IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3G-P24

Date/Time: July 23 Poster

Pyroclastic gas escape structures
Jack Holt1 , Tim Kluge1 , Mark A Gilbertson1 , Alison C Rust2 , Katharine V Cashman2
1

Department of Mechanical Engineering, University of Bristol, UK, 2 School of Earth Sciences,
University of Bristol, UK
E-mail: m.gilbertson@bristol.ac.uk

Pyroclastic flows and vent deposits consist of mixtures of particles of different sizes and densities, which canseparate from each other resulting in grading or vertical pipe structures from which the finer particles have been
removed. Sometimes these elutriation pipes are associated with logs or large clasts, and it is hypothesised thatthe release of volatiles induces an overlying pipe. The origins and conditions of formation of these structures havebeen explored through laboratory experiments with mixtures of solid particles placed in a planar container throughwhich dry air is passed. The particle beds were well defined mixtures of spherical particles or material from naturaldeposits.

A variety of structures formed in the experiments: pipes of various lengths can grow either upwards ordownwards; horizontal layers of fine or coarse particles can lie at the bottom or the top of a bed; there are areas
where particles are mixed well. Several mechanisms enable this segregation and mixing of particles of different
sizes and densities. First, fine particles are able to move, or percolate, through the pores between larger particles.
In a binary bed, this process requires that the size ratio between the finer and coarser particles is less than acritical value called the percolation limit. The upper bound of the percolation limit, calculated for regularly packed
coarse particles, is 0.41, which is consistent with experimental results.

A second mechanism for mixing and segregation of particles is fluidisation.

The vertical gas velocity at

which the drag matches the particles weight is the minimum fluidisation velocity umf . At this point particles areable to move freely and rearrange themselves. umf is dependent on the size distribution of particles and how they
are packed; so, heterogeneities in the bed lead to heterogeneities in umf . This can lead to complex behaviour as
the segregation of particles and the value of umf are closely coupled. The experiments demonstrate that specificconfigurations of vertical or horizontal sorting depend on precise conditions for formation such as gas velocity,
proportion of fine particles, and size and density ratios, suggesting that pipes and other structures seen in naturaldeposits may be linked to the particular processes that led to them.

A third process by which segregation can take place is through the presence of an embedded object substantiallylarger than the particles that make up the bulk of a deposit. The effect of the embedded object depends on thecomposition of the bed. For bed clast size ratios less than the percolation limit, the presence of a larger object does
not significantly affect pipe formation; however, above the limit then a large, single pipe forms above the obstacle.
These results indicate large clasts and logs may initiate pipe above them due to their effect on gas flow, and are
not necessarily significant gas sources.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

985