An analogue tensile experiment of particle and bubble suspension producing various shapes of pyroclasts

Fragmentation is an essential process in explosive eruptions breaking a continuous magma into small pieces, generating pyroclastic materials, and enabling the rapid expansion. Fragmentation of basaltic and basaltic andesitic magmas with low viscosity produces pyroclasts in various shapes, such as scoria, pumice, volcanic ash, and Pele’s hair. The shapes of pyroclasts from low-viscosity magma may preserve their deformation history. Brittle fracture of low-viscosity magma is unlikely; instead, fluid-dynamical deformation during the eruption tears off the magma. Crystallization in ascending magma increases its effective viscosity, thereby extending the relaxation time, and potentially leading to brittle fragmentation. However, rheology measurements of crystal-bearing magma indicate that large strains can unjam the crystals, leading to deformation instead of brittle fracture.

To understand how the shapes of pyroclasts generated from low-viscosity magma are determined, uniaxial tensile experiments were conducted using a magma analogue with three phases: liquid, solid, and gas. These experiments simulate processes in a lava fountain, an elongation of a magma parcel. In our experiments, fluids with particle volume fractions >0.3 tend to tear off at a low strain and exhibit rough fracture surfaces. A high volume fraction of undeformable solid particles reduces the thickness of the deformable liquid region, resulting in easy rupture of thin liquid films. Therefore, the entire fluids are broken at a small strain. On the other hand, bubbly fluid without solid particles generates thin threads of the liquid phase. Bubbles can coalesce into larger bubbles and deform, elongating vertically, splitting the fluid longitudinally to form fibers. The fracture mechanism associated with high particle fraction may generate scoriae with irregular shapes. The mechanism forming thin threads due to bubble deformation may produce Pele’s hair. Our experimental results indicate that bubbles and crystals included in magma affect the fracture manners of magma as well as its physical properties, and that this determines the shapes of pyroclasts variously.

 

Reference

Oda, S. & Namiki, A. An analogue experiment showing varying shapes of pyroclasts by including crystals and bubbles. Journal of Volcanology and Geothermal Research, in press.