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USGS2019

Mini-columns and ghost columns in Columbia river lava

The master joints bounding the columns that make up the basal colonnade of large lava flows of the Columbia Plateau are, in places, flanked by sub-horizontal mini-columns that have grown normal to the master joints. The secondary mini-columns grow into the main columns and are clearly younger than them. They are small adjacent to the master joint, but merge together and thicken away from the fracture toward the master column interior. Commonly the mini-columns are one-half meter in length and 2–12 cm in diameter. Where the horizontal mini-columns grow longer they intersect toward the middle of the master joints. This plexus of joints changes the aspect of the original master columns making them almost unrecognizable producing ghost columns . The basalt flow may acquire an entablature-like appearance where the ghost column outlines disappear due to extensive secondary fracturing. At the time that the hot flow center had cooled sufficiently below the brittle-plastic transformation, the primary vertical basal colonnade joints growing up from the bottom connected with those in the upper colonnade growing down. This allowed steam trapped beneath the flow to be released to the surface and ushered in a change from a conduction-cooling regime to a convection-cooling regime. The steam beneath the flow was formed and sustained by heat from the lava that boiled the groundwater in the underlying substrate. Large volumes of the rising steam was on average much cooler than the hot fractures in the flow interior through which it passed, causing contraction of the master column walls to produce the secondary horizontal mini-columns. The presence of mini-columns indicates emplacement of lava over moist ground and are absent where the lavas advanced across arid areas or flowed over recently-erupted lava. The extreme shattering that forms ghost columns by late stage convective cooling can produce a flow layer of considerable thickness, a layer that can later serve as an aquifer with high porosity and permeability.

Journal of Volcanology and Geothermal Research↗
USGS2019

3-D seismic tomographic study of Sinabung Volcano, Northern Sumatra, Indonesia, during the inter-eruptive period October 2010-July 2013

We estimated 3D P-wave velocity (Vp), S-wave velocity (Vs), and Vp/Vs tomographic structures in and around the Sinabung Volcano area, Northern Sumatra, Indonesia during the intereruptive period between October 2010 and July 2013 using high-quality P- and S-wave phases from > 600 local volcano tectonic (VT) earthquakes and more than 5000 phase picks. We used the iteritive damped-least-squares method of SIMULPS12 to simultaneously invert for the velocity structure and hypocenter relocations. We find that the majority of the VT earthquakes locate beneath Sinabung volcano to depths of about 8 km, with distal hypocenters dominantly to the north and northwest of the summit. We find six anomalous regions with the most notable including an area of low seismicity, high Vp/Vs and low Vp at depths of around 5 km slightly south of the summit. We believe this anomaly most likely represents hot material with some partial melt that may be the source for the 2010 phreatic eruptions and continued unrest throughout the study period. We also find a region below the summit with high seismicity, high Vp, high Vs and low Vp/Vs that we believe likely represents old intrusive material that has been fractured by the processes of the 2010 phreatic eruptions. Other shallow anomalous values of Vp, Vs and Vp/Vs are consistent with the known geology of Sinabung: anomalies of high Vp and Vs likely represent old, well-consolidated rock, and are observed to the SW and NE of Sinabung; while low Vp, low Vs and high Vp/Vs likely represent increased fracture densities, temperatures, or gas contents in geothermal features to the west of the summit. Finally we compare our results to those of the tomographic study during the October to November 2013 eruptive period in Nugraha et al. (this issue) and find evidence that suggests magma rose from the lower region imaged in our study to within a few kilometers of the surface.

