Thursday, February 2, 2012

Comminution in Pyroclastic Density Currents

In explosive volcanic eruptions the grain size distribution strongly controls eruptive behavior. Examples include modifying the residence time of ash, as was illustrated in the Eyjafjallajökull eruptive episode, and altering the dynamics of pyroclastic density currents. Recently our lab has been exploring how modifications to grain size during an eruption can alter the dynamics, residence times, and ultimately the dispersal and inundation area for volcanic flows. There are numerous ways grain sizes can be modified, but we can roughly group these processes into aggregation (combining smaller particles to make bigger particles) and comminution (breaking larger particles into smaller particles). In this post we will explore some of our recent work on comminution, and in particular focus on low energy comminution. It is important to note that our experiments indicate that the style of particle breakup is energy dependent with low energy collisions breaking the bubble walls and septa in pumice. During low energy collisions and frictional grinding the particles removed from pumice are fine-grained ash leaving a progressively rounder pumice with more collisions. High-energy collisions break up particles in a macroscopic fashion and produce angular fragments.

Pyroclastic flow deposit from Mount St. Helens in 1980 showing some rounding of pumice clasts. USGS photograph.

You may be pondering, why consider particle breakup? While at face value the topic may appear somewhat obscure, the answer to this question really comes in two parts. First, particle breakup, especially low energy breakup, alters the textures of volcanic particles, and these textures can be measured in deposits. For instance, particles in pyroclastic density currents are typically rounder than fall deposits because small ash particles have been removed from the original pumice, little by little, to produce rounded pumice. By measuring the roundness of many particles we can constrain the dynamics in these currents, something we currently cannot do in real time due to the obvious hazard of placing equipment in the path of pyroclastic density currents. So particle breakup provides a metric for integrated energy in a flow.

Perhaps more importantly though, particle breakup manifestly changes the dynamics of volcanic flows. Pyroclastic density currents are mixtures of pumice, ash and volcanic gases that are denser than the surrounding atmosphere. You can typically think of them as hot, granular avalanches. They are particularly hazardous to those who live on the flanks of volcanoes because they are very energetic and fast moving. Particles in these currents can interact due to prolonged contact with each other (frictional contact) or through collisions. Both interactions are likely common in most pyroclastic density currents. The generation of greater amounts of fine particles increases the amount of particles that can be suspended by turbulence, enhances pore pressure in flows, creates larger granular drag on larger clasts, and ultimately makes flows go further. Moreover, fine particles can later be introduced to higher levels in the atmosphere due to secondary plumes.

To better understand the fate of fine ash in pyroclastic flows our group used information obtained from particle collision and frictional grinding experiments to create a constitutive relationship for ash production; basically a rule for the amount of ash produced based on the local concentration of particles and the energy of their collisions. The collision experiments used compressed gas to propel pumice at a target piece of pumice with a ‘pumice gun’, and used high-speed video to calculate the energy of the impact. A rotating cylinder filled with pumice is used to understand the prolonged frictional contact of particles. The constitutive relationship was then introduced into a computer simulation to understand the distance the flow travels and changes in particle concentration in pyroclastic density currents. We found that these numerous low energy collisions produce rounding like that observed at Mount St. Helens pyroclastic density current deposits, and the ash makes the flows travel further (an example simulation is shown below).

To learn more about this work see the following papers:

Dufek, J.  and  Manga, M. (2008) The In-Situ Production of Ash in Pyroclastic Flows. Journal of Geophysical Research, 113, B09207, doi:10.1029/2007JB005555.

Manga, M., Patel, A., and Dufek, J. (2011) Rounding of pumice clasts during transport: field measurements and laboratory studies. Bulletin of Volcanology, 73(3), 321-333.