Pyroclastic Density Current Deposits Research Articles

Transport and deposition of the 39.8 ka Campanian Ignimbrite large-scale pyroclastic density currents (Italy)

Processes of transport and deposition of large-scale pyroclastic density currents, which transport 102−103 km3 of material, are still poorly understood. The volcanic explosivity index (VEI) 7 Campanian Ignimbrite pyroclastic density current is one such poorly understood case that is subject to lively scientific debates about its origin and processes. The Campanian Ignimbrite is related to the 39.8 ka eruption that formed the partly submarine and active Campi Flegrei caldera in Italy. The related pyroclastic density current extended over an area of more than 6000 km2 on land around the caldera, with a final runout of ∼75 km, and surmounted mountain ridges higher than 1000 m above sea level, even at distal reaches. We used physical characteristics of the ignimbrite to understand the transport and emplacement dynamics of the Campanian Ignimbrite pyroclastic density current. Thickness, mass, and clast-size distributions across paleotopography were used to infer that the main transport system was a dilute, density-stratified pyroclastic density current, which was at least 1.5 km thick, which was short-lived (possibly as short as ∼20 min), and which had a mass flow rate of ∼1011 kg/s. At its base, high-concentration undercurrents formed and interacted with the rough paleotopography, laying down a low-aspect-ratio ignimbrite sheet when on flat plains, but generating back-flows from ridges and channeling in paleovalleys. The deposit is predominantly a massive valley-pond ignimbrite facies, even across rough mountain terrains. The absence of veneer facies on steep slopes reflects the near-Newtonian rheology of the undercurrents, interpreted as fluidized dense granular flows. This work advances the interpretations of pyroclastic density current deposits, which do not necessarily directly reflect conditions in the transport system.

A Hidden Eruption: The 21 May 2023 Paroxysm of the Etna Volcano (Italy)

On 21 May 2023, a hidden eruption occurred at the Southeast Crater (SEC) of Etna (Italy); indeed, bad weather prevented its direct and remote observation. Tephra fell toward the southwest, and two lava flows propagated along the SEC’s southern and eastern flanks. The monitoring system of the Istituto Nazionale di Geofisica e Vulcanologia testified to its occurrence. We analyzed the seismic and infrasound signals to constrain the temporal evolution of the fountain, which lasted about 5 h. We finally reached Etna’s summit two weeks later and found an unexpected pyroclastic density current (PDC) deposit covering the southern lava flow at its middle portion. We performed unoccupied aerial system and field surveys to reconstruct in 3D the SEC, lava flows, and PDC deposits and to collect some samples. The data allowed for detailed mapping, quantification, and characterization of the products. The resulting lava flows and PDC deposit volumes were (1.54 ± 0.47) × 106 m3 and (1.30 ± 0.26) × 105 m3, respectively. We also analyzed ground-radar and satellite data to evaluate that the plume height ranges between 10 and 15 km. This work is a comprehensive analysis of the fieldwork, UAS, volcanic tremor, infrasound, radar, and satellite data. Our results increase awareness of the volcanic activity and potential dangers for visitors to Etna’s summit area.

Stratigraphy and structure of Chachahuén volcanic complex, southern Mendoza province, Argentina

The Chachahuén Volcanic Complex is an eroded Upper Miocene arc to back-arc volcanic system that was emplaced in the north-eastern edge of the Neuquén Basin, southern Mendoza province, Argentina. It was formed as the result of shallowing followed by steepening of the Nazca plate during Miocene-Pliocene, and is characterized by a volcanism which ranges from trachydacites to basalts. Thanks to erosion, the stratigraphy and the shallow plumbing systems of the volcano are well-exposed. A new evolution for the volcano stratigraphy is proposed, based on new field observations, satellite imagery, and incorporating the radiometric and petrological data of previous works. The stratigraphy is constrained around one main explosive event, recorded by a thick dacitic pyroclastic density current (PDC) deposit (the Corrales Ignimbrite) (>150m). The main units forming the Chachahuén Volcanic Complex are (1) the pre-dacite PDC deposits (trachydacitic to rhyolitic) Vizcachas Formation, (2) the thick Corrales Ignimbrite, (3) a post-dacite trachyandesitic PDC deposits (dominated by block and ash (BAF) deposits) and thick lava flows, and (4) thin mafic basaltic lava flows. The collapse of a large elliptical caldera occurred during the Trachyandesite phase. The main magma transport channels and feeders are sub-vertical dykes, which exhibit a radial distribution centred on the main caldera depression. Moreover, cryptodomes mainly preserved outside the rim of the caldera accommodated the transport and emplacement of the more silicic magma during the Trachyandesite phase.

