Formation of high-grade ignimbrites Part II. A pyroclastic suspension current model with implications also for low-grade ignimbrites |
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| Authors: | Armin Freundt |
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| Institution: | (1) GEOMAR, Wischhofstrasse 1–3, D-24148 Kiel, Germany e-mail: afreundt@geomar.de Fax: +49 431 600 2924, DE |
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| Abstract: | Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as
dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic
flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents
are derived from column-collapse modeling for magmas with an initial H2O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or
radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature,
bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents
to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80%
of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 107-1012 kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining
currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the
fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the
prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent
if eruption temperature is To>1.3.Tmw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation
of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle
aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout.
As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic
currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially
leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate
occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite
masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite
mass.
Received: 10 May 1998 / Accepted: 21 October 1998 |
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| Keywords: | Ignimbrite Pyroclastic suspension current Column collapse Physical modeling Welding Particle aggregation Co-ignimbrite ash |
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