Journal of Volcanology and Geothermal Research↗
USGS2019

Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy

The dacite of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene tephra eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and phenocryst assemblage establish that certain conspicuous dissected pumice exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene Plinian eruption . Deposit thicknesses and pumice sizes support an eruption on the order of low Volcanic Explosivity Index (VEI) 5, atypically explosive for dominantly lava-producing Mount Rainier. Statistically permissible 40 Ar/ 39 Ar plateau ages of plagioclase phenocryst separates are 138 ± 20 ka and 101 ± 11 ka (2σ). A previously published result of 206 ± 11 ka is herein shown to result from a sample selection error. Zircon from the pumice yields a U-Th crystallization age of 147 ± 8 ka if the isochron is required to pass through the tephra U-Th isotopic composition . In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved 40 Ar/ 39 Ar ages of stratigraphically associated lavas high on Mount Rainier, and is the best estimate of the pumice's true eruption age. Inclusions of undegassed melt (glass) in the plagioclase separates are proposed as biasing apparent 40 Ar/ 39 Ar plateau ages to old values through coupling of undegassed magmatic excess Ar with radiogenic Ar that accumulated post-eruptively from relatively K-rich glass. U-Th ages record zircon growth prior to eruption, consistent with a possible complex history of advanced solidification followed by remobilization . The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate Glaciation (MIS 6) and Last Glacial Maximum (MIS 2). This eruption age also provides an important time marker for glacial and other sedimentary deposits in southern Puget Sound lowland that, excepting the Vashon Drift (MIS 2), generally lack reliable age determinations.

Washington↗
USGS2019

A process-based model of pre-eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes

Using long-term data sets, the eruptive size, timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns. The USAID-USGS Volcano Disaster Assistance Program (VDAP) utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size, style and onset times, as well as changes within ongoing eruptions. From millions of seismic event observations, we show that seismic data, a proxy for rate-dependent strain changes in the magmatic system, combined with a process-based conceptual geologic model enables eruption forecasting, especially for stratovolcanoes dormant >20 years. We show that this model often works for some open systems, ones that sufficiently reseal to plug the conduit, and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun. This model is divided into four seismicity and geologic stages. We describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage: Stage 1. Deep intrusion and seismicity occurring at 10 to 40 + km depth below the volcanic edifice; Stage 2. Distal VT seismicity induced at distances from 2 to >30 km laterally from the summit by magma intruding into and inflating the upper-crustal magma storage region; Stage 3. Vent-clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity. Stage 4. Repetitive event seismicity associated with the final ascent of magma from 2 km to the surface. We present 36 examples of this seismic progression from 26 volcanoes. We believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions.

Journal of Volcanology and Geothermal Research↗
USGS2019

A global synthesis of lava lake dynamics

Active lava lakes represent a variety of open-vent volcanism in which a sizeable body of lava accumulates at the top of the magma column, constrained by the vent and/or crater geometry. The longevity of lava lakes reflects a balancing of cooling and outgassing occurring at the surface by input of hot and gas-rich magma from below. Due to their longevity and relative accessibility, lava lakes provide a natural laboratory for studying fundamental volcanic processes such as degassing, convection and cooling. This article examines all seven lakes that existed at the time of writing, located in the Pacific, Antarctica, Africa, and South and Central America. They span all tectonic environments, and a range of magma compositions. We focus on analysis of the lake surface motion using image velocimetry, which reveals both similarities and contrasts in outgassing and lake dynamics when comparing the different lakes. We identify two categories of lake behavior: Organized (Erta ’Ale, Nyiragongo, K¯ılauea after 2011, and Erebus) and Chaotic (Villarrica,Masaya, Marum). This division does not map directly to lake size, viscosity, gas emission rate, or temperature. Instead, when examined together, we find that the lakes follow a linear relationship between average surface speed and the ratio of total gas flux to lake surface area. This relationship points to the importance of both flux and lake size in addition to the total volumetric outgassing rate, and suggests a shared deep mechanism controls the supply of heat and gas to all lakes. On the other hand, the differences between Chaotic and Organized lakes highlight the important role of the geometry of the conduit-lake transition, which superimposes a shallow signal on that of the deep circulation. The spatial patterns of surface motion we document suggest the release of gas bubbles at Chaotic lakes is more efficient (i.e., bubbles are less likely to be retained and recycled) compared with Organized lakes. In addition, the data presented here indicate that the solidified crust of Organized lakes plays a role in regulating convection and outgassing in lava lakes.

