The Keanakāko‘i Tephra offers an exceptional window into the explosive portion of Kīlauea’s recent past. Once thought to be the products of a single eruption, the deposits instead formed through a wide range of pyroclastic activity during an ~300 yr period following the collapse of the modern caldera in ca. 1500 CE. No single shallow conduit or vent system prevailed during this period, and most of the deposits are confined to distinct sectors around the caldera. Vent position shifted abruptly and repeatedly throughout this time period. This combination of circumstances, influenced by prevailing wind direction, led to rapid lateral changes in the stratigraphy. We define and describe 12 units, several of which are subdivided into subunits or beds, and place them in a framework that reflects volcanologic processes as well as temporal succession.

Eruption style and intensity are exceptionally diverse for a basaltic shield volcano. Bulk tephra volumes range from 10⁶ to 10⁷ m³, and the volcanic explosivity index (VEI) ranges from 1 to 3. The most intense activity included high Hawaiian fountaining eruptions, probably associated with caldera-confined lava flows, and subplinian and phreatoplinian explosions. There was also a wide range of less intense phreatomagmatic activity characterized by different magma/water ratios, with products ranging from ballistic block falls, to cross-bedded pyroclastic density current deposits, to fine-grained ash falls commonly bearing accretionary lapilli.

Resumption of a Keanakāko‘i style and pattern of volcanism, which seems possible given events unfolding in May–July 2018, has serious implication in terms of future volcanic risk. The hazards associated with every style of explosive activity at Kīlauea summit are quite distinct from the dominantly effusive style of the past 200 yr and should be factored into any future evaluation of risk.