Pyroclastic Density Current Deposits Research Articles

Age and context of the oldest known hominin fossils from Flores.

Recent excavations at the early Middle Pleistocene site of Mata Menge in the So'a Basin of central Flores, Indonesia, have yielded hominin fossils attributed to a population ancestral to Late Pleistocene Homo floresiensis. Here we describe the age and context of the Mata Menge hominin specimens and associated archaeological findings. The fluvial sandstone layer from which the in situ fossils were excavated in 2014 was deposited in a small valley stream around 700 thousand years ago, as indicated by (40)Ar/(39)Ar and fission track dates on stratigraphically bracketing volcanic ash and pyroclastic density current deposits, in combination with coupled uranium-series and electron spin resonance dating of fossil teeth. Palaeoenvironmental data indicate a relatively dry climate in the So'a Basin during the early Middle Pleistocene, while various lines of evidence suggest the hominins inhabited a savannah-like open grassland habitat with a wetland component. The hominin fossils occur alongside the remains of an insular fauna and a simple stone technology that is markedly similar to that associated with Late Pleistocene H. floresiensis.

A chemostratigraphic study of the Campanian Ignimbrite eruption (Campi Flegrei, Italy): Insights on magma chamber withdrawal and deposit accumulation as revealed by compositionally zoned stratigraphic and facies framework

Petrochemical analyses of juvenile samples from twenty stratigraphic sections of the Campanian Ignimbrite medial deposits, located from 30 to 79km from the vent, are presented here. Sampling has accurately followed a well-defined stratigraphic framework and the new component facies scheme. The Campanian Ignimbrite succession is formed by a basal plinian pumice fall deposit, overlain by a complex architecture of pyroclastic density current deposits emplaced from a single sustained pyroclastic density current through a mechanism of vertical and lateral accretion. The deposit is broadly zoned, from more evolved trachyte at its base to less evolved trachyte at its top, and is similarly less evolved with increasing distance from the area of emission. Irregular chemical trends are locally observed and interpreted to represent only a limited, “patchy” record of the entire vertical geochemical trend. The petrochemical variation observed horizontally was ascribed to changes in the flow dynamics and interaction between the advancing flow and the underlying topography. The results of this study were used to propose a unified volcanological-petrological model for the Campanian Ignimbrite eruption, taking into account the emplacement of both the proximal (i.e., the “Breccia Museo” formation) and medial deposits.

Dome growth, collapse, and valley fill at Soufrière Hills Volcano, Montserrat, from 1995 to 2013: Contributions from satellite radar measurements of topographic change

Frequent high-resolution measurements of topography at active volcanoes can provide important information for assessing the distribution and rate of emplacement of volcanic deposits and their influence on hazard. At dome-building volcanoes, monitoring techniques such as LiDAR and photogrammetry often provide a limited view of the area affected by the eruption. Here, we show the ability of satellite radar observations to image the lava dome and pyroclastic density current deposits that resulted from 15 years of eruptive activity at Soufriere Hills Volcano, Montserrat, from 1995 to 2010. We present the first geodetic measurements of the complete subaerial deposition field on Montserrat, including the lava dome. Synthetic aperture radar observations from the Advanced Land Observation Satellite (ALOS) and TanDEM-X mission are used to map the distribution and magnitude of elevation changes. We estimate a net dense-rock equivalent volume increase of 108 ± 15M m3 of the lava dome and 300 ± 220M m3 of talus and subaerial pyroclastic density current deposits. We also show variations in deposit distribution during different phases of the eruption, with greatest on-land deposition to the south and west, from 1995 to 2005, and the thickest deposits to the west and north after 2005. We conclude by assessing the potential of using radar-derived topographic measurements as a tool for monitoring and hazard assessment during eruptions at dome-building volcanoes.