Journal of Volcanology and Geothermal Research↗
USGS2019

3D electrical conductivity imaging of Halema‘uma‘u lava lake (Kīlauea volcano)

Before the 2018 collapse of the summit of Kīlauea volcano, a ca. 200 m in diameter lava lake inside of Halema‘uma‘u crater was embedded in a very active hydrothermal system. In 2015, we carried out an electrical conductivity survey and the data were inverted in 3D. The lack of conductivity contrast precludes distinguishing the lava column from the surrounding hydrothermal zones. Laboratory measurements on samples from the lava lake show that the conductivity of magma is significantly lower than that of hydrothermal zones but the high vesicularity of the upper part of the lava lake may decrease its macroscopic conductivity. Based on the 3D conductivity model, we distinguish at least two types of hydrothermal circulations: 1) one guided by the collapse faults of Halema‘uma‘u crater and by the magmatic column of the lava lake, and 2) another guided by previous caldera faults and fractures related to intrusions. We observe that the location of the faults formed during the 2018 collapse of the summit was greatly influenced by the hydrothermally altered zones.

Hawaii↗
USGS2020

Understanding melt evolution and eruption dynamics of the 1666 C.E. eruption of Cinder Cone, Lassen Volcanic National Park, California: Insights from olivine-hosted melt inclusions

Cinder Cone is the youngest scoria cone volcano in the continental United States. Erupted in 1666 C.E. within what is now Lassen Volcanic National Park, Cinder Cone is an un-vegetated scoria cone with well-preserved lava flows and tephra deposits that display complex geochemical variability. In this study, we utilize the volatile (H 2 O, CO 2, Cl), major, and trace element chemistry of olivine-hosted melt inclusions from the tephra deposit of Cinder Cone to better understand the sub-surface evolution of magmas that erupt to produce scoria cones. High-Fo olivine phenocrysts from all erupted units contain melt inclusions that are more primitive in composition than the erupted material. The evolved compositions of the lava and bulk tephra and the abundance of quartz xenocrysts within the deposits suggest the basaltic parental magmas were rapidly contaminated by granitic material in the middle to upper crust, after melt inclusion entrapment. Distinct compositional variability between early and late erupted units suggests two different mantle-derived basaltic magmas were tapped and erupted sequentially as two distinct eruptive phases. The CO 2 concentrations in the melt inclusions, after correction for the presence of vapor bubbles, suggest minimum entrapment depths of ~9.5–20 km and show no resolvable differences between early and late erupted units at the time of olivine crystallization. Diffusion modeling of Ni and Fo gradients in olivine rims indicates that olivine residence times in an evolving magma were on the order of weeks to years, similar to those calculated for longer-lived scoria cone eruptions, such as Jorullo, in Mexico. Additionally, geochemical evidence suggests that the evolution of parental magmas was likely driven by the partial melting, disaggregation, and assimilation of granitic material in the upper crust. Our combined results provide new insight into the complexities of short-lived monogenetic eruptions.

California↗
USGS2019

Hydrothermal fluid migration due to interaction with shallow magma: Insights from gravity changes before and after the 2015 eruption of Cotopaxi volcano, Ecuador

On August 14, 2015 Cotopaxi Volcano (Ecuador) erupted with several phreatomagmatic explosions after nearly 135 years of quiescence. Unrest began in April 2015 with an increase in the number of daily seismic events and inflation of the flanks of the volcano. Time-lapse gravity measurements started at Cotopaxi volcano in June 2015. Although minor gravity changes were detected prior to eruptive activity, however, the largest gravity variations at Cotopaxi were measured between October 2015 and March 2016, when other geophysical parameters had reached background levels. Inverse modelling of GPS data suggests a deep intrusion prior to the eruptive activity, while inverse modelling of post-eruptive gravity changes suggests variations in the volcano hydrothermal system. Deformation, seismicity, and gravity changes are consistent with the intrusion of a deep magmatic source between April and August 2015. Part of the magma rose from depth and interacted with the hydrothermal system, causing the phreatomagmatic activity and pushing hydrothermal fluids from a deep aquifer into a shallow perched aquifer.

