Analog Study of Clastes Transport and Sedimentation in Pyroclastic Density Currents
Abstract
Analog experiments using an aqueous lock-exchange flume were carried out to study the transport and sedimentation processes occurring in pyroclastic density currents (PDC) generating extensive sheet-like ignimbrites. Those experimental flows scaled to turbulent PDCs, focusing on : (i) a very wide range of particle sizes, and (ii) particles of two different densities (the water-immersed densities of which scaled to those of lithics and pumice in gas). A suspension, containing either one or two components of different densities (SiC:2600 kg.m-3 and LB:1440 kg.m-3), with a log-normal particle size distribution, produced flows of various initial concentration (0.6 to 23 vol.%). Hydrodynamic equivalence was respected in the two-component suspension. Development of stratification and particle segregation was followed in the moving flows. Originally in a homogeneous suspension, the dense component in the one- or two-components flow displayed vertically and laterally a normal grading in the flow, latter observed in the deposit. Its mass loading and grain size distribution was insensitive to the initial concentration of the flow. Deposit from a one-component flow provides little information on particle concentration of the parental flow. The transport and sedimentation behaviour of the light component in the two component experiments was extremely concentration sensitive. For dilute currents, dense and light particles settled respecting their hydrodynamic equivalence whereas they segregated efficiently for initial flow concentration higher than a few percent. In the moving flow of high initial concentration, the light component exhibited a reverse grading vertically and laterally. Their resulting deposit showed reverse grading and the site of their maximum accumulation was progressively displaced downstream, due to some increasing buoyancy effects. Segregation between pumice (light component) and lithics (dense component) occur in a PCD, the density of which is lower than that of the pumice. Based on the study of the aerodynamic equivalence between pumice and lithics, we conclude that the principal transport system for the large ignimbrites is of low concentration. The sharp variation in the decrease of the maximum size of the lithics versus distance is explained by the distinct value of the Pn of the settling particles around Pn critical value, ruling out the hypothesis of a deflation zone. The proximal co-ignimbrite lag-breccia results from the settling of coarse clastes from the overloaded PDC.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V12F..04C
- Keywords:
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- 8400 VOLCANOLOGY;
- 8404 Ash deposits;
- 8499 General or miscellaneous