ABSTRACT 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 10 6 to 10 7 m 3 , 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.

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.

ABSTRACT The golden pumice deposit (unit K1) represents one of the latest episodes of Hawaiian fountaining in the Keanakāko‘i Tephra and is the product of the first high fountaining eruption at Kīlauea summit in ~300 yr, since the caldera formed in ca. 1500 CE. We present a new physical characterization of the deposit based on over 200 field sites, all affected by severe erosion, alteration, and silicic encrusting. We detail the deposit geometry, stratigraphic and structural relationships, and componentry to constrain its volume and reconstruct the eruptive sequence. The deposit is then discussed and set against other young episodes of high fountaining at Kīlauea. We interpret the golden pumice as the product of a days-long eruptive sequence with a source located inside a caldera much deeper than that of today. The eruption probably started along a NE-SW–oriented fissure and migrated toward a single vent in the southwestern part of the caldera, where at least two high Hawaiian-style fountains produced a tephra deposit of ~6 × 10 6 m 3 . Stratigraphic contacts reveal that erosion occurred not only between, but also during the fountaining episodes, suggesting heavy rainfall during deposition. Field observations during this study also led to the discovery of the first stratigraphic evidence that the eastern pumice postdates the golden pumice, which contributes to the new definition of the stratigraphy of the Keanakāko‘i Tephra presented in this volume.

Fontana Tephra was erupted from the Masaya area in west-central Nicaragua in the late Pleistocene. This basaltic-andesitic Plinian eruption evolved through (1) an initial sequence of short, highly explosive pulses emplacing thinly stratified fallout lapilli, (2) emplacement of a surge to the southwest while fallout took place in the northwesterly dispersal sectors, (3) a series of quasi-steady Plinian episodes depositing massive fallout beds of highly vesicular scoria lapilli, and (4) a terminal phase of the eruption comprising numerous subplinian eruption pulses in which varying amounts of external water were involved, forming a well-stratified sequence of lapilli beds. The Plinian episodes were repeatedly interrupted by phreatomagmatically affected pulses, evidenced by layers of higher lithic contents and scoria clasts with quenched rims, as well as by proximal cross-bedded fine to medium lapilli pyroclastic surge deposits, which left pale ash partings at distal locations. Erupted tephra volumes, column heights, and wind velocities have been estimated for three different vent scenarios because no firm source location could be identified. The minimum total erupted tephra volume is between 1.4 and 1.8 km 3 , much lower than previous estimates for this eruption. Eruption column heights ranging from 24 to 30 km for the Plinian eruptive phases were obtained by comparing lithic and scoria distribution data with modeling results. Consistent results from different approaches suggest that these models, which were developed for dacitic to rhyolitic Plinian eruptions, also provide good approximations for basaltic Plinian eruptions considering all sources of uncertainty.

The pyroclastic deposits, known as Tierra Blanca Joven, underlie most of metropolitan San Salvador and other areas surrounding Lake Ilopango. The Tierra Blanca Joven deposits are products of a complex sequence of pyroclastic flows and falls that occurred during the A.D. 430 eruption of Ilopango Caldera. Very fine, compact white ash-lapilli predominates in both flow and fall units. Laboratory tests carried out on high-quality, undisturbed Tierra Blanca Joven samples show negative pore-water pressures and weak cementation. They also reveal how the strength and compressibility of these sediments can change significantly when the suction and bonding are lost upon soaking or remolding. Thick Tierra Blanca Joven deposits contribute to landslide risk during heavy rainfalls and strong earthquakes in the region.