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[1] The 1883 eruption of Krakatau volcano in Indonesia was one of the most explosive volcanic events in history. It was a marine caldera-forming eruption that resulted in voluminous ignimbrite deposits and huge tsunamis. We have used numerical simulations to investigate three major mechanisms for tsunami generation: caldera collapse, phreatomagmatic explosion, and pyroclastic flow, and have constrained the source parameters. Computed tsunami characteristics for each hypothesis are compared with observations at locations along the coasts of the Sunda Strait, where tsunami data were obtained immediately after the eruption. For the pyroclastic flow hypothesis, two types of two-layer shallow water models, dense- and light-type models, were used under different initial conditions. Pyroclastic flows are erupted from a circular source following a sine function that assumes waning and waxing phases. Caldera collapse was performed using a simple piston-like plunger model, in which collapse duration was assumed to be up to 1 h. The phreatomagmatic explosion hypothesis was examined using simple empirical models for underwater explosions in shallow water, with explosion energy between 1016 and 1017 J. The results show that when a pyroclastic flow with a volume of >5 km3 and an average discharge rate of the order of 107 m3/s enters the sea, the computed tsunami heights are broadly consistent with historical records in coastal areas, including a tide gauge record at Batavia (now Jakarta). We conclude that a pyroclastic flow entering the sea is the most plausible mechanism of the 1883 Krakatau tsunami.

[1] Kasatochi volcano in the central Aleutian Islands erupted unexpectedly on 7–8 August 2008. Kasatochi has received little study by volcanologists and has had no confirmed historical eruptions. The island is an important nesting area for seabirds and a long-term biological study site of the U.S. Fish and Wildlife Service. After a notably energetic preeruptive earthquake swarm, the volcano erupted violently in a series of explosive events beginning in the early afternoon of 7 August. Each event produced ash-gas plumes that reached 14–18 km above sea level. The volcanic plume contained large amounts of SO2 and was tracked around the globe by satellite observations. The cumulative volcanic cloud interfered with air travel across the North Pacific, causing many flight cancelations that affected thousands of travelers. Visits to the volcano in 2008–2009 indicated that the eruption generated pyroclastic flows and surges that swept all flanks of the island, accumulated several tens of meters of pyroclastic debris, and increased the diameter of the island by about 800 m. Pyroclastic flow deposits contain abundant accidental lithic debris derived from the inner walls of the Kasatochi crater. Juvenile material is crystal-rich silicic andesite that ranges from slightly pumiceous to frothy pumice. Fine-grained pyroclastic surge and fall deposits with accretionary lapilli cover the lithic-rich pyroclastic flow deposits and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions. Pyroclastic deposits completely cover the island, but wave erosion and gully development on the flanks have begun to modify the surface mantle of volcanic deposits.

[1] The geologic origin of the Medusae Fossae Formation (MFF) has remained a mystery despite three decades of research. To better constrain its formation, an in-depth analysis of observations made in the literature was combined with a new survey of over 700 Mars Orbiter Camera narrow-angle images of the MFF to identify morphologic characteristics and material properties that define this formation as a whole. While previous work has identified clear agreement on some characteristics, our analysis identifies yardangs, collapse features, and layering as pervasive features of the MFF. Whereas collapse features and layering may implicate several different physical and chemical processes, yardangs provide vital information on material properties that inform about mechanical properties of the MFF lithology. Aspect ratios of megayardangs range from 3:1 to 50:1, and slope analyses reveal heights of up to 200 m with cliffs that are almost vertical. Other yardangs show lower aspect ratios and topographic profiles. These characteristics coupled to the presence of serrated margins, suggest that MFF lithology must be of weakly to heavily indurated material that lends itself to jointing. The characteristics and properties of the MFF are inconsistent with those of terrestrial pyroclastic fall deposits or loess, but are in common with large terrestrial ignimbrites, a hypothesis that explains all key observations with a single mechanism. Yardang fields developed in regionally extensive ignimbrite sheets in the central Andes display morphologic characteristics that correlate with degree of induration of the host lithology and suggest an origin by pyroclastic flow for the MFF.

