Spread or Splash: The Ubiquitous Role of Droplets in Mafic Explosive Eruptions

Abstract

Magma fragmentation is an essential process driving explosive volcanic eruptions, generating a distribution of pyroclasts with characteristic shape and grain size. These characteristics are often used to inform on the energetics of magma fragmentation and the associated eruption style and intensity. However, a portion of these pyroclasts, droplets when still in the molten state, are likely to be generated through impact mechanisms (i.e., collisions), and subsequent secondary fragmentation (i.e., splashing). Here, we successfully apply and dynamically scale concepts and findings of liquid droplet impacts in engineering to magma fragmentation processes in volcanology. We compile and model physical data for two mafic melt compositions (kimberlite and basalt) and use specific eruption examples from Igwisi Hills, Kīlauea and Stromboli volcanoes to define composition-specific impact dynamics. Pyroclast impact dynamics have a direct control on in-conduit processes, eruption dynamics, and ash dispersal. For low viscosity mafic melts such as kimberlite and basalt, pyroclast impacts can lead to both splash and deposition on the conduit wall, resulting either in conduit clearing or conduit narrowing, respectively. In both cases, shifting the impact regime toward surface deposition will lead to an inexorable decay in explosiveness, potentially switching the eruption style to effusive behavior. This has direct consequences for the transport of volcanic ash at the surface and inferring magma fragmentation processes (e.g., energies) from the depositional record.