A history of explosive eruptions at Young Damavand volcano, Iran

Young Damavand volcano is a steep isolated volcano in the Central Alborz Mountains, Iran, that is characterized by extensive lava flows and pyroclastic successions and is placed on the eroded remains of Old Damavand. Pyroclastic fall and density current deposits on all flanks of the volcano were sampled and studied. From a tephrostratigraphic and geochemical study of the proximal-medial sequences, 14 phases of explosive volcanic activity are identified, characterized by a combination of ash and pumice fall deposits, and/or dense and dilute pyroclastic density current deposits. Some explosive phases of Damavand are thought to have happened one after the other with only short time intervals, since almost no traces of paleosol can be identified between them. We also show that the intensity of the eruptions varied between different phases, confirmed by relative thickness and grain size of the different fall deposits and their distance from the crater. The evidence of numerous explosive eruptions, five of which with Volcanic Explosivity Index 4 in the history of the activity of Young Damavand volcano, shows the importance of assessing its potential volcanic hazards.

Complex facies architecture of unusual pyroclastic density current deposits at the Rincón de la Vieja Volcano, Costa Rica

Pyroclastic Density Currents (PDCs) constitute a complex, multiphase, and multiscale volcanic process generated in a wide spectrum of eruptive scenarios and being among the most deadly and destructive volcanic hazards. Over the north flank of the Rincón de la Vieja Volcano, Costa Rica, previous works identified PDC deposits related to the plinian Río Blanco eruption, which is the largest Holocene explosive event recorded at this volcano. However, so far, there have been no in-depth studies aimed at understanding the eruptive style, and dynamics of the parental PDCs. This work presents, for the first time, a detailed facies analysis and architecture reconstruction (plus a geochemical characterization and novel C14 age determinations) of these outstanding deposits. The presence of cauliflower bombs, juvenile mega-blocks, scorias, and pumices, coupled with the scarcity of accidental and/or accessory lithics, is interpreted here as evidence of an unusual eruptive style. We propose that the Río Blanco eruption was a single complex eruption consisting of two successive phases, each involving the drainage of distinct magma batches. The PDCs were related to the sudden spill out of a lava-pool (Arenal-type PDC), during the collapse of an ultraproximal pyroclastic cone that was built in an initial plinian phase. The collapse of this ultraproximal cone produced a leading-edge pumice-rich PDC, which increased its mobility through fragmentation-induced fluidization. This leading edge PDC was followed by successive high concentrated, mafic, modified granular flow pulses derived from the lava-pool spill out. These modified granular flows were highly channelized in proximal zones, but when they reached the main break in slope, the basal underflow froze en masse while its upper, more diluted part started to decouple and move in a more unconfined manner. After the decoupling, the material feeding towards the basal zone of the surviving current produced a new underflow by rapid suspension sedimentation (“surge-derived pyroclastic flows”). Finally, our results indicate that the generation of Arenal-type PDCs is an underestimated scenario that should be considered in future short-term hazard and risk assessments at the Rincón de la Vieja Volcano, as well as in many other stratovolcanoes worldwide.