Journal of Volcanology and Geothermal Research↗
USGS2019

Interferometric synthetic aperture radar study of recent eruptive activity at Shrub mud volcano, Alaska

Shrub mud volcano is one of three large mud volcanoes that comprise the Klawasi Group in the Copper River Basin of southcentral Alaska. Except for minor discharges in the mid-1950s when the group was first described, Shrub was dormant prior to its reactivation in summer 1996. From 1997 to 1999, Shrub vigorously erupted more than 5 x 105 cubic meters of saline mud and carbon dioxide-rich gas at temperatures as high as 54 degrees C. Thereafter, activity waned but continued at least through 2015. We analyzed 192 interferograms derived from 106 synthetic aperture radar (SAR) images acquired by the JERS-1 (L-band), ERS-1/2 (C-band), RADARSAT-1 (C-band), and ALOS PALSAR (L-band) satellites to characterize ground deformation at Shrub before, during, and after its reactivation. Collectively, the interferograms span 1992–2000 and 2006–2011. We fit the observations with two deformation sources: a deflating, steeply dipping, pipe-like body under the summit area and an inflating, shallow-dipping, sill-like body under the southwest flank. Both sources are shallow, with centroids less than 1 km beneath the summit. Prior to reactivation, the flank source inflated ~0.35 x 105 cubic meters per year from July 1992 to May 1996. During eruptive activity, the summit source deflated at higher rates that peaked at ~8.71 x 105 cubic meters per year during May–November 1997 and continued at ~0.95 x 105 cubic meters per year during the 2006–2011 observation window. Cumulative source-volume loss is comparable to the volume of mud erupted. We interpret the summit source as the volcano’s feeder conduit that pressurized prior to the first SAR observation in 1992. Also before 1992, the conduit ruptured to feed a lateral intrusion of mud under the southwest flank, perhaps along a bedding plane in underlying glaciolacustrine deposits. The growing sill caused the southwest flank to inflate while it accommodated the mud supply from depth, which explains why we observed pre-eruptive inflation of the flank but not the summit. The summit began deflating when the conduit ruptured to the surface at the onset of eruptive activity. The flank source did not deflate concurrently because the weight of the thin overburden was insufficient to collapse the sill. There is a suggestion in the modern topography that lateral intrusions under Shrub’s southwest flank are a common feature of activity there.

Alaska↗
USGS2020

Influence of grain size and shape on volcanic ash electrical conductivity

Few studies have examined the electrical properties of volcanic ash or considered the effects of physical characteristics, such as grain size and shape on its electrification. This study measures the resistivity of eight volcanic ash samples, three milled-samples and five natural ashfall samples from Alaska, U.S.A., using a current amplifier and examines the influence of particle size and particle shape on calculated grain conductivity. Grain conductivities are calculated by applying spherical, oblate, and prolate ellipsoidal shape parameters within a general effective media equation. Volcanic ash is an electrically insulating material with average resistivity values ranging from 1.732E 7 to 1.606E 12 Ωm. Results show that grain size has a limited impact on resistivity. Calculated grain conductivities remained within the same order of magnitude between the three different shape calculations ranging between 1.367E − 7 and 2.196E − 12 S/m using the spherical shape parameter. Electrical measurements of heterogeneous powders are complex and challenging. This study aims to expand the community's dataset and understanding of the electrical properties of volcanic ash and the impact of different physical characteristics. This has implication for understanding ash hazards to electrical infrastructure and volcanic lightning.

