Gravity and pyroclastic flows on sea coasts: analytical solutions and estimations for the Caribbean
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Abstract On Venus, relatively young deposits near volcanic and coronal summits with unique radar characteristics have been proposed to be emplaced by pyroclastic density currents (PDCs). The proposed units are laterally extensive, long‐runout deposits showing moderate to high radar backscatter and circular polarization ratio in 12.6 cm wavelength synthetic aperture radar data. Previous studies have hypothesized that a recent resumption of volcanic activity in the form of PDC‐forming eruptions could have emplaced these deposits. We model the dynamics of dense PDCs using a 2D, depth‐averaged framework focusing on regions where stereo‐derived topography coverage is available; this includes the flanks of Irnini Mons, Anala Mons, Didilia Corona, and Pavlova Corona. Two different mechanisms of initiation which includes impulsive collapse of an eruption column and sustained pyroclastic fountaining are considered. The results emphasize the importance of pyroclastic flow fluidization via high pore pressure in emplacing long‐runout deposits along gently sloping (<2°) volcanic flanks. We also show that collapse of columns >1.2–1.4 km tall as well as pyroclastic fountains lasting >400 s with fountain heights of 50 m are capable of generating pyroclastic flows that could emplace some of the smaller deposits studied. For the large deposits at Irnini Mons, more energetic flows resulting from taller column heights would be necessary; the dynamics of such flows under Venus's conditions are not well understood. Distinguishing between the two initiation styles, that is, column collapse and sustained fountaining is not feasible with currently available datasets and would require higher resolution imagery and topography data.
Pyroclastic fall
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Abstract Emplacement of small‐volume (<0·1 km 3 ) pyroclastic flows is significantly influenced by topography. The Arico ignimbrite on Tenerife (Canary Islands) is a characteristic small‐volume pyroclastic flow deposit emplaced on high relief topography. The pyroclastic flow flowed down pre‐existing valleys on the southern slopes of the island. In proximal areas deep (up to 100 m) valleys acted as efficient conduits for the pyroclastic flow, which was mostly channelled; in this particular area the ignimbrite corresponds to a homogeneous, moderately welded deposit, consisting of flattened pumices in an abundant ashy matrix with a relatively low lithic fragment content. In intermediate zones significant changes occur in the steepness of the slope and, although still channelled, here the pyroclastic flow was influenced by hydraulic jumps. In this area, two different units can be clearly distinguished in the ignimbrite: the lower unit is composed of a lithic‐rich ground‐layer deposit that formed at the turbulent, highly concentrated head of the flow; the upper unit consists of a well welded pumice‐rich deposit that occasionally reveals a basal layer formed by shearing with the lower part. This division into two units is maintained as far as distal areas near the present‐day coastline, where the slope is very gentle or null and the ignimbrite is not channelled. The ground layer is not found in distal areas. The ignimbrite here only consists of the upper unit, which is occasionally repeated due to a surging process provoked by the lower flow speed, as the pyroclastic flow spread out of the channelled zone. A theoretical model on how topography controlled the deposition of the Arico ignimbrite is derived by interpreting the observed lithological and sedimentological variations in terms of changes in topography and bedrock morphology. This new model is of general applicability and will help to explain other deposits of similar characteristics.
Pumice
Shearing (physics)
Pyroclastic fall
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Abstract The morphology and structure of the 1999 lava flows at Mount Cameroon volcano are documented and discussed in relation to local and source dynamics. Structures are analysed qualitatively and more detailed arguments are developed on the processes of levee formation and systematic links between flow dynamics and levee–channel interface geometry. The flows have clear channels bordered by four main types of levees: initial, accretionary, rubble and overflow levees. Thermally immature pahoehoe lava units with overflow drapes define the proximal zone, whereas rubble and accretionary levees are common in the distal region bordering thermally mature aa clinker or blocky aa flow channels. Pressure ridges, squeeze-ups and pahoehoe ropes are the prevalent compressive structures. Standlines displayed on clinkery breccias are interpreted to represent levee–channel interactions in response to changing flow levels. These data complement previous knowledge on lava flow morphology, thus far dominated by Etnean and Hawaiian examples.
Rubble
Lava dome
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We congratulate [Grunewald et al. (2000)][1] on their very detailed study of pseudotachylite-bearing friction marks on andesitic blocks from block-and-ash flow deposits of Soufriere Hills volcano, Montserrat. Their observations contribute substantially to the current discussion on flow models of
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Dome (geology)
Deposition
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Pyroclastic fall
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A refined Taylor instability model is developed to describe the surface morphology of rhyolite lava flows. The effect of the downslope flow of the lava on the structures resulting from the Taylor instability mechanism is considered. Squire's (1933) transformation is developed for this flow in order to extend the results to three‐dimensional modes. This permits assessing why ridges thought to arise from the Taylor instability mechanism are preferentially oriented transverse to the direction of lava flow. Measured diapir and ridge spacings for the Little and Big Glass Mountain rhyolite flows in northern California are used in conjunction with the model in order to explore the implications of the Taylor instability for flow emplacement. The model suggests additional lava flow features that can be measured in order to test whether the Taylor instability mechanism has influenced the flow's surface morphology.
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Numerical simulation of pyroclastic density currents has developed significantly in recent years and is increasingly applied to volcanological research. Results from physical modeling are commonly taken into account in volcanic hazard assessment and in the definition of hazard mitigation strategies. In this work, we modeled pyroclastic density currents in the Phlegrean Fields caldera, where flows propagating along the flat ground could be confined by the old crater rims that separate downtown Naples from the caldera. The different eruptive scenarios (mass eruption rates, magma compositions, and water contents) were based on available knowledge of this volcanic system, and appropriate vent conditions were calculated for each scenario. Simulations were performed along different topographic profiles to evaluate the effects of topographic barriers on flow propagation. Simulations highlighted interesting features associated with the presence of obstacles such as the development of backflows. Complex interaction between outward moving fronts and backflows can affect flow propagation; if backflows reach the vent, they can even interfere with fountain dynamics and induce a more collapsing behavior. Results show that in the case of large events (≥10 8 kg/s), obstacles affect flow propagation by reducing flow velocity and hence dynamic pressure in distal regions, but they cannot stop the advancement of flows. Deadly conditions (in terms of temperature and ash concentration) characterize the entire region invaded by pyroclastic flows. In the case of small events (2.5 × 10 7 kg/s), flows are confined by distal topographic barriers which provide valuable protection to the region beyond.
Caldera
Volcanic hazards
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