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Pyroclastic flows are avalanches of hot fragments and gas down the slopes of volcanoes, typically at 20–50 m s − 1. The terminology of pyroclastic flows is complicated, reflecting multiple origins of flows (Wright et al., 1980). Most pyroclastic flows form by collapse of an eruption column, collapse of a dense slug of debris erupted just a ...
They may be subdivided according to how they formed: “pyroclastic” rocks form by volcanic explosivity; “epiclastic” rocks form by sedimentary denudation; “autoclastic” rocks are fragmented by non-explosive magma movement, say in a dyke or lava flow, and “cataclastic” volcaniclastic rocks form by faulting of volcanic rocks.
Pyroclastic density currents (PDCs) or pyroclastic flows and surges as end-member types, are probably the most destructive and complex volcanic phenomena with respect to the dynamics of flow (Sigurdsson et al., 1987, Druitt, 1998, Branney and Kokelaar, 2002). They have also been the most difficult to observe directly, due to the extreme hazard ...
Pyroclastic flows feature large propagation speeds and inviscid behavior. •. Fluidization is responsible for the large mobility of pyroclastic flows. •. Fluidization is due to slow escape of interstitial gas during solids fall-out. •. Fluidization may be self-sustained by air entrainment at the head of the flow. •.
Pyroclastic density currents (PDCs) are mixtures of two components, namely solid particles and fluid (gas) phase. They macroscopically behave as dense, multiphase gravity currents (flowing pyroclastic mixtures of particles and gas) immersed in a less dense, almost isotropic fluid (the atmosphere). As for other natural phenomena, their study ...
Hence, whether pyroclastic flows and surges represent two truly distinct phenomena remains unresolved. The density discontinuities reproduced in experiments of fluidization [17] and high-speed two-phase flow decompression [18], as well as the marked facies diversity of the deposits, are cited in support of a discontinuity between flow and surge.
The term, pyroclastic flow, in the past has been used to refer to dense particle mixtures that typically produce massive (subtle to no structure) deposits that tend to thicken in topographic lows. The use of the term, pyroclastic density current, is now preferred to refer to all these flows as individual currents can have behavior that was ...
In general, the size of the resulting caldera is proportional to the volume of erupted pyroclastic flow material, but caldera diameters may range from 2 to 30 km. Pyroclastic flow deposits are typically 1 to 10 m thick and massive. Their internal structure is characterized by very poor sorting, with pyroclastic fragments, such as pumice and ...
Nevertheless, the identification of areally extensive air-fall deposits (Hynek et al., 2003) or pyroclastic flow deposits is important as the thin atmosphere and lower gravity on Mars should favor magma fragmentation and, hence, the production of friable pyroclastic materials (Wilson, 1984; Wilson and Head, 1994).
PELE is a unique facility that synthesizes nature-scaled PDCs of up to 6 t of pyroclastic material and air, which reach velocities of 7–30 m s −1, flow thicknesses of 2–4.5 m, and runout lengths of >35 m (Lube et al., 2015). Particle–gas currents are generated by the controlled gravitational collapse of concentrated suspensions of ...