Eruption and emplacement dynamics of coarse-grained, wall rock–rich beds in the Keanakāko‘i Tephra, Kīlauea, Hawai‘i

ABSTRACT A series of coarse-grained, relatively well-sorted, but wall rock–rich pyroclastic deposits within Unit H of the Keanakāko‘i deposits at Kīlauea Volcano, Hawai‘i, is the focus of this study. These “c” subunits within Unit H consist of alternations between very coarse and relatively well-sorted pyroclastic fall deposits and products of relatively concentrated pyroclastic density currents. They are associated with both accretionary lapilli–bearing ash falls (a beds) and cross-bedded, fine-grained pyroclastic density current deposits (b beds). The Unit H sequence is related to phreatomagmatic explosions from multiple sources in the modern caldera, and we infer that most vents for the c subunits were located near the southern part of the caldera. The c beds contain varying proportions of dense, outgassed juvenile bombs and hydrothermally altered wall rock that suggest, along with coarser grain size and good sorting, that fragmentation conditions were relatively dry for phreatomagmatic eruptions and were perhaps aided by the release of magmatic gases from a deep magma source. The c fall subunits, with thinning half distances of 200–300 m, are more widely dispersed than both the most powerful Hawaiian fountaining eruptions and the well-documented historical explosive eruptions at Kīlauea, with proximal dispersal rates similar to historical subplinian eruptions at other volcanoes. The c pyroclastic density currents were erosive and of a style that represents a threat that is underrated at Kīlauea.