An analog experiment showing varying shapes of pyroclasts by including crystals and bubbles

Low-viscosity basaltic and basaltic andesitic magmas form pyroclastic materials with various shapes, such as scoria, volcanic ash, and Pele’s hair, through fracturing. Brittle fracture of low-viscosity magma is unlikely. Instead, fluid dynamical deformation during the eruption may cause the magma to tear off. The various morphologies of pyroclast shapes should be determined by their deformation history. High crystallinity in magma increases its effective viscosity, potentially enabling brittle-like fracture. To understand how the shape of pyroclastic material produced by low-viscosity magma is determined, we conducted uniaxial extension experiments using a magma analogue mixture of liquid, solid, and gas phases. Our experiments demonstrate that fluids with a high solid particle fraction ( > 0.3) tend to tear off at a small strain, thereby generating a rough fracture surface. The solid particles were undeformable, and a high particle fraction reduced the thickness of the deformable liquid region. Thin liquid films are easily disrupted, and thus a small strain can break the entire fluid. In contrast, a particle-free bubbly fluid generates thin threads. Bubbles can coalesce to create a large bubble and deform. Vertically elongated bubbles split the fluid in the longitudinal direction, forming threads. The former mechanism may generate the irregularly shaped scoriae, and the latter forms Pele’s hair. Our experimental results indicate that suspended crystals and bubbles in magma affect not only the physical properties of magma but also the fracture manner, thereby determining the shapes of pyroclasts. • We conducted a series of uniaxial extensional experiments of magma analogues. • A strain for breakage decreases in size for a magma analogue with more crystals and bubbles. • When a high crystallinity magma analogue breaks up, it forms a rough surface. • A small strain can break thin melt films located between the crystals. • This mechanism forms rough surfaces of the pyroclasts of basaltic magmas.