[1] During the A.D. 79 eruption of Vesuvius, Italy, the Roman town of Pompeii was covered by 2.5 m of pyroclastic fall pumice and then partially destroyed by pyroclastic density currents (PDCs). Thermal remanent magnetization measurements performed on the lithic and roof tile fragments embedded in the PDC deposits allow us to quantify the variations in the temperature (Tdep) of the deposits within and around Pompeii. These results reveal that the presence of buildings strongly influenced the deposition temperature of the erupted products. The first two currents, which entered Pompeii at a temperature around 300–360°C, show drastic decreases in the Tdep, with minima of 100–140°C, found in the deposits within the town. We interpret these decreases in temperature as being the result of localized interactions between the PDCs and the city structures, which were only able to affect the lower part of the currents. Down flow of Pompeii, the lowermost portion of the PDCs regained its original physical characteristics, emplacing hot deposits once more. The final, dilute PDCs entered a town that was already partially destroyed by the previous currents. These PDCs left thin ash deposits, which mantled the previous ones. The lack of interaction with the urban fabric is indicated by their uniform temperature everywhere. However, the relatively high temperature of the deposits, between 140 and 300°C, indicates that even these distal, thin ash layers, capped by their accretionary lapilli bed, were associated with PDCs that were still hot enough to cause problems for unsheltered people.

[1] A large, 154 km diameter dark annular ring, located in the southwestern part of the Orientale Basin along the Montes Rook ring, was first discovered and documented in Soviet Zond 8 images. Clementine UV-visible multispectral data indicate that the dark ring consists of material similar to the pyroclastic glasses collected by the Apollo astronauts, with the glasses being more closely related to the orange glass beads that comprise the Aristarchus Plateau than to the crystallized black beads typical of Taurus-Littrow. This implies relatively rapid cooling times for the eruption products. We propose that the dark ring is the manifestation of a pyroclastic eruption originating at a fissure vent, an elongate 7.5 km by 16 km depression, located near the center of the ring. The event producing the eruption began with a dike rapidly emplaced from subcrustal depths to within ∼3–4 km of the surface. The dike stabilized and degassed over ∼1.7 years to form an upper foam layer which then penetrated to the surface to cause an eruption, lasting ∼1–2 weeks. The eruption produced a ∼38 km high symmetrical spray of pyroclasts into the lunar vacuum at velocities of ∼350 to ∼420 m/s, and the pyroclastic material accumulated in a symmetrical ring around the vent. The geometry of eruption caused the deposits to accumulate preferentially in a ring representing the material ejected at 45°. The paucity of pyroclastic rings of this type on the Moon can be attributed to the low probability of a dike stalling at just the right depth (∼3–4 km) to create these eruption conditions. The detailed characteristics of this ring provide important new insight into the emplacement of pyroclastics in the more regionally continuous lunar dark mantle deposits, suggesting that their sources are dominated by effusive and Hawaiian-style eruptions. The Orientale dark ring deposit has similarities to the pyroclastic rings on the Galilean satellite Io. Both the Orientale dark ring and Ionian eruptions involve acceleration of small particles to a high velocity in an expanding gas stream, silicate pyroclasts derived from the disrupted foam layer in the lunar case, and a mixture of silicate pyroclasts and “snowflakes” of condensing SO2 solids on Io. In both cases the silicate pyroclasts ejected from the vent are accelerated by the gas until they become decoupled and continue on ballistic trajectories controlled only by gravity. Differences in mass flux and cloud opacity between the Moon and Io are the direct result of the origin and mass fraction of the volatile phase: the accumulation of a magmatic foam layer on top of a silicate intrusion in the lunar case and the intimate mixing on Io of a steadily erupting magma and liquid SO2. The interpretation of the Orientale dark mantle ring as a pyroclastic eruption provides an alternative to the hypothesis that the dark ring represents the presence of an ancient pre-Orientale impact structure and that the Orientale Basin cavity of excavation must therefore lie within the Outer Rook Mountain ring.

Many pyroclastic flows have runout distances longer than expected from basic frictional arguments. In these cases the ratio of the height descended H and the runout L are as small as ∼0.2, a feature which has been attributed to fluidization. However, significant fluidization by gases is now considered unlikely during emplacement of debris avalanches, where H/L and volume are inversely related. We determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout. H/L and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits, as well as 15 pyroclastic flow deposits. Systematic differences in the volume versus H/L plots were explored by statistical analysis. Results show an inverse relationship between H/L and volume for both nonvolcanic and volcanic debris avalanche deposits, as expected, and also for pyroclastic flow deposits. Regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non-volcanic debris avalanche line is parallel to the other two. We argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms. Therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable H/L versus volume, it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism. Based on the data in hand, we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large-volume debris avalanches.