Concentration of hydroxyl defects in quartz from various rhyolitic ignimbrite horizons: results from unpolarized micro-FTIR analyses on unoriented phenocryst fragments

Hydroxyl defect concentrations of quartz phenocryst fragments from various rhyolitic pyroclastic density current deposits from the Bukk Foreland Volcanic Area, Hungary, were determined by using micro-FTIR spectrometry. In addition trace-element analysis and SEM cathodoluminescence imaging were performed on the same crystals. Hydroxyl defect-content (expressed in water equivalent) of volcanic quartz ranges from 0.9±0.1 to 2.8±0.4 wt. ppm, which is lower than those in quartz of plutonic (granitic), metamorphic and hydrothermal origin. The incorporation of hydroxyl defect is mainly due to H+ + Al3+ substitutions into Si-tetrahedral vacancies of quartz. Furthermore, the presence of molecular water probably in (nano-)inclusions was proven. The post-eruptive diffusive loss of hydroxyl defects during cooling seems to be the main factor causing the very low concentration of hydroxyl defects. This may be also manifested in the almost homogeneous distribution of hydroxyl defects regardless of the considerable zonation in Al-content, although complete diffusive loss of hydroxyl defects was possibly hindered by the cooling effect of phreatomagmatism (interaction of excess water with magma during eruption) and deposition in a shallow submarine environment. Moreover, a reasonable linear correlation is observed between the integrated area of Si–O bands (between 2110 and 1440 cm−1) and sample thickness up to ~300 μm by studying unoriented quartz phenocryst fragments. This may enable the quantitative analysis of hydroxyl defects in separated quartz crystals without the need for preparing oriented thin sections.

Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits

The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan, was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30–50 wt% of fine ash (silt–clay component). The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, and by applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of the PDC at the summit area were estimated as 2 × 10−4–2 × 10−3 and 24–28 m/s, respectively. The tephra thinning trend in the proximal area shows a steeper slope than in similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics.

Topographic controls on pyroclastic density current dynamics: Insight from 18 May 1980 deposits at Mount St. Helens, Washington (USA)

Our ability to interpret the deposits of pyroclastic density currents (PDCs) is critical for understanding the transport and depositional processes that control PDC dynamics. This paper focuses on the influence of slope on flow dynamics and criticality as recorded in PDC deposits from the 18 May 1980 eruption of Mt. St. Helens (USA). PDC deposits are found along the steep flanks (10°–30°) and across the pumice plain (~5°) up to 8km north of the volcano. Granulometry, componentry and descriptions of depositional characteristics (e.g., bedform morphology) are recorded with distance from source. The pumice plain deposits are primarily thick (3–12m), massive and poorly-sorted, and represent deposition from a series of concentrated PDCs. By contrast, the steep flank deposits are stratified to cross-stratified, suggesting deposition from PDCs where turbulence strongly influenced transport and depositional processes. We propose that acceleration of the concentrated PDCs along the steep flanks resulted in thinning of the concentrated, basal region of the current(s). Enhanced entrainment of ambient air, and autofluidization from upward fluxes of air from substrate interstices and plunging breakers across rugged, irregular topography further inflated the currents to the point that the overriding turbulent region strongly influenced transport and depositional mechanisms. Acceleration in combination with partial confinement in slot canyons and high surface roughness would also increase basal shear stress, further promoting shear and traction transport in the basal region of the current. Conditions along the steep flank resulted in supercritical flow, as recorded by regressive bedforms, which gradually transitioned to subcritical flow downstream as the concentrated basal region thickness increased as a function of decreasing slope and flow energy. We also find that (1) PDCs were erosive into the underlying granular substrate along high slopes (>25°) where currents were partially confined in steep slot canyons, suggesting that basal shear stress is an important control on erosive capacity, and (2) bedform amplitude, wavelength and the presence of regressive bedforms increase with increasing slope and proximity to source along the steep flank, suggesting a link between bedform morphology, flow velocity, and flow criticality. While our results indicate that slope and irregular topography strongly influence PDC dynamics, criticality and erosive capacity, the influence of these conditions on ultimate flow runout distance is unclear. The work here also highlights the issue that relationships between the controls on bedform size and morphology in density stratified flows remain poorly constrained, limiting our ability to extract important information about the currents that produced them. These final two points warrant further exploration through the combination of field, experimental and numerical approaches.

