5:15 PM - 6:45 PM
[SVC31-P04] A tensile deformation experiment of analog low-viscosity magma containing bubbles and crystals: visual observation and force measurement of fracture behavior
Keywords:Magma, Fragmentation
Explosive eruptions tend to cause great damage, and it is important to understand their mechanisms. The mechanism that causes explosive eruptions involves magma fragmentation, by which continuous magma breaks and becomes gas and pyroclasts. Low-viscosity magma generally causes effusive eruptions, but its properties change when crystals are included in the magma and brittle fracture becomes possible (Moitra et al., 2018). Here, magma also contains bubbles, but the role of bubbles needs to be clarified. Tensile experiments for magma analogue containing both bubbles and crystals have yet to be performed. To clarify how bubbles and crystals affect the way magma fragments, we conducted analog tensile experiments on low-viscosity magma containing bubbles and crystals.
Eight types of fluids were used: pure liquid, bubbly fluid, crystal-bearing fluid, and crystal-bearing bubbly fluid, using starch syrup with two types of viscosities (~10 Pa s and 250-500 Pa s) as the liquid phase. Plastic particles with a diameter of ~100 μm were used to analog the crystals. We extended these suspensions at a constant speed, measured the force, and observed fracture behavior.
The fluid containing crystals breaks with a short displacement, while the crystal-free fluids make a filament-like structure and do not break. For bubbly fluids, the fluid at 250-500 Pa s extends like filaments and does not fracture, while the fluid at ~10 Pa s breaks during elongation and then forms a droplet due to surface tension. For crystal-bearing fluids, the fluid at 250-500 Pa s fractures with a short extension due to cracks, while the fluid at ~10 Pa s does not form cracks and breaks with a long extension. The force required to elongate the fluids decreases as the displacement increases. The fluids with a liquid phase viscosity of 250-500 Pa s need a larger force than those with a liquid phase viscosity of ~10 Pa s even if the contents in the suspensions are the same. We thus conclude that the viscosity of the liquid phase has a critical role in determining the behavior of tensile deformation, and fragmentation.
Our experiments revealed that suspensions with more particles and bubbles break with a short displacement. To explain this result, we consider a liquid phase thickness δ, existing between bubbles and/or plastic particles, which is estimated using the volume fraction of bubbles and crystals. Our measurements clearly show that the larger the thickness δ of the liquid phase requires larger displacement for the fracture. That is, the film thickness becomes thin enough to rapture with a small displacement. The particle-free bubbly fluid with a liquid phase viscosity of 250-500 Pa s did not break with a similar δ, but those with a liquid phase viscosity of ~10 Pa s did. This may be because of the difference in the Capillary number, which is the ratio of viscous stress and surface tension. We also found an intermediate fracture between brittle and ductile, which may be attributed to bubbles in the fluid. Our experiments suggest that combining bubbles and crystals content and the melt phase viscosity can explain the variety of the morphologies of pyroclasts such as scoria, Pele’s hair, and tear.
Eight types of fluids were used: pure liquid, bubbly fluid, crystal-bearing fluid, and crystal-bearing bubbly fluid, using starch syrup with two types of viscosities (~10 Pa s and 250-500 Pa s) as the liquid phase. Plastic particles with a diameter of ~100 μm were used to analog the crystals. We extended these suspensions at a constant speed, measured the force, and observed fracture behavior.
The fluid containing crystals breaks with a short displacement, while the crystal-free fluids make a filament-like structure and do not break. For bubbly fluids, the fluid at 250-500 Pa s extends like filaments and does not fracture, while the fluid at ~10 Pa s breaks during elongation and then forms a droplet due to surface tension. For crystal-bearing fluids, the fluid at 250-500 Pa s fractures with a short extension due to cracks, while the fluid at ~10 Pa s does not form cracks and breaks with a long extension. The force required to elongate the fluids decreases as the displacement increases. The fluids with a liquid phase viscosity of 250-500 Pa s need a larger force than those with a liquid phase viscosity of ~10 Pa s even if the contents in the suspensions are the same. We thus conclude that the viscosity of the liquid phase has a critical role in determining the behavior of tensile deformation, and fragmentation.
Our experiments revealed that suspensions with more particles and bubbles break with a short displacement. To explain this result, we consider a liquid phase thickness δ, existing between bubbles and/or plastic particles, which is estimated using the volume fraction of bubbles and crystals. Our measurements clearly show that the larger the thickness δ of the liquid phase requires larger displacement for the fracture. That is, the film thickness becomes thin enough to rapture with a small displacement. The particle-free bubbly fluid with a liquid phase viscosity of 250-500 Pa s did not break with a similar δ, but those with a liquid phase viscosity of ~10 Pa s did. This may be because of the difference in the Capillary number, which is the ratio of viscous stress and surface tension. We also found an intermediate fracture between brittle and ductile, which may be attributed to bubbles in the fluid. Our experiments suggest that combining bubbles and crystals content and the melt phase viscosity can explain the variety of the morphologies of pyroclasts such as scoria, Pele’s hair, and tear.