The Uwekahuna Ash Member of the Puna Basalt: product of violent phreatomagmatic eruptions at Kilauea volcano, Hawaii, between 2800 and 210014C years ago

Kilauea volcanoʹs reputation for relatively gentle effusive eruptions belies a violent geologic past, including several large phreatic and phreatomagmatic eruptions that are recorded by Holocene pyroclastic deposits which mantle Kilaueaʹs summit area and the southeast flank of adjacent Mauna Loa volcano. The most widespread of these deposits whose original distribution can be reconstructed is the Uwekahuna Ash Member of the Puna Basalt, a basaltic surge and fall deposit emplaced during two or more eruptive episodes separated by a few decades to several centuries. The first episode occurred between 2770 ± 70 and 2265 ± 5014C yr ago. It included two major pyroclastic surges, each preceded by unusually vigorous lava fountaining from a vent near the volcanoʹs summit. Before the second eruptive episode, 2110 ± 12014C yr ago, plants had re-colonized the rainforest environment northeast of the summit, and at least two lava flows from Mauna Loa had buried parts of the first-episode deposits. The second episode also began with vigorous lava fountaining, followed by widespread lithic ashfall, a third major surge and finally a fourth fountaining event. Before the final pumice deposit could be significantly reworked, it was partly buried by picritic basalt flows that are unusual in Kilaueaʹs summit area. ximal areas, the Uwekahuna Ash Member is more than 1 m thick (locally > 5 m) and includes lithic blocks up to 0.8 m in diameter. Coarse, primarily lithic debris was deposited around the vent by laterally expanding surges; fallout deposits accumulated preferentially to the northeast under the influence of high-altitude counter-tradewinds. The area devastated by surges and originally buried by at least 15 cm of the Uwekahuna was about 420 km2. The bulk volume of the deposits was approximately 0.3 km3, including less than 0.1 km3 of juvenile material. Juvenile constituents are olivine-tholeiitic basalts similar in major-element composition to typical Kilauea summit lava flows, but variations in both major elements and trace elements suggest that the eruptions tapped more than a single, uniform source region. er that the eruptions which produced the Uwekahuna were driven by water interacting with a fluctuating magma column. Magma withdrawal episodes may have been accompanied by large-volume submarine effusive eruptions and by summit collapse. The volume, extent and character of the Uwekahuna deposits underscore the hazards posed by relatively infrequent but potentially devastating explosive eruptions at Kilauea, as well as at other basaltic volcanoes.