Geochemical and isotopic composition of volcanic rocks of the heterogeneous Miocene (~ 23–19 Ma) Tepoztlán Formation, early Transmexican Volcanic Belt, Mexico

We present the first geochemical data (major and trace elements, as well as Sr, Nd, and Pb isotopes) on volcanic rocks from the Tepoztlán Formation in the central Transmexican Volcanic Belt. The Tepoztlán Formation is up to 800m thick and comprises a wide range of primary volcanic rocks (lavas, pyroclastic density current deposits, pyroclastic fall deposits), and their secondary reworked products due to mass flow (lahars) and fluvial processes. Magnetostratigraphy combined with K/Ar and Ar/Ar geochronology suggests an age of Early Miocene (23–19Ma) for this Formation. Lava flows, pyroclastic rocks, dykes and volcanic clasts range from basaltic andesite to rhyolite, with a clear dominance of andesites and dacites. All samples are subalkaline and hy-normative. These rocks show homogeneous REE patterns with LREE enrichment and higher LILE concentrations with respect to HFSE, notably the typical negative anomaly of Nb, Ta, and Ti, suggesting a subduction-related magma genesis. Major and trace element concentrations show that either assimilation of heterogeneous continental crust or crustal recycling by subduction erosion and fractional crystallization are important processes in the evolution of the Tepoztlán Formation magmas. Isotopic compositions of the Tepoztlán Formation samples range from (87Sr/86Sr)t=0.703693 to 0.704355 and (143Nd/144Nd)t=0.512751 to 0.512882, falling within the mantle array. All geochemical characteristics indicate that these rocks originated from a heterogeneous mantle, modified and evolved through assimilation of country rock and fractional crystallization in the upper crust.

Distal ash hurricane (pyroclastic density current) deposits from a ca. 2000 yr B.P. Plinian-style eruption of Mount Pelée, Martinique: Distribution, grain-size characteristics, and implications for future hazard

Plinian-style activity has not occurred on any volcano in the Lesser Antilles since European settlement; however, pumice-rich deposits from this eruptive style are found throughout the Lesser Antilles, and such eruptions were witnessed by pre-Columbian Native American populations. One of these eruptive events from Mount Pelee on Martinique, occurring between 1800 and 2200 yr B.P., produced pumice-and-ash flow deposits that followed the main drainage channels on the volcano, ash hurricane deposits that both mantle the volcano’s flanks (slopes up to 25°) and extended at least 20 km from the crater. We examined the latter in the context of modern developments in understanding transport and depositional mechanics of pyroclastic density currents. Based on grain-size analysis, they are subdivided into proximal and distal deposits. The distal deposits become progressively more depleted in fines and enriched in crystals with distance from the crater. We conclude these are deposits of dilute, or inertia-dominated pyroclastic overcurrents, which decoupled from their underflows as they surmounted the topographic barrier formed by the older Pitons du Carbet volcanic center. A combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff, and resulting extreme crystal enrichment. Their areal distribution suggests much of the northern half of Martinique must have been severely affected, and the zone of devastation extended at least to the northern outskirts of the capital Fort-de-France, although their final runout distance may have extended significantly further south. Major coignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation.

Experimental volcanic ash aggregation: Internal structuring of accretionary lapilli and the role of liquid bonding

