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Shape evolution of pumice during granular flow

Carolina Figueiredo 1, Ulrich Kueppers1, Lisa Depauli1, Sarp Esenyel1, Luiz Pereira1, Donald B. Dingwell.1

  • Affiliations:  1 Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, 80333 Munich, Germany. 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 176

  • Programme No: 3.9.12

  • Theme 3 > Session 9

Abstract

Explosive volcanic eruptions are a major geo-hazard. Direct observation of key phenomena is essential for improving hazard assessment. However, this is often unfeasible and information is generally inferred from volcanic deposits. During explosive eruptions, felsic magma fragments into angular, porous pyroclasts, transported into a buoyant eruption plume or into ground-hugging currents of variable particle concentration and flow turbulence. Depending on granular temperature, clasts change size and shape. The in-situ production of ash contributes to flow's mobility, enlarging areas at threat. To understand their transport and modification during granular flow, we performed three types of tumbling experiments using 2 kg lapilli from the 13 ka Laacher See eruption. At 5 experimental increments, the starting material was sieved at 2 mm. Ash generation was as high as 48 wt.% after 120 minutes. Shape evolution was investigated for 100 painted clasts with known petrophysical properties (volume, porosity) via four morphological (Axial Ratio, Convexity, Form Factor, and Solidity) parameters. Size and roughness evolved and allowed for quantifying volcanic ash generation and shape changes framing them in a specific relaxation theory. We have framed the analysis of the experimental findings in terms of effective relaxation times, whereby pyroclasts display a decelerating rate of change, on their approach to a time-invariant state of shape and roughness. This quantification of the susceptibility of porous pyroclasts to shape and size changes enhances our understanding of transport processes. Thorough field characterization of volcanic deposits will aid upscaling the experiments to natural processes, serving thereby to help mitigate volcanic hazards.