Journal of Volcanology and Geothermal Research↗
USGS2020

Seismic tomography of compressional wave velocity and attenuation structure for Makushin Volcano, Alaska

Located in the central portion of the Alaska-Aleutian subduction zone, Makushin Volcano is among the most active volcanoes in the United States and has been classified as a very high threat by the National Volcano Early Warning System based on its eruptive history and proximity to the City of Unalaska and international air routes. In 2015, five individual seismic stations and three mini seismic arrays of 15 stations each were deployed on Unalaska island to supplement the Alaska Volcano Observatory permanent seismic network. This temporary array was operational for one year. Taking advantage of the increased azimuthal coverage and the array’s increased earthquake detection capability, we developed body-wave Vp and Vp/Vs seismic images of the velocity structure beneath the volcano. Our results show a complex structure with the upper 4 km of the crust. The shallow high-Vp features possibly delineate remnant magma pathways or conduits. Low-Vp regions are found east of the caldera at approximately 4-8 km depth. This is in agreement with previous seismic tomographic work and InSAR-based geodetic models, which had identified this region as a possible long-term source of magma. A three-dimensional compressional wave attenuation model complemented the velocity models. In general, we observe regions of corresponding low P-wave velocity and high attenuation east of the summit caldera between 5 and 7 km depth, supporting the presence of a melt-rich zone. The absolute Vp values (5.5-6.0 km/s) in this region are consistent with the presence of a magma body with a mafic signature. We suggest that a triggering mechanism that involves mafic injections into a more evolved shallow magma chamber is representative of the current magmatic system below the volcano, similarly to what petrological studies have inferred for past eruptions.

Alaska↗
USGS2020

High-resolution imaging of hydrothermal heat flux using optical and thermal Structure-from-Motion photogrammetry

Quantifying hydrothermal heat flux at meter-scale resolution over N0.25 km2 is required to bridge in-situ heat flux and satellite-based measurements. We advance a methodology that blends ground-based daytime optical and nighttime thermal infrared (TIR) imagery using Structure-from-Motion photogrammetry to map radiant hydrothermal heat flux over these scales at sites with low signal-to-noise ratios that would otherwise be difficult to characterize using, for example, unmanned aerial systems. The improved method uses a computerized telescopic mount to relocate and align daytime optical acquisitions with nighttime TIR imagery, thereby enabling TIR acquisition from multiple camera orientations positioned throughout a study area. This facilitates mapping of thermal features at sites of varying size and complexity and helps to ameliorate topographic occlusion effects and geometric distortions that can bias radiant hydrothermal heat flux estimates derived from the resulting orthorectified thermal maps. We assessed detection thresholds of this method at three sites across central California, which range in size, topography, and heat flux conditions. We found that blending of optical and thermal acquisitions successfully detected anomalous heat flux, even in cases where temperatures were only slightly greater than background. This approach might be applied to a variety of volcanic and hydrothermal systems to quantify the spatial distribution of heat flux, and how this may relate to factors such as the distribution of ground fractures and lava flow rheology.

Journal of Volcanology and Geothermal Research↗
USGS2020

A post-eruption study of gases and thermal waters at Okmok Volcano, Alaska

We report here on the first focused study of gas discharges and thermal spring waters at Okmok Volcano since the 2008 phreatomagmatic eruptions. Results include the first compositional gas data from Okmok with minimal air contamination and the first data on magmatic carbon in Okmok spring waters. Chemical and isotopic analyses of the waters and gases are used to assess the character of Okmok fluids eight years after the eruptions ceased. Gases from vents on intracaldera Cone C have high concentrations of H2 and contain H2S rather than SO2, demonstrating the influence of a hydrothermal system, while isotope values of carbon ( 10.2 to 8.9‰) and helium (~8 RA) confirm the presence of magma-derived volatiles. Estimates of equilibrium temperatures for the Cone C gas are ~230 ± 30 ºC. A much cooler reservoir with a maximum temperature of ~55 ºC feeds the intracaldera warm springs. Based on discharge measurements of creeks draining the caldera, the total heat output of the warm springs is estimated to be about 32 MW. Gas data from a single location of steaming ground at the Geyser Bight geothermal area southwest of the Okmok Caldera are given. The gas is typical of geothermal gases with high concentrations of H2S and an air-corrected helium isotope ratio of 7.15 RA.

