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.

ABSTRACT Two small scoria vents were discovered in the Koa‘e fault system, an extensional regime connecting the east and southwest rift zones of Kīlauea that was previously considered to be noneruptive. The chemical composition of the scoria suggests an early to middle nineteenth-century age. The vents prove that magma can intrude several kilometers into the central part of the Koa‘e fault system from the nearest rift zone, supporting previous seismic and geodetic inferences of intrusions into the Koa‘e fault system in the twentieth century. Geodetic studies for the past 50 yr document widening of the Koa‘e fault system at a time-averaged rate of ~4.5 cm/yr, involving mostly coseismic strains, but also creep and displacement related to dike intrusions. These rates are consistent with a longer-term widening rate for the past ~700 yr calculated from crack widths in a lava flow of about that age. The Koa‘e fault system blends into, and is a structural continuation of, the east rift zone. We interpret the locus of intrusion in the east rift zone to have migrated ~6.5 km SE during the past 100,000–125,000 yr, as estimated from linear extrapolation of measured displacement rates across the Koa‘e fault system and east rift zone. The inception of migration is consistent with the onset of the tholeiitic stage at Kīlauea as interpreted by previous studies. As the rift zone moved away from the summit, a marked curvature in the transport pathway developed in order for the rift zone to maintain its connection to the summit magma reservoir. The migration resulted in development of the SE-trending east rift connector, a term we prefer instead of the upper east rift zone. The connector supplies magma to the ENE-trending rift zone from the summit storage complex but is not itself the site of significant magma storage or eruption. The Koa‘e fault system merges into the southwest rift zone, which has been migrating southeastward for an uncertain period of time. Some magma that enters it passes from the summit reservoir complex through the southwest rift connector (seismic southwest rift zone), analogous to the east rift connector. Both connectors reflect the response of magma-transport pathways to asymmetric volcano spreading away from a relatively fixed summit magma reservoir. The ENE structural grain of the Koa‘e fault system and east rift zone pervades Kīlauea’s entire edifice. Most eruptions take place along this trend. The major exception is the southwest rift zone, which may reflect the stresses of Mauna Loa spreading and the Ka‘ōiki fault system. The dominant ENE grain emphasizes the importance of SSE-directed volcano spreading in controlling most of Kīlauea’s tectonic and eruptive behavior.

The previously accepted estimates for the areal extent (200,000 km 2 ) and volume (325,000 to 382,000 km 3 ) of the Columbia River Basalt Group (CRBG) have, upon reevaluation, been found to be too large. New area and volume estimates for 38 units that compose most of the CRBG indicate that it once covered an area of approximately 163,700 ± 5,000 km 2 and has a volume of approximately 174,300 ± 31,000 km 3 . Our work further suggests that the volume of individual flows is huge, on average exceeding hundreds of cubic kilometers. The maximum known volume of an individual flow exceeds 2,000 km 3 , and some flows may have volumes on the order of 3,000 km 3 . Typically such huge-volume flows (here termed “great flows”) were able to travel hundreds of kilometers from their vents, with some flows known to have advanced more than 750 km. The eruption of great flows generally ceased with the end of Wanapum volcanism. The extent and volume of great flows qualifies them as the largest known terrestrial lava flows.

Abstract Mount St. Helens, in the Cascade Range of southwest Washington, is the centerpiece of the Mount St. Helens National Volcanic Monument. Windy Ridge, in the monument about 4.2 mi (7 km) northeast of the volcano, offers the only readily accessible (in 1986) viewpoint for observing many of the effects of the 1980–85 eruption. A series of paved and graveled roads, narrow and curvy but suitable for buses, leads to the ridge (Fig. 1). The area west of Windy Ridge is closed to public access in 1986; for details, contact monument headquarters (U.S. Forest Service, Rt. 1, Box 369, Amboy, WA 98601, telephone 206-247-5473). Roads between Randle and Swift Reservoir are usually closed by snow between early November and early June. Construction of an overlook on Johnston Ridge, 4.8 mi (8 km) northwest of the volcano,(in is planned for the late 1980s; this overlook, which offers a better view of the crater than does Windy Ridge, will be accessible by road from Castle Rock but probably not from Windy Ridge.

Abstract Mount St. Helens, in the Cascade Range of southwest Washington, is the centerpiece of the Mount St. Helens National Volcanic Monument. Windy Ridge, in the monument about 4.2 mi (7 km) northeast of the volcano, offers the only readily accessible (in 1986) viewpoint for observing many of the effects of the 1980–85 eruption. A series of paved and graveled roads, narrow and curvy but suitable for buses, leads to the ridge (Fig. 1). The area west of Windy Ridge is closed to public access in 1986; for details, contact monument headquarters (U.S. Forest Service, Rt. 1, Box 369, Amboy, WA 98601, telephone 206-247-5473). Roads between Randle and Swift Reservoir are usually closed by snow between early November and early June. Construction of an overlook on Johnston Ridge, 4.8 mi (8 km) northwest of the volcano,(in is planned for the late 1980s; this overlook, which offers a better view of the crater than does Windy Ridge, will be accessible by road from Castle Rock but probably not from Windy Ridge.

Abstract This chapter treats two areas of the northwestern United States characterized by great late Cenozoic outpourings of basaltic lava. The western part of this region is underlain by flood basalt of the Columbia River Basalt Group. As discussed below by Waitt and Swanson, this area constitutes the Columbia Plain. The laterally extensive, thick cooling units of the Columbia River Basalt Group contrast with the thinner, less extensive flows of the Snake River Plain, which lies to the southeast. Lavas of the Snake River Plain were emplaced coincident with Pliocene-Quaternary rifting, from about 5 Ma to present. Much of the Columbia Plain is overlain by tens of meters of Pleistocene loess, which is extensively dissected to form the rolling topography of the Palouse Hills. In the northeast, broad channels were carved into the Palouse loess and underlying basalt by cataclysmic Pleistocene floods. The characteristic flood erosion of the basalt impressed early settlers as a scaring of the earth by removing its protective soil, and the term “scabland” was applied to it. The best known geomorphic studies in the Columbia Plateau centered on the role of cataclysmic flooding in the origin of its landscape. The central figure in this research for over half a century was J Harlen Bretz (Fig. 1), a glacial geologist at the University of Chicago. Bretz (1923a) used the name “Channeled Scabland” to describe the area of loess-mantled northeastern Columbia Plain that was scoured by flood channels. In 20 major articles and monographs, mostly published between