Controls on juvenile ash morphologies in lava fountains: insights from laboratory experiments

Hawaiian lava fountains produce a wide variety of pyroclasts, including achneliths, i.e., fluidal juvenile fragments. These range from nearly perfect spheres to very elongate Pele’s hairs, but the controls on such variations are not yet entirely clear. We therefore conduct laboratory-scale experiments using magmas of three different compositions (olivine-melilitite with 38 wt.% SiO₂, alkali basalt with 45 wt.% SiO₂, and basaltic trachyandesite with 54 wt.% SiO₂). These magmas are ejected from a crucible using two different gas driving pressures (3 and 10 MPa) which correspond to low and high exit velocities. All magmas are at the same initial temperature of 1200 °C, and each run is somewhat comparable to a very short (< 1 s) lava fountain or Strombolian pulse. We collect and sieve the artificial ejecta and focus on two ash fractions, 0.71–0.5 mm (narrow + 1ɸ) and 88–63 µm (narrow + 4ɸ). We measure the componentry, morphometric parameters, and internal textures of these particles. We find two end-members in terms of fluidal ash morphologies: (1) olivine-melilitite ejected at low velocity mostly generates spheres and other equant shapes; (2) basaltic trachy-andesite ejected at high speed mostly generates Pele’s hairs and other elongate shapes. Natural achneliths from the 1959 Kīlauea Iki eruption (Hawaii, USA) are most similar in shape to the artificial ones generated with the alkali basalt ejected at high speed, and mostly consist of fluidal elongate grains and Pele’s tears. We analyze shape-controlling processes using high-speed video recordings of the experiments, and filament thinning theory. When hydrodynamic fragmentation occurs, surface tension acts to reshape clasts towards a sphere. Opposing factors can extend filament thinning timescales, and the two most relevant ones here are magma viscosity and ejection speed. This, along with the effects of rapid cooling, largely explains the observed morphological variety in artificial and natural fluidal-shaped juvenile ash.