Alaska↗
USGS2020

Geophysical characterization of the Northwest Geysers geothermal field, California

The Clear Lake Volcanic Field in northern California is the youngest and northern-most part of a long-lived volcanic system that has produced recent (~10 ka) eruptions. Adjacent to the Clear Lake Volcanic Field is the worlds largest energy producing geothermal field, The Geysers. The hottest part of The Geysers geothermal field is in the northwest where temperatures reach ~400 C at 3 km depth. Low permeability, high thermal gradients, and low steam saturation prescribed development of an enhanced geothermal system (EGS) in the Northwest Geysers to increase energy producing capacity. Though the Northwest Geysers is known to be the hottest part of the field, geophysical methods have failed to adequately image any inferred heat source. This project aims to image the heat source of the Northwest Geysers using newly collected gravity and magnetotelluric (MT) measurements. Gravity data were jointly modeled with existing magnetic data along a two-dimensional profile aligned with an existing geologic cross-section. The key feature of the potential field model is a low-density, low-susceptibility body at 5 km depth (bmsl) under the EGS. MT data were modeled in three-dimensions to characterize subsurface resistivity structure, where the upper 3 km of the resistivity model agrees well with existing data. Lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data. Below 3 km depth (bmsl), the resistivity model images a possible young intrusion under the EGS. A possible zone of partial melt (<5%) below 7 km depth (bmsl) in the northwestern part of the field is also imaged which extends northeast towards the main part of the Clear Lake Volcanic Field.

California↗
USGS2020

Post-1978 tumescence at Long Valley Caldera, California: A geophysical perspective

Long Valley Caldera has been restless since at least 1978. Prominent symptoms of this unrest include earthquake swarms and tumescence (inflation) centered on the resurgent dome. Over the years, interpretations of physical processes underlying this unrest have varied considerably. Results from a collection of geophysical studies infer the presence and/or active intrusion of magma in the crust. Geologic evidence, however, does not support recent magmatic activity in the caldera, leading to an interpretation that the caldera volcanic system is moribund, and the current unrest is a result of second boiling (aqueous fluids released during crystallization of the rhyolitic magma that produced the Bishop Tuff). Here, we examine the collective constraints provided by geophysical studies over the past four decades. Although the current geophysical evidence does not conclusively discriminate between unrest driven by recent crustal magmatic intrusion versus second boiling, it does provide evidence that a large volume of partial melt persists within the mid- and lower crust. This implies that the Long Valley Caldera system as a whole is long-lived, and the magma reservoir remains at least partially molten. In the shallow crust, the possibility of small pockets of magma remains. In aggregate, the data suggest commonality with other large, long-lived silicic caldera systems, such as Yellowstone and Campi Flegrei. Although the possibility of eruption within Long Valley Caldera remains unlikely, the geophysical evidence argues that Long Valley has an active magmatic system at depth and we must retain the possibility of eruptive hazards.

California↗
USGS2020

Cyclic shear zone cataclasis and sintering during lava dome extrusion: Insights from Chaos Crags, Lassen Volcanic Center (USA)