Explosive volcanic eruptions can release vast quantities of pyroclastic material into Earth's atmosphere, including volcanic ash, particles with diameters less than two millimeters. Ash particles can cluster together to form aggregates, in some cases reaching up to several centimeters in size. Aggregation alters ash transport and settling behavior compared to un-aggregated particles, influencing ash distribution and deposit stratigraphy. Accretionary lapilli, the most commonly preserved type of aggregates within the geologic record, can exhibit complex internal stratigraphy. The processes involved in the formation and preservation of these aggregates remain poorly constrained quantitatively. In this study, we simulate the variable gas-particle flow conditions which may be encountered within eruption plumes and pyroclastic density currents via laboratory experiments using the ProCell Lab System® of Glatt Ingenieurtechnik GmbH. In this apparatus, solid particles are set into motion in a fluidized bed over a range of well-controlled boundary conditions (particle concentration, air flow rate, gas temperature, humidity, liquid composition). Experiments were conducted with soda-lime glass beads and natural volcanic ash particles under a range of experimental conditions. Both glass beads and volcanic ash exhibited the capacity for aggregation, but stable aggregates could only be produced when materials were coated with high but volcanically-relevant concentrations of NaCl. The growth and structure of aggregates was dependent on the initial granulometry, while the rate of aggregate formation increased exponentially with increasing relative humidity (12–45% RH), before overwetting promoted mud droplet formation. Notably, by use of a broad granulometry, we generated spherical, internally structured aggregates similar to some accretionary pellets found in volcanic deposits. Adaptation of a powder-technology model offers an explanation for the origin of natural accretionary pellets, suggesting them to be the result of a particular granulometry and fast-acting selective aggregation processes. For such aggregates to survive deposition and be preserved in the deposits of eruption plumes and pyroclastic density currents likely requires a significant pre-existing salt load on ash surfaces, and rapid aggregate drying prior to deposition or interaction with a more energetic environment. Our results carry clear benefits for future efforts to parameterize models of ash transport and deposition in the field.

Climatic, tectonic and volcanic controls of sediment supply to an Oligocene Foredeep basin: The Val d'Aveto Formation (Northern Italian Apennines)

The Val d'Aveto Formation is a turbidite sequence deposited in the Northern Apennines during the Oligocene (32–29Ma). The occurrence of calcareous, siliciclastic, volcaniclastic and carbonatoclastic deposits makes the Val d'Aveto Formation a natural laboratory to study the interplay among climate, tectonics and volcanism in a subduction-related context. Eight logs have been studied in order to reconstruct the stratigraphic architecture of the succession. Ten different facies (one limestone senso lato facies, four siliciclastic facies, four volcaniclastic facies and one carbonatoclastic facies) have been recognized on the basis of their different sedimentary features, microtextures and matrix compositions (through thin section analysis and X-ray powder diffraction). For the first time, syn-volcanic facies (pyroclastic density current and epiclastic deposits) have been distinguished from post-volcanic facies in the volcaniclastic facies. Compositional trends of sandy to muddy detritus, together with image analyses on gravel dimensions, highlight spatial and temporal persistence of the syn-depositional volcanic activity across the source-to-sink system. Lithological and petrographic analyses on the gravel size fraction of the siliciclastic and volcaniclastic detritus point out a SE France provenance and reveal the strong influence that tectonics has on sediment production. The comparison between the geodynamic evolution of the source area and the stratigraphic evolution of the Val d'Aveto Formation can help unravel the importance of climate on the redistribution of tectonically produced sediments and shaping of the resulting submarine fan.

Application of actualistic models to unravel primary volcanic control on sedimentation (Taveyanne Sandstones, Oligocene Northalpine Foreland Basin)

This work is focused on the Taveyanne Sandstones (Grés de Taveyanne), an Oligocene volcaniclastic turbidite sequence cropping out in the Northern Alpine Molassa between SE France and Central Switzerland, with the aim to investigate the temporal relationship between volcanic activity and sediment supply. Detailed stratigraphic, sedimentological, and petrographic (XRD analyses on mudstones and point counts on sandstones) studies conducted on three sections (Col de l'Oulette and Flaine in SE France, Taveyanne in SW Switzerland) allow a discrimination of three main facies, among which only one is extremely enriched in volcaniclastic detritus and characterized by features similar to those of disaggregated pyroclastic density current deposits. The other two facies are characterized by variable to no volcanic detritus but supplied by crystalline and sedimentary detritus. Such sediment trends are similar to those of modern, volcanically controlled source-to-sink systems. This allows a reinterpretation of the Taveyanne Sandstones as a syn-volcanic turbidite system, episodically supplied by large amounts of volcanic detritus, which periodically modified the drainage paths. Moreover, the well-known temporal and spatial persistence of such modifications in modern settings leads to conciliate the syn-volcanic supply with the location of the volcanic centers in the internal part of the Alps, without invoking particular climatic and tectonic conditions controlling foreland sedimentation.

