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.

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.

Hazards and Risks at Gunung Merapi, Central Java: A Case Study

2005 ◽  
Author(s):  
Jean-Claude Thouret ◽  
Franck Lavigne
Keyword(s):  
Pyroclastic Flows ◽  
Pyroclastic Density Currents ◽  
Hazard Map ◽  
Density Currents ◽  
Danger Zone ◽  
Merapi Volcano ◽  
Central Java ◽  
Dome Extrusion ◽  
Risk Areas

Of the 1.1 million people living on the flanks of the active Merapi volcano in Java (average population density: 1140 inhabitants per km2), 440 000 live in relatively high-risk areas prone to pyroclastic flows, surges, and lahars. The sixty-one reported eruptions since the mid-1500s killed about 7000 people. For the last two centuries the activity of Merapi has alternated regularly between long periods of lava dome extrusion and brief explosive episodes with dome collapse pyroclastic flows at eight- to fifteen-year intervals. Violent explosive episodes on an average recurrence of twenty-six to fifty-four years have generated pyroclastic flows, surges, tephra falls, and subsequent lahars. The current hazard zone map of Merapi (Pardyanto et al. 1978) portrays three areas, termed the forbidden zone, first danger zone, and second danger zone, based on progressively declining hazard intensity. Revision of the hazard map has been carried out because it lacked the details necessary to outline hazard zones with accuracy (in particular the valleys likely to be swept by lahars), and excluded some areas likely to be devastated by pyroclastic density currents, such as the 22 November 1994 surge. In addition, risk maps were developed in order to incorporate social, technical, and economic elements of vulnerability (Lavigne 1998, 2000) in the decision-making progress. Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, based on eruptive behaviour and scenarios combined with existing models and preliminary numerical modelling (Thouret et al. 2000). The reconstructed past eruptive activity and related damage define the extent and frequency of pyroclastic flows, the most hazardous phenomenon (Camus et al. 2000; Newhall et al. 2000). Pyroclastic flows travelled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Camus et al. 2000; Newhall et al. 2000).

scholarly journals Volcano–glacier interactions on composite cones and lahar generation: Nevado del Ruiz, Colombia, case study

2007 ◽  
Vol 45 ◽  
pp. 115-127 ◽  
Author(s):  
J.C. Thouret ◽  
J. Ramírez C. ◽  
B. Gibert-Malengreau ◽  
C.A. Vargas ◽  
J.L. Naranjo ◽  
...  
Keyword(s):  
Pyroclastic Flows ◽  
Pyroclastic Density Currents ◽  
Density Currents ◽  
Long Term Effects ◽  
Ice Cap ◽  
The North ◽  
Potential Source ◽  
Ice Field

AbstractThe catastrophic lahars triggered by the 13 November 1985 eruption of the ice-clad Nevado del Ruiz volcano, Colombia, demonstrate that the interaction of hot pyroclasts with snow and ice can release 30–50 millionm3 of meltwater in 30–90 minutes. The 1985 eruption caused a 16% loss in area and a 9% loss in volume of snow, firn and ice. Turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0.5–1.6mms–1. Laboratory experiments suggest that turbulent, fluidized pyroclastic density currents exert mechanical and thermal scour, thereby efficiently transferring heat from hot pyroclasts to snow. Ice cap loss at Nevado del Ruiz continued between 1985 and 2000, representing a ∽52% decline in area and a ∽30% fall in volume. Ice 60–190m thick caps the east and southeast summit plateau, whereas an ice field < 30m thick and devoid of snow is retreating on the north, northeast and west edges. This asymmetrical distribution of ice reflects combined long-term effects of the 1985 eruption and of the post-1985 ice cap retreat. Should volcanic activity resume, steep-sided glaciers can fail and pyroclastic flows and surges can sweep the snowpack and generate mixed avalanches and lahars. Although the potential source of meltwater has decreased since 1985, extensive debris at the ice cap margins can be incorporated to future lahars.

Determination of the phase structure of polymerblends by electron microscopy

1978 ◽  
Vol 36 (1) ◽  
pp. 492-493
Author(s):  
Dr. G. Kaemof
Keyword(s):  
Screw Extruder ◽  
Electron Microscopic ◽  
Thin Sections ◽  
Single Screw Extruder ◽  
Transmission Electron ◽  
The Matrix ◽  
Twin Screw ◽  
Special Case ◽  
Microscopic Investigations

A mixture of polycarbonate (PC) and styrene-acrylonitrile-copolymer (SAN) represents a very good example for the efficiency of electron microscopic investigations concerning the determination of optimum production procedures for high grade product properties.The following parameters have been varied:components of charge (PC : SAN 50 : 50, 60 : 40, 70 : 30), kind of compounding machine (single screw extruder, twin screw extruder, discontinuous kneader), mass-temperature (lowest and highest possible temperature).The transmission electron microscopic investigations (TEM) were carried out on ultra thin sections, the PC-phase of which was selectively etched by triethylamine.The phase transition (matrix to disperse phase) does not occur - as might be expected - at a PC to SAN ratio of 50 : 50, but at a ratio of 65 : 35. Our results show that the matrix is preferably formed by the components with the lower melting viscosity (in this special case SAN), even at concentrations of less than 50 %.

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

&lt;p&gt;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 &amp;#8220;distance&amp;#8221; steps (from 0.2 to 6&amp;#160;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.&lt;/p&gt;

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