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

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, K & imacr;lauea 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. During explosive eruptions, magma is ripped apart to form a distribution of incandescent fragments, termed pyroclasts, of different sizes and shapes. These characteristics are often used to forensically reconstruct eruption dynamics. For explosive volcanic eruptions of low viscosity magmas, fragmentation dynamics often resemble water spraying with the generation of droplets. In volcanoes, and under certain conditions, magma droplets can impact a surface or each other and further fragment, splashing into finer droplets. Such liquid impact dynamics have been long researched in engineering but seldom considered in volcanology. Here, we apply and scale the concepts of liquid impacts from engineering to volcanic processes. We compiled data for ultramafic and mafic magma compositions, which are characterized by different physical properties that influence the impact outcomes (e.g., splash vs. surface deposition). Droplet impacts may lead to splashing or sticking against the conduit wall. For example, under prolonged deposition conditions, progressive narrowing of the conduit could lead to eruption termination or switch to pure lava effusion. Finally, we show that droplet impacts are likely to control the final size of pyroclasts produced during ultramafic and mafic explosive eruptions, and therefore must be considered when interpreting the rock record from related past events. Liquid droplet impacts are relevant to magma fragmentation processes in volcanology Magma composition and ejection velocity are the main factors controlling pyroclast impact dynamics Transitions between pyroclast splash and deposition can contribute to eruption style transitions and eruption dynamics