The ground layer of the Campanian Ignimbrite: an example of deposition from a dilute pyroclastic density current

A thin, fines-poor, and lithic- and crystal-rich layer locally present, from proximal to distal areas, at the base of the pyroclastic density current deposits of the Campanian Ignimbrite eruption (39 ka), sourced from Campi Flegrei (Italy), is interpreted as a ground layer (GL). It has an average lithic and crystal content of 57 and 25 wt%, respectively. The GL rests on a paleosol or an early fall deposit and is capped by a stratified ash deposit. It is bounded by erosion surfaces and its thickness does not change systematically from the source. The occurrences of GL in places separated from the source by a stretch of sea and the angular to sub-angular shape of the lithic clasts are consistent with deposition from a dilute current. The presence of the GL up to 971 m above sea level and beyond 1400–1500-m-high mountain ridges, allows estimation of the thickness of the PDC as having been at least 1.5 km. To investigate the parameters influencing the transport and emplacement mechanisms that produced the GL, we use field and sedimentological data (grain size and componentry), from samples collected between 30 and 70 km from the source. The progressive decrease in maximum lithic clast and median grain-size with distance from the vent and increasing altitude corresponds to lateral and vertical grading of lithic clasts within the current. The coarser clasts moved toward the basal part of the current, settling in more proximal areas and at low altitude. Farther from the vent (45–50 km), the lower part of the pyroclastic current impacted against the reliefs surrounding the Campanian Plain, causing the emplacement of coarser and more poorly sorted deposits on slopes oriented toward the source.

Tracing the evolution of 2010 Merapi volcanic deposits (Indonesia) based on object-oriented classification and analysis of multi-temporal, very high resolution images

ABSTRACT We compare identification, delineation and recording of freshly erupted deposits around active volcanoes from very high resolution optical images with that done by traditional geologic mapping. Object-oriented classification (OOC) and normalized difference spectral indices of vegetation, moisture and soil redness (NDVI, NDWI and NDRSI) have been applied to sub-metric GeoEye-1 and Pleiades images to identify and map pyroclastic and lahar deposits and trace their spatio-temporal evolution over two years, following the 2010 eruption of Merapi Volcano, Indonesia. We could identify several categories of pyroclastic depositional areas, and also map the damaged forest and destroyed cultivated terraces and settlements in the Gendol and Opak River basins on the south flank of the volcano. More than 75% of erupted deposits were delineated semi-automatically unlike the ground geological map based on photo-interpretation of the 2010 GeoEye image and field observations. A temporal image analysis, using bivariate scatter diagrams of the three spectral indices between 2010 and 2012 and a combination of NDWI and NDVI, separated areas affected by surges from unscathed vegetation. Use of NDRSI and NDWI allowed us to differentiate overbank Pyroclastic Density Current deposits from wet lahar deposits. NDRSI values close to 0 refer to scoria-rich pyroclastic deposits. About 40% of the devastated upper catchment was recolonized by vegetation between 2010 and 2012. The recovery also took place in the forested valley margins affected by ash-cloud surges. The morphometric analysis of the initial drainage network, digitized from the 2011–2012 images, demonstrated (1) the resurfacing of pristine 2010 PDC deposits by runoff and (2) incision or remobilization by lahars. It took two years following the eruption in the rugged upper catchment devastated by high-energy surges to fully develop the hydrographic network. It is, however, still rudimentary on gently sloping fans created by overbank PDC deposits in the middle valley, thus suggesting the importance of slope gradient, grain size, permeability and thickness of deposits. As much as 35% of the 2010 PDC deposits, emplaced in the vicinity of the river channels, were remobilized by lahars over the two post-eruption rainy seasons and also by constant mining activities. Studies on the erosion of the pyroclastic deposits after 2012 need to concentrate on the upper reach of the catchment on the south flank.