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

Gedemsa caldera is a peralkaline volcanic depression located in the Central sector of the Main Ethiopian Rift. An integrated volcanological study of stratigraphic sections was carried out in order to constrain the eruptive history of Gedemsa caldera volcano (GCV), Ethiopia. Textural analyses on plagioclase crystals together with field observations shed light on magma chamber processes feeding the volcanic complex. A multi-vent eruption is ascribed to the pre-caldera volcanic products, as can be seen from the varying caldera wall sequences comprising lavas and pyroclastic deposits. At least three major caldera-forming eruptions are identified producing: 1) a lower ignimbrite, 2) extensive pumice fall and pyroclastic density current deposits, and 3) an upper ignimbrite exposed in different areas of the caldera, indicating a sector collapse. We identified 20 individual pyroclastic deposits found outside and within the caldera that erupted after the climactic caldera collapse. The most recent volcanic activity at Gedemsa is characterized by rhyolitic lava domes and pyroclastic deposits from eruptions through numerous vents aligned with WNW-ESE pre-existing cross-cutting structures. Macrocryst disequilibrium textures such as fine-scale oscillatory zoning (FOZ), sieve textures, glomerocryst, and synneusis indicate that magma reservoir is characterized by repeated magma injection, convection flows, and mixing.

Plio-Pleistocene rear-arc volcanism in the Southern Volcanic Zone: Eruptive styles of the Varvarco Volcanic Field

The Varvarco Volcanic Field (VVF) is located in the southern part of the Las Loicas Trough, as part of the Late Pliocene-Early Pleistocene rear-arc volcanic belt in the Transitional Southern Volcanic Zone (34.5–37°S). Its volcanic products show an elliptical distribution, elongated parallel to NW-SE main structures that regionally controlled the Las Loicas Trough. A detailed field and petrographic study was carried out to identify main lithofacies and establish its eruptive styles.The VVF magmatic evolution is initially characterized by a voluminous explosive stage represented in the area by dense and dilute pyroclastic density currents (PDC) deposits (massive lapilli tuffs, cross-stratified lapilli tuffs and diffuse-stratified tuffs). Afterwards, it evolved into an effusive stage represented by basaltic lava flows (coherent basalts), associated with Hawaiian to Strombolian eruptive style, which constitutes most of the VVF volume. The final stage of the VVF history was linked to a stratovolcano-type activity where both effusive (coherent basalts and andesites, and rhyolitic coulees) and explosive lithofacies (such as massive lithic breccias and massive lapilli tuffs) are described. Within this stage, the uppermost effusive levels were intruded by dacitic and rhyolitic domes and basaltic dykes. Available ages allow to conclude that the VVF emplacement was developed during Plio-Pleistocene times, linked to the re-steepening of the Nazca plate, after the Late Miocene Payenia shallow subduction regime.

Physical properties of pyroclastic density currents: relevance, challenges and future directions

Pyroclastic density currents (PDCs) are hazardous and destructive phenomena that pose a significant threat to communities living in the proximity of active volcanoes. PDCs are ground-hugging density currents comprised of high temperature mixtures of pyroclasts, lithics, and gas that can propagate kilometres away from their source. The physical properties of the solid particles, such as their grain size distribution, morphology, density, and componentry play a crucial role in determining the dynamics and impact of these flows. The modification of these properties during transport also records the causative physical processes such as deposition and particle fragmentation. Understanding these processes from the study of deposits from PDCs and related co-PDC plumes is essential for developing effective hazard assessment and risk management strategies. In this article, we describe the importance and relevance of the physical properties of PDC deposits and provide a perspective on the challenges associated with their measurement and characterization. We also discuss emerging topics and future research directions such as electrical charging, granular rheology, ultra-fine ash and thermal and surface properties that are underpinned by the characterization of pyroclasts and their interactions at the micro-scale. We highlight the need to systematically integrate experiments, field observations, and laboratory measurements into numerical modelling approaches for improving our understanding of PDCs. Additionally, we outline a need for the development of standardised protocols and methodologies for the measurement and reporting of physical properties of PDC deposits. This will ensure comparability, reproducibility of results from field studies and also ensure the data are sufficient to benchmark future numerical models of PDCs. This will support more accurate simulations that guide hazard and risk assessments.

