Features of broken crystals reveal the fracturing and healing of basaltic magmas during explosive volcanic eruptions

The eruption of fragmental magma of basaltic composition is the most frequent type of explosive volcanism on Earth and in the Solar System, with eruptions spanning from centuries of persistent, weak explosions, through days to months of lava fountains and ash emissions, to rare, global-scale catastrophic events. The mechanism through which continuous magma fragments into volcanic particles is central in governing eruption dynamics and the ensuing hazards. However, the mechanism of fragmentation of basaltic magmas is still disputed, with both viscous and brittle mechanisms having been proposed for different eruptive styles. Here we carry out textural analysis of the products of ten eruptions from seven volcanoes by Scanning Electron Microscope. We find broken crystals surrounded by intact glass and other features that testify to the brittle fragmentation of basaltic magmas during explosive activity differing in style, intensity and magnitude. We then replicated the natural textures of broken crystals in laboratory experiments where variably crystallised basaltic melt was fragmented by rapid deformation. The experiments reveal that crystals are broken by the propagation of a network of fractures through magma, and that afterwards many of the fractures heal by viscous flow of the melt. Fracturing and healing affect gas mobility, stress distribution, and bubble and crystal size distributions in magma. Our results challenge the idea that the grain size distribution of basaltic eruption products reflects the density of the fractures that initially fragmented the magma. Unrecognised broken crystals and accompanying textures appear in previous literature covering many eruptions spanning Hawaiian lava fountains, catastrophic Plinian events, and more. We conclude that brittle fragmentation and subsequent healing are not specific to some eruption style or pyroclast type, but are ubiquitous factors controlling basaltic explosive volcanism.