Combining historical and 14C data to assess pyroclastic density current hazards in Baños city near Tungurahua volcano (Ecuador)

Pyroclastic density currents (PDCs) from Tungurahua volcano, Ecuador, recurrently rush down the northern Vazcún valley and threaten the small tourist city of Baños located on its mouth. Estimating PDC hazards, i.e. PDC return rate and extent, is difficult in Baños and Vazcún because data from the literature are rare and debated, while geological exposures are few and vegetated. To improve our awareness of PDC hazards in the Baños area, we examined historical documents, conducted new geological research in the Vazcún valley, and obtained additional 14C data. We highlight complex radiocarbon results that may lead to difficulties when interpreting the PDC chronology, and offer recommendations to improve sampling strategies for 14C age determinations of PDC deposits. Overall, our results show that the 1640, 1773, and 1886 PDC-forming eruptions are recorded in the valley, while products of the 1918 and 2006 events were likely removed. Through considering recorded/unrecorded bias in the reconstruction of past PDC emplacement in Vazcún, we calculate a minimal average return rate of 18 PDC-forming events for pre-Columbian and historical eruptions since the devastating 3 ka BP eruption, which translates into a maximum average return rate of 150–200 years in the valley. However, the archives suggest that most PDCs did not affected the now-urbanized Baños area (e.g. 1918, 2006) and we estimate that the city is impacted on average every 350–500 years, a duration that is likely perceived as fairly long for people living in the area. The Baños case-study thus raises questions on how to communicate on uncertain threats and impacts, and how to improve alert messages in a town where economic incomes are essentially based on tourist activities. To better address these issues we plan in future works to focus on cognitive perception of risk and volcanic hazard judgment by populations of Baños and nearby rural communities.

Dynamics of the major plinian eruption of Samalas in 1257 A.D. (Lombok, Indonesia)

The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores. It is suspected to have impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40 km3 dense rock equivalent (DRE) of deposits, consisting of (i) 7–9 km3 DRE of pumiceous plinian fall products, (ii) 16 km3 DRE of pyroclastic density current deposits (PDC) and (iii) 8–9 km3 DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6 × 108 kg/s, a maximum plume height of 43 km and a dispersal index of 110,500 km2, the 1257 A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.

Lab-scale ash production by abrasion and collision experiments of porous volcanic samples

In the course of explosive eruptions, magma is fragmented into smaller pieces by a plethora of processes before and during deposition. Volcanic ash, fragments smaller than 2mm, has near-volcano effects (e.g. increasing mobility of PDCs, threat to human infrastructure) but may also cause various problems over long duration and/or far away from the source (human health and aviation matters). We quantify the efficiency of ash generation during experimental fracturing of pumiceous and scoriaceous samples subjected to shear and normal stress fields. Experiments were designed to produce ash by overcoming the yield strength of samples from Tenerife (Canary Islands, Spain), Sicily and Lipari Islands (Italy), with this study having particular interest in the <355μm fraction. Fracturing within volcanic conduits, plumes and pyroclastic density currents (PDCs) was simulated through a series of abrasion (shear) and collision (normal) experiments. An understanding of these processes is crucial as they are capable of producing very fine ash (<10μm). These particles can remain in the atmosphere for several days and may travel large distances (~1000s of km). This poses a threat to the aviation industry and human health. From the experiments we establish that abrasion produced the finest-grained material and up to 50% of the generated ash was smaller than 10μm. In comparison, the collision experiments that applied mainly normal stress fields produced coarser grain sizes. Results were compared to established grain size distributions for natural fall and PDC deposits and good correlation was found. Energies involved in collision and abrasion experiments were calculated and showed an exponential correlation with ash production rate. Projecting these experimental results into the volcanic environment, the greatest amounts of ash are produced in the most energetic and turbulent regions of volcanic flows, which are proximal to the vent. Finest grain sizes are produced in PDCs and can be observed as co-ignimbrite clouds above density currents. Finally, a significant dependency was found between material density and the mass of fines produced, also observable in the total particle size distribution: higher values of open porosity promote the generation of finer-grained particles and overall greater ratios of ash. While this paper draws on numerous previous studies of particle comminution processes, it is the first to analyze and compare results of several comminution experiments with each other in order to characterize these mechanisms.

