# Shape evolution of pumice during granular flow

## TL;DR

This study uses lab experiments to show how pumice rocks change shape during volcanic flows, with most changes happening in the first 15 minutes.

## Contribution

The study introduces a new method to quantify shape evolution of pumice in granular flows using morphological parameters and relaxation timescales.

## Key findings

- Most shape change in pumice occurs within the first 15 minutes of tumbling, generating up to 48 wt.% ash.
- Pumice clasts evolve toward a time-invariant shape with a decelerating rate of change.
- The susceptibility of porous pyroclasts to shape changes is quantified, aiding understanding of volcanic transport processes.

## Abstract

Explosive volcanic eruptions are a major geo-hazard. Given the energetic nature of eruptive processes, direct observation is limited, making the study of deposits and pyroclast textures essential for understanding eruption dynamics. Experimental constraints therefore provide a vital contribution to improving hazard assessment. We performed tumbling experiments using pumice lapilli from the Laacher See eruption (Eifel, Germany) to investigate ash generation and pyroclast shape evolution. Before and after each experimental step, samples were sieved, and the volume and four morphological parameters (axial ratio, convexity, form factor, solidity) of 100 clasts were measured. Most shape change happened before the first 15 min (first experimental step) and produced up to 48 wt.% ash. We frame our analysis in terms of effective relaxation timescales, whereby pyroclasts display a decelerating rate of shape change towards a time-invariant morphology. This quantification of the susceptibility of porous pyroclasts to changes enhances our understanding of transport processes from clast generation to sedimentation.

Pumice clasts within volcanic granular flows evolve towards a time-invariant shape with most shape change occurring during the first minutes of flow, according to laboratory-based tumbling experiments.