The April 2015 Calbuco eruption pyroclastic density currents: deposition, impacts on woody vegetation, and cooling on the northern flank of the cone

The 22-23 April 2015 eruption of the Calbuco volcano (Southern Andes, Chile) led to extensive pyroclastic density currents (PDCs) interactions with vegetation. We seek to describe the PDCs which affected both Tepu and Frío rivers, northern Calbuco, from their timing and deposition to cooling and erosion, as well as their impacts on forests. Our investigation is based on field stratigraphy, forest disturbance assessment, and geothermometry from degassing pipes and charcoal. These PDCs reached at least ~540-603 °C, as estimated from fumaroles, and consisted of both concentrated and dilute PDCs during the first pulse (22 April) at Tepu and mainly during the second pulse (23 April) at Frío. Effects of PDCs on forest vegetation recorded in Tepu consisted of heating, abrasion, burial, and impact force. On the valley floor, trees were buried with up to 4 m of deposits from the concentrated PDCs, and all trees in this deposition zone died with no subsequent sprouting. Conversely, in the margins of the valley, defoliated fallen trees and standing shrubs indicate scorching due to the passage of dilute PDCs, and some of them were later sprouting. Estimated impact forces required to produce toppling range from 1.5 to 3.7 kPa, and PDC velocities reached up to 36 m s-1. Charring of the buried wood involved an emplacement temperature of 400-550 °C within PDC deposits. The rapid watershed formation may have facilitated infiltration, decreasing the temperature in the basal part within the deposits at the Tepu river. Runoff during the subsequent months triggered lahars and caused the rivers to incise the deposits and transport sediment downstream. This set of observations provides valuable insights into how the interaction between volcanic phenomena and margine forest on the valley floors informs eruptive processes, dynamics, and impacts. Our study is also relevant to interpret the thermal history and potential hazards of PDCs.

Complex styles of phreatomagmatic explosions at Kīlauea Volcano, Hawaii, controlled by magma structure

Explosive eruptions at basaltic volcanoes remain poorly understood. Kīlauea Volcano is a type locality for basaltic eruptions and is well-known for effusive activity. However, more than 7 m of phreatomagmatic Keanakākoʻi Tephra unit D deposits from explosive eruptions crown the southern rim of the summit caldera and provide a stark reminder of Kīlauea’s explosive past and future potential. We used detailed field observations as well as granulometric and morphological analysis of 100 samples from two proximal sections to assess the eruption style and fragmentation mechanism. The deposits can be divided into four subunits, six different lithofacies, and contain three juvenile tephra components. Each juvenile component shows distinct shape variability resulting from molten fuel-coolant interaction (MFCI) explosions of magma of variable vesicularity. Fragmentation of dense glass generates olive-green ash, fragmentation of low to moderately vesicular magma generates a dark gray ash-lapilli component, and fragmentation of highly vesicular magma generates light-yellow pumice. Our work shows that magma structure impacts MFCI explosion efficiency. Small-scale planar bedding throughout most of the deposit points to a general eruption style of small, frequent explosions generating low plumes. Thicker beds of accretionary lapilli of fine-extremely fine ash are related to very efficient magma-water mixing. Pyroclastic density current (PDC) deposits in the upper part of the stratigraphy contain at least three flows but show no significant dune or cross-bedding structures. We suggest that this is a function of the vent being situated in a caldera that was then ∼600 m deep, where the caldera wall acted as a barrier and changed the flow dynamics to very dilute overspills and co-PDC plume falls over the wall. Deconvolution modeling of the polymodal grain size distributions is used to assess grain size changes of each juvenile component for this deposit, which greatly improves interpretation of lithofacies generation and eruption dynamics. Size-correlated shape parameters show that shape data across a wide size range are needed to accurately track grain shapes. This study demonstrates how careful examination of grain size and shape of juvenile tephra clasts can help volcanologists understand how effusive basaltic volcanoes can become violently explosive.

Dynamics and Deposits of Pyroclastic Density Currents in Magmatic and Phreatomagmatic Eruptions Revealed by a Two‐Layer Depth‐Averaged Model

AbstractA pyroclastic density current (PDC) is characterized by its strong stratification of particle concentration; it consists of upper dilute and lower dense currents, which generally control the dynamics and deposits of PDCs, respectively. To explain the relationship between the dynamics and deposits for magmatic and phreatomagmatic eruptions in a unified way, we developed a two‐layer PDC model considering thermal energy conservation for mixing of pyroclasts, external water, and air. The results show that the run‐out distance of dilute currents increases with the mass fraction of external water at the source (wmw) owing to the suppression of thermal expansion of entrained air. For wmw ∼ 0.07–0.38, the particle concentration in the dilute current becomes too low to generate the dense current so that the deposits directly form at the bottom of the dilute current in the entire area. These results capture the diverse features of natural PDCs in magmatic and phreatomagmatic eruptions.