The historical (218 ± 14 aBP) explosive eruption of Tutupaca volcano (Southern Peru)

The little known Tutupaca volcano (17° 01′ S, 70° 21′ W), located at the southern end of the Peruvian arc, is a dacitic dome complex that experienced a large explosive eruption during historical times. Based on historic chronicles and our radiometric data, this eruption occurred 218 ± 14 aBP, probably between 1787 and 1802 AD. This eruption was characterised by a large sector collapse that triggered a small debris avalanche (<1 km3) and an associated pyroclastic eruption whose bulk volume was 6.5–7.5 × 107 m3. Both units were emplaced synchronously and spread onto the plain situated to the northeast of Tutupaca volcano. The spatial and temporal relationship between the debris avalanche and the pyroclastic density current deposits, coupled with the petrological similarity between the juvenile fragments in the debris avalanche, the pyroclastic density current deposits and the pre-avalanche domes, indicates that juvenile magma was involved in the sector collapse. Large amounts of hydrothermally altered material are also found in the avalanche deposit. Thus, the ascent of a dacitic magma, coupled with the fact that the Tutupaca dome complex was constructed on top of an older, altered volcanic sequence, probably induced the destabilisation of the hydrothermally active edifice, producing the debris avalanche and its related pyroclastic density currents. This eruption probably represents the youngest debris avalanche in the Andes and was accompanied by one of the larger explosive events to have occurred in Southern Peru during historical times.

Syn-eruptive, soft-sediment deformation of deposits from dilute pyroclastic density current: triggers from granular shear, dynamic pore pressure, ballistic impacts and shock waves

Abstract. Soft-sediment deformation structures can provide valuable information about the conditions of parent flows, the sediment state and the surrounding environment. Here, examples of soft-sediment deformation in deposits of dilute pyroclastic density currents are documented and possible syn-eruptive triggers suggested. Outcrops from six different volcanoes have been compiled in order to provide a broad perspective on the variety of structures: Soufrière Hills (Montserrat), Tungurahua (Ecuador), Ubehebe craters (USA), Laacher See (Germany), and Tower Hill and Purrumbete lakes (both Australia). The variety of features can be classified in four groups: (1) tubular features such as pipes; (2) isolated, laterally oriented deformation such as overturned or oversteepened laminations and vortex-shaped laminae; (3) folds-and-faults structures involving thick (&gt;30 cm) units; (4) dominantly vertical inter-penetration of two layers such as potatoids, dishes, or diapiric flame-like structures. The occurrence of degassing pipes together with basal intrusions suggest fluidization during flow stages, and can facilitate the development of other soft-sediment deformation structures. Variations from injection dikes to suction-driven, local uplifts at the base of outcrops indicate the role of dynamic pore pressure. Isolated, centimeter-scale, overturned beds with vortex forms have been interpreted to be the signature of shear instabilities occurring at the boundary of two granular media. They may represent the frozen record of granular, pseudo Kelvin–Helmholtz instabilities. Their recognition can be a diagnostic for flows with a granular basal boundary layer. Vertical inter-penetration and those folds-and-faults features related to slumps are driven by their excess weight and occur after deposition but penecontemporaneous to the eruption. The passage of shock waves emanating from the vent may also produce trains of isolated, fine-grained overturned beds that disturb the surface bedding without occurrence of a sedimentation phase in the vicinity of explosion centers. Finally, ballistic impacts can trigger unconventional sags producing local displacement or liquefaction. Based on the deformation depth, these can yield precise insights into depositional unit boundaries. Such impact structures may also be at the origin of some of the steep truncation planes visible at the base of the so-called "chute and pool" structures. Dilute pyroclastic density currents occur contemporaneously with seismogenic volcanic explosions. They can experience extremely high sedimentation rates and may flow at the border between traction, granular and fluid-escape boundary zones. They are often deposited on steep slopes and can incorporate large amounts of water and gas in the sediment. These are just some of the many possible triggers acting in a single environment, and they reveal the potential for insights into the eruptive and flow mechanisms of dilute pyroclastic density currents.

