile, moving seaward tent with vents in a deep caldera where magma and external water several centimeters a year (Swanson et al., 1976; Montgomery-Brown et al., 2009). could interact. Each unit had explosive eruptions later in its develop- ment that may not have involved much external water (Keanakākoʻi ending the eruption. Incremental collapse might be favored if the layer 6, Kulanaokuaiki units 1, 3, and 5)—perhapsasthecalderafill be- magma reservoir consisted of separate bodies (Fiske and Kinoshita, came thick enough to minimize phreatomagmatic activity. 1969), as in a sill-dike complex, rather than a single balloon-like The two examples of prolonged explosive periods suggest that a chamber often used in modeling. caldera may be necessary at Kīlauea to enable such periods. If so, We are left with an unsatisfactory explanation for the origin of the rapid filling of the modern caldera following the 1790 explosive the caldera. Yet understanding the cause(s) of caldera formation eruption (Mastin, 1997) may have sharply minimized the chance at Kīlauea takes on more relevance than might first appear, for the for explosive eruptions to resume. The latest Keanakākoʻi eruption, ongoing Puʻu ʻŌʻō eruption is in its 29th year (in 2011), about half in the early 1800s (Sharp et al., 1987) produced “dry” pumice from the duration of the ʻAilāʻau eruption. What will happen if the current a tall fountain; by that time the caldera had possibly filled sufficiently activity persists for as long as that eruption? The caldera was subsiding to inhibit external water involvement. for the first 20 years of the Puʻu ʻŌʻō eruption and by 2004 had dropped Minor explosive eruptions can take place at Kīlauea's summit about 2 m. This could be interpreted as sagging above an emptying under specific circumstances, even with a full caldera. The phreatic reservoir. The subsidence ended in 2004, however, and has not re- eruptions in 1924 resulted from draining of a lava lake in Halemaʻu- sumed at this writing. maʻu to below the water table (Jaggar and Finch, 1924; Decker and Christiansen, 1984), and one tiny explosive eruption at Halemaʻumaʻu 8. Relation of explosive activity to calderas at Kīlauea in 2008 was apparently caused by choking of a gas vent (Houghton et al., 2011; others in 2008 and 2011 were triggered by rock falls into Two prolonged periods of episodic explosive activity are now lava). But it seems to us that another prolonged period of major ex- reasonably well defined at Kīlauea, represented by the Keanakākoʻi plosive activity will have to await the formation of a new caldera. Tephra and the Uwēkahuna Tephra, including its best studied member, From a short-term hazards perspective, this may be reassuring. In the Kulanaokuaiki Tephra (Fiske et al., 2009). The Keanakākoʻi lasted the long term, however, caldera collapse is likely to occur.

9. Conclusions

The Keanakākoʻi Tephra was produced by multiple eruptions throughout a 300-year period starting in about 1470– 1510 CE. This conclusion, supported by several characteristics of the physical stra- tigraphy and by 97 new AMS 14C ages, differs from the previous view that the Keanakākoʻi was formed during a single eruption in 1790. A challenge being tackled by on-going work is relating single depositional units or packages to changing vent locations and erup- tion styles during the 300 years of explosive activity. Kīlauea's caldera formed between the end of the 60-yr-long ʻAilāʻau eruption in about 1470 CE and the start of the Keanakākoʻi eruptions. This is consistent with oral tradition but 300 years older than commonly assumed. Caldera formation took place during essen- tially the same time period as the initiation of Keanakākoʻi eruptions, but stratigraphic relations indicate that the caldera formed first by collapse. Physical characteristics of the Keanakākoʻi reticulite, and veneers Fig. 17. Reticulite lapilli exposed at Hawaiian Volcano Observatory (Fig. 3, HVO). Note of early ash plastered against today's caldera walls, indicate a depth minor amounts of achneliths and ash. Scale in centimeters and inches. of at least a few hundred meters for the caldera soon after it formed. 24 D.A. Swanson et al. / Journal of Volcanology and Geothermal Research 215-216 (2012) 8–25

Fig. 18. Map showing that the Koaʻe fault system (KFS) and most of the east and southwest rift zones form the landward boundary of the mobile south flank. The east rift zone trends southeastward from the caldera to its intersection with the Koaʻe fault system, where it bends east-northeastward. The Koaʻe fault system is a noneruptive extension of the east rift zone and blends into the southwest rift zone. The Hilina fault system is a series of normal faults in the south flank. Lines are ground cracks and faults north of the south flank, simplified from Neal and Lockwood (2002) and Wolfe and Morris (1996).

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