Ash aggregate-rich pyroclastic density currents of the 431 CE Tierra Blanca Joven eruption, Ilopango caldera, El Salvador

The VEI 6, Tierra Blanca Joven pyroclastic sequence (30–90 km3 DRE volume), erupted from Ilopango caldera, El Salvador, in 431 CE, is the product of one of the largest eruptions of the last two millennia. The eruption devastated Central America's Mayan civilization. The eruption began with a short-lived phase of ash and pumice fall deposition and transitioned to a ‘wet’ explosive phase during which pyroclastic density currents flowed >40 km from the caldera. Detailed field and sedimentological analyses are provided for the deposits of ash-aggregate-rich pyroclastic density currents generated during early phases of the eruption. The first phase of pyroclastic density current inundation incinerated forests and deposited up to 30 m of, non-welded, ash-rich ignimbrite in proximal regions, along with ash fall layers of co-ignimbrite origin. Following fallout of a thin layer of pumice and lithic lapilli, a second phase of pyroclastic density current inundation and co-ignimbrite ash fall commenced. A range of ash aggregate types is present in the pyroclastic density current deposits and interbedded co-ignimbrite ash fall layers. Whole and broken concentrically layered ash aggregates (accretionary lapilli) reach >50 vol% in some horizons within some beds in the pyroclastic density current deposits. The evidence indicates that the ash aggregates grew within overriding co-ignimbrite ash plumes and subsequently fell into ground-hugging currents. Our findings suggest that the aggregate-rich nature of the pyroclastic density current deposits originated through incorporation of lake water into eruptive plumes, which in turn triggered rapid, pervasive aggregation within ash clouds and co-ignimbrite plumes.

Hydrovolcanic eruptions of the Paraná Igneous Province: Insights from mafic volcaniclastic deposits in Sertanópolis, Paraná, Brazil

In many continental flood basalt provinces (CFBPs), primary volcaniclastic deposits formed by explosive hydrovolcanism have been shown to be significant counterparts of better-known effusive facies, a notion that challenged traditional views of CFB evolution. For a long time, the only exception to this was the Paraná Igneous Province in South America. In order to better characterize and understand this setting, detailed geological mapping and petrographic studies were carried out on interflow beds at Sertanópolis, a 250 km2 region in northwest Paraná. Four interflow units were identified at the transition between the high and low TiO2 volcanic sequences. Fine-grained interflow facies include vesiculated clastic rocks, with structures ranging from diffuse to well defined stratification, decimeter-scale dune-bedding with preserved backset and foreset laminations. They are formed mainly by fine accessory (non-juvenile) siliciclastic ash (20–70%) and extremely fine ash or mud (10–50%), followed by fine hydrothermal mica (<5–40%), commonly hydrated (clay-mineral-rich) ash aggregates (<5–20%) and lesser amounts of juvenile and accessory glass and basalt fragments (ash to block-sized, <5–30%). In the eastern portion of the research area, a 4 × 2.5 km region with outcrops of scoriaceous basaltic breccias was mapped. These are mainly matrix to clast-supported breccias with finely laminated matrix similar to the fine-grained units. These rocks are interpreted as accessory-rich mafic pyroclastic beds, and their features suggest that multiple pyroclastic density currents (PDCs) formed each interflow unit, with minor discrete fallout contribution in medial successions. Pyroclastic breccias represent proximal PDC and ballistic deposits that help outline a possible conduit in which magmatic-like fragments are dominant in the framework. Stratified tuff-breccia strata in medial deposits represent progressive dilution of PDCs and continuous reworking of scoriaceous fragments by multiple currents. The local stratigraphy is consistent with repeated explosive hydrovolcanic eruptions in a maar-diatreme-like system similar to hydromagmatic volcanoes of other CFBPs.

Volcaniclastic sedimentation in a closed, marginal rift basin: the case of the Melka Kunture area (upper Awash, Ethiopia)

Abstract The upper Awash runs across a volcano-sedimentary succession dated from the Early to Middle Pleistocene and located on the western margin of the northern portion of the Main Ethiopian Rift. The succession lies above Late Miocene to Pliocene lava flows, domes and large volume ignimbrites. The succession formed within a fluvial system that developed within transversal rift faults. The stratigraphy consists of primary volcanic deposits interbedded with reworked sediments emplaced in a low-energy floodplain developed in a subsiding area. The lower part of the volcaniclastic sequence is dominated by a 1.2 Ma old, low aspect ratio, pyroclastic density current deposit (Kella ignimbrite). This eruption was followed by an eruptive stasis and reorganization of the drainage system. Tephra deposition in the floodplain climaxed again between 0.9 and 0.7 Ma and was associated with extensive tephra reworking. Sedimentation rates significantly decreased after 0.6 myr ago, probably owing to declining volcanic activity. Dynamic interaction between tectonics and volcanic activity created a complex sedimentary environment preserving numerous artefacts (lithic tools), animal and hominin remains. Stratigraphic correlation is based on the interpretation of the basin evolution and has relevance for the reconstruction of the palaeoenvironment and the interpretation of the palaeontological and archaeological data.

Eruptive history of the last-1300-years activity of Kelud volcano, Indonesia: Inferences from stratigraphy, chronology, sedimentology, componentry, and geochemistry

Kelud is considered one of the deadliest volcanoes in Indonesia due to its proximity to three cities in East Java (Malang, Kediri, and Blitar) and violent eruptions over the last century. Regardless of the frequently witnessed eruptions, the volcanic stratigraphy for the historical ages has been poorly understood. Here, we provide the first documentation of volcanic stratigraphy for the last 1300 years of activity at Kelud. Componentry and 14C data are coupled with detailed field observations to develop a comprehensive stratigraphy and characterize the main eruption types. The volcanic succession can be subdivided into ten bed-sets composed of pyroclastic density current (PDC) and tephra fallout deposits. A total of twelve fallouts and ten individual PDC units were identified between ∼1.3 and 0.1 cal. ka B.P. One of these units may be attributed to the 1481C.E. eruption. The fallout layers are generally thin, derived from short-sustained eruption columns. By contrast, PDC deposits are massive to stratified, suggesting variable particle concentrations typically resulting from low-column collapses. This complex stratigraphy suggests that Kelud is capable of producing a wide spectrum of eruptions with an increasing eruptive frequency over time and shorter dormancy periods than those between witnessed eruptions (from once /∼100 years to once/∼10 years). This stratigraphic framework, which documents extended records from the oldest witnessed eruption in Kelud (1000C.E.), sheds new insights into Kelud's activity during historical time. Since the products share similar characteristics with those from the most recent activities at Kelud (e.g., 1990, 2007–2008, and 2014), similar eruptions may also occur in the future.

Thermobarometry, vesicularity and microlite populations in pyroclastic density current deposits from the last 4500 years of activity at Galeras, Colombia

Vulcanian eruptions during the 19th and 20th Centuries at the Galeras volcano have produced small eruption columns and bomb fields around the active vent. Four pyroclastic density current (PDC) deposits from the early history of the currently active Galeras cone which have ages between 4500 and 2300 years before present have a much wider distribution. The first two (ML 1 and ML 2) were deposited from hot, dilute, pumice-rich pyroclastic density currents. Amphibole thermobarometry indicates that the magma for these eruptions tapped reservoirs between 5 and 28 km (ML 1) and 33–41 km (ML 2) below the crater. Debse, degassed magma formed a conduit plug which persisted for up to several weeks before the eruptions based on the thickness of breakdown rims around the amphibole. Amphibole-bearing pumice came from a reservoir 5–26 km below the crater and since it lacks breakdown rims it must have reached surface in <8 days. The vesicularity of the pumice indicates two phases of bubble nucleation and growth which coincides with growth of plagioclase and orthopyroxene microlites. The upper two layers (ML 3 and ML 4) are block and ash flows. ML3 shows significant hydrothermal alteration of the dense lithic clasts and has only minor vesicular scoria. The magma for the ML 3 eruption came from a reservoir 17–29 km below the crater and the presence of 20 μm thick breakdown rims on the amphibole implies that the degassing plug was stable for ∼20 days, though the degree of hydrothermal alteration of the clasts appears to require a significantly longer time. The uppermost layer (ML4) is relatively unaltered with a large proportion of pumice. Dense lithics indicate plug formation from magma that had its source at ∼17 km below the crater and remained stable during degassing for ∼20 days. The absence of amphibole from the pumice in ML 4 implies a distinct magma composition was emplaced after the plug.

Geomorphological Classification of Monogenetic Volcanoes and Its Implication to Tectonic Stress Orientation in the Middle Atlas Volcanic Field (Morocco)

The Middle Atlas Volcanic Field (MAVF) covers an area of 1500 km2, with a total erupted volume of solid products (e.g., Dense Rock Equivalent or DRE) estimated to be more than 80 km3. The MAVF comprises 87 monogenetic basaltic volcanoes of Tertiary-Quaternary age as scoria cones (71%) and maars (29%). These monogenetic basaltic volcanoes have various morphologies (e.g., circular, semi-elliptic, elliptic in map views). They can be isolated or form clustered monogenetic complexes. They are largely grouped in the Middle Atlas, in an intraplate geotectonic context forming two distinct major alignments (N160–170° and N40–50°), each closely associated with regional structural elements. By the best estimates, the preserved bulk pyroclastic products do not exceed 0.7 km3, and they show large textural and componentry diversity (e.g., bedded/unbedded, coarse/fine, dense/scoriaceous fallout and pyroclastic density current deposit, etc.). Lava flows also demonstrate great variety of preserved surface textures, including pāhoehoe, ‘a’ā, and clastogenic types. Morphostructural features of lava flows linked to lava flow dynamics have also been recognized, and the presence of hornitos, columnar jointed basaltic flow units, lava tubes, tumuli, and clastogenic lava flows have been recognized and mapped. Some half-sectioned dykes expose interior parts of magmatic shallow feeding pipes. The current morphology of the volcanoes of the MAVF reflects various syn- and post-eruptive processes, including (1) erosional features due to weathering, (2) gravitational instability during and after volcanic activity, (3) vegetation impact, and (4) successive burial of lava flows. The documented volcanic features of this typical monogenetic volcanic field form the core of the region’s geoheritage elements and are considered to be unique in the new African geoheritage context. Hence, they will likely form the basis of future geotourism, geoeducation, and geoconservation ventures.

Integrating LiDAR to unravel the volcanic architecture and eruptive history of the peralkaline Tūhua (Mayor Island) volcano, New Zealand

Tūhua (Mayor Island), located off New Zealand's Bay of Plenty coastline, is a compound, shield-like volcano dominated by peralkaline rhyolite lavas and pyroclastic rocks. Previous studies established a chronostratigraphic framework for the island, yet detailed geomorphic interpretation has been hindered by the rugged, shrub covered terrain. Here we present a revised geological map, stratigraphic framework and geological history based on existing work, a 1 m digital elevation model, 100 m bathymetry and new field observations. Three caldera/crater structures are recognised; the 36 ka C1 caldera, 9.2 ka C2 caldera and a newly identified 7.6 ka C3 crater. Our analysis suggests that the 7.6 ka Tūhua eruption is not related to collapse and formation of the C2 caldera, as interpreted by previous studies. Instead, we attribute the Tūhua eruption to the C3 crater and associated ejecta ring identified in this study. Geomorphic assessment, field observations and volume estimates suggest the C2 caldera collapse event occurred at c. 9.2 ka. The upper Taumou Pa and 9.2 ka spatter-fed lava flows are both observed to be flowing into and outward of the C2 caldera, suggesting they were both active during the collapse event. The lack of volumetrically significant eruptions at the time of the two collapse events is leading us to attribute caldera collapses to magma inflation and drawdown at Tūhua. Coastal exposures include thick cone building pyroclastics and deposits of pyroclastic density currents alternating with lava flows exhibit the dominance of hydrovolcanism for explosive activity. Consequently, explosive interaction between magma and water is the most probable scenario for the initial stage of future eruptions.