Nature and velocity of pyroclastic density currents inferred from models of entrainment of substrate lithic clasts

Deposits of pyroclastic density currents (PDCs) often contain accidental lithic clasts of typical size of 0.1–1 m captured from an underlying substrate by the parent flows at distances up to several tens of kilometers from the eruptive vent. In order to gain insights into the nature of PDCs, this study investigates the conditions required for entrainment of particles from a granular substrate by a gas–particle density current, with special emphasis to ignimbrite-forming currents whose dynamics are controversial. The two types of physics of emplacement of PDCs proposed in literature are considered. The first model deals with a hydraulically rough, dilute turbulent PDC of bulk density of ∼1–10 kg/m3 and considers that entrainment through both traction and saltation is controlled by a Shield criterion at high (>104) particle Reynolds number. The second model considers entrainment by a PDC consisting of a dense basal flow of bulk density of the order of 103 kg/m3 and with high interstitial gas pore pressure. This model involves uplift of substrate particles, caused by an upward pressure gradient at the flow-substrate interface, and then transport and deposition on the aggrading basal deposit of the flow as demonstrated by recent laboratory experiments. Results show that a dilute PDC can entrain blocks of maximum size of ∼10–15 cm (for a block density of 2000–3000 kg/m3) if maximum current velocities up to ∼100 m/s are taken into account. This, in turn, suggests that larger (heavier) blocks found in deposits were captured by PDCs if these had a dense basal flow. The dense flow model predicts that PDCs have the potential to entrain metric blocks, whose maximum size (up to ∼2–5 m) decreases with decreasing atmospheric pressure (i.e. increasing altitude). Application of the model considering published data on the characteristics of accidental blocks in several well-studied ignimbrites indicates that the velocity of the parent dense PDCs was up to ∼25–30 m/s.

High-spatial-resolution imagery helps map deposits of the large (VEI 4) 2010 Merapi Volcano eruption and their impact

The 26 October–23 November 2010 eruption is Merapi’s largest event (VEI 4) over the past 140 years. We used high-spatial-resolution (HSR) imagery from GeoEye, Pleiades, IKONOS, and SPOT5 satellites to assess the extent and effects of the pyroclastic density currents (PDCs) and subsequent lahars. We have tracked the geomorphic and structural (fracturing) changes of Merapi’s summit crater and dome between 2008 and 2012. The 4 September 2011 GeoEye image shows that due to the explosive eruption, the summit area lost about 10 × 106 m3. The eruption enlarged the SSE-trending Gendol breach to be 1.3 × 0.3 × 0.2 km. The 2010 tephra and PDC deposits covered about 26 km2 in the two catchments of Gendol and Opak Rivers on Merapi’s south flank, i.e., 60–75 % of the total PDC deposit area, with a total bulk volume of 45 × 106 m3. The tephra-fall deposit covered an area of about 1300 km2 with a range in volume of 18–21 × 106 m3. Supervised and object-oriented classification on HSR imagery enables us to map in detail the PDC deposits across the Gendol-Opak catchment. We delineated 16 spectrally and/or texturally distinct units of PDC deposits and compared them with previously published results. They encompass high-energy surge deposits within ca. 8 km of the summit, valley-confined PDC deposits channeled as far as 16.5 km in the Gendol River, and widespread overbank PDC with ash-cloud surge deposits on valley margins. Additional high-resolution data are provided to map and analyze flooded areas due to lahar activity in 2011–2012 on the south and west flanks. Subsequent overbank lahars impacted selective small areas in the populated ring plain, devastating villages along the Putih River on the southeast flank and the Gendol River. We have analyzed the morphometric parameters (channel cross-sectional capacity, longitudinal rate of channel confinement, and channel sinuosity) of the Gendol-Opak River channels that govern overbank and avulsion of lahars in the ring plain. The paper demonstrates the potential of HSR satellite imagery to outline tephra, PDC, and lahar deposits; map the geomorphic and structural evolution of the summit area of persistently active composite cones; and thus improve hazard assessment for Merapi, a volcano whose summit, slopes, and drainages have changed more since October 2010 than at any other time since 1931.