The Glass Mountain volcanic complex represents the oldest rhyolites of the Long Valley magma system. At least 30 high-silica rhyolites erupted in two major episodes: a minimum of 4.8 km3 (DRE) of lava erupted sporadically between 2.1 and 1.2 Ma, and a minimum of 10.5 km3 erupted at a greater volumetric rate 1.1–0.8 Ma. The rhyolite complex was truncated as a result of caldera collapse on eruption of the Bishop Tuff at 0.73 Ma. Early Glass Mountain lavas are biotitebearing and were cooler and less fluid than later ones, some of which contain pyroxene. This correlates with a crude, long-term trend with time toward less evolved rhyolite compositions; all Glass Mountain lavas are more evolved than the late-erupted Bishop Tuff. A large, integrated, magma chamber may not have been present until ∼1.1 Ma, as evidenced by the greater chemical homogeneity of lavas of the younger eruptive episode. Glass Mountain is flanked by a pyroclastic apron containing ash-flow tuffs and pumice fall deposits, and correlatives of some of the latter have been found as far away as Ventura County, California, and Beaver County, Utah. Explosive activity occurred throughout the eruptive history of Glass Mountain and is not restricted to more evolved compositions. Although apparent volumetric eruptive rates increased about six fold after 1.1 Ma, the eruptions apparently were not large enough to depressurize the magma chamber and initiate a caldera-forming eruption. As Long Valley is a tectonically active area, release of pressure on the magma chamber due to rupture along the Hilton Creek-Hartley Springs fault zone seems the most likely mechanism for triggering the catastrophic eruption of the Bishop Tuff.

The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7–10 km depth at a rate of 1.94×107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920°–940° C), and confining pressure (190–250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95-m-diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model-derived vent diameters based on 0.1-MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.

About 22,000 years ago a series of large-scale pyroclastic eruptions produced the Aira caldera 20 km×20 km wide at the northern end of Kagoshima Bay in southern Kyushu. It started with a Plinian pumice eruption (Osumi pumice fall, 98 km3) followed by oxidized, fine-grained Tsumaya pyroclastic flow (13 km3), both erupted from a vent located at the present site of Sakurajima volano, 8 km south of the caldera center. After a very short pause, violent explosive ejection of the basement rock fragments and pumiceous materials occurred at the central vent, gradually changing itself to a huge eruption column rapidly collapsing to form the Ito pyroclastic flow about 300 km3 in volume. The earliest phase produced up to 30-m-thick Kamewarizaka breccia developed along the caldera rim and charged with basement (lithic) fragments up to 2 m across. The breccia is a near-vent variety of the bottom concentration zone of lithics in the Ito deposit. Various textural features and monotonous petrologic character indicate that the main part of the Ito pyroclastic flow was emplaced by a simple, short-lived eruptive mechanism. The Aira-Tn ash, a fine-grained counterpart of the Ito pyroclastic flow, covered a wide area more than 1000 km from the vent. Evacuation of more than 110 km3 of rhyolitic magma produced a funnel-shaped collapse structure with the center of the magma chamber about 10 km deep. Like many other Japanese Quaternary calderas, the Aira caldera is considered to heave formed not by a piston cylinder-type subsidence utilizing a ring fracture but by coring and high-angle slumping of the wall rocks into a funnel-shaped central vent. The outline of the caldera was strongly controlled by the faults bounding the volcano-tectonic graben forming Kagoshima Bay.

Strongly peralkaline volcanic complexes tend to be shieldlike due to coalescence of low-viscosity lavas from numerous vent areas and blanketing by pyroclastic flows and falls. Dense welding and rheomorphism commonly mask the nature of pyroclastic units; welded pumice falls are common. Relatively low eruptive columns produce poorly sorted pumice falls that are difficult to distinguish from topography mantling pyroclastic flows. Welded fall deposits are characterized by multiple, fine-scale, welding reversals, welding zones that correspond with stratification, and by flattening of fiamme parallel to underlying slopes, whereas welded topography-mantling ignimbrites may have fine-grained basal layers, lithic lenses, imbricate fiamme, and variations in crystal content and fiamme size with topographic position. Calderas are relatively small, typically 3–9 km in diameter, with little evidence for single-stage subsidence in excess of a few hundred meters. Many strongly peralkaline volcanic centers show evidence of two or more nested calderas, and partial reactivation of older ring fractures is common. Replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures, resulting in multicyclic collapses that reuse these structures. In several centers, the caldera-forming unit is zoned from pantellerite to more crystal-rich trachyte. Following collapse, trachyte lavas commonly erupt from a central vent on the caldera floor, building a cone that nearly fills the caldera. Reestablishment of isostatic equilibrium may be accomplished both by eruption of these lavas and by flow of trachytic and more-mafic magma into the root zone beneath the volcano. Resurgent doming sensu strictu has not been documented in strongly peralkaline systems; at Pantelleria and Mount Suswa, however, the central portions of the cauldron blocks were uplifted and tilted. Magma reservoirs that feed explosive eruptions generally are shallow, as indicated by pyroclastic units that contain lithic fragments of the volcanic edifice (commonly including cognate syenite) but lack subjacent crustal lithologies. In addition, the overwhelming predominance of alkali feldspar as a phenocryst phase and the scarcity of quartz are consistent with pantellerite liquidus relations only at pressures