The ascent and extrusion of crystal-rich magma is commonly facilitated by deformation partitioned within annular, conduit-parallel shear zones. The physical properties and textures of the shear zone materials, where exposed at surface, provide a record of ascent and eruption dynamics. We describe the shear zone developed in Dome C, part of Chaos Crags in the Lassen Volcanic Center (California, USA). The extruded shear zone comprises volcanic fault gouge and variably densified cataclasites. The competent cataclasites evidence deep-seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent. Textural, geochemical and mineralogical data identify solid-state sintering as the densification mechanism. At the temperatures and pressures in the volcanic conduit, solid-state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening. Longer dwell times (i.e., slower ascent) allow for more sintering, producing stronger, denser and less permeable cataclasites. At Chaos Crags, we use the extent of sintering, quantified by residual porosity, to recover minimum in-conduit dwell times necessary to produce the observed cataclasites. Our analysis of the Dome C cataclasites suggests a maximum linear ascent rate of 10 m/day and a minimum ascent time of 100 days. We evaluate the consequences of shear zone lithification by solid-state sintering for the eruption of other crystal-rich, glass-poor magmas. Chaos Crags cataclasites preserve evidence of multiple cycles of fracturing, cataclasis and (re-)sintering suggesting a mechanism for transitions between effusive and explosive phases of dome-building eruptions.

California↗
USGS2020

The 1951 eruption of Mount Lamington, Papua New Guinea: Devastating directed blast triggered by small-scale edifice failure

The catastrophic explosion of Mount Lamington volcano, Papua New Guinea on January 21, 1951 produced a devastating pyroclastic density current (PDC) that knocked down dense tropical rainforest over an area of 230 km 2 and killed approximately 3000 people. We present results of a field reinvestigation of the 1951 PDC deposit combined with an analysis of the available photographs and eyewitness accounts of the eruption first published in the fundamental work of G. A. M. Taylor (1958). We have concluded that the six-days-long pre-climactic activity before the 1951 eruption (which included felt local seismicity, frequent ash-laden explosions of vulcanian type, bulging of the volcano slope accompanied with landslides) was associated with shallow-level intrusion of a highly viscous magma body (cryptodome/dome) of andesitic composition with a volume of approximately 0.01 km 3 . This intrusion destabilized Mount Lamington's prehistoric intra-crater lava dome. On January 21 the destabilized dome gravitationally collapsed and produced a relatively small-volume debris avalanche, the deposit of which was not recognized during Taylor's original investigation. The debris avalanche had a volume of approximately 0.02–0.04 km 3 , travelled a distance (L) of 8.5 km and had the ratio of vertical drop (H) to runout (L) of 0.14. The edifice collapse decompressed the intruding cryptodome and triggered its explosive fragmentation. Photographs of the climactic explosion show that the eruptive cloud initially rose vertically but subsequently collapsed upon the terrain around the vent, and formed a PDC which flowed radially outward. The enhanced northward propagation of the PDC to a maximum distance of 13 km reveals that the northern breach in the ancient crater's high walls influenced the distribution of the deposit. In the studied NE-N-NW sector of the devastated area, in the zone proximal to the volcano, the PDC emplaced a normally graded layer of coarse ash and lapilli mixed in the base with picked-up soil and plant fragments. The layer gradually becomes thinner and finer-grained with distance from the volcano. The PDC deposit has a volume of approximately 0.025 km 3 and consists of approximately 80% juvenile rock fragments derived from the explosively fragmented cryptodome. The remaining 20% consists of accidental clasts derived from the old volcanic edifice. The juvenile material is crystal-rich andesite with a unimodal vesicularity distribution (4 to 36%). The reconstructed eruption sequence, the PDC tree blowdown pattern and characteristics of the PDC deposit are similar to those of catastrophic laterally-directed blasts of volcanoes Bezymianny in 1956, Mount St.Helens in 1980, and Soufriere Hills, Montserrat in 1997. In contrast to the cases of these “classic” lateral blasts, the blast cloud of Lamington was initially vertically-directed before collapsing to produce a PDC. We speculate that the climactic explosion of Mount Lamington was initially vertical because the rupture surface of the triggering sector collapse intersected the apex of the intruding cryptodome (it exposed a subhorizontal surface of the cryptodome apex), while at Bezymianny, Mount St.Helens, and Soufriere Hills the rupture intersected the main body of the cryptodome/dome, and exposed their steeply inclined surfaces.

Journal of Volcanology and Geothermal Research↗