5:15 PM - 7:15 PM
[SCG62-P14] A tensile deformation experiment of analog low-viscosity magma containing bubbles and crystals: A thinner film ruptures by small displacement
Keywords:Magma, Fragmentation
Since the explosive eruption of volcanoes can cause serious damage, it is important to understand the mechanisms of it. One of the factors that causes explosive eruptions is the fragmentation of magma. Fragmentation is the phenomenon in which a continuous magma breaks up into small pieces, becoming gas phases and pyroclasts. Low-viscosity magma tends to cause effusive eruptions, but explosive eruptions of low-viscosity magma have also been reported. However, the mechanisms of explosive eruption and fragmentation of low-viscosity magma are unclear. Previous studies have shown that low-viscosity magma fragments through hydrodynamic deformation in a manner different from brittle fracture (Gonnermann, 2015; Jones et al., 2019), and that crystallization of low-viscosity magma can increase its effective viscosity and promote fragmentation by brittle fracture (Moitra et al., 2018). In this study, analog deformation and fragmentation experiments of low-viscosity magma containing bubbles and crystals were conducted to understand the fragmentation mechanisms of low-viscosity magma.
We made analog magmas using starch syrup as a melt phase and plastic particles as crystal. We added CO2 bubbles to the fluids by chemical reaction of sodium bicarbonate and citric acid. 20 types of analog magma were prepared, with five different liquid viscosities (40 Pas, 100 Pas, 200 Pas, 500 Pas, 1000 Pas) and four suspension types (liquid only, liquid + bubbles, liquid + crystals, liquid + bubbles + crystals). The fluids were deformed at a constant speed (10 mm/s), the applied force was measured, and the state of fracture was observed.
This study found a unique fracture manner; a viscous melt film ruptures in a small displacement, generating fragments with complex shapes. When the fluids contain bubbles, multiple thin threads are formed during tensile deformation. The larger fraction of suspending particles/bubbles shortens the critical displacement to cause fracture. The required force to deform the suspension is approximately determined by its effective viscosity.
It is plausible that low-viscosity magma fractures when the melt film between bubbles and/or crystals thins with deformation and reaches a critical thickness. The shape of pyroclasts formed from low-viscosity magma varies with the volume fraction of bubbles and/or crystals and the viscosity of the liquid phase.
We made analog magmas using starch syrup as a melt phase and plastic particles as crystal. We added CO2 bubbles to the fluids by chemical reaction of sodium bicarbonate and citric acid. 20 types of analog magma were prepared, with five different liquid viscosities (40 Pas, 100 Pas, 200 Pas, 500 Pas, 1000 Pas) and four suspension types (liquid only, liquid + bubbles, liquid + crystals, liquid + bubbles + crystals). The fluids were deformed at a constant speed (10 mm/s), the applied force was measured, and the state of fracture was observed.
This study found a unique fracture manner; a viscous melt film ruptures in a small displacement, generating fragments with complex shapes. When the fluids contain bubbles, multiple thin threads are formed during tensile deformation. The larger fraction of suspending particles/bubbles shortens the critical displacement to cause fracture. The required force to deform the suspension is approximately determined by its effective viscosity.
It is plausible that low-viscosity magma fractures when the melt film between bubbles and/or crystals thins with deformation and reaches a critical thickness. The shape of pyroclasts formed from low-viscosity magma varies with the volume fraction of bubbles and/or crystals and the viscosity of the liquid phase.