The Keanak??ko'i Tephra at K??lauea Volcano has previously been interpreted by some as the product of a caldera-forming eruption in 1790 CE. Our study, however, finds stratigraphic and 14C evidence that the tephra instead results from numerous eruptions throughout a 300-year period between about 1500 and 1800. The stratigraphic evidence includes: (1) as many as six pure lithic ash beds interleaved in sand dunes made of earlier Keanak??ko'i vitric ash, (2) three lava flows from K??lauea and Mauna Loa interbedded with the tephra, (3) buried syneruptive cultural structures, (4) numerous intraformational water-cut gullies, and (5) abundant organic layers rich in charcoal within the tephra section. Interpretation of 97 new accelerator mass spectrometry (AMS) 14C ages and 4 previous conventional ages suggests that explosive eruptions began in 1470-1510 CE, and that explosive activity continued episodically until the early 1800s, probably with two periods of quiescence lasting several decades. K??lauea's caldera, rather than forming in 1790, predates the first eruption of the Keanak??ko'i and collapsed in 1470-1510, immediately following, and perhaps causing, the end of the 60-year-long, 4-6km 3 'Ail??'au eruption from the east side of K??lauea's summit area. The caldera was several hundred meters deep when the Keanak??ko'i began erupting, consistent with oral tradition, and probably had a volume of 4-6km 3. 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 'Ail??'au eruption, causing episodic collapses before the final, largest downdrop took place. Two prolonged periods of episodic explosive eruptions are known at K??lauea, the Keanak??ko'i and the Uw??kahuna 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 Halema'uma'u Crater) or the formation of a new caldera. ?? 2011.
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Keanak??ko'i Tephra produced by 300years of explosive eruptions following collapse of K??lauea's caldera in about 1500CE