Radar Backscatter and Emissivity Models of Proposed Pyroclastic Density Current Deposits on Venus

AbstractMagellan synthetic aperture radar observations of Venus revealed a small number of deposits in the highland regions that were suggested to have formed from pyroclastic density currents. Studying these deposits is useful for understanding the nature of pyroclastic activity and eruptive history on Venus. The proposed pyroclastic deposits occupy the uppermost unit in local stratigraphy and are found near exceptionally high reflectivity (∼0.6) units in the highlands. Their radar properties include high copolarized backscatter (∼−8 to −15 dB) and moderate emissivity values (∼0.70–0.88) in the 12.6 cm wavelength Magellan data acquired at incidence angles between ∼15° and 45°. We aim to characterize the structure of these deposits by modeling the observed backscatter and emissivity as a function of different physical and dielectric properties and shallow subsurface stratigraphy. Three different physical scenarios focusing on three different scattering mechanisms—surface scattering, subsurface scattering from buried dielectric horizons, and volume scattering from buried, distributed scatterers—are considered. By comparing the model results to Magellan observations, we narrow down likely pyroclastic deposit structures. We show that the deposits are likely analogous to dense, welded ignimbrites with high surface roughness. We also investigate other possible but less likely scenarios of a thin, low‐density, low‐loss mantling pyroclastic deposit on top of high reflectivity units and a thick, low‐density, low‐loss deposit with ∼5–10 volume % of scatterers of sub‐wavelength size. Future multiwavelength, multipolarization radar observations from VERITAS and EnVision may enable unambiguous characterization of these deposits.

Collisional interactions and the transition between lava dome sector collapse and pyroclastic density currents at Tutupaca volcano (Southern Peru)

We describe sedimentological variations of the block-rich debris avalanche deposits and associated pyroclastic density current deposits emplaced around 1802 CE from Tutupaca volcano in southern Peru. We use these exceptionally well-preserved features to document the collisional shearing contact between the avalanche and coeval pyroclastic density currents. Furthermore, we show how the first stages of the edifice collapse and syn-cataclastic emplacement process affect the block-size distributions.With field observations, we describe imbricated block clusters, jigsaw cracks and striations related to elongated ridge structures on the deposit surface. Sedimentological and statistical methods (Fourier Shape analysis and Shape Preferred Orientation measured on 208 blocks and 566 mesoscale structures) help us to characterize the cataclastic gradient and establish the collisional relationships between different units. We determine that the proximal impacted deposits and block lithofacies from ridges may be related to distal block units around ∼10 km run-out distance. Different block clusters indicate a kinematic transition between avalanche units to pyroclastic density currents. Block shape parameters help to differentiate rounded blocks resulting from matrix abrasion with and striated blocks from ridges related to proximal imbricated block clusters. From the statistical dataset, a few equations have been developed indicating a common cataclastic origin with a co-genetic evolution of block lithofacies during sequential syn-cataclastic emplacement.The dome collapse is associated with a specific granular flow regime between avalanche and pyroclastic density currents with secondary reworking. Cyclic impact waves contribute to block cluster growth. Clusters are disaggregated during shock propagation. The inherited shapes of the block lithofacies with a/b = 1.2–2 and ellipse = 0.2–2.5 indicate the reworking by impact waves. A multidirectional switch to mass spreading in the median zone between 2 and 6 km may be considered with secondary flow and segregation waves. A basal frictional regime with striations is differentiated from collisional cataclastic flow, generating polymodal grooves during peak velocity at the flow front. Impact forces around ∼15.7 × 1010 N are implied by suggested clast velocities around 8.86 m.s−1 and the transitional regime between avalanche units and pyroclastic density currents between 15.5 and 39.6 m.s−1. An extensional disaggregation with the fractal dimensions (D) of the surrounded matrix between 0.6 and 2.8 characterizes the granular transport. A collisional shearing contact probably operated between avalanche units and pyroclastic density currents, which contribute to co-genetic evolution of block clusters from median to frontal distal zones. In the distal zone, abraded block clusters and tilted blocks are related to frontal reworking by impact wave.The cataclastic gradient of avalanche units is correlated with the pyroclastic flow regime. Semi-quantitative analysis of block clusters provides information about syn-emplacement processes during sequential impact waves related to volcanic debris-avalanche units and pyroclastic density currents.