scholarly journals Pompeian hiatuses: new stratigraphic data highlight pauses in the course of the ad 79 eruption at Pompeii

2020 ◽  
Vol 157 (4) ◽  
pp. 695-700
Author(s):  
Claudio Scarpati ◽  
Annamaria Perrotta ◽  
Alberta Martellone ◽  
Massimo Osanna
Keyword(s):  
Second Phase ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Pyroclastic Deposits ◽  
The Town

AbstractA new stratigraphic survey of the pyroclastic deposits blanketing Pompeii ruins shows departures from prior reconstruction of the events that occurred inside the town during the two main phases (pumice fallout and pyroclastic density currents) of the ad 79 Vesuvius eruption. We document the depth and distribution of subaerial erosion surfaces in the upper part of the pyroclastic sequence, formed during two short-lived breaks occurring in the course of the second phase of the eruption. These pauses could explain why 50% of the victims were found in the streets during the pyroclastic density currents phase.

scholarly journals Mapping of Pyroclastic Density Currents Hazards and Assessment of Related Risks by AMS Technique in the West-Cameroon Highlands: Case of Bambouto and Bamenda Volcanoes

2020 ◽  
pp. 39-60 ◽  
Author(s):  
Merlin Gountié Dedzo ◽  
Ghislain Zangmo Tefogoum ◽  
Boris Chako Tchamabé ◽  
Eric Martial Fozing ◽  
Emmanuel Njonfang ◽  
...  
Keyword(s):  
Pyroclastic Flows ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Magnetic Studies ◽  
Pyroclastic Deposits ◽  
Thin Sections ◽  
Magnetic Lineations ◽  
Field Indicators ◽  
Magnetic Foliation

Ignimbritic flow deposits which derived from pyroclastic density currents (PDCs) are mostly observed in West-Cameroon Highlands located in the central portion of the Cameroon Volcanic Line (CVL), especially in Bambouto (21.12 - 0.50 Ma) and Bamenda (27.40 - 0 Ma) volcanoes. These deposits covering approximately 27% (≈195 km2) of the volcanoes surface with thickness ranging from 30 to 200 m representing a total volume estimated at 20 km3. Because of the intense weathering of the ignimbritic formations after their setting up and being buried by basaltic and trachtytic flows, the initial volume of these pyroclastic deposits is really much larger. Soil fertility has fostered an important population growth (more than 1,200,000 people) in these volcanoes. The economic and agropastoral activities on the flanks and inside the caldera of the volcanoes are estimated at about $US7.5 billion. In this paper, we evaluate and realize cartography of the hazards associated to ignimbritic eruptions which are most disastrous in term of volcanic process in this region. Magnetic studies, specifically, Anisotropy of Magnetic Susceptibility (AMS) method has been utilized for the determination of flow directions in visually nearly isotropic ignimbritic deposits outcrops. The AMS data reported from the Bamenda and Bambouto volcanoes ignimbrites produced significant informations about the depositional scheme of the PDCs. In most sites, magnetic lineations and principally magnetic foliation are reliably parallel to downhill directions, frequently with an upslope imbrication. Inferred palaeoflow directions based on the field indicators, orientation of minerals and other objects in oriented thin sections and the directional AMS data show that Bambouto caldera, Oku crater and Santa-Mbu caldera are the sources of main PDCs of Bambouto and Bamenda volcanoes. These AMS results have aided us to produce a hazard and risks maps related to potential future pyroclastic flows on these volcanoes. The assessment of risks in these volcanoes was based on populations in the study area, infrastructures (houses and roads) and average income of breeding activity.

Experimental constraints on volcanic ash generation and clast morphometrics in pyroclastic density currents and granular flows

2021 ◽  
Author(s):  
Adrian Hornby ◽  
Ulrich Kueppers ◽  
Benedikt Maurer ◽  
Carina Poetsch ◽  
Donald Dingwell
Keyword(s):  
Experimental Approach ◽  
Direct Evidence ◽  
General Description ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Shape Factors ◽  
Material Density ◽  
Bed Height ◽  
Rotary Drum ◽  
Basal Flow

<p>Pyroclastic density currents (PDCs) present perhaps the greatest proximal primary hazard of volcanic activity and produce abundant fine ash that can present a range of health, environment and infrastructure hazards. However, direct, fully quantitative observation of ash production in PDCs is lacking, and little direct evidence exists to constrain the parameters controlling ash generation in PDCs. Here, we use an experimental approach to investigate the effects of starting mass, material density and ash removal on the efficiency of ash generation and concurrent clast rounding in the dense basal flow of PDCs. We employ a rotary drum to tumble pumice and scoria lapilli clasts over multiple transport “distance” steps (from 0.2 to 6 km). We observe increased ash generation rates with the periodic removal of ash during the experiments and with increasing starting mass. By scaling to the bed height and clast diameter we obtain a general description for ash production in all experiments as a function of flow distance, bed height and average clast diameter. We confirm that changes in lapilli shape factors correlate with the ash fraction generated and that the grain size of ash produced decreases with distance. Finally, we estimate shear rate in our experiments and calculate the inertial number, which describes the ratio between clast-scale and flow-scale rearrangement during flow. We show that, under certain conditions, fractional ash production can be calculated accurately for any starting mass solely as a function of the inertial number and the flow distance. This work sheds light on some of the first systematic and generalizable experimental parameterizations of ash production and associated clast evolution in PDCs and should advance our ability to understand flow mobility and associated hazards.</p>

The numerical reconstruction of three past eruptions at Gede volcano (Indonesia)

2021 ◽  
Author(s):  
Eleanor Tennant ◽  
Susanna Jenkins ◽  
Annie Winson ◽  
Christina Widiwijayanti ◽  
Hendra Gunawan ◽  
...  
Keyword(s):  
Epistemic Uncertainty ◽  
Source Parameters ◽  
Frictional Resistance ◽  
Collapse Mechanism ◽  
Eruption Dynamics ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Basal Friction ◽  
Column Collapse ◽  
Hazard And Risk

<p>Understanding past eruption dynamics at a volcano is crucial for forecasting the range of possible future eruptions and their associated hazards and risk. In this work we reconstructed pyroclastic density currents and tephra fall from three eruptions at Gede volcano, Indonesia with the aim of gaining further insight into past eruptions and identifying suitable eruption source parameters for future hazard and risk assessment. Gede has the largest number of people living within 100 km of any volcano worldwide, and has exhibited recent unrest activity, yet little is known about its eruption history. For pyroclastic density currents, we used Titan2D to reconstruct geological deposits dated at 1200 and c. 1000 years BP. An objective and quantitative multi-criteria method was developed to evaluate the fit of over 300 pyroclastic density current (PDC) model simulations to field observations. We found that the 1200 years BP geological deposits could be reproduced with either a dome collapse or column collapse as the generation mechanism although a relatively low basal friction of 6 degrees would suggest that the PDCs were markedly mobile. Lower basal frictions may reflect the occurrence of previous PDCs that smoothed the path, reducing frictional resistance and enabling greater runout for the reconstructed unit. For the 1,000 years BP PDC, a column collapse mechanism and higher basal friction was required to fit the geological deposits. In agreement with previous studies, we found that Titan2D simulations were most sensitive to the basal friction; however, we also found that the internal friction – often fixed and considered of low influence on outputs - can have a moderate effect on the simulated average deposit thickness. We used Tephra2 to reconstruct historic observations of tephra dispersed to Jakarta and other towns during the last known magmatic eruption of Gede in 1948. In the absence of observable field deposits, or detailed information from the published literature, we stochastically sampled eruption source parameters from wide ranges informed by analogous volcanic systems. Our modelling suggests that the deposition of tephra in Jakarta during the November 1948 eruption was a very low probability event, with approximately a 0.03 % chance of occurrence. Through this work, we exemplify the reconstruction of past eruptions when faced with epistemic uncertainty, and improve our understanding of past eruption dynamics at Gede volcano, providing a crucial step towards the reduction of risk to nearby populations through volcanic hazard assessment.</p>

scholarly journals The hazards of unconfined pyroclastic density currents: a new synthesis and classification according to their deposits, dynamics, and thermal and impact characteristics

2021 ◽  
Author(s):  
Geoffrey Lerner ◽  
Susanna Jenkins ◽  
Sylvain Charbonnier ◽  
Jean-Christophe Komorowski ◽  
Peter Baxter
Keyword(s):  
Volcanic Hazards ◽  
High Energy ◽  
Significant Loss ◽  
Lessons Learned ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Loss Of Life ◽  
New Synthesis ◽  
End Members ◽  
Impact Characteristics

Pyroclastic density currents (PDCs) that escape their confining channels are among the most dangerous of volcanic hazards. These unconfined PDCs are capable of inundating inhabited areas that may be unprepared for these hazards, resulting in significant loss of life and damage to infrastructure. Despite their ability to cause serious impacts, unconfined PDCs have previously only been described for a limited number of specific case studies. Here, we carry out a broader comparative study that reviews the different types of unconfined PDCs, their deposits, dynamics and impacts, as well as the relationships between each element. Unconfined PDCs exist within a range of concentration, velocity and temperature: characteristics that are important in determining their impact. We define four end-member unconfined PDCs: 1. fast overspill flows, 2. slow overspill flows, 3. high-energy surges, and 4. low-energy detached surges (LEDS), and review characteristics and incidents of each from historical eruptions. These four end-members were all observed within the 2010 eruptive sequence of Merapi, Indonesia. We use this well-studied eruption as a case study, in particular the villages of Bakalan, 13 km south, and Bronggang 14 km south of the volcano, which were impacted by slow overspill flows and LEDS, respectively. These two unconfined PDC types are the least described from previous eruptions, but during the Merapi eruption the overspill flow resulted in building destruction and the LEDS in significant loss of life. We discuss the dynamics and deposits of these unconfined PDCs, and the resultant impacts. We then use the lessons learned from the 2010 Merapi eruption to assess some of the impacts associated with the deadly 2018 Fuego, Guatemala eruption. Satellite imagery and media images supplementing fieldwork were used to determine the presence of both overspill flows and LEDS, which resulted in the loss of hundreds of lives and the destruction of hundreds of buildings in inundated areas within 9 km of the summit. By cataloguing unconfined PDC characteristics, dynamics and impacts, we aim to highlight the importance and value of accounting for such phenomena in emergency management